the CONTOUR-CHECK method of ORCHARD IRRIGATION J. B. BROWN Revised by J. C. MARR ~^Vtt?%, ■HHBMHHHMHaMBMBBMMHM| DECEMBER CALIFORNIA AGRICULTURAL EXTENSION SERVICE 1949 COLLEGE OF AGRICULTURE • UNIVERSITY OF CALIFORNIA • BERKELEY THE CONTOUR-CHECK METHOD „ a practical and inex . pensive means of irrigating orchards. IT HAS THESE ADVANTAGES: Within prescribed limits of size of irrigation head and slope of land, it is flexible in its application; It is of temporary design so as not to interfere with cultivating and harvesting; In first cost and operation it is simple and economical. BUT IT ALSO HAS LIMITATIONS: It requires a larger stream of water than is available for many orchards. 300 gallons per minute is the minimum flow that is satisfactory; It is not adapted to foothill orchards. The general slope should not exceed 2|/2 f eet P er 100 feet CULAR gives specific directions for laying out a contour- check system. It tells, with rules of thumb and examples: Page What conditions make this system feasible ... 5 How to use the level and target rod to establish the contours 8 How to mark the trees in the rows 9 How to figure the proper area and number of trees in the checks 13 How to calculate the amount of water to apply . . 14 THE AUTHORS: Mr. Brown was Associate in the Experiment Station, Division of Irrigation, Davis. (Deceased.) Mr. Marr is Lecturer in Irrigation and Associate Specialist in the Experi- ment Station, Davis. what is the Contour- Check Method? A contour check is a basin usually long and irregular in shape, formed by two levees or ridges along adjacent contours and short cross levees at either end. A basin so formed permits all of its surface to be ponded, but the depth of water will be greatest along the downhill contour levee. Furrows run at right angles to the contour levees sometimes are used to help spread the irrigation water. On an uneven piece of land contours are far apart in places and close together in others, depending upon the slope. Con- tour checks on such land similarly vary in width. This accounts for their irregular shape. Elevation. The standard interval in elevation of the ground surface between adjacent contour levees is 0.2 foot or 2.4 inches. This difference in elevation will give five levees for each foot fall in the land surface. If the slope of the land is such that the 0.2-foot difference makes the checks too wide for proper irrigation, a smaller difference may be used with- out detracting from the method. On the other hand, if the topography is such that more than 0.2-foot difference in elevation between the bases of the con- tour checks is required it is the practice to increase the difference, especially for crossing rough spots in the land surface. In extreme cases the 0.2 foot may be doubled or more. However, differences of 3 inches (0.25 foot) and more are avoided as much as possible as they are likely to lead to dif- ficulties in constructing proper levees. Construction. If the contour checks are to be furrowed, the operation is com- pleted before construction of levees is started. The levees are built substantially to guard against undermining by rodents or overtopping by irrigation water. They are usually constructed with rounded tops to a height above the general ground level of 12 to 14 inches and a bottom width of 4 or 5 feet. The size of these levees and Fig. 1 . This peach orchard is a good example of the use of contour-check irrigation on the slight slope. Fig. 2. Levees are built to a height of 12 to 14 inches above the general ground level, and should have broad, rounded tops. the space required to manipulate the im- plements for building them, limit the number that can be placed between tree rows. Where levels indicate that more than two or possibly more than three should be built between the tree rows, the extra levees are deadended against the adjacent upstream ridge. In some cases three levees are built in one tree row. Four small ones have been constructed in a 24-foot row, but this number is imprac- tical with levees large enough to be satis- factory. The size of check varies with the topography, the soil, and the head of water available for irrigation. It is the aim to include several trees in each check. Topography is one of the princi- pal factors in limiting the size of check. In steep and restricted places, only 3 or 4 trees might be included in each check. In flat places, maximum numbers of 100 or 150 might be included. Less than the maximum usable head of water further limits the size of check that can be prop- erly irrigated. Porous soils tend to reduce the size of check that can be