2008-02-07_GENERAL DOCUMENTS - C1981008

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} IRRIGATIOIJ MANAGEMENT S E R I E S Using Evapotranspiration Reports for Furrow Irrigation Scheduling !Many H. Rogers Extension Agricultural Engineer Cooperative Xxtenslon Service Manhattan, Kansas Efficient irrigation with surface- irrigation systems is a tricky task-so most flood irrigators usually can improve irrigation efficiency. One management opportunity that should improve overall water-use etBciency, is through irrigation scheduling with the use of local or regional evapotranspira- tion data. WHAT IS EVAPOTRANSPIRATION? Evapotranspiration is the term coined to describe the amount of water used by a growing crop (not the amount applied by an irrigation sys- tem). It is the combination of two words: evaporation and transpiration, and it is often referred to as ET. Any .eater, whether deposited by dew, pre- cipitation, or irrigation, can be con- sumed by the crop to fUlfill the ET requirement. The amount of ET that occurs is influenced by climatic or atmospheric conditions such as temperature, relative humidity, wind, and solar radiation. In addition, crop conditions such as stages of growth and plant health affect ET. Procedures to calculate ET based on weather and crop data have been devel- oped for many Kansas crops and are available for usu. In western Kansas, ET information may be available through radio and newspaper reports. However, if these sources are not available, other meth- ods exist to estimate ET and to aid in developing a crop water budget and irrigation schedule. HOW TO USE ET INFORMATION Irrigation scheduling using ET information is like a checkbook accounting procedure. ET is the amount of crop water withdrawal that mast be balanced against water deposits of rainfall and irrigation. The water balance must be kept within the limits of crop stress as determined by the field condition, irrigation capacity, and crop varlwy. Through the schedul- ing procedure, the amount of watei application required and the tine of application can be determined. 73 IRRIGATION SCHEDULING To schedule flood irrigation using figures, you must follow these steps. 1. Determine the total crop water use (M since the last update of soil water status; 2. Determine the total effective rain- fall and irrigation amount since the last update of soil water status; 3. Update the soil water status; and 4. Begin irrigation when the soil water depletion equals or exceeds the net irrigation application amount. This procedure will maintain soil water at or above the allowable soil water depletion if the irrigation system has the capacity to supply water demanded by the crop at its peak rate of water use_ If the system capacity is less, the crop will reduce soil water reserves unless rainfall replaces the deficiency. Periodic soil water sam- pling is recommended to ensure ade- quate soil water is being maintained. This should be done at various sites within a field. The timing' of the initial application of flood irrigation is not easily deter- mined. Only a large application nor- mally can be applied uniformly, and often the first itrigation must begin early in order to water the final set before it reaches critical toil water depletion. This means several of the first irrigation sets may be over- irrigated because of the early irrigation, and often full root development has not becu achieved, reducing the effective total soil water storage. Before scheduling can begin, the following preliminary information about field conditions must be determined. A. DETERMINE THE ACTIVE CROP ROOT ZONE. The active root zone of the crop is dependent on crop type, its stage of maturity, and soil conditions. Soil con- ditions, such as hard pans, may restrict root development. This example assumes corn grown on good soil and uses a managed root zone of 3 feet. The root zