UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA Irrigation Water Requirement Studies of Citrus and Avocado Trees in San Diego County, California, 1926 and 1927 S. H. BECKETT, HARRY F. BLANEY, and COLIN A. TAYLOR BULLETIN 489 April, 1930 UNIVERSITY OF CALIFORNIA PRINTING OFFICE BERKELEY, CALIFORNIA 1930 CONTENTS PAGE Introduction 3 General description of the area 3 Crops, soil types, and location of experimental plots 5 Methods of procedure 7 Selection of plots 7 Measurement of water 9 Soil sampling and computations 9 Meteorological observations 10 Use of water by citrus and avocado trees, October, 1925 to October, 1926 15 Use of water by citrus and avocado trees, March 15, to October 25, 1927 23 Wilkins plot (Eureka lemons), Escondido, season of 1927 24 Clemens plot (Eureka lemons), Fallbrook, season of 1927 27 Treat plot (Eureka lemons), Vista, season of 1927 29 Hinrichs plots (Valencia oranges), Escondido, season of 1927 33 Red Mountain plots (Navel oranges), season of 1927 39 Treat avocado plot, season of 1927 42 Summary of results, season of 1927 46 Efficiency of irrigation 48 Conclusions 49 Acknowledgments 51 Irrigation Water Requirement Studies of Citrus and Avocado Trees in San Diego County, California, 1926 and 1927 S. H. BECKETT,t HARRY F. BLANEY,2 and COLIN A. TAYLOR3 INTRODUCTION This bulletin presents the results of a two years' study dealing with the irrigation water requirements of citrus and avocado trees in the coastal plains area and interior valleys of northern San Diego County, California, and is the first report on an extended investi- gation of the economic field duty of water in southern California. The study has for its primary objects the determination of the quantity of irrigation water required for successful crop production in the principal irrigated areas of that section and the ascertainment of the safe economic water requirement under which irrigation dis- tricts and mutual water companies may be organized. The investigations of the economic field duty of water in southern California, of which this bulletin is the first of a series of reports, are conducted cooperatively by the Division of Irrigation Investigations and Practice of the College of Agriculture; the Division of Agricul- tural Engineering, Bureau of Public Roads, United States Depart- ment of Agriculture ; and the Division of Water Resources, California State Department of Public Works. During the conduct of the investigation the Director and several of the specialists of the Citrus Experiment Station at Riverside were frequently consulted. GENERAL DESCRIPTION OF THE AREA The part of San Diego County covered by this report includes the areas lying within the San Dieguito, San Luis Rey and Santa Marga- rita watersheds and along the intervening and adjacent minor coastal streams. In general, the area is mountainous and rugged, containing numerous sharply outlined interior valleys, fringed by a narrow coastal plain. Within this area, profitable crop production is almost entirely dependent upon irrigation. 1 Professor of Irrigation Investigations and Practice and Irrigation Engineer in the Experiment Station. 2 Irrigation Engineer in the Division of Agricultural Engineering, Bureau of Public Roads, United States Department of Agriculture. 3 Assistant Irrigation Engineer in the Division of Agricultural Engineering, Bureau of Public Roads, United States Department of Agriculture. 4 University of California — Experiment Station The present irrigation water supply is dependent upon storage on the three major rivers and upon a limited additional underground supply made available by pumping from the valleys and bottom lands bordering these rivers and the minor coastal streams. The problem of storage is complicated by wide fluctuations in seasonal precipita- tion which give rise to erratic, ' flashy, ' and widely fluctuating stream flow. Stored water is distributed through organized irrigation dis- tricts and mutual water companies ; the major portion of the pumping is done by private enterprise. The soils of the area have been classified 4 into three groups, which include: (1) the residual soils (loams, sandy loams, etc.) of the Sierra, Holland, and Aiken series derived through the disintegration or weathering in place, of consolidated rocks; (2) soils derived through the weathering and other modifications of old, unconsolidated, water- laid deposits, including the Montezuma adobes, the Kimball sandy loams, and the various types of the Las Flores, Placentia, and Redding series; (3) recent alluvial loams, sandy loams, and sands of the Yolo, Hanford, and Foster series, which are deposited at or near the present stream channels or as alluvial fans. The area as a whole includes some 870,000 acres, 230,000 of which may be classed as agricultural land. Of the latter approximately 100,000 acres is irrigable and of such quality as to warrant considera- tion in future development and utilization of the water supply. A survey of the area in 1928 showed a total of 17,400 acres under irrigation. The climate is featured by a rainless season extending from the middle of April to about the middle of November, the normal irri- gation season extending to about the middle of October. The winter rainfall averages 10 to 12 inches on the coastal plain area and 16 to 17 inches in the interior valleys. Moderate temperatures prevail in the summer, with occasional hot desert winds in the interior valleys. Table 1 shows a comparison of temperatures and rainfall at Ocean- side, Escondido, and Fallbrook for the years 1900 to 1924. The principal crops are citrus fruits, avocados, grapes, truck crops, and bulbs. Of the citrus fruits, lemons occupy most of the present acreage, although in recent plantings oranges predominate. The citrus fruits are mainly confined to the interior valley areas and rolling foothills adjacent to the coastal plain. Avocados are fast coming into prominence, especially on the newly developed areas of the coastal 4 Holmes, L. C. Reconnaissance soil survey of the San Diego region, Cali- fornia. Bureau of Soils, U. S. Dept. of Agricuiture. 1918. Bul. 489] Irrigation of Citrus and Avocado Trees plains and adjacent foothills. Truck crops (celery, tomatoes, beans, peas, etc.) and bulbs are confined principally to the coastal plain. The area is characterized by an abundance of irrigable land and a limited irrigation water supply. In its agricultural development the available water supply should be utilized to the maximum and the yearly water allowance should be based, as far as possible, on the actual requirements of the crops, with the irrigation water applied under reasonably efficient irrigation practice. TABLE 1 Summary of Temperature and Rainfall Records at Oceanside, Escondido, and Fallbrook, 1900-1924 Temperature (degrees Fahrenheit) Seasonal rai inches afall. 25-year average High- est re- corded Low- est re- corded Locality Maximum Minimum Mean Max- imum Min- imum Aver- Range Aver- age Range Aver- age Range Aver- age age, 25 years Oceanside 61 to 75 64 to 89 63.5 76.2 45 t,o 64 35 to 56 53.6 45 1 54 to 69 50 to 72 61.4 60.8 106 113 21 13 19.8 28.4 28.6 5.7 . 7.9 8.7 12.9 16.4 17.1 CROPS, SOIL TYPES, AND LOCATION OF EXPERIMENTAL PLOTS During the 1926 season, the crops selected were oranges, lemons, and avocados. In 1927, the observations were continued with these same crops on a more intensive scale. With the exception of truck crops and grapes these are the predominating irrigated crops of the area. Deciduous fruits, because of their smaller returns, are not favored. Alfalfa, field crops, and dairying occupy a very limited acreage. The predominating soils of agricultural value within the area are the residual sandy loams and loams of the Sierra, Holland, and Aiken series, and the Kimball and Placentia series derived through the weathering and uplifting of the old water-laid deposits, The investi- gations were therefore confined to these types. In order to obtain a wide range in climatic conditions, experi- mental areas were selected in the Fallbrook, Escondido, and Vista localities. The first two are representative of conditions found in the interior valleys, while the Vista section is typical of the rolling foothill areas adjacent to the coastal plain. Figure 1 shows the general location of the 1926 and 1927 experi- mental fields. University of California — Experiment Station Fa I I brook trrigatio/. District INDEX Or FIELDS TkllbrooJ 3 o FARM 1 RED MOUNTA/N 2 CLEMENS 3 MZCORMAC 4 BOWMAN at MARSH - ** CRUICKSNANK 6 TREAT 7 TREAT WHETSTONE 9 HINRICHS 10 GREAVES 11 WILKINS CROP SEASON ORANGES 1926-27 LEMONS /926~27 AVOCADOS /926 LEMONS 1926 LEMONS /926 LEMONS /927 AVOCADOS /927 LEMONS /926 ORANCES 1926-27 ORANGES 1926 LEMONS /927 Y 2£ Oceans/de Mutual Hbter Co. Arec l/ista irrigation District cscondido Mutual \WjterCb. s' f2 o \ Area ■ J u icoivdi sJan Dieguito irrigation District *(& f Ror?c/?o \s5bnto fe & \Lcike Hodges Santo re 'irrigation District/ Sorreivto Fig. 1. — Northern San Diego County, showing location of experimental fields, seasons of 1926 and 1927. Bul. 489] Irrigation of Citrus and Avocado Trees METHODS OF PROCEDURE Typical orchards were selected in which the quantity of water used, the frequency of irrigation, the method of irrigation, and other factors represented standard practice of the locality. The quantity of water delivered to each farm at each irrigation was measured, and by frequent sampling of the soil for moisture content in the selected plots, a full season's record of the soil-moisture fluctuations and the rate of moisture extraction from the soil at various depths was obtained. The records of water measurements and of soil-moisture determi- nations were used as a. basis in determining the following for each grove and for each plot: (1) the rate of use of water and the soil depths from which this water was taken ; (2) the monthly and seasonal use of water by the crop under observation; (3) the percentage of water applied which could be accounted for in soil-moisture increase ; (4) the monthly and seasonal irrigation requirements; and (5) the required frequency of irrigation, and depth of water to be applied at each irrigation on various soil types. Selection of Plots. — The following factors governed the selection of field plots: (1) evenness of topography and uniformity of soil; (2) general condition of the grove, and uniformity of trees within the plot; (3) conditions of water delivery, especially facilities for meas- urement; (4) absence of underground water table; (5) willingness of the water-user to cooperate. It was originally planned that the area of each of the experimental plots should be one-quarter acre, and should include eight permanently located points of soil sampling within each plot, with such additional sampling as might be necessary to obtain a complete seasonal record of soil moisture fluctuations, The analysis of the data obtained by this method in 1926 showed certain inconsistencies in the soil-moisture determinations. These were attributed to variations in soil type within the quarter-acre plots and to the fact that the samples taken were insufficient in number to counteract these variations. As a result, at the beginning of the season of 1927 the area of each plot was reduced to one-fortieth acre, with from 17 to 30 points of sampling within the plot. In each case, the experimental plot was chosen after a very careful examination of the soil, uniformity of depth and texture being the determining factors. Moisture-equivalent determinations were used as a final check on the uniformity of the texture. Figure 2 shows the general arrangement of the plots and points of sampling within the plots during 1926 and 1927. University of California — Experiment Station Q.7 (J: :b: •j _________ furrows*? , fi ^l4-- / S—/G /1 -^/7—/6—/9tr^20--Z/-e3 ? ?