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Table 2. Estimated seasonal water requirement (consumptive use) In western Colorado* (inches /season). *Colorado Irrigation Guide, 1988. Net irrigation requirement is the difference between crop consumptive use and effective precipitation. Figure 3: Example of daily ET during the growing season. Limited Water Management - Reduced Allocations When producers are faced with reduced surface water storage, they have three man- agement options. They can: 1.) reduce ir- rigated acreage, 2.) reduce irrigation to the entire field, or 3.) include different crops that require less irrigation. Option I will idle potentially productive ground while option 2 will reduce yields for the irrigated acres unless precipitation is above normal. Option 3 incorporates the use of crops that require less irrigation for maximum production to apply the "saved water" for traditionally irrigated crops. An example in Longmont would be irrigating all corn or irrigating some corn and dry beans. Corn requires 17.3 inches of irrigation (85 percent efficiency) and dry bean requires 10.4 inches. If the alloca- tion from the ditch limits a producer to 14 inches of water, he or she could raise 80 percent of their acres to irrigated corn and the remainder in dryland production or idle. They could also raise 100 percent of available acres to corn and apply only 80 percent of the ir- rigation required for maximum produc- tion. The final option would be to raise 50 percent of available acres to dry bean and 50 percent to corn and maintain maxi- mum production on all acreage. Limited Water Management - Low Capacity Systems When managing for maximum production, irrigation systems must have minimum capacities that meet crop water requirements during peak water use pe- riods. (See Fact Sheet 4.704, Center -pivot Irrigation Systems.) If irrigation system capacities are below what is normally required, reduced yields are expected with normal precipitation. Management strate- gies to compensate for low capacity include pre - irrigation and beginning irrigation at higher soil moisture contents. These strate- gies may maintain yields in above normal precipitation years but do not help as much in below normal precipitation years. Man- agement strategies to alleviate this problem include splitting systems into two or more crops that have different peak water needs, thus reducing the rate of water require- ments during both peak periods. Crop rotations also spread the irriga- tion season over a greater time period as compared to a single crop. When planting multiple crops such as corn and winter wheat under irrigation, the irrigation sea- son is extended from May to early October as compared to continuous corn, which is predominantly irrigated from June to early September. Crops such as corn, soybean and wheat have different timings for peak water use (Figure 3). With low capacity wells, plant- ing multiple crops with smaller acreages allows for water to be applied at amounts and times when the crop needs the water. The net effect of irrigating fewer acres at any one point in time is that ET demand of that crop can be better met. If capacities are increased by splitting acres into crops that have different water timing needs, management can be done to replace stored soil moisture rather than maintaining soil moisture near field capacity in anticipa- tion of crop ET since the system will not meet ET. Another option is to plant the entire pivot or field to a single crop. Irrigation management with low capacity systems requires that a producer maintain soil moisture at or near field capacity when ET is less than what the system can apply. When the ET for the crop is greater than the capacity of the system, plants will use stored soil moisture to maintain ET. This type of management is necessary to insure that moisture will be available for plants when they reach the reproductive growth stage. However, if precipitation is less than anticipated, soil moisture may be less than 50 percent of available during the reproductive growth stage and yields will be reduced. Colorado State University, U.S. Department of Agriculture and Colorado counties cooperating. CSU Extension programs are available to all without discrimination. No endorsement of products mentioned is intended nor is criticism implied of products not mentioned. Canon City Cortez Durango Gunnison Fruita Meeker Monte Vita Norwood Salida Walden Alfalfa 39.69 29.36 27.49 17.99 36.22 23.55 23.58 23.58 24.83 12.89 Grass hay /pasture 33.49 24.74 23.17 17.12 31.44 21.43 19.85 20.40 20.90 13.61 Dry beans 19.93 Corn, grain 25.12 Corn, silage 22.21 17.98 