efficiently watered. Thus, if the soil is relatively im- pervious, the checks will be relatively large, and if it is sandy they will be small. Marking Trees. Orchard manage- ment may call for leveling the contour ridges several times each year, requir- ing that their locations be permanently marked. This is done by painting on the adjacent trees a system of dashes or bands in several colors or by nailing to the trees painted pieces of tin. Adjacent trees on the upper side only of the con- tour are marked. Orchards which are contour check ir- rigated are quite easily identified by the [4] vivid colors used for marking the trees and by the irregular alignment of the levees. Three typical orchard layouts are illustrated in figures 3, 4, and 5. CONDITIONS SUITABLE FOR CONTOUR-CHECK SYSTEM Orchard irrigation by the contour-check method is feasible provided: 1— the irrigating stream is suffi- ciently large; 2— the slope of the land is within the allowable limits ; 3— flooding does not prove harmful to the trees. Size of Stream Needed The irrigating stream should be large enough to fill each of the contour checks before excessive, uneven percolation oc- curs. This factor becomes increasingly more important where the soil ranges from clay to loam to sand. Irrigation heads of 300 gallons per minute are the minimum which would be adequate. If the irrigating stream is of sufficient size it is feasible to use the contour-check method in place of any of the common systems of flood irrigation, such as single-tree basins, square basins containing a number of trees, 1 or strip checks. 2 Maximum Grade Permitted Slope is another limiting factor. Gen- erally speaking, the method is not ap- plicable to foothill orchards. The amount of slope that orchard land may have and still be successfully irrigated by the con- tour-check method is governed by the permissible interval in elevation between levees and the number of satisfactory levees that can be constructed between tree rows. On an average the maximum grade should be no more than 2% feet vertical to 100 feet horizontal. However, it is entirely feasible to take care of an 1 Fortier, Samuel, Orchard Irrigation, U.S. D.A. Bui. No. 1518, 1927 (Revised, 1940). 2 Fortier, Samuel, The Border Method of Ir- rigation, U.S.D.A. Bui. 1243, 1922. occasional swale through an orchard where the slope approaches 5 feet per 100 feet. As an extreme example one orchard in Butte County which has such a swale may be cited. The slope of the swale in this orchard is 5.4 feet in 88 feet. In this case the standard contour interval of 0.2-foot difference in elevation between levee bases could not be maintained. By drop- ping two intermediate contours, it was possible to carry the levees through on a 0.6-foot difference of elevation. With such differences of elevation the levees must be extra large. The contour checks in such cases have the appearance of broad furrows. Tree Damage Possible Where the size of the irrigating stream is sufficient, and the slope is not too great, the contour-check method may also be used in some furrow-irrigated orchards. 3 An important consideration in making such a change is whether or not furrow- irrigated trees can withstand water com- ing in contact with their trunks. Tree damage from such treatment is said to have occurred in some instances. Where trees suffer crown rot or similar disease, or the soil remains wet for long periods, it is, perhaps, unsafe to flood irrigate. This damage probably will not occur if the trees are healthy and the soil is per- meable. The method is particularly adaptable to the irrigation of orchards where no plan was made for irrigation at the time of planting and where little or no leveling and smoothing was done. ADVANTAGES . . . AND OTHERWISE There are some notable advantages in irrigating by means of contour checks, when it is possible to do so, rather than 3 Huberty, M. R., and Brown, J. B. Irrigation of orchards by contour furrows. Calif. Agr. Ext. Cir. 16:1-16. 