^;SL3—34 30 y -/?<9®- -27®- ---2S< 31 29 26 -/o- ^-- /'---/1k--- / £-?, — '£ m Season of 1926 28 29 30 26 i f -^r. 2S. 27 K T3 .'.i «- 4> " li# ; 1 .-jT Sri. V^f __» /*: = -jw?) was used. In this formula P represents the moisture loss in percentage for the period of time under consideration; v is the apparent specific gravity (often referred to as volume weight) ; d is the depth of soil in inches; and D is the equivalent depth of water in inches. Bul. 489] Irrigation of Citrus and Avocado Trees 11 a 2 o o p rv a a a a M j . CB o* w ! to — O0>-'0000^00>-. on : Ol Ol Ol Ol Ol Oh p WOl00U0J00l|i*O>CD co : >*•. tOOOtocOOlOOhP-OstOOO H-» : Go Ml^Ml^WNlOltSMH Oi : Ol Ol Ol Ol Ol 4~ 4- tO O0 Ol OOtOOOGOOO^JCOGOCO On ; en tO>-»OlcD0000COtOCOO0 oo ! ^ — 'OOOOtOtOOtOt-'^ Oi : Ol Ol Ol Ol Ol tO -4 Ol OS OS p ■O^OOtOCOCOtOCOCO*- 00 : 00 -o>£».osococooioococo a* >*>. : Oi t-^OCOH-OOOh-OO Oi : Ol OS Ol Ol Ol OS l— > OO CO CO > OCOOltOOO^I^OCOOS t^ : oo Osi—^OOOOcOOOtOh- 00 j Ol rf*- Ol o to & S n> -4 : OS P a oooooooo OS : £73 Oi Oj Oj 0-> % 2 S ; £2 oooosoooostooo COOltOtOtOl-'tOK- p << *>■ CD a GO : OO M i tO o a i^aiKoioisooi S3 : a p a i a'' vl S S Ol Ol «H 3 O O tO 00 00 a a- era 3 m Ol -J H * oo 4- ^ — , 00 »*- to OS Ol oo H» "< CD a o ~J -J -J OS OS !> — ' l—i to CO CO <£ g 2 **i Oi ^) to i£ CO to o Ol to — OS »**■ rf^ -4 p cr f> OS OS Ol o o o os os a SP 31 Oi OO 0O N 00 OS 00 a W Ol H o o Ol o <| OS CO to CO =*• r 1- w (n oi iti O0000000>*»000 OS OS OS OS OS OS OS o o 00 Ol o Ol o< tt) (D M aoooiOMoioiMttS ■*- to H K W Ji « ^ OO tOOtOi— ►— >— O i-» O O OS OS Ol OS On OS Oi 3 o >3COtOtOrf»OSCOtOCOCO to co OOOOOOOOSh-OOCOOSrf^ o o S3 tO »-» Ol CO : to Wh-bOH-tOtf^h-h-ik— *- Ol Ol Jl Jk Ol Ol Ol oo OOOSOSOOOSOO^I^I-vI O CO ^1 OS O0 GO I— > O I-* Ol ■"■' to Ol Si 00 (*«. h- -s] ^ MOlNHNilkPOllO* SB : oo £IOlt-OMOS|tk*.OOM 00 ^1 Ol 00 >*»• oo pa 12 University of California — Experiment Station In table 2, a comparison is made between the meteorological obser- vations obtained at Escondido, Vista, and Fallbrook during 1926 and 1927, and the mean values obtained from a 25-year record of adjacent observation stations. Table 2 indicates that during the 1926 and 1927 summer periods the average temperatures showed but a slight departure from the mean of the 25-year period. The two winter periods were abnormal in that the rainfall of each was more than 25 per cent excessive. In each year, fortunately, the rainfall was so distributed that at the beginning of the summer period the soil in all plots was thoroughly moistened to the full depth of root penetration of the trees. Notwith- standing the heavy seasonal rainfall of 1925-26, the distribution was such that a winter irrigation was necessary in January, 1926. 1 10 ?0 31 BiOaOiOJIOJOJIlUfflJI 10 20 3 D K) » 31 10 20 31 10 ffl 30 10 JO 31 10 20 30 10 ao 31 Mill 1! '-.-- +J - -1- -- - / 1 1 H-1L — 1 — — C L I, IJU iL ± _ 1 i - . . 1 t"-"~ -"-I---T- ~:::::::::i::::: t _:i t 2 i _T _L _[_ I i 1 /--=d- — -h- w fr--z-±. 130 Days 1 .-J| $ l i-flLlj , 1 1 Ji 1 lllllli 1 I $ ■*$---- ----------- :i::::::::: :::::: $±__ !_,__, I «?<5 COy.5 J C o_ jlJl.jk _1 ^l^J Mill II . .. i , c 2 J 216 Days 1- ,J\ J| - o :::::-::::::i::il £::::::::: :::::: lllllli .. L , 1. Fig. 3. — Occurrence of rainfall and periods between effective rains, Escondido, California, 1920 to 1924. Shaded blocks show the amount of daily rainfall, unshaded blocks the accumulated rainfall for each storm. Rainfall records, when plotted as in figure 3, clearly show the wide variation in seasonal rainfall and in its monthly distribution. In this 5-year period the interval between the last effective rain 7 in the spring and the first effective rain in the fall ranged from 130 to 296 days. Over a 25-year period, as shown in table 3, the similar interval ranged from a maximum of 332 days to a minimum of 130 days, averaging 236 days. 7 An effective rain is here considered as being of such an amount that it materially adds to the moisture supply in the soil area which sustains tree growth. Bul. 489 Irrigation of Citrus and Avocado Trees 13 Table 3 shows that during this 25-year period, where winter cover crops were grown, there were 6 years in which two winter irrigations would have been needed, 8 years in which one winter irrigation was necessary, and 11 years in which no winter irrigation was required. TABLE 3 Occurrence of Effective Rains at Escondido, 1903-1927, and Number of Irrigations which Theoretically Would Have Been Needed in Good Practice, If Cover Crops Had Been Grown Dates of Period between effective rains* Number of irrigations neededt Year Last effective spring rain First effective winter rain Summer Winter Total 1903 293 266 181 207 302 263 228 285 332 244 232 223 212 209 268 238$ 215 296 130 256 226 216 163 228 196 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2 3 3 3 2 3 3 2 1 2 1 2 2 1 1 2 2 1 1 1 1 5 1904 March 29 . 4 1905 May 8 3 1906 April 29 3 1907 March 27 5 1908 April 23.. . 4 1909 March 28 . 3 1910 March 28 5 1911 April 11 March 1 5 1912 May 9 4 1913 3 1914 March 30 . 3 1915 3 1916 March 5 April 16 3 1917 4 1918 March 20 5 1919 March 22 October 23 .... 3 1920 March 27 5 1921 April 23 3 1922 March 18 4 1923 April 18 3 1924 April 7 4 1925 April 23 October 3 3 1926 April 10 3 1927 April 12.. . October 25 3 April 9 236 * Eainfall of one-half inch or more is considered as being effective. t For soils of 3 feet or more in depth, each irrigation being sufficient to moisten the soil to the depth of root penetration. t No rain December 21, 1918, to March 13, 1919. Similar information obtained from a 45-year record at Fallbrook, beginning with 1875, shows the average interval between the last effective rain in the spring and the first effective rain in the fall to have been 231 days, extending from April 9 to November 25. This interval ranged from a maximum of 309 days in 1876 to a minimum of 129 days in 1921. During this 45-year period, if winter cover crops had been grown, there were 20 years in which no winter irrigation would be needed, 15 years requiring one winter irrigation, and 10 years requiring two. 14 University of California — Experiment Station «M k fl a CO .2 .2 o 3 o 'E 'E 'E 'E 'E 'E 'E 'E 'E Method water me urement plots CO CO CO CO CO CO CO CO ~co a a a a a a a a a 3 3 a 3 3 3 3 3 3 0000000 >>>>>>> > > -d fl "o « 03 rZ bo bi bfi b£ ■ i T5-J3 c 3 3 £ IS fe 3 £ £3 ^33 1 fe co E h "C u E "E 5 "E 'E E ° 3 E 3a33a3aa %-, 3 hcotafecofecoco fe 6 6 6 j>> O O ; O "a t- (- tn ■p (1) UJ -P CO a CO -"-= -^ 3 "3 'E .2 'E .=* -^ 'E -S S. <£. -s 3 * a _o 3 3 3 *o 03 ■•5 ^ g '43 03 03 » co bO bO O O bfi 3 [rri ing [rri did did [rri mg ing did o ^a^cc^aa c co " 3 ^ B 8 " 3 ' g '£■ p* '£■ H W j> p* & H CO v , erag istur uiva ent O»0c0t^W5T-cOC OOOlNmMiflPSCC > o n*— ' ^ a « Mgi tonn^soioc CM cs'S > ^> lO^lOUJlfllOiOC- lO r-i .5? 09 bfi S. CO bfi j OOUiOOiOOC Avera soil dept feet lOio-tiiOkC^cciir «« 00«0»00»000 CM ■*^N(N'*miOTt a : a : a : a a a § § § £ a c3 o3 c3 '"^ ■""' •—■' c 03 CO a >> ,0 .2 >> >> >> c ~ 7* . 13 TJ "d 7 >i >l >> C rj r- > g -d -d "d 5 5 5 t -d cj fl C C «i 05 ai C 3 5 S g ■ s •§ •§ ■§ a B g g g cS eS (S » ! 03 J CO ! 03 T^coCocjOOOq 2 -. t - 1 C dJSno*^^ 2 i-5 » 13 .a g £.231 3 03 >^>HH> fet ' H : ^ : c : 03 ■ -a a : 09 : M : i J CO : '3 : 1- ': ? 1 S II! i 3 hetsi leme arsh- inric reave owm edM j r3 g j < $ a ; c a 5 a H ^ 5 s: Bul. 489] Irrigation of Citrus and Avocado Trees 15 USE OF WATER BY CITRUS AND AVOCADO TREES, OCTOBER, 1925, TO OCTOBER, 1926 During the winter 1925-1926, eight experimental plots were selected for study. A thorough soil sampling of each plot was made on October 15, a second on January 15, and a final one on April 1, just prior to the heavy rainfall of April 2 to 8 of that year. In order to make allowance for evaporation losses, rainfall of less than 0.5 inch was not considered in the final computation of winter use. This figure was obtained by observing the depth of moisture penetration after each rain and observing the rapidity of loss by evaporation from the wet soil surface. Table 4 shows the main physical features of each of these experi- mental plots. Tables 5 to 21, inclusive, show the average moisture content at each sampling, the dates and quantities of water applied at each irrigation, the calculated use of water, and the total seasonal require- ment for the period from October, 1925, to October, 1926. TABLE 5 Average Moisture Content and Dates and Amount of Irrigation, Whetstone Plot (Eureka Lemons), Vista, California, October 12, 1925, to October 13, 1926 Dates of sampling Average moisture content of the soil, per cent First foot Second foot Third foot Fourth foot Fifth foot Dates of irrigation Amount of irrigation water applied, acre-inches per acre Whole plot 1925 October 12 1926 January 9 March 22 April 24 May 24 June 25 July 2 August 2 August 31 September 8.... October 13 June 25 October 5 14 1 14.1 14.8 15.1 9.9 12.6 12.6 12.7 13.3 13.3 16.1 17.1 17.1 14.9 16.7 17.5 13.7 15.6 16.9 13.0 12.1 14.4 17.1 16.7 15.9 January 9 0.90 Irrigated section 15 6 8.6 7.5 14.7 7.9 14.5 17.2 17.9 11.5 14.8 16.6 11.5 14.1 16.2 13.2 15.1 17.1 11.4 14 2 15.8 June 25, '29 August 31 0.84 2.50 Unirrigated section 9.0 6.2 13.7 9.8 14.5 12.4 15.9 14.4 15.8 14.1 16 University of California — Experiment Station TABLE 6 Computed Seasonal Use of Water, Whetstone Plot (Eureka Lemons), Vista, California, October, 15, 1925, to October, 15, 1926 Winter use of water by trees and cover crop Summer use of water by trees Soil- Effec- mois- Soil Equiv- tive ture Total mois- alent rainfall loss or use, ture loss in Num- plus gain, acre- Num- loss, 30 days, Dates ber of irriga- acre- inches Dates ber of acre- acre- days tion, inches per days inches inches inches per acre* acre per acre per acre Oct. 15 to Jan. 9 86 1.32 -2.85 4.17 Apr. 1 to May 24 54 1.84 1 02 Oct. 15 to Jan. 15 92 4 46f 32 1.05 0.98 Jan. 10 to Mar. 22.... 