2008-02-07_GENERAL DOCUMENTS - C1981008

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beyond 3 feet can be considered a safety factor. Early in the season, the root zone may be less than 3 feet and should be properly accounted for within the scheduling procedure. Production handbooks, available through your county extension agents, for the major Karrsas crops can help detennine vari- ous root development ranges.. 0. DETERMINE THE AMOUNT OF SOIL WATER STORAGE CAPACITY IN THE ROOT ZONE. The soil texture influences the water-holding capacity of the soil; the coarser the texture, the less the holding capacity. Holding capacities of soiree common Kansas sots are available in KSLI Extension bulletin LA04, Soil. Water and Plant Relation.vhl^ county soil surveys, or the SCS Kansas Irrigation Guide. Contact a county extension agent for assistance. For this example, assume a silt loam type with a 2-inch per foot water-holding capac- ity. The total soil water available would be 2 inches per foot times a 3-foot root zone or 6 inches. C. DETERMINE THE AMOUNT OF ALLOW- ABLE SOIL WATER DEPLETION BEFORE IRRIGATION IS STARTED, Crops have differing levels of water depletion tolerance. Too much deple- tion stresses the crop and depresses yields; too frequent watering wastes water, fuel, and fertilizer and could also depress yields. A general irrigation guideline for most field crops is to maintain at least 50 percent of avail- able soil water during the bulk of the growing season. Depletion of 60 to 70 percent late in the season may be permissible without yield loss. The allowable depletion for this example will be 50 percent of 6 inches total allowable storage for a 3-foot root zone, which equals 3 incites. In addition, it is highly recom- mended that soil water monitoring be used as a backup to supplement the ET information and the effective amount of rainfall-ensuring soil water reserves are being maintained. General irrigation guidelines for various crops are available in Extension publications. For additional information on sails and soil water monitoring, ask a county extension agent about bulletins from the Irrigation Water Management Series and crop production handbooks. 0. DETERMINE THE IRRIGATION APPLIGA. TION AMOUNT. The irrigation application amount can be calculated by the following formula: Gross Irrigation Application (inches) _ X T 430 x where; Q - flow rate, gpm+ T - length of application, hours A = area being irrigated, acres 450 is a conversion constant: 450 gpm - I acre-inch/hour gpm - gallons per minute The area being irrigated, A, is deter- mined using the following formula; A - set size or area (acres) NxRxL 43,560 where: N = number of wetted furrows R = width between wetted furrows, feet L = row length, feet 43,560 is a conversion constant: 43,560 square feet = i acre Gross application represents the total amount of irrigation water deliv- ered to the set. The important amount in scheduling is the net irrigation amount, which is gross irrigation minus losses such as deep percolation and runoff. Use the best estimate available for system cfftcnency (Net irrigation = gross irrigation x system efficiency), Periodic soil water sam- pling can be used to adjust the soil water figures and help determine sys- tem efficiency. Table I lists expected irrigation efficiency ranges for various levels of surface irrigation management. EXAMPLE CALCULATION FOR IRRIGATION APPLICATION AMOUNT Required infomtation: Flow rate - Q = 1125 gpm Row width - 30 indocs or 2.5 feet Alternate rows are watered therefore R = 5 feet Number of gates open - N - 50 Set time - T - 12 hours Length of run = L - 1300 feet Surge flow without reuse: Efficiency estimate- 75% Field size = 150 acres Set Size Area = A-NxRxL143560 = 50 x 5 x 1300143560 7.5 acres Gross Irrigation = (Q x T)1(450 x A) R (1125 x 12)1(450 x 75) 4 inches Net Irrigation - Gross Irrigation x Efficiency -4 x 0.75 - 3 inches Another important itein to note is the total time to complete one irrigation. In this example, 7.5 acrestset are irrigated and two setslday, can be complcted. Therefore, for the 150 acre field, 10 days are needed to complete one irrigation. E. COMPARE THE NET IRRIGATION APPLI- CATION AMOUNT TO THE ALLOWABLE SOIL WATER DEPLETION. The net irrigation amount should be no more than the allowable depletion. If it is larger, the set times trust be reduced or the area irrigated increased to reduce the application anwunt. This may require other management changes to accomplish a uniform appli- cation of a. smaller irrigation. Possible actions might be furrow smoothing and packing prior to irrigating or the use of surge irrigation. SCHEDULING EXAMPLE After all the preliminary informa- tion has been determined and filled in at the top of the "Soil Water Balance Worksheet' (Table 2), irrigation sched- uling can begin. Step T. Determine the total ETsince the. Fast update ojthe soil water status. Obtain the amount of ET that occurred during this period. This can be done on a daily basis and recorded on a water balance sheet as shown in Table 2. The amount of ET may he reported as either reference ET (ETr) or actual ET. If actual ET information is obtained, record it directly into the column marled crop 6-T on Table 2, and ignore the colutruts marked ETr, Stage of Growth, and Crop Coefficient. ETr is the expected ET from a uniform, green. actively growing reference crop (i.e. atfalfa) due to atmospheric demand. Actual ET is usually less than ETr since plant characteristics of other 74