1928 (Revised, 1932) (Out of print) . [5] by other methods that might be used. A smaller amount of levee-construction work is required than is the case with the rectangular check and the single-tree basin method. Less land leveling or smoothing is necessary than is required for furrow irrigation or for satisfactory basins on sloping land. It permits the use of large flows of water under control of the irrigator at the entrance to a single check; hence it does not involve the ad- justment of numerous small streams as in furrow irrigation, nor the large amount of shoveling necessary to control water in basins containing only one tree. Less labor is required than is involved in mov- ing pipe for a portable sprinkler system. When the vertical interval between the contour levees is small, the uniformity of irrigation is probably better than is pos- sible with furrow irrigation, and only slightly poorer than when level, single- tree basins are used. The distribution of water is more uniform than by single-tree basins if the slope of the land is such that two contour levees of the contour-check systems pass through one of the basins of the single-tree basin system. Why System Might Not Work The chief drawback to using the con- tour-check method of irrigating orchards arises from the occasional poor design of a system and from careless irrigation. In such cases more water is used than is necessary, and there may be maldistribu- tion of water with respect to the inlet and far ends of the checks. Invariably, the less watching and work required during the application of water to an orchard the more frequently it is forgotten and neglected. Since the con- tour-check method does not require con- stant attention it is subject to this kind of abuse. More often, faulty irrigation is due to poor design of the system. Some orchards are intentionally laid out in checks that are unreasonably large so the water will require changing less often. Other or- chards are laid out without considering the insufficiency of the irrigating stream or excessive porosity of the soil. In any of these cases it may be impossible to fill the checks with water fast enough to pre- vent gross ununiform distribution of water with respect to the inlet and far ends of the checks. Since water percolates into the soil dur- ing the process of filling, measurement of the depth of water in the contour basins after they become filled may be far from a true measure of the total number of acre-inches per acre applied. This may be illustrated by the following test. The time required to fill to the same depth three adjoining checks containing 50, 50, and 150 trees with identically the same size stream was 1 hour, 1 hour, and 5 hours, respectively. The first two checks of equal areas received the same amount of water; but the third, although only three times as large as either of the smaller checks, received 5 times as much water, or 1% times as much per unit area, as the smaller checks. For all practical pur- poses uniformity of distribution of water throughout each individual check can be secured by adjusting check size to flow of water and type of soil. (See three examples of contour-check layouts on pages 10, 11, and 12.) [6 toying Out the contour-check system Preparing the Ground Since contour levees are constructed for the most part at 0.2-foot intervals in elevation or less, it is essential that they be located fairly accurately with respect to the general ground surface. It is usually necessary to remove the minor irregu- larities of the ground surface by disking and floating before a satisfactory survey for staking the levees can be made. Or- chards which have been plowed year after year toward the trees or away from them should be disked and floated crosswise to any ridges or depressions that may have been formed by such cultivation. It is much easier for a surveyor to lay out contour checks for a deciduous orchard during the time of the year when the trees are bare. If the operation is planned far enough in advance the ground surface can be prepared before the fall rains start and the survey can be made at any time from late fall to early spring when the ground surface happens to be sufficiently firm. Use of Contour Maps In evergreen orchards with dense foli- age, such as citrus, it may be impossible to locate contour ridges directly in the field by moving from point to point on the contours with the rod. In such cases a contour map is made first. The map locations of the levees are then marked on the trees in the orchard. The location of contour levees from a contour map is, in general, not to be rec- ommended, because the positions of con- tours on a map are usually interpolated from fewer level readings than are taken in the direct location of contours. It is well for the owner to have a map of his orchard even though it is not needed for laying out and marking the positions of the contour levees. A simple drawing on ordinary rectangular coordi- nate paper, with the positions of the trees indicated by the intersections of the cross lines and showing the positions of the va- rious contour levees, will suffice for his purposes. Such a map is often a great help in locating field ditches and cross levees where checks are too