72 5 51 +1 57 3 94 June 26 to July 31 36 1.82 1.51 Jan. 16 to Mar. 31.... 75 4 llf Aug. 1 to Aug. 31 Sept. 1 to Oct. 15 31 0.52 0.50J Oct. 15 to Mar. 31.... 167 8 57t 45 1.85 1.23 Apr. 1 to Oct. 15 198 7 08 * Minus (— ) sign indicates soil-moisture loss during interval. Plus (+) sign indicates soil-moisture gain during interval. t Values obtained by interpolation. % Trees suffered from drought during this period. TABLE 7 Average Moisture Content and Dates and Amount of Irrigation, Marsh- Cruickshank Plot (Eureka Lemons), Vista, California, October 12, 1925, to October 22, 1926 Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation Dates of sampling First foot Second foot Third foot Fourth foot water applied, acre-inches per acre Whole plot 1925 October 12 11.2 4.2 8.8 13.8 10.2 9.2 8.0 12.1 4.8 9.4 14 5 12 4 11 5 10.9 9.6 5.4 9.3 13.6 12 1 12 5 11 9 10 3 5.8 1926 1.25 April 13 Mav 20 June 2-3 1.25 July 12 July 12-13 Irrigated section July 15 13.9 8.7 12.2 7.8 10.1 6.4 12.7 12.0 12.6 10.7 10.3 8.6 12.0 1.30 August 17-18 1.14 September 10 September 27 October 22 Sept. 22-23 0.83 Unirrigated section July 12 6 8 4.7 10 1 7.4 10.8 7.8 October 2 Bul. 489] Irrigation of Citrus and Avocado Trees 17 TABLE 8 Computed Seasonal Use of Water,, Marsh-Cruickshank Plot (Eureka Lemons), Vista, California, October 15, 1925, to October, 15, 1926 Winter use of water by trees and cover crop (grass and weeds) Summer use of water by trees Dates Num- ber of days Effec- tive rainfall plus irriga- tion, inches Soil mois- ture loss or gain, acre- inches per acre Total use, acre- inches per acre Dates Num- ber of days Soil mois- ture loss, acre- inches per acre Equiv- alent loss in 30 days, acre- inches per acre Oct. 15 to Jan. 15. 92 1.32 -4.05 5.37 April 1 to June 1 62 2.25 1 08 Jan. 16 to Mar. 25. 69 5.86 +2.44 3.42 June 2 to July 12 41 1 04 0.76 Jan. 16 to Mar. 31. 75 3.74* July 13 to Aug. 17 36 1.07 0.89 Oct. 15 to Mai. 31. 167 9.11* Aug. 18 to Sept. 22.... 36 1.32 1.10 Sept. 23 to Oct. 15 23 0.72 0.94 April 1 to Oct. 15 198 6 40 Values obtained by interpolation. TABLE 9 Average Moisture Content and Dates and Amount of Irrigation, Clemens Plot (Eureka Lemons), Fallbrook, California, October 13, 1925, to October. 4, 1926 Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation Dates of sampling First foot Second foot Third foot Fourth foot Fifth foot water applied, acre-inches per acre 1925 October 13 1926 January 8 March 20 9.4 4.8 5.8 8.8 5 1 6.2 10.3 8.1 7.9 11.0 8.0 7.4 9.4 10.4 10.9 12.4 10 7 8.4 10.1 9.2 12.4 12.5 11.3 9.2 8 9 8.4 10.7 12.2 10.0 Nov. 12, 1925 Jan. 26, 1926 1.29 1 54 April 28 June 23 June 24 2 05 October 4 July 31 1 86 TABLE 10 Computed Winter, Use of Water, by Trees and Cover Crop (Purple Vetch), Clemens Plot (Eureka Lemons), Fallbrook, California, October 15, 1925, to March 31, 1926 Dates Number of days Effective rainfall plus irrigation, inches Soil moisture loss or gain, acre-inches per acre Total use, acre-inches per acre October 13 to January 8 88 92 71 75 167 3.33* — 1 35 4.68 October 15 to January 15 4.90f January 9 to March 20 6.80 +1.38 5.42 January 16 to March 31 5.73f October 15 to March 31 10 63t Includes one rain of less than 0.5 inch. f Values obtained by interpolation. 18 University of California — Experiment Station TABLE 11 Average Moisture Content and Dates and Amount of Irrigation, Hinrichs Plot (Valencia Oranges), Escondido, California, October 19, 1925, to October 28, 1926 Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation Dates of sampling First foot Second foot Third foot Fourth foot water applied, acre-inches per acre Whole plot 1925 October 19 10.3 4.7 7.6 12.0 7.5 13.5 7.6 11 4 14.1 12.0 13.6 10.7 12.1 14.1 12 7 12 9 11 5 13.0 13.5 11 4 1926 2 32 March 23 April 24 June 10-12 2«34 Irrigated section 13.3 6.4 12 5 5.8 11 8 5.8 15 11.4 12.9 10.0 13.7 9.8 14 4 13.3 14.2 13.4 14.8 12.2 12 2 13.2 13 6 11 9 13 9 13.0 July 20 July 21-23 1.80 July 26 September 2-4 2 01 September 8 October 28 Unirrigated section 8.4 5 12.9 7.8 12.8 11.1 12.8 12.3 October 12 TABLE 12 Computed Seasonal Use of Water, Hinrichs Plot (Valencia Oranges), Escondido, California, October 15, 1925, to October 15, 1926 Winter use of water by trees and cover crop (purple vetch) Summer use of water by trees Soil Effec- mois- Soil Equiv- tive ture Total mois- alent Num- rainfall loss or use, Num- ture loss in Dates ber of plus gain, acre- Dates ber of loss, 30 days, days irriga- acre- inches days acre- acre- tion, inches per inches inches inches per acre acre per acre per acre Oct. 19 to Jan. 12.. 85 1.66 -3 21 4.87 April 1 to June 9 70 3 01 1.28 Oct 15 to Jan. 15.. 92 5.26* June 10 to July 20 41 1 61 1 20 Jan. 13 to Mar. 23.. 69 7.83 +2.03 5.80 July 21 to Sept. 1 43 1.63 1.14 Jan 16 to Mar. 31.. 75 6.22* Sept. 2 to Oct. 15 44 1 39 0.95 Oct 15 to Mar 31 167 11 48* April 1 to Oct. 15 198 7 64 Values obtained by interpolation. Bul. 489] Irrigation of Citrus and Avocado Trees 19 TABLE 13 Average Moisture Content and Dates and Amount of Irrigation, Ked Mountain Plots (Navel Oranges), Fallbrook, California, April 15 to October 9, 1926 Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation Dates of sampling First foot Second foot Third foot Fourth foot Fifth foot waterapplied, acre-inches per acre Trees 30 years old April 15 13 8 7.2 6.6 6.0 9 5 9 13.9 10.3 9.0 7.4 10.0 6.6 14 9 12.1 10.6 8.9 10.1 7.6 14.3 12.3 12.0 10.3 10.5 8.9 14 1 13.0 12.6 11.5 May 25 June 23 July 22 July 26-28 2 62 August 17 October 9 October 9 3 40 Trees 6 y8ars old April 15 13.2 7.8 7.8 6.5 9.3 6.5 14.8 11.6 10.6 9.7 11.8 7.8 14.9 13.1 11.6 11.3 12.6 9.0 14.2 13.7 12.9 11.2 12.2 11.0 14 1 12.0 10.8 9.9 May 25 June 23 July 22 July 26 2 62 October 9 3 40 TABLE 14 Computed Seasonal Use of Water, Eed Mountain Plots (Navel Oranges), Fallbrook, California, April 1 to October 15, 1926 Summer use of water by old trees, acre-inches per acre Summer use of water by young trees, acre-inches per acre Dates Num- ber of days Soil- moisture loss Equiva- lent loss in 30 days Dates Num- ber of days Soil- moisture loss Equiva- lent loss in 30 days April 1 to June 23 June 24 to July 26 July 27 to Oct. 15 April 1 to Oct. 15 "84 33 81 188 4.21 1.39 3.63 9 23 1.50 1.26 1.35 April 1 to June 23 June 24 to July 26 July 27 to Oct. 15 April 1 to Oct. 15 84 33 81 198 3.55 1.07 3.27 7 89 1.27 0.98 1.21 20 University of California — Experiment Station TABLE 15 Average Moisture Content and Dates and Amount of Irrigation, Greaves Plot (Valencia Oranges), Escondido, California, October 19, 1925, to October 29, 1926 Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation Dates of sampling First foot Second foot Third foot Fourth foot Fifth foot waterapplied, acre-inches per acre 1925 October 19 12 5 7.2 5 6 12.8 13.9 8.5 9.2 12.3 13 7 10 9 10.9 11.1 13.2 11.0 11.0 12.3 11.6 10.1 10.0 11.2 1926 January 16 1 42 March 23 April 14 May 26 1 07 July 2 2 15 1 48 October 29 9.1 9.2 9.5 10 7 110 September 20 2 36 TABLE 16 Computed Winter Use of Water by Trees and Cover Crop (Purple Vetch). Greaves Plot (Navel Oranges), Escondido, California, October 15, 1925, to April 1, 1926 Dates Number of days Effective rainfall plus irrigation, inches Soil moisture loss or gain, acre-inches per acre Total use, acre-inches per acre 89 92 79 75 167 1 66 -3.38 5.04 5 21* 6.93 —0.21 7 14 6 77* 11 98* * Values obtained by interpolation. TABLE 17 Average Moisture Content and Computed Use of Water, by Trees, Wilson Plot (Eureka Lemons), Escondido, California, August 11 to October 13, 1926 Average moisture content of the soil, per cent Use of water by trees Dates of sampling First foot Second foot Third foot Dates Num- ber of days Soil moisture loss, acre- inches per acre Equiv- alent loss in 30 days, acre- inches per acre 12.9 6.5 5.9 14 4 8.8 7.0 9.8 6.3 6.1 13.0 9.8 7.3 8.0 5.8 5.5 11.2 9.3 7.3 Aug. 11 to Aug. 23 Aug. 24 to Sept. 7 13 15 14 17 1.10 0.21 0.99 0.66 2.54 0.42* September 8 September 15. ...... September 29 Sept. 15 to Sept. 28 Sept. 29 to Oct. 15 2.11 1.16f * Heavy deficiency in soil moisture during this period. t Deficiency in soil moisture during last part of this period. Bul. 489] Irrigation of Citrus and Avocado Trees 21 TABLE 18 Average Moisture Content and Dates and Amount of Irrigation, McCormac Plot (Avocados), Fallbrook, California, October 14, 1925, to October, 6, 1926 Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation Dates of sampling First foot Second foot Third foot Fourth foot waterapplied, acre-inches per acre 1925 12 2 8.1 9 8 13 4 9.1 7.3 7 3 6.7 5.4 11.3 9.5 7.2 9 2 7.9 10.6 12.0 9 5 7.6 7.7 7.0 5.6 10.7 9.4 7.4 7.5 7.2 11.8 12.4 10.0 8.5 8.0 7.3 5.2 9.9 8.9 7.2 7.5 6.7 10.4 10.3 8.8 8.7 8.0 7.6 5.9 9.8 8.6 7.2 1926 1 66 March 16 March 16 1 65 April 15 May 13 1 94 June 29 June 30 1 92 July 19 . . July 20 1 39 August 23-27 7.90 October 6 October 8-9 2.64 TABLE 19 Computed Seasonal Use of Water, McCormac Plot (Avocados), Fallbrook, California, October 15, 1925, to October 15, 1926 Winter use of water by trees (no cover crop) Summer use of water by trees Soil Effec- mois- Soil Equiv- tive ture Total mois- alent Num- rainfall loss or use. Num- ture loss in Dates ber of plus gain, acre- Dates ber of loss, 30 days, days irriga- acre- inches days acre- acre- tion, inches per inches inches inches per acre acre per acre per acre Oct. 14 to Jan. 8 ... 87 1.72* — 1.26 2.98 April 1 to May 13 43 2.43 1.70 Oct. 15 to Jan. 15 .. 92 3.18J 32 1 02 0.96f 1.92 Jan. 9 to Mar. 16 .... 67 6.83 2.07 4.76 June 15 to June 29 15 0.96 Jan 16 to Mar. 31 75 5.34J June 30 to July 19 .... 20 1 16 1.74f 1.39t 1.86 Oct 15 to Mar. 31 167 8 52J July 20 to Aug. 23 .... Aug. 24 to Sept. 13 35 1 62 21 1.31 Sept. 14 to Oct. 15 32 1.70 1.59 April 1 to Oct. 15 198 10 20 * Includes one rain of less than 0.5 inch. t Deficiency in soil moisture during this period. X Values obtained by interpolation. 