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Colorado State University Extension Seasonal Water Needs and Opportunities for Limited Irrigation for Colorado Crops Fact Sheet No. 4.718 by J. Schneekloth and A. Andales` Crop water use, consumptive use and evapotranspiration (ET), are terms used interchangeably to describe the water con- sumed by a crop. This water is mainly used for cooling purposes; a negligible amount is retained by the crop for growth. For more information on ET refer to Colorado State University Extension Fact Sheet 4.715, Crop Water Use and Growth Stages. Water requirements for crops depend mainly on environmental conditions. Plants use water for cooling purposes and the driv- ing force of this process is prevailing weather conditions. Different crops have different wa- ter use requirements, under the same weather conditions. Crops will transpire water at the maximum rate when the soil water is at field capacity. When soil moisture decreases, crops have to exert greater forces (energy) to extract water from the soil. Usually, the transpiration rate doesn't decrease significantly until the soil moisture falls below 50 percent of avail- able water capacity. Knowing seasonal crop water require- ments is crucial for planning your crop plant- ing mixture, especially during drought years. For example, in the Greeley area, the seasonal water use of sugar beets is 30 inches, while corn uses only 22 inches of water. That means to fully irrigate sugar beets you need to apply 36 percent more water as compared to corn. These water requirements are net crop water use, the amount that the crop will use (not counting water losses) in an average year, given soil moisture levels don't fall below critical levels. Under ideal conditions, this net water requirement is reduced by the effective rain, which for the Greeley area is 7 inches for the growing season. The rest of the crop water requirement must be supplied by irrigation. No irrigation system is 100 percent efficient, so to apply the V. Schneekloth, Colorado State University Extension water resource specialist, U. S. Central Great Plains Research Station, Akron, Colorado; A. Andales, Colorado State University assistant professor, soil and crop sciences. 912009 Crop Series I Irrigation net water requirement to the entire field, in- crease the amount of water or multiply by the efficiency (or inefficiency) of the irrigation system. Looking at the above example, the net water requirements, after subtracting ef- fective rain, are 23 inches for sugar beets and 15 inches for corn. If the irrigation system is 85 percent efficient, apply 27 inches (gross irrigation amount) to the sugar beets crop and 17.6 inches to the corn crop to store the net water requirement in the crops' root zone. Now the difference between the seasonal gross water requirements of sugar beets and corn is 52 percent. The difference in the gross irrigation requirement amounts increases as the irrigation system efficiency decreases. Net Crop Water Requirement Net crop water requirement is estimated using models that are based on weather vari- ables. Estimate seasonal crop water require- ment by using these models and averaging weather conditions over many years. This will create an average weather year. Tables 1 and 2 are a summary of net water requirements of different crops and effective precipitation for different locations in eastern Colorado and western Colorado, respectively. To figure the net irrigation requirement, subtract the effective rain (Average Effective Precipita- tion from Tables 1 and 2) from the net crop water requirement. The gross irrigation water requirement is the net irrigation requirement divided by the irrigation system efficiency (fraction of one). For example, corn for grain in Burlington requires 26 inches of water. Effective precipitation is 11.28 inches for the season; therefore the net irrigation require- ment is 14.72 inches. The gross irrigation requirement for a center pivot with 80 per- cent irrigation efficiency is 18.4 inches. For a furrow irrigation system with 55 percent irrigation efficiency, the gross irrigation requirement is 26.7 inches. Quick Facts • Knowing seasonal crop water requirements is crucial for planning your crop mixture. • Net crop water requirements are estimated using models, based on weather variables. • To irrigate for the greatest return, producers need to understand how crops respond to water, how crop rotation enhances water availability, and how changes in agronomic practices affect water needs. © Colorado State University Extension. 4/03. Revised 9/09. www.ext.colostate.edu q; o — o