large to be irri- gated as one unit. Also it serves as a record which may be referred to when needed for replacing tree markers when an occasional one becomes obliterated or lost. MAKING THE SURVEY The services of an engineer are not necessary to lay out a system of contour levees. Those who wish to make their own survey can do the job satisfactorily if they follow closely the following instruc- tions. A three-man crew can be used to best advantage in making the survey: a levelman, a rodman, and one man to paint the tree markings along the contour ridges. The positions of contour ridges are de- termined by means of an engineer's level, and a target rod divided into feet and hundredths of a foot. With the target set at the proper reading for a given contour, the levelman directs the rodman to move up or down the slope until the cross wire of the level intersects the horizontal cross line on the target. It is sufficiently ac- curate if the cross hair of the instrument intersects the cross hair of the target to within % inch. The foot of the rod is then at the elevation of the required con- tour, and the adjacent tree, or trees, on the upper side of the contour are indi- cated to the painter by the rodman for marking. The rodman then moves along to the next space between trees. The rod should be held in both the longitudinal and the cross-tree rows. Some difficulty may be experienced in making the survey because of sighting [7] through the trees. A bright, clean target will make it easier to see the rod through the instrument when the orchard is in full foliage. A short tripod is sometimes an advantage as it allows the instrument to be set low enough to see under the branches of the trees. In some locations it is possible to set the level on a ditch bank or other high point and by using a long rod sight over the tree tops. Portable towers for setting the level above the tree tops have been used. Surveying Equipment. It is best to use an engineer's level and target rod for this purpose. However, a farm level will serve if the sight distance is kept short (200 feet or less) and extra care is taken to keep it level at all times. Regardless of which instrument is used it must be in good adjustment. The rod which usually accompanies a farm level is divided in feet, inches, and fractions of an inch, which makes it difficult and unsatisfac- tory to use when level notes must be kept. A satisfactory cheap substitute for the regular engineer's target rod is a printed cloth strip, called "flexible leveling rod" which is tacked to a long, narrow, thin board. Such a rod has no target. How- ever, a red rag tied around the board, which may be moved up and down to the required rod reading, can be used instead. Instructions for Leveling and Note Keeping (Follow the sample notes given below) Start surveying at the high point of the orchard. A starting point with an assumed elevation is necessary. This point is called bench mark, and desig- nated B.M. Drive a nail into the base of a tree, mark a spot on a concrete struc- SAMPLE LEVEL NOTES (left-hand page of field book) John Doe Land Company May 11,1949 (right-hand page of field book) J.B.B.— Level E.F.S.— Rod S.M.D.— Paint (1) (2) (3) (4) (5) Eleva- (6) Notes (1) Station + H.I. — tion Color (2) B.M. 1.82 101.82 100.00 Top of concrete stand, S.E. Cor. assumed. (3) . . . . 2.40 99.4 Elevation high point in orchard. (4) . . . . 2.62 99.2 Y Fixed elevation for highest con- (5) .... 2.82 99.0 B tour. (6) .... 3.02 98.8 W (Letters in Column 6 indicate (7) .... 3.22 98.6 R color of contour.) (8) . . . . 3.42 98.4 Y (Minus sights to hundredths in (9) . . . . 3.62 98.2 B column 4 are target settings for (10) .... 3.82 98.0 W various contours.) (ll)T.P. 5.73 103.17 4.38 97.44 Stake on E. Bound.-200' N. of S.E. Corner. (12) .... 5.17 98.0 W Continuation of 98.0 W. Contour. (13) .... 5.37 97.8 R (14) .... 5.57 97.6 Y [3] ture, or select any other solid, permanent, convenient point for the purpose. Give this point an arbitrary elevation, such as 100 feet. All other elevations which will be established during the survey will be based on this assumed elevation. Set tripod legs of the level firmly in the ground and level the instrument. Sight on the rod held on the bench mark. Sup- pose the rod reading is 1.82 feet. This means the elevation of the line of sight of the instrument is 1.82 feet higher than the bench mark. (See line 2, level notes.) Record 1.82 in column 2, line 2. Add 1.82 to 100.00 and record the sum, 101.82 in column 3, line 2. The column heading is H.I., which means height of instru- ment. Now, in order to determine the