22 University of California — Experiment Station TABLE 20 Summary of Winter Use of Water, by Trees and Cover Crops, October 15, 1925, to April 1, 1926 Crop Cover crop Use of water, acre-inches per acre Farm Oct. 15 to Jan. 15 Jan. 16 to April 1 Oct. 15 to April 1 Purple vetch (medium) Weeds and grass (medium).... Purple vetch (medium) Purple vetch (medium) 4.46 5.37 4.95 5.26 5.21 4.11 3.74 5.73 6.22 6.77 6.88 3.34 8 57 9 11 10 68 Oranges 11 48 11 98 Red Mountain Oranges Avocados 3.18 8 52 TABLE 21 Summary of Probable Use of Water and Estimated Irrigation Eequirements of Eight Typical San Diego County Groves from April 1 to October 15, 1926 Farm Wilson Whetstone Marsh-Cruickshank Clemens Hinrichs Red Mt. Plot A Red Mt. Plot B McCormac Location Escondido Vista Vista Fallbrook. Escondido Fallbrook . Fallbrook . Fallbrook. Crop Lemons.. Lemons... Lemons... Lemons... Oranges... Oranges... Oranges... Avocados Age of trees, years 20+ 11 12 10 6 30 Rel- ative size of trees, per cent* 100 67 50 60 50 90 50 80 Probable use of water for each 30 days, acre-inches per acre Apr. 1 to June 30 2 00 1.00 1.00 1.10 1.20 2.00 1.10 1.80 July 1 to Aug. 31 2.50 1.50 1.20 1.30 1 20 2.25J 1.30J 2.00J Sept. 1 to Oct. 15 2.20 1 25 1.10 1.20 1 00 2.00J 1.20 1.75 Probable total use of water, April 1 to October 15, acre-inches per acre 14.30 7.90 7.00 7.70 7.50 13.50 7.70 12.00 Farm Wilson Whetstone Marsh-Cruickshank Clemens Hinrichs Red Mt. Plot A Red Mt. Plot B McCormac Carried over from winter rainfall, inches 2 10 2.00 1.80 2.00 2.40 3.50 2.50 3.00 To be sup- plied by irrigation, acre-inches per acre 12 20 5.90 5.20 5.70 5.10 10.00 5.20 9.00 Seasonal Re- irrigation quired require- number ment of acre- irriga- inches tions per acref 20.0 6 10 4 8.5 4 9.5 3 8.5 3 16.5 3 8.5 3 15 3 Average quantity of water required at each irrigation, acre-inches per acre Interval between irriga- tions, days 25 to 30 35 35 40 45 45 45 45 Estimated seasonal require- ment at maturity, acre-inches per acre * With reference to probable size at maturity. This is based on a comparison of the averages of the outside areas of the trees included in each plot. t On basis of 60 per cent efficiency in irrigation. % Estimated values. Bul. 489 J Irrigation op Citrus and Avocado Trees 23 Table 22 shows the apparent root development in the various soil types at depths to which soil samples were taken in the different groves. TABLE 22 Apparent Boot Development in Various Soil Depths as Determined prom the Bate of Water Extraction Crop Soil type Depth of soil, feet Apparent root development,* in percent Farm First foot Second foot Third foot F'rth foot Fifth foot Sierra sandy loam 3.0 5.0 3.0 4.0 5.0 4 4.0 53 55 60 48 35 50 36 29 21 24 28 28 30 24 18 13 16 13 19 16 22 Whetstone 6 11 13 4 18 5 Marsh-Cruickshank. . . Sierra sandy loam Sierra sandy loam Holland sandy loam... Holland sandy loam.. Sierra sandy loam Hinrichs Red Mt. Plot A 5 Red Mt. Plot B Avocados * It is assumed that the rate of soil-moisture loss from the various soil depths is a measure of the root development at those depths. The intervals during which soil moisture was available at all times are the only ones considered in obtaining these values. As an example, in the Wilson plot, during the intervals of August 11 to 24, and September 15 to 29, the total soil moisture taken from the first, second, and third feet was 12.0, 6.7, and 4.1 per cent, indicating an apparent root activity of 53 per cent in the first foot, 29 per cent in the second foot, and 18 per cent in the third foot. USE OF WATER BY CITRUS AND AVOCADO TREES, MARCH 15 TO OCTOBER 25, 1927 At the beginning of the season of 1927, it was decided to reduce the area of the plots and materially increase the number of samples. Each plot was confined to an area cornered by four adjacent trees of uniform size, located at points in the groves where the soil type and depth were as nearly uniform as possible, and situated where con- ditions were favorable for uniform applications of water. Where the furrow method of irrigation was used and where only a portion of the soil mass was wet by irrigation, the permanent points of sampling were located as shown in figure 2. Where the whole soil mass was moistened, either by furrow irrigation or by sprinkling, holes 14, 20, 22, 25, and 28 were eliminated, leaving 25 holes from which average moisture percentages and losses might be determined. The results obtained in 1926 also show that because of a shortage of water, the trees in each plot suffered from drought at some time during the season; hence the quantity of water used by the trees depended upon the quantity of water available, and was not a true measure of what the use would have been had water been available in the soil at all times. 24 University of California — Experiment Station The plots selected for study in 1927 were so located that, in so far as possible, soil-moisture deficiencies would be avoided throughout the season. The following is a brief description of each grove, with tables and diagrams showing the results obtained: Tables 23 to 38 summarize the results of soil sampling and the computations of the quantities of water used in the intervals between irrigations. Figures 4 to 15 inclusive show, by means of diagrams, the seasonal variation in moisture content and the seasonal consump- tive use of water in acre-inches per acre for each month during the irrigation season. A final summary of the use of water by months and the estimated irrigation requirements for each grove is contained in tables 39 and 41. Wilkins Plot (Eureka Lemons), Escondido, Season of 1927. — This grove is located 2.5 miles east of Escondido, and contains 4.85 acres of mature trees. The trees are fully developed, the grove is well cared for, and the use of water should represent the requirement of a mature grove in this locality. The soil is classed as a Sierra sandy loam, ranging in depth from 2.5 to 5 feet. Irrigation water is obtained from the Escondido Mutual Water Company, being distributed through a concrete pipe line and measured by means of a weir. The trees are planted on the square with a spacing of 24 feet. Furrow irrigation is used with 5 furrows spaced 3.5 feet apart, the outer furrows being 5 feet from the tree row. At the time of the first sampling (March 15), determinations of apparent specific gravity were made by the soil-tube method. Samples were also taken at this time from which moisture-equivalent values were determined. These gave the following results: Depth of Soil ■■? -■ ■■ First foot Second foot Third foot Fourth foot Average 1.52 9.9 1 45 9.9 1.58 11 3 1.68 13.1 1 56 110 Bul. 489] Irrigation of Citrus and Avocado Trees 25 TABLE 23 Kesults of Soil Sampling and Irrigation Data, Welkins Plot (Eureka Lemons), Escondido, California, Season of 1927 Dates of sampling March 15 April 6 April 19 May 11 May 31 June 9 June 21 July 7 July 22 August 5 August 30 September 10. September 22 September 30 October 11 October 19 October 25 Average moisture content of the soil, per cent First foot Second foot Third foot Fourth foot 12.9 13.3 13.3 14.1 11.7 11.1 12.3 13.9 11 3 10.5 11.9 13.3 9.5 9.1 10.5 12.7 7.2 7.5 9.3 12.0 6.5 6.8 8.9 11.7 11 1 12.1 12.8 13.9 8.6 9.5 11.1 13.3 6.9 7.3 9.1 12.4 11.5 12.2 13.0 13.8 7.3 7.9 9.6 12.6 6.0 6.4 9.0 12.2 12.0 12.7 13.1 14.1 10.7 11 5 12.7 13.9 8.7 9.1 11.2 13.0 8.0 8.3 10.6 12.9 6.9 7.6 9.9 12.2 Dates of irrigation June 9 and 10. July 24 to 27.. Sept. 11 to 14.. Amount of irrigation water applied, acre-inches per acre 40 40 Per cent of soil mass moistened 100 100 IS 10 5 o i p-§lli==iii=-iE=i==§l==i liM "in rrn -1=E5=-==;=E==5S==E-| Zfis^ ^J, _±^__ — =-=11== — i iVr 1 \°~ r—-- O , ..i 10 | ' s — ■ o i M 4-H — ■ ; E .33 — | mm 1 ml 1 i i TrlUL 111 rrrU-LL I -*•-=£ -||| ■*-M- 1 1 TTwU 1 1 1 1 II 1* 10 20 31 10 20 30 March April 10 20 Moy io 20 June 30 IO 20 31 10 20 31 10 20 30 K) 20 31 July August September October Fig. 4. — Seasonal variation in moisture content, Wilkins plot, season of 1927. 26 University of California — Experiment Station TABLE 24 Quantities of Water Used in Intervals Between Irrigations, Wilkins Plot, Season of 1927 Number of days Soil-moisture loss, acre-inches per acre Interval First foot Second Third foot foot Fourth foot Total Equivalent loss in 30 days 57 29 45 49 44 0.93 0.55 1.00 1.37 1.23 0.66 0.40 1.14 1.37 1.17 0.57 0.29 0.93 1.03 0.83 0.36 0.18 42 0.45 0.52 2.52 1.42 3.49 4.22 3.75 1 33 1 47 June 9-July 24 2 33 July 24-Sept. 11 Sept. 11-Oct. 25 2.58 2.55 March 15-Oct. 25 224 5.08 4.74 3.65 1.93 15.40 J s? si 1 — ^r: ^ £8 io 20 3i io eo 30 io ao 3i io eo so io eo 31 10 eo 31 10 20 30 k> eo 3/ March April May June Uuly August September October y — 1.33 — 4* — i* 8 — -t — 7 - eo ~H" — S4S — T — * se — t~ e - S7 — *r sss — H Total seasonat use, April 1 to October 15 = A3. 39 acre- inches per acre. Fig. 5. — Seasonal use of water in acre-inches per acre per month, Wilkins plot, season of 1927. An inspection of figure 4 shows that the field capacity of this soil is very close to 14 per cent, and that the moisture content of the top 2 feet of soil can be reduced to 7 per cent without an apparent change in the rate of use of water. Furthermore, the trees showed no evidence of lack of moisture throughout the season. If this is correct, at least 1.25 acre-inches per acre is available in each foot of the top 2 feet of soil. By the time this available moisture in the top 2 feet of soil has been used, 1.00 acre-inch per acre will have been taken from the third foot, and 0.50 acre-inch per acre from the fourth foot. Hence the total available supply is approximately 4 acre-inches per acre. With a total seasonal water requirement from April 1 to October 15 of 13.4 acre-inches per acre (fig. 5) 9.4 acre-inches per acre would have to be supplied by irrigation. On a basis of 60 per cent efficiency this would require that 16.0 acre-inches per acre of irrigation water be available to carry the grove to October 15. Bul. 489] Irrigation of Citrus and Avocado Trees 27 If we assume that winter rainfall had been sufficient to leave the soil at field capacity on April 1, the first irrigation would be needed about June 20. With an average monthly use of 2.50 acre-inches per acre for the remainder of the summer, the maximum period between irrigations would be 45 to 50 days, the 45-day period probably being necessary under the rotation system of delivery. The average use of water during the 45-day period would be 3.75 acre-inches per acre ; and in order to supply this, 6.25 acre-inches per acre would have to be applied at each of the summer irrigations. Three such irrigations would carry the grove well into the month of November. Clemens Plot (Eureka Lemons), Fallbrook, Season of 1927. — Because of the lack of uniformity of soil in the experimental plot in this grove in 1926, a new plot was selected equivalent in area to that occupied by a single tree. At the beginning of the season, two single-tree plots -were selected, on one of which a heavy cover crop of vetch was growing, the other being kept clean by cultivation. During the spring the first of these plots was used in determining the use of water by cover crops, the second plot being used in determining the summer use under clean cultivation. Water was obtained by pumping from a dug well, dis- tribution being made through portable sprinklers, Throughout the season, the sprinklers were set directly under the trees, except during the irrigation of July 11 to 14, when an extra setting of the sprinklers was made in the center of the area between tree rows. At the beginning of the season the following determinations of the moisture equivalent 8 and apparent specific gravity were made : Depth of Soil First foot Second foot Third foot Fourth foot Plot 1, cover-cropped: 1 45 7.3 1.46 1.43 8.6 1.42 1 55 9 9 1 47 1.58 10 9 Plot 2, clean cultivated: 1 48 s The moisture equivalent was not determined for plot 2. 