16.06 22.67 17.34 Corn Melons Orchards w/o cover crop 27.12 Orchards w/ cover crop 25.71 Potatoes 16.49 Small vegetables 18.06 6.79 Sorghum, grain Soybeans Spring grains 13.51 14.79 16.73 19.61 15.46 12.66 11.38 18.04 (barley, wheat) Sugarbeets 31.58 Wheat, winter 18.70 20.13 18.83 18.95 Av. Precipitation 12.99 12.90 18.59 11.00 8.30 17.06 7.25 15.73 11.37 9.56 Av. Effective Precipitation 9.28 5.09 8.34 3.80 3.98 6.19 3.93 6.05 5.66 3.02 *Colorado Irrigation Guide, 1988. Net irrigation requirement is the difference between crop consumptive use and effective precipitation. Figure 3: Example of daily ET during the growing season. Limited Water Management - Reduced Allocations When producers are faced with reduced surface water storage, they have three man- agement options. They can: 1.) reduce ir- rigated acreage, 2.) reduce irrigation to the entire field, or 3.) include different crops that require less irrigation. Option I will idle potentially productive ground while option 2 will reduce yields for the irrigated acres unless precipitation is above normal. Option 3 incorporates the use of crops that require less irrigation for maximum production to apply the "saved water" for traditionally irrigated crops. An example in Longmont would be irrigating all corn or irrigating some corn and dry beans. Corn requires 17.3 inches of irrigation (85 percent efficiency) and dry bean requires 10.4 inches. If the alloca- tion from the ditch limits a producer to 14 inches of water, he or she could raise 80 percent of their acres to irrigated corn and the remainder in dryland production or idle. They could also raise 100 percent of available acres to corn and apply only 80 percent of the ir- rigation required for maximum produc- tion. The final option would be to raise 50 percent of available acres to dry bean and 50 percent to corn and maintain maxi- mum production on all acreage. Limited Water Management - Low Capacity Systems When managing for maximum production, irrigation systems must have minimum capacities that meet crop water requirements during peak water use pe- riods. (See Fact Sheet 4.704, Center -pivot Irrigation Systems.) If irrigation system capacities are below what is normally required, reduced yields are expected with normal precipitation. Management strate- gies to compensate for low capacity include pre - irrigation and beginning irrigation at higher soil moisture contents. These strate- gies may maintain yields in above normal precipitation years but do not help as much in below normal precipitation years. Man- agement strategies to alleviate this problem include splitting systems into two or more crops that have different peak water needs, thus reducing the rate of water require- ments during both peak periods. Crop rotations also spread the irriga- tion season over a greater time period as compared to a single crop. When planting multiple crops such as corn and winter wheat under irrigation, the irrigation sea- son is extended from May to early October as compared to continuous corn, which is predominantly irrigated from June to early September. Crops such as corn, soybean and wheat have different timings for peak water use (Figure 3). With low capacity wells, plant- ing multiple crops with smaller acreages allows for water to be applied at amounts and times when the crop needs the water. The net effect of irrigating fewer acres at any one point in time is that ET demand of that crop can be better met. If capacities are increased by splitting acres into crops that have different water timing needs, management can be done to replace stored soil moisture rather than maintaining soil moisture near field capacity in anticipa- tion of crop ET since the system will not meet ET. Another option is to plant the entire pivot or field to a single crop. Irrigation management with low capacity systems requires that a producer maintain soil moisture at or near field capacity when ET is less than what the system can apply. When the ET for the crop is greater than the capacity of the system, plants will use stored soil moisture to maintain ET. This type of management is necessary to insure that moisture will be available for plants when they reach the reproductive growth stage. However, if precipitation is less than anticipated, soil moisture may be less than 50 percent of available during the reproductive growth stage and yields will be reduced. Colorado State University, U.S. Department of Agriculture and Colorado counties cooperating. CSU Extension programs are available to all without discrimination. No endorsement of products mentioned is intended nor is criticism implied of products not mentioned.