ele- vation of any other point, it is necessary to read the rod held on that point and subtract the rod reading from the eleva- tion of the line of sight, or H.I. For example, line 3 shows that the rod reading (to the nearest 0.1 foot) at the high point in the orchard is 2.4 feet (line 3, column 4). Then 101.82 - 2.4 = 99.4, the elevation of the high point (line 3, column 5) . Dropping down 0.2 foot (the contour interval), the elevation of the highest contour is fixed at 99.2 feet. The correct target setting to give an elevation of 99.2 feet with an H.I. of 101.82 feet is 2.62 feet. With the target set at 2.62 feet, locate the 99.2 Y (yellow) contour (line 4). Run the 99.2 Y contour out to the boundaries of the orchard. Raise the target 0.2 foot to 2.82, which is the correct setting for the next lower, or 99.00 B (blue) contour. Run this con- tour to the boundaries of the orchard. For each successive lower contour raise the target 0.2 foot. Use of a Turning Point. Assume that when he comes to the 98.0 W contour the levelman cannot see through the trees and cannot run it out to the boundary (line 10) . This requires that the instrument be moved forward and that a turning point (T.P.) be used. Since it is necessary to carry the same elevations all the way through the survey, the rod is carefully read on some fixed point, such as a stake, and the elevation of its top determined. Such a point should be preserved during the survey, as it may be used several times. Points used for carrying levels forward are called turn- ing points and recorded as T.P. in the notes (column 1, line 11). Read the rod on the T.P. and record the reading, 4.38, in column 4, line 11. Subtract reading from H.I., 101.82 - 4.38 = 97.44, which equals elevation of T.P. (column 5, line 11) . Move the instrument and relevel. Sight on the rod held on the T.P. and record new reading, 5.73, in column 2, line 11. Treat as if starting from a new bench mark, that is, add reading to elevation of T.P. stake for a new H.I., 97.44 + 5.73 = 103.17 (line 11, columns 2, 3, and 5). Continuing the 98.0 W contour, the correct target setting for elevation 98.0 from an H.I. of 103.17 is 5.17 feet (line 12, columns 4 and 5). For contours 97.8 R and 97.6 Y, the corresponding target settings are 5.37 and 5.57 (lines 13 and 14). One or more permanent bench marks should be established for each field, and during the process of surveying, turning- point stakes should be protected so that there will be points of known elevation to refer to in case they are needed. Instructions for Marking Trees Mark adjacent trees on the upper side of the contour. The band of paint is usu- ally 3 to 4 inches in width. Extend it far enough around the trunk of the tree to be readily seen by the tractor operator when approaching from either direction along the contour while he constructs the levees. Where two or more contour ridges pass between two trees, mark the tree with two or more colors representing the corresponding contour ridges. Put the color representing the ridge of highest [9] elevation at the top of the series of color bands. Various color schemes are used to mark the location of contour ridges. Some employ only three colors while others employ four. The use of four colors, such as red, white, blue, and yellow, is generally preferred, as the fourth color serves to make the sequence of various series of colors a little more pronounced. Using the colors, red, white, blue, and yellow, in ascending elevation, the lowest con- tour ridge would be indicated by red, the next above by white, the next by blue, and the highest by yellow. The contour next above the yellow would be indicated by red, and so on. In descending order from the high point of an orchard, the sequences would be yellow, blue, white, red, yellow, blue, et cetera. When pieces of tin are used to mark the position of contour ridges, the tins are attached to the trees by shingle nails. Use care in nailing so that the nails are not driven tightly against the tree, but are left with % inch to % inch of head ex- posed. When the nail is driven in tightly the growth of the tree will pull the tin over the nailhead. Various painting schemes have been devised to indicate dropped contours, but such schemes are not discussed in this circular. Painting Materials. A good quality of paint should be used. One containing a small amount of varnish or enamel, to give a somewhat glossy surface, is better than a flat-drying paint. A brush 1% inches in width is a con- venient size. A small box with a leather handle, arranged to hold four pint-sized fruit jars containing various colored