28 University of California — Experiment Station TABLE 25 Results of Soil Sampling and Irrigation Data, Clemens Plot (Eureka Lemons), Fallbrook, California, Season of 1927 Dates of Average moisture conteDt of the soil, per cent Dates of irrigation Amount of irrigation water applied, acre-inches per acre Per cent sampling First foot Second foot Third foot Fourth foot of soil mass moistened PLOT 1 (cover-cropped) March 19 March 25 April 4 April 18 May 9 June 2 10.1 7.9 9.2 8.5 7.4 6.4 11.6 10.9 10.3 9.6 9.4 11.9 11.7 10.4 10.3 10.0 9.6 PLOT 2 (clean cultivated) 7.4 6.2 9.0 6.8 5.6 7.3 5.2 4 5 6.2 5.8 5.9 7.7 7.8 9.5 8.5 7.8 8.5 6.8 6.2 6.5 6.0 6.1 9.9 9.3 10.1 9.5 9.2 9.6 8.8 8.1 8.7 7.9 8.0 11.1 10.6 11.0 10.9 10.7 11.1 9.8 9.9 10.2 9.8 9.7 2.31 June 28 July 19 July 11 to 14 4.64 100 2.31 September 26 September 29 1.93 13.5 October 15 15 \ 1 ■-j --« • S rt ~ ~TT lb | | 1 I 10 i s* S^ r, I 1 1 I* 1 1 I Mill £ \'° f^l >"«i. — ■ ■ , L -~ 5 ** 15 | | 1 i 10 -j- | a. 3i. .* 64 in Intervals Between Irrigations, Clemens Plot, Season of 1927 Number of days Soil-moisture loss, acre-inches per acre Interval First foot Second foot Third foot Fourth foot Total Equivalent loss in 30 days 16 63 18 . 33 47 26 0.16 0.54 0.23 0.83 0.72 0.21 0.25 40 0.16 40 57 0.19 0.23 0.28 10 0.23 34 0.15 0.31 15 0.07 0.10 1.69* 1.37 0.57 1 56 3 19t 0.65 June 10-June 28 0.95 July 11-August 13 1.42 Aug. 13-Sept. 29 Sept. 29-0ct. 25 0.07 0.15 1.70 70 1.09 80 Includes 0.74 inch rainfall. t Heavy loss because of cover crop. During* the summer, this grove received only one irrigation in which the whole soil surface was covered. With the exception of the one period between July 11 and August 13, which shows an average rate of use of 1.42 acre-inches per acre per 30 days, the rates shown in table 30 are not representative of what they would have been if sufficient moisture had been available to meet the demands of the trees throughout the season. If an average of two-thirds of the soil mass was moistened in the irrigations between June 13 and October 6, the average monthly use would be 0.96 acre-inch per acre. With 53 per cent of the root activity in the top foot of the soil, the interval between irrigations should be 40 days, and on a basis of moistening two-thirds of the soil mass, and with an efficiency of irrigation of 60 per cent, 2.0 acre-inches of water per acre is required at each irrigation. Four irrigations are thus needed to meet the seasonal requirement of 8.0 acre-inches per acre. With 100 per cent of the soil mass irrigated at each application, four irrigations of 3.0 acre-inches per acre each, totaling 12.0 acre inches per acre, would be necessary. Treat Plot {Eureka Lemons), Vista, Season of 1927 m — This prop- erty is located on the main highway one-half mile west of Vista. The soil is typical Sierra sandy loam, ranging in depth from 2.5 feet at the higher elevations to 4.0 feet on the lower portions. The grove, containing 2.84 acres of mixed avocados and lemons, is 13 years old; but because of water shortage prior to 1926, the trees had not attained normal size for their age. The trees are planted on the contour, with an average spacing of 22.5 feet between tree rows. The furrow method of irrigation is used, with four furrows rather closely spaced in the center of the area between the rows. This spac- ing did not permit the moistening of more than 50 per cent of the soil mass occupied by the roots of the trees, and at each irrigation 30 University of California — Experiment Station two extra furrows were placed 5 feet from the tree row through the experimental plot. Irrigation water was obtained from the Vista Irrigation District and was available on the growers' request. The following determinations of the moisture equivalent and apparent specific gravity were made at the beginning of the season : Depth of Soil Apparent specific gravity Moisture equivalent, per cent First foot 1.44 10.9 Second foot 1.43 11 4 Third foot 1.47 7.4 TABLE 27 Eesults of Soil Sampling and Irrigation Data, Treat Plot (Eureka Lemons), Vista, California, Season of 1927 Dates of sampling Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation water applied, acre-inches per acre Per cent First foot Second foot Third foot of soil mass moistened Irrigated section 10.6 9.5 13.7 11.8 8.1 7.8 12.1 9.1 11.2 8.8 7 9.7 7.5 6 6 10.5 9.2 8.1 7.4 7.0 6.8 6.6 12.0 11 2 12.2 11.9 10.4 10.6 12.4 11.2 12.0 11.6 9.5 9.7 9.2 8.3 10.7 10 9.3 8.9 8.4 8.6 8.2 12.8 12.4 12.9 12.4 11.3 11.3 11.8 11.8 12.4 12.1 9.9 9.9 9.0 8.6 11.9 10.2 9.7 10.2 8.4 8.7 8.2 April 20 May 9 May 5 to 6 1.68 40 May 16 1.93 70 July 5 July 14 to 16 3.27 69 July 23 Aug 13 2.34 69 Sept 7 Sept 13 Sept. 13 2.17 65 Sept 21 Sept 28 Oct 6 Oct 14 Oct 17 Oct 20 Unirrigated section 8.8 8.5 7.8 6.7 6.6 6.7 6.3 5.6 5.1 5.1 4 9 5.1 5.2 11.0 11.1 10.2 9.8 9.8 9.5 9.2 8.7 8.4 7.8 7.1 7.7 7.2 12.0 12.4 11.4 10.8 11.6 11.3 10.6 10.0 9.6 8.8 8.6 8.6 8.5 May 16 June 20 July 5 Tulv 23 Aug 13 Aug. 23 Oct 7 Oct. 17 Bul. 489] Irrigation of Citrus and Avocado Trees 31 15 *»l ■* s. ■V to 1 *>> - £*■, 6 1 * , IS ° V 1* •- 1 ^ •fr. p %/0 a.. 4> II f," I ^ AS 10 . -4-- 8 o- ? \, 93 3.27 Ac in/oc. Ac in/oc Ac m/ac Ac in/oc. 1 1 1 t ii I 1 1 1 1 1 1 \ II 1 31 io so 3i io so March /Ipril /o so May ao eo 30 /o so Uune July IO 20 31 IO SO 30 10 SO i August September October Fig. 7a. — Seasonal variation in moisture content of irrigated section, Treat lemon plot, season of 1927. Dotted portions of lines represent the assumed moisture condition between irrigation and subsequent sampling. ■6- m 61 rii ! ^ > V \" i ° r 1 10 s U 30 10 SO 31 10 SO 30 IO SO 31 IO SO 31 10 SO 30 10 SO 31 May Uune July August September October Fig. 7b. — Seasonal variation in moisture content of unirrigated section Treat lemon plot, season of 1927. 32 University of California — Experiment Station TABLE 28 Quantities of Water Used from Irrigated and Unirrigated Soil Sections in Intervals Between Irrigations, Treat Plot, Season of 1927 Interval Num- ber of days Irrigated soil section, soil-moisture loss, acre-inches per acre Unirrigated soil section, soil-moisture loss, acre-inches per acre Total acre-inches per acre Equivalent loss in 30 days, First foot Second foot Third* foot First foot Second foot Thirdf foot acre-inches per acre 35 39 31 32 29 42 42 0.46 0.74 0.75 0.51 0.70 0.33 0.15 0.31 0.42 0.21 0.33 0.06 0.03 0.04 04 0.05 0.10 0.81 1 06 1.21 1.34 87 1 13 70 0.22 0.04 0.04 0.02 0.11 05 0.05 0.05 0.09 0.03 0.04 0.03 81 June 13-July 14 July 14-Aug. 15 Aug. 15-Sept. 13 Sept. 13-Oct. 25 1.17 1.25 0.90 0.81 2.0 to 2.33 feet. t 2.0 to 2.77 feet. ST / 10 20 31 10 20 30 10 20 31 10 20 30 lO 20 31 10 20 31 10 20 30 IO 20 31 March April Mau June July . August September October y. o 70 — 4. — o.8o — 4" — l0S — i — L25 1 — '- 20 — *r — ° 90 — i — °- 7S — H Total seasonal use April 1 to October 15 '6.27 acre-inches per ocre. Fig. 8. — Seasonal use of water in acre-inches per acre per month, Treat lemon plot, season of 1927. For this grove, figure 8 shows the total seasonal use from April 1 to October 13 to be 6.3 acre-inches per acre, with a maximum monthly requirement of 1.25 acre-inches per acre during July and August. The soil moisture curves in figures 7a and 7b show this soil to have a field capacity of 12 per cent and a wilting point of 6 per cent. With an average depth of soil of 2.5 feet, and with 63 per cent of the root activity in the top foot of soil, 1.65 acre-inches per acre would be available in the soil in the periods between irrigations. With a maximum rate of use of 1.25 acre-inches per acre per month during July and August, the proper interval between irrigations during the period would be 35 days. This interval could be extended to 45 days during the spring and to 40 days in the fall. With two-thirds of the soil mass moistened, and on a basis of 60 per cent efficiency in irrigation, four irrigations averaging 2.0 acre-inches per acre would be needed, giving a total seasonal requirement of 8.0 acre-inches per acre. Assuming that this Bul. 489 Irrigation of Citrus and Avocado Trees 33 grove is 76 per cent grown, the water requirement at maturity under the present practice of moistening two-thirds of the soil mass would be 10.5 acre-inches per acre. At maturity and with 100 per cent of the soil mass moistened at each irrigation, the seasonal irrigation requirement would be 16 acre- inches per acre. Hinrichs Plots (Valencia Oranges), Escondido, Season of 1927. — During 1927, soil sampling and measurement of irrigation water were continued in this grove, the experimental plots being moved to a more favorable location having a more uniform soil type than that found in the 1926 plots. The grove is irrigated by the furrow method, a set of three furrows 21 inches apart being placed oh each side of each tree row for the June irrigation, and a similar set 3 feet apart for the July and September irrigations. An average of 64 per cent of the soil mass was moistened at each irrigation. The favorable location of the plot and the uniformity of the soil afforded an excellent oppor- tunity for accurate determination of the efficiency of irrigation and the percentage of the soil mass moistened at each irrigation, as well as a measure of soil-moisture loss from irrigated and unirrigated soil sections. Figure 2 shows the location of furrows and points of soil sampling in this grove during 1927. Beginning with the first irrigation on June 8 a second plot (B) was located, in which 90 per cent of the soil mass was moistened at each irrigation. This offered an opportunity to compare the rate of loss when different percentages of the soil mass were irrigated. The following apparent specific gravity and moisture equivalent determinations were made on plot A at the beginning of the season : Depth of Soil First foot Second foot Third foot Fourth foot Apparent specific gravity 1.49 11.3 1.52 14.1 1.54 14.8 1.58 14.5 34 University of California — Experiment Station 19 ffl t- =- s ■fe £ J? ic o 15 R 1 f> P« ^ W; k& | 1 F yo if* u IS f^z « 10 1 5 — Qjin V e.49 3.0'- - a. 7 a o 1.0*- 0.7S" .1 - 1 1 ■ 1 1 1 1 II i 10 SO 31 10 20 30 10 20 31 10 20 30 10 21 March April May June -July 31 10 20 31 10 20 30 /O 20 31 August September October Fig. 9a. — Seasonal variation in moisture content of irrigated section, Hinrichs grove (plot A), season of 1927. /5 X \ JO S IS <3 r> 10 P „* *J S p v "*. 15 1 IS « t> * .5 o IS i i 10 s o *J/ 10 20 30 10 20 31 IO 20 31 10 20 30 10 20 J/ ■June Uulu August September October Fig. 9b. — Seasonal variation in moisture content of unirrigated section, Hinrichs grove (plot A), season of 1927. Bul. 489] Irrigation of Citrus and Avocado Trees 35 TABLE 29 Besults of Soil Sampling and Irrigation Data, Hinrichs Grove (Plot A, Valencia Oranges), Escondido, California, Season of 1927 Dates of Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation water applied, acre-inches per acre Per cent sampling First foot Second foot Third foot Fourth foot of soil mass moistened Whole plot Mar 16 13.5 12.6 12.4 11.8 10.9 9 4 7.7 14 6 14.2 13.7 13 .1 12 6 12.0 11.0 14.4 13.9 13 6 13.1 12.7 12 1 115 14.8 14.2 14 .0 13.5 13.4 12.9 12.3 Mar. 24 . April 9 . April 22 May 11 May 28 June 17 . 2.49 62 June 17-19 Irrigated section June 27 12 9 14.2 12.6 11 8 13 5 12.6 11 4 13 3 13.2 12.2 11 .6 11.5 13.9 12.8 12 2 13 1 12.6 11 7 12.7 13 11.7 11 9 11.8 14 .3 13.7 13.2 13.6 13.0 12 5 13 4 13.6 12.7 12.7 12.9 July 16 9 8 12 9 8 12 11 9 9 8 6 2 4 2 2 3 6 1 S July 30 July 30 3.01 64 Aug. 8 Aug. 27 Sept. 8 Sept. 8 2.74 67 Sept. 19 Sept. 29 Oct. 10 Oct. 19 Oct. 25 Unirrigated section June 27 7.8 7.4 6 8 6.6 6.6 6.4 6 2 6 4 11.3 10.6 10 2 10.0 9.7 9.4 8.9 9.5 11 .6 11.2 10.9 11.0 10.8 10 3 10 10.5 12.4 12.3 12.0 12.3 12.2 11.6 11.8 12.2 July 16 July 30 Aug. 8 Sept. 8 Sept. 19 . . Oct. 19 Oct. 25 36 University of California — Experiment Station TABLE 30 Quantities of Water, Used from Irrigate© and Unirrigated Soil Sections in Intervals Between Irrigations, Hinrichs Grove, (Plot A), Season of 1927 Interval Num- ber of days Irrigated soil section, soil-moisture loss, acre-inches per acre Unirrigated soil section, soil-moisture loss, acre-inches per acre Total acre- inches per acre Equiva- lent loss in 30 days, acre- 1st foot 2nd foot 3rd foot 4th foot 1st foot 2nd foot 3rd foot 4th foot inches per acre March 16-April 9 24 69 44 39 47 20 0.95 0.67 0.60 0.61 0.16 0.49 0.34 0.32 0.33 17 0.39 0.26 0.20 0.15 0.15 0.27 0.17 0.17 0.10 1.22* 2.10 1.76 1.36 1.27 1.59t April 9-June 17 .0.91 June 17-July 31 0.12 0.01 0.01 0.09 0.03 0.03 0.07 0.02 0.03 0.04 0.01 0.01 1.20 July 31-Sept. 8 1.05 Sept. 8-Oct. 25 0.81 Includes 0.53 inch rainfall t Heavy loss due to cover crop. An inspection of figures 9a and 9b shows that the field capacity of this soil is from 13.5 to 15 per cent and that the wilting point is probably about 7.0 per cent, the latter being indicated in figure 9b by the break in the moisture curve of the top foot of the unirrigated soil mass. With 47 per cent of the root system in the top foot of soil, which is figured from rate of soil moisture loss, irrigation water should be applied when the top foot reaches the wilting point. Under these conditions, and with the top 4 feet of soil at field capacity on April 1, about 2.5 acre-inches of water per acre would be available for use before irrigation was necessary. The soil-moisture curves shown in figure 9a indicate that this point would be reached on or shortly after June 20. During the irrigation season, with two-thirds of the soil mass moistened at each irrigation, 1.65 acre-inches per acre would be available, and at an average rate of use of 1.0 acre-inch per acre per month, the proper interval of irrigation would be 45 days. On a basis of 60 per cent efficiency in irrigation, 2.50 acre-inches per acre would be applied at each irrigation. Under the above conditions, three seasonal irrigations, totaling 7.50 acre-inches per acre, would be required. On a basis of two-thirds of the soil mass being moistened at each irrigation, this grove being assumed to be 40 per cent grown, the seasonal requirement at full growth would be 18 acre-inches per acre. In connection with the soil-moisture diagram shown in figures 9a and 9b, there is no indication either in the irrigated or in the unirri- gated soil sections that the rate of moisture extraction decreases as the moisture per cent decreases. This would indicate that as long as the moisture content of the soil is above the wilting point, the soil water is as readily available as when the moisture content is near or Bul. 489] Irrigation of Citrus and Avocado Trees 37 at field capacity. It is further noted that when the moisture content of the unirrigated soil section reaches the wilting point, there is no apparent increase in the rate of moisture extraction from the irrigated soil zone. TABLE 31 Kesults of Soil Sampling and Irrigation Data, Hinrichs Grove (Plot B, Valencia Oranges), Esconmdo, California, Season of 1927 Dates of Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation water applied, acre-inches per acre Per cent of soil mass sampling First foot Second foot Third foot * moistened Mar. 15 12.0 5 2 10 9 5.0 9.6 5 5 10 8 7.8 5.4 12.2 7.6 12 2 8 1 12.2 8 6 12 8 10 5 8.2 12.5 9.6 12.6 11.1 13 110 13.5 11.7 10.1 June 8 to 9 2.49 90 July 20 July 30 July 20 3 01 90 Sept. 1 2.74 90 Sept. 12 Sept. 29 Oct 24 2.0 to 2.8 feet. IS IO s %' 5 \io -v. - ^ — - i— 5=* . "~ s ^ - "■"■* ^ ^ - ^r -*- H ^ 15 IO 5 O — ~ +- - - - = == — '•-J 15 tO 20 31 IO 20 JO 10 20 '-rch April Mou to BO JO June 10 20 31 July IO 20 Jl IO 20 30 IO 20 31 August September October Fig. 10. — Seasonal variation in moisture content, Hinrichs grove (plot B), season of 1927. TABLE 32 Quantities of Water Used in Intervals Between Irrigations, Hinrichs Grove (Plot B), Season of 1927 Number of days Soil-moisture loss, acre-inches per acre Interval First foot Second foot Third foot Total Equivalent loss in 30 days 60 41 33 53 0.87 0.72 0.75 1.04 0.59 0.60 0.66 84 022 0.19 0.20 37 1.68 1.51 1 62 2.25 0.84 June 8-Julv 20 1.10 July 30-September 1 1 47 1 28 38 University of California — Experiment Station Figure 10 shows the field capacity and wilting point of the soil of plot B, averaging 2.58 feet in depth, to be approximately the same as that for plot A, which has an average depth of 4 feet, During the interval from April 1 to June 15, plot A showed a loss of 2.34 acre-inches per acre and plot B a. loss of 2,10 acre-inches per acre, with a loss for the remainder of the season of 4.51 acre- inches per acre from plot A and 5.18 acre-inches per acre from plot B. Considering the differences in use by the two plots between April 1 and June 15, and for the soil moisture loss from the unirrigated soil section of plot A from June 15 to October 15, the losses from the two plots are approximately in direct proportion to the average percentage of the soil mass moistened at each irrigation. 8* So. IO 20 March eo 30 April - das 4- — a $2 Plot A • 64%. soil mass irrigated. Total seasonol use, April 1 to October J6~ -6.85 acre -inches 10 20 3o /o 20 September October — o.sr — A — a 7a — per acre I*. i Q \ ^^^__ ___ __^__ 10 20 31 O 20 JO IO 20 March April May \ — o. 73 -J- — o. as ■ 31 10 BO JO IO 20 Jl IO 20 31 IO 20 30 10 20 31 June Uulu August September October 3-90% of soil mass irrigated. Total seasonal use, April i to October AS"- 7.26 acre-incher per acre Fig. 11. — Seasonal use of water in acre-inches per acre per month, Hinrichs grove (plots A and B), season of 1927. In plot A, July was the month of maximum use, while in plot B maximum use is shown in August. In plot A, during July there was still available moisture in the unirrigated soil zone, while during the remainder of the summer the monthly losses were a measure of the water available in the moistened zone. The total seasonal loss from plot A is a measure of the quantity of water available rather than a measure of the quantity which would have been used if a full moisture supply had been available throughout the season. Plot B, with 90 per cent of the soil mass moistened, represents the probable water requirement of this grove with approximately a full Bul. 489] Irrigation of Citrus and Avocado Trees 39 supply of moisture available throughout the summer. With this quantity of water applied, the period between irrigations would remain at 45 days and under 60 per cent efficiency, 3.5 acre-inches per acre should be applied at each irrigation. Red Mountain Plots {Navel Oranges), Season of 1927. — The results obtained from this grove during 1926 yielded little information con- cerning the total seasonal water requirement of either the old or young trees. Throughout the season there was a deficiency in irrigation water applied ; and, in consequence, the results obtained were a measure of the water available rather than of the water requirement. In the continuation of the work of 1927, arrangements were made whereby irrigation water would be applied at the proper intervals and in proper quantities to keep an ample supply of moisture available to meet the needs of both young and old trees throughout the season. The following determinations of the moisture equivalent and apparent specific gravity were made at the beginning of the irrigation season : Depth in Feet First foot Second foot Third foot Fourth foot Fifth foot 1 45 13.5 1.33 13.9 1 45 14.0 1.63 15.1 1.73 16.9 TABLE 33 Kesults of Soil Sampling and Irrigation Data, Red Mountain Grove (Plot A, 31- Year-Old Navel Oranges), Fallbrook, California, Season of 1927 Dates of sampling Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation water applied, acre-inches per acre Percent of soil mass First foot Second foot Third foot Fourth foot Fifth foot moistened Mar. 17 13.5 11 8 11 9.6 8.9 8.4 11.9 9 4 8.1 7.4 11.7 9.2 8.4 11 2 12 1 10.3 9.2 9.0 13.7 12.7 12.0 11 3 10 2 9.7 11 8 10.8 9.4 8.6 13.0 11.5 9.9 11.6 12.8 12.2 111 11 1 14.0 13.4 12.6 12 3 11 4 10.8 11.8 11 4 10.3 9.8 13.5 12.5 11.3 12.2 13.7 12.8 12 5 12 1 14.6 13.8 13.3 13.0 12 5 12.2 12.6 12 4 11.6 11.5 13.8 12.8 12.3 12.8 14.0 12.9 13 5 12 8 14.8 14 6 14.2 14.2 13.9 13.7 13.8 13.7 13.5 13.5 13.9 13.7 13.9 14.2 14.5 14.0 14 4 14 April 8 April 21 May 10 May 25 June 3 4.28 100 June 29 July 15 July 25 July 27. 