paints, is handy for use in spotting trees. A separate brush should be used for each color, and all brushes should be very carefully cleaned after each day's work. Grade — [10] o o o i o o o o I o o I o o o I o t o I o o jo o J o o ! o o \ dv^ o o\ o o o\^o o x l\o cm oYs>o oX^ o o\o o o o\o o o\o s Vo'»o v^ O I0< o o o o o 33 \ o o o o "5\ o o\o o o o o o o o \p o o o o o o ,o o' o o o o o o o o o » Example: Refer again to the situation given above, where the 900-gallon flow is used on sandy soil. Suppose the or- chard in question had trees square planted 25 feet apart. As shown in table 1, this would be 70 trees to the acre. Since 1/10 acre is the area of the check, 7 trees can be included in the check. In table 2 are tabulated a few sample calculations of sizes of contour checks for different soils and irrigating streams. A comparison with these figures will indi- cate if a system is correctly designed. On lands with very slight slopes, a contour interval of 0.2 foot may enclose a check too large in area to be irrigated as a single basin. If, after applying the above test to area, soil type, and flow of water, it is decided to divide the check into smaller areas, this division may be accomplished either by reducing the in- terval between contour ridges, or by run- ning cross levees between the upper and lower contour ridges. The cross levees may be indicated by marking trunks of adjacent trees with white crosses. Table 1 . Number of Trees per Acre in Square and Hexagonal Plantings Number of trees apart (feet) Square Hexagonal planting planting 18 134 154 20 109 125 21 99 114 22 90 103 24 75 86 25 70 80 30 43 55 35 36 41 40 27 31 45 22 25 50 17 19 60 12 14 Water Supply to Interior Checks To convey water to interior checks it may be necessary to use field ditches, concrete pipe lines, or portable slip-joint pipe. (See figures 3 and 5.) Field ditches are formed by running parallel ridges at proper distances apart to carry the stream. When field ditches are run inside the orchard, their position may be marked by double bars on the trunks of adjacent trees. On slopes too steep for ordinary field ditches, where erosion of the soil may Table 2. Sample Calculations of Sizes of Contour Checks Flow of water delivered to a single check (measured in different units) Number of trees to a check (based on 100 trees per acre) Gallons per min. Southern California miner's in. Statutory miner's inches Cubic feet per second Sandy soils Loams Clays 450 900 1,350 50 100 150 40 80 120 1 2 3 5 10 15 20 40 60 50 100 150 [13] Fig. 6. A field ditch may be used to convey water to interior checks. occur, concrete pipe lines may be re- quired for conveying water to the interior checks. Short lines of portable slip-joint pipe are useful in reaching small checks on steep slopes. Types of Ridgers Many types of ridgers are in common use. Any type may be selected as long as a proper ridge is constructed— that is, watertight, of sufficient height, and with a broad, rounded crown. The common types of ridgers are as follows : Blade, or Open-V, Ridger. This type of ridger has steel or wooden blades joined together in the shape of an open- ended letter V. It is drawn with the large opening forward. Such a ridger may be used in light soils, but the resulting levee is likely to be too low in height and too peaked at the top. Single-Disk Ridgers. These ridgers are of the wheel type, with two large single disks set to throw to the middle. Some tools are constructed for use only as ridgers, while others have tool bars to which the disk mountings are attached, and to which other tools for various or- chard operations may be attached. Im- plements with tool bars provide ready adjustment of distance between disks, while tools constructed only as ridgers are not so readily adjustable. The single- disk ridgers are satisfactory in light and medium soils, but do not work up heavier soils into fine enough particles to make good levees in a single operation. By go- ing over levees more than once, the soil is settled and pulverized. Multiple-Disk Types. One type of ridger is a special tool constructed in local machine shops. It consists of sets of three disks, of varying sizes, attached to a heavy plate-metal frame. Each set of disks is composed of 26-inch, 22-inch, and 18-inch disks set with the largest disk to the inside. These ridgers are heavy and rather wide. They are not readily adjust- [14] able. They make a satisfactory levee in heavy soils, as the smaller disks behind the large inner disks work the soil into sufficiently fine particles. The front half of an ordinary heavy orchard disk also makes a satisfactory ridging tool. The disks are set to throw the soil inward and upward. In heavy soils, the tool must be weighted. If this tool is used, it is necessary to go over levees twice and in some cases in heavy soils, three times in order to get a com- pact, watertight levee of sufficient height. Orchard disks are readily adjusted in pitch and angle. Ordinarily, levees are left up for more than one irrigation. If the soil is such that much cracking takes place, it is necessary to run over the levees with a disk once before each succeeding irrigation. Where weed growth is heavy levees should be worked down between irrigations. Cost of Layouts Cost of laying out the system would be for surveying and marking. Usually a two-man party, engineer and rodman, are employed to run the contours. The orchard owner or one of his regular em- ployees can do the marking. The time required for the surveying varies with the topography and the visi- bility through the orchard trees. Prog- ress of from one to three acres per hour can be expected. The rougher the topog- raphy, the more contours there are to run. The thicker and lower the foliage, the slower is the progress in locating the contours. Cost will vary according to the local engineering fee. In some cases the mini- mum is the going charge per day. Thus the cost for surveying a very small or- chard may be relatively high. The permanent marking which is placed on the trees is done simultaneously with the surveying, and the time required will be the same as for the surveying. Cost of marking depends on how the operation is done. The wage of a farm laborer will be the main cost if he uses scrap tin dipped in the paint. If the trees are painted it will require from % to 1/10 gallon of paint per acre of orchard. Using THE CONTOUR-CHECK SYSTEM How to Irrigate Irrigation by means of contour checks should start at the upper check and con- tinue successively to the lower. This method is necessary to protect the or- chard from serious washing in case of breaks in the levees. If a break occurs at the top, there are always empty checks below to hold the water. The whole flow should be turned into each check, in turn, and the check cut off when it is filled. This individual filling of checks is the principal departure from the system used in rice irrigation, where water is spilled from upper to lower checks. In most orchard soils, irrigation by spilling from one check to another would result in the upper checks receiv- ing very much more water than the lower ones. How Much Water? In applying water to the soil, a knowl- edge of the amount of water necessary to wet different types of soil to various depths will aid in determining the total amount of water to apply. A convenient rule of thumb is the following : An acre-inch per acre of water, or water 1 inch in depth over a given area, will wet clay soil to a depth of 4 to 5 inches; loam soils, 6 to 10 inches; and sandy soils, 12 inches or more. (See formulas next page) [15] Formulas for Computing Amounts of Water 1. Water measured in cubic feet per second: Number of second-feet x hours run acre-inches per acre Number of acres ~ or inches of depth applied. 2. Water measured in statute miner's inches: Number miner's inches x hours run acre-inches per acre 40 x number of acres = or inches of depth applied. 3. Water measured in common or southern California miner's inches: Number of miner's inches x hours run acre-inches per acre 50 x number of acres = or inches of depth applied. 4. Water measured in gallons per minute: Number of gallons per minute x hours run acre-inches per acre 450 x number of acres " = °r inches of depth applied. Calculation of Amount of Water to Apply The following problem illustrates the method for finding out the right amount of water to apply : Problem: How many acre-inches per acre of water are applied to a check con- taining 24 trees, planted 22 feet apart in a hexagonal system, in 70 minutes, by a pump discharging 640 gallons per minute? 22-foot hexagonal planting = 103 trees per acre (from table 1) . 24/103 = 0.233 acre in check. 70 minutes = 1.17 hours. Applying Formula 4 above: Gallons per minute x hours 640 x 1.17 450 x acres 450 x 0.233 7.1 acre-inches per acre. For other units of water measurement, the applicable formula above should be selected. Cooperative Extension work in Agriculture and Home Economics, College of Agriculture, University of California, and United States Department of Agriculture cooperating. Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. J. Earl Coke, Director, California Agricultural Extension Service. 20m-12,'49(B6653) [16]