7.50 100 Aug. 6 Aug. 24 Sept. 5 Sept. 5 Sept. 22 No information No information Sept. 15 Oct. 1 Oct. 13 .... Oct. 21 . Oct. 26 40 University of California — Experiment Station IS s 1 1 IO I S- -4»» "~- "*1M 1 1 .^. N $.'3 __ s 1 dfV - — 3 = "■ IS — 7.5 4.28 Ac.in./ac 1 | 1 | M 6 1 1 1 1 III 1 I .?/ IO 20 31 10 20 30 IO 20 31 10 20 30 IO 20 31 IO 20 31 10 20 30 lO 20 31 March April May Uune -July August September October Fig. 12. — Seasonal variation in moisture content, Red Mountain grove (plot A), season of 1927. TABLE 34 Quantities of Water Used in Intervals Between Irrigations, Red Mountain Grove (Plot A), Season of 1927 Num- ber of days Soil-moisture loss, acre-inches per acre Interval First foot Second foot Third foot Fourth foot Fifth foot Total Equhalent loss in 30 days March 15-June 4 June4-July 27 July 27-September 5 October 1-October 25 81 53 40 24 29 0.87 0.89 79 55 0.58 65 61 0.53 0.29 0.36 55 0.42 0.60 0.25 0.29 0.48 31 48 0.18 0.21 0.24 0.11 05 0.07 2.79 2.34 2.45 1 34 1 44 1 03 1 32 1.84 1 68 1.49 TABLE 35 Results of Soil Sampling and Irrigation Data, Red Mountain Grove (Plot B, 7-Year-Old Navel Oranges), Season of 1927 Dates of Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation water applied, acre-inches per acre Per cent sampling First foot Second foot Third foot Fourth foot of soil mass moistened Mar 15 13.3 9.0 11 8 9.9 8.3 13.2 10.8 9.7 13.3 13 9 10.6 12 2 11.3 10.2 14.3 12 9 11.9 14.3 14.1 10.9 11.8 11.0 10.1 13.5 12 4 12.2 14.2 12 9 10.5 10.2 9.5 9.3 12.7 11.4 11.2 12.8 2.50 100 June 13 July 6 July 29 Aug. 6 Sept. 5 Sept 22 July 29 5.00 100 Sept. 22 5 00 100 Oct 5 Bul. 489] Irrigation of Citrus and Avocado Trees 41 TABLE 36 Quantities of Water. Used in Intervals Between Irrigations, Bed Mountain Grove (Plot B), Season of 1927 Number of days Soil-moisture loss, acre-inches per acre Interval First foot Second foot Third foot Fourth foot Total Equivalent loss in 30 days 81 46 47 0.75 0.61 0.61 0.52 0.30 0.38 0.55 0.30 0.23 0.41 0.23 0.27 2.23 1.44 1.49 0.83 June 13-July 29 94 August 6-Sept. 22 0.95 t x— .__, a^_ f C ^ ' IS, / IO £0 SO IO 20 SO K> 20 31 IO 20 30 10 SO 31 10 20 31 rO 20 JO 10 BO 31 March April May June July August September October y-I.OO -J- l.ld 1 I.BO -| /.SO «| ISO •}— 175 -|— 1.65 -j Plot A. Jl year old Nave/ Orange grove. Total seasonal use April 1 to Oct. 15 =9.2 acre-inches per ocre t * IO 20 March Plot B 31 10 SO 30 IO SO 31 , April May | — oeo — 4-— a as \ 7 year old Navel Orange grove. IO June - 090 ao 30 10 20 31 IO 20 31 10 20 30 10 20 31 , August , September October , -\ 95 1 09Z — *\ — o. da - L — \ Total seasonal use April 1 to Oct. 15 ■ 5.80 acre-inches per acre JuIl - 0.93 Fig. 13. — Seasonal use of water in acre-inches per acre per month, Eed Mountain grove (plots A and B), season of 1927. Vfith the soil in plot A having a field capacity of 12.5 per cent and a wilting point of 7 per cent, there would be available 3.10 acre- inches of water per acre before the top 2 feet of soil would have reached the wilting point. With an average summer use by the trees of 1.70 acre-inches per acre per month, the correct interval between irrigations would be slightly over 50 days. Allowing for a reasonable margin of safety, good practice would call for a 45-day period between irrigations. During this period 2.55 acre-inches of water would have been used, and on a basis of 60 per cent efficiency and with 100 per cent of the soil mass moistened, a depth of 4.25 acre-inches per acre should be applied. Three irrigations would bring the total seasonal requirement of this grove to 12.75 acre-inches per acre. 42 University of California — Experiment Station In plot B, with the same soil type and the same water-holding capacity as found in plot A, there would be available 2.35 acre-inches per acre before the top foot of soil would reach the wilting point. With an average use of water during the summer of 0.90 acre-inch per acre, a 45-day interval between irrigations would provide ample water to meet the transpiration use of the grove. This use, with 100 per cent of the soil mass moistened and an efficiency of 60 per cent, would require a depth of application of 2.25 acre-inches per acre at each irrigation, with a total seasonal requirement of 6.75 acre-inches per acre for three irrigations. If we assume this grove to be 36 per cent grown, the water requirement at full growth would be 18.5 acre-inches per acre. Treat Avocado Plot, Season of 1927. — At the beginning of the season of 1927, a new plot was chosen for study of use of water by avocados. This was located one-quarter mile west of Vista and con- tained 3 x /2 acres of mixed varieties. The trees, which were 13 years old and of somewhat uneven size, were planted on the contour with an average spacing of 27 feet and were irrigated by the furrow method. The soil is decomposed granite (Sierra sandy loam), its average depth in the experimental plot being 2.5 feet, In the main portion of the grove, two irrigation furrows spaced 2 feet apart were placed on each side of the tree row. After the first irrigation, in order to moisten a greater percentage of the soil mass, six furrows were used in the experimental plot, the inner furrows being about 6 feet from the tree row. At the beginning of the season the following determinations of the moisture equivalent and apparent specific gravity were made: Depth of Soil First foot Second foot Third foot 1 46 10.3 1 36 10.1 1.43 8.4 Bul. 489] Irrigation of Citrus and Avocado Trees 43 TABLE 37 Kesults of Soil Sampling and Irrigation Data, Treat Plot (Avocados), Vista, California, Season of 1927 Dates of Average moisture content of the soil, per cent Dates of irrigation Amount of irrigation water applied, acre-inches per acre Per cent sampling First foot Second foot Third foot* of soil mass moistened Irrigated section Mar. 12 . 12 5 12 8 11.2 11.8 11.0 13 4 12 1 9.9 9.3 12.2 10 1 11 1 9.2 7.8 9.8 7.7 7.2 10.8 9.0 8.2 7.5 6.4 10 8 10.7 12.6 11.9 14 4 12 9 10.3 9 6 12.2 11.3 10.5 9.7 8.1 8.1 8 6.8 8.7 9.0 7.5 7 1 6.6 Mar. 26 10 10 9 13 10 7 7 11 8 10 7 6 9 7 6 10 8 7 6 5 3 9 6 7 9 8 5 5 3 1 8 5 9 8 8 5 2 6 April 5 April 20 May 9 1.68 38 May 16 . . June 6 June 13 June 13 to 14 1.93 67 June 20 July 5 July 14 to 16 3.27 66 July 23 Aug. 1 Aug. 13 Aug. 23 Aug. 15 to 17 2 34 66 Sept. 7 Sept. 13 Sept. 21 . Sept. 13 to 14 2.17 62 Sept. 28. ... Oct. 6 Oct. 17 Oct. 24 to 25 1.86 Oct. 24 Unirrigated section May 9 9.2 8 8 7.0 6.5 6.2 5.8 5.7 5 4.5 4 5 4 7 10 5 10.6 8 6 7 9 7.8 6.8 6 9 6.2 5.6 5.8 5.8 11 10.7 9.0 9 9.7 8.0 8.0 6 4 6 4 6.1 6.3 May 16 June 13 June 20 . . July 5 July 23 Aug. 13 Aug. 23 Sept. 13 Oct. 17 2.0 to 2.33 feet. 44 University of California — Experiment Station m i ►, IO ,^^ , X ,_. .^k - v --k , ~ J Ni ^s ts "«« ^S _p+>-« J3 — *0 fi p.. f "*" tv -j- 1 8 <«^ d* ^15 to p = O s „ V? i 27 Ac in/oc Ac in/oc Wc. in/oc. — Ac ,,,/i k i 1 1 i 1 1 1 1 1 ■ 1 1 1 li 1 10 20 31 IO 20 30 IO 20 31 IO 20 30 IO 20 31 IO 20 31 IO 20 30 IO 20 March April Man June July Auoust September October Fig. 14. — Seasonal variation in moisture content of irrigated section, Treat avocado plot, season of 1927. TABLE 38 Quantities of Water, Usex> in - Intervals Between Irrigations, Treat Plot (Avocados), Season of 1927 Interval Num- ber of days Irrigated soil section, soil-moisture loss, acre-inches per acre Unirrigated soil section, soil-moisture loss, acre-inches per acre Total acre- inches per acre Equivalent loss in 30 days, acre-inches per acre First foot Second foot Third* foot First foot Second foot Thirdf foot 15 39 31 32 29 23 18 0.22 0.37 0.78 0.63 0.64 0.50 23 12 0.26 0.48 45 0.39 24 0.18 0.05 0.10 14 16 0.09 0.04 0.04 0.39 1 30 1 58 1.44 1.12 0.78 45 0.79 0.26 0.06 0.06 0.18 0.07 08 13 0.05 0.06 1 00 June 13-July 14 ..., 1.53 July 14-Aug. 15 1.35 Aug 15 Sept 13 1.15 Sept 13 Oct 6 1.02 Oct 6 Oct 24 0.76 * 2.0 to 2.33 feet, t 2.0 to 2.68 feet. Bul. 489] Irrigation op Citrus and Avocado Trees 45 The use-of -water curves shown in figure 15 indicate that there was a deficiency in soil moisture in the latter part of each of the periods between irrigations after midsummer. The probable use-of-water curve shows the quantity of water which would have been taken from the irrigated section if a full supply had been available during the entire period between irrigations; while the actual curve is a measure of the water which was available for use. J* ■5 . io bo 3i to bo 30 10 20 31 10 bo 30 March April Mau Uune f— a as IO BO 31 10 BO 30 IO BO 31 September October I.SS — 4* — <-*o H Total loss, April 1 to October 15 - 9. IS acre-inches per acre ! ! 1 r : 1 10 eo 31 10 ao 30 10 bo 31 10 bo 30 10 bo si 10 bo 31 10 bo 30 10 bo 31 March April Mau June Julu , August September October V — 080 4- O. 90 4— /. 35 1 /.SO 1 I.B5 — -f — 'OS total loss, April 1 to October 15* 7.20 c 7S acre inches per acre Fig. 15. — Top, probable seasonal use of water with a full supply of soil moisture available throughout the season, based on use during periods when ample moisture was available; bottom, the actual seasonal use with a defici- ency in soil moisture during the latter part of the season. Treat avocado plot, season of 1927. Assuming that the 'probable' curve represents the seasonal water requirement of this grove, a total of 9.15 acre-inches of available water in the soil per acre would have to be supplied by rainfall and irrigation to meet the needs of this grove. With 54 per cent of the root activity in the top foot of this shallow soil, irrigation water should be applied at the time the moisture content in the top foot reaches the wilting point. Since the soil has a field capacity of 12.5 per cent and a wilting point of 7.0 per cent, 1.7 acre-inches of water per acre would be available in it in the period between irrigations. The monthly uses, as obtained from the probable use-of-water curve, indicate that the first irrigation would be applied about June 1. If the average monthly requirement for the remainder of the season is 1.6 acre-inches per acre per month, and if 1.7 acre-inches per acre is available, the proper interval between irrigations would be 30 days, five irrigations being required during the season. 46 University of California — Experiment Station On a basis of 60 per cent efficiency, 3 acre-inches per acre should be provided for at each irrigation, making a total seasonal require- ment of 15 acre-inches per acre. At maturity, with 67 per cent of the soil mass moistened at each irrigation, the total seasonal requirement would be 17 acre-inches per acre, while with the whole soil mass moistened the requirement would be increased to 25 acre-inches per acre. SUMMARY OF RESULTS, SEASON OF 1927 TABLE 39 Monthly Use of Water* by Six Groves under Observation in San Diego County, April 1 to October 15, 1927 Farm Crop Age of trees, years Apr. May June July Aug. Sept. Oct.t Total 16 13 7 31 7 13 Acre-inches per acre 1 33 70 0.85 1 00 0.80 85 1.38 0.80 0.S2 1 10 0.85 1.20 1 80 1 05 1.15 1 20 0.90 1.45 2 45 1 25 1.20 1 50 93 1 70 2.58 1.20 1.08 1.80 0.95 1.70 2.57 0.90 0.87 1 75 0.92 1 55 1 27 0.37 0.39 0.82 44 0.70 13 39 Treat Lemons Valencia oranges... Navel oranges Navel oranges 6 27 6 85 Red Mt., Plot A.... Red Mt., PIotB... Treat 9.20 5.80 9 15 Taken from seasonal use-of-water t October 1 to October 15 only. Table 40 shows the apparent root development in the various soil types at depths to which soil samples were taken in the different groves. Here, as in 1926, the rate of soil-moisture loss at the various soil depths is taken as a measure of the root development at those depths. TABLE 40 Apparent Boot Development in Various Soil Depths for the Various Orchards Studied Crop Soil type Depth of soil, feet Root activity, per cent Farm 1st foot 2nd foot 3rd foot 4th foot 5th foot Wilkins Sierra sandy loam Sierra sandy loam Sierra sandy loam Holland sandy loam.. Holland sandy loam. . Sierra sandy loam 4 2 5 4 5 4.0 2.5 33 63 47 35 40 54 31 31 24 25 23 35 24 6 17 20 20 11 12 12 16 17 Red Mt plot A 4 Red Mt. plot£ Avocados Based on total seasonal moisture loss from each foot in depth. Bul. 489] Irrigation of Citrus and Avocado Trees 47 H 5. Red Mt. Plot A Red Mt. Plot5 VVilkins Clemens Treat I T ^Fallbrook < ^Fallbrook n < % f i : 8 3d •3 3 a- 3 p 0' 3 > •5 1 oranges Navel oranges Navel < D 3 a P t" 1 3 o 7. e 3 5 — 1 Ti 3 D 3 D Crop CO ^J t-* ~4 W M O) sis 3- s 2, CO CO ~4 4k --J Oi O © o> oo o 02 Rel- ative size of trees* CO Ol CO OS OS ~J CO tO 00 tO OO -c O 4- April 1 to Oct. 15, acre- inches per acre •-• to CO to h- tO 4k S cc Cn OS t-- cn Amount supplied by rainfall, inches S W OS 4k 4*. 4k CO Cn 1^ tO CO -J CO 4k Cn Amount supplied by irriga- tion, acre-inches per acre Cn CO CO CO 4k 4k CO Re- quired number of irri- gations 6.25 2.0 2 2 5 4.25 2.25 3.0 Average required depth of each irri- gation, inches CO 4k 4*. 4*. CO 4> 4k O cn Cn cn Cn Cn Average interval between irriga- tions, days 18.75 8.0 8.0 7.5 12.75 6 75 15 Total seasonal require- ment, acre-inches per acre 18 5 13 5 10 5 18 16 5 18.5 17.0 Estimated seasonal requirement of grove at maturity, under present irriga- tion practice, acre-inches per acre o o 27 16 5 18.5 3! JO ^1 Estimated requirement at maturity with 100 per cent of soil mass moistened at each irrigation, acre-inches per acre 48 University of California — Experiment Station EFFICIENCY OF IRRIGATION As previously stated, efficiency of irrigation is denned as the per- centage of the water applied that is shown in soil-moisture increase in the soil mass occupied by the principal rooting system of the crop. Tables 42 and 43 show the results of a number of observations made during the irrigation seasons of 1926 and 1927. The results contained in tables 42 and 43 show a wide variation in the efficiency of irrigation, ranging from a maximum of 73 per cent to a minimum of 26 per cent, and averaging 52 per cent, for the 40 observations made during the two years. TABLE 42 Efficiency of Irrigation", Season of 1926 Method of Soil mass moistened, per cent Water applied Water accounted for Per cent irrigation Acre-inches per acre per irrigation efficiency 2.32 1.51 65 50 2.06 1.49 72 50 2.02 1 19 59 90 57 63 50 2.50 1 42 57 50 1.30 75 58 50 1 14 0.62 54 1.10 0.61 55 40 1 86 1.06 57 2 52 1.16 46 60 2.84 1.22 43 25 2.41 0.94 39 20 2.20 66 30 Spray J 100 2.36 2.62 1.47 1.72 62 66 I 100 1 95 1 20 61 55 4 Under conditions characterized by rolling topography and where the furrow method of irrigation was used, low efficiencies generally resulted from excessive run-off from the ends of the furrow. Other losses include those from deep penetration at the heads of the furrows, the unavoidable losses due to evaporation from the water surface during irrigation, and those incurred in moistening the soil mulch. Where shallow depths of water are applied at frequent intervals, the evaporation from the water surface during irrigation and evaporation from the soil mulch may represent a major portion of the total. Bul. 489] Irrigation of Citrus and Avocado Trees 49 As a result of the measurements and observations made during the two years, the conclusion is drawn that under existing conditions, as found in the areas under observation, the average efficiency which may be expected under good irrigation practice is about 60 per cent. TABLE 43 Efficiency of Irrigation, Season of 1927 Method of Soil mass moistened, per cent Water applied Water accounted for Per cent irrigation Acre-inches per acre per irrigation efficiency 90 6 40 3.70 58 100 6.40 3.84 60 100 6 80 4 46 66 64 2.49 1.67 67 64 3 01 1.17 39 67 2.74 1 23 45 90 2.49 1.80 72 90 3.01 2 20 73 Furrow 90 40 2.74 1 68 1 98 0.54 72 32 70 1 93 1.10 57 69 3.27 1.00 31 69 2.34 61 26 65 2 17 0.92 42 38 1.68 0.55 32 67 1.93 1.28 66 66 3.27 0.99 30 66 . 2.34 1 05 45 62 2.17 1.08 50 4.28 1.50 35 100 7.50 2.50 3.50 1 16 47 46 100 5.00 2.82 56 100 5.00 2.11 42 49.6 CONCLUSIONS 1. The winter water requirement of citrus groves in northern San Diego County in which cover crops of vetch or grass and weeds are grown, varied from 8.6 acre-inches per acre to 12.0 acre-inches per acre, depending upon the size of the trees and condition of the cover crop. 2. In the areas of northern San Diego County covered by this report, normal rainfall when properly distributed is adequate to meet the winter needs of both trees and cover crops. 50 University of California — Experiment Station 3. Periods of drought of more than six weeks' duration during the winter, even if preceded by heavy rains, will, under conditions of cover cropping, require an application of irrigation water. 4. Analysis of rainfall records at Escondido and Fallbrook shows, that because of deficiency in seasonal rainfall or lack of normal dis- tribution, in at least three years out of ten, one winter irrigation, and in two years out of ten, two winter irrigations, should be provided to meet normal winter requirements of trees and cover crops. 5. With 60 per cent efficiency in irrigation and with 90 to 100 per cent of the soil mass moistened at each irrigation, mature citrus groves in the Escondido and Fallbrook areas have a net seasonal summer irrigation requirement of 18 acre-inches of water per acre. Similar groves in the Vista area under similar conditions require at least 15 acre-inches per acre. In fully mature groves where smaller quantities than these are available and where furrow irrigation is practiced, a correspondingly smaller percentage of the soil mass should be moistened at each irrigation. 6. Citrus groves, 6 to 8 years of age and 40 to 50 per cent of their probable ultimate size, will have a net seasonal summer water require- ment of 6 to 8 acre-inches per acre. 7. In the Sierra and Holland sandy loams 4 to 6 feet deep, the interval between irrigations should not exceed 45 days. As the depth of soil becomes less, the interval should be shortened, soils of 2 to 3 feet in depth requiring irrigation every 30 to 35 days, with smaller amounts of water applied at each irrigation. 8. In properly laid out groves on rolling topography, and where care is used in the application of water, 60 per cent of the water delivered to the grove should be accounted for in soil-moisture increase in the soil mass occupied by the major rooting system of the trees. 9. As long as the soil moisture is above the wilting point, the moisture content has no measurable effect on the rate of moisture extraction ; that is, moisture is as readily available when the moisture content is one-third or two-thirds of the way between field capacity and the wilting point as it is in the thoroughly moistened soil after irrigation. 10. In the experiments thus far completed, indications are that when the available moisture in the unirrigated portions of the soil has been exhausted, there is no apparent increase in the rate of extrac- tion from the irrigated portions; and when the available moisture in the top foot of soil has been exhausted, there is no increase in the rate of extraction from the lower depths. Btil. 489] Irrigation of Citrus and Avocado Trees 51 11. Some evidence was obtained from the Hinrichs grove at Escondido that the quantity of water used by citrus trees may be to some extent dependent on the percentage of soil mass moistened. While this is contrary to previously accepted principles, the evidence referred to seems to warrant further investigation, and this is being arranged for. 12. In mature citrus groves in soils 5 feet or more in depth, an average of not more than 5 per cent of the moisture extracted was taken from the fifth foot, the mature trees having a greater range of root activity than is found in the partly matured groves, In soils less than 3 feet in depth, 50 to 60 per cent of the root activity is in the top foot of soil. 13. Under the same soil conditions, a greater apparent moisture absorption by roots is found in the lower soil depths with avocados than is found with citrus. ACKNOWLEDGMENTS The authors of this bulletin wish to express their grateful apprecia- tion for the hearty cooperation of the growers on whose property these investigations were conducted. They are especially grateful to the manager and directors of the Vista Irrigation District for furnishing office and laboratory space during the two years in which the field work was being carried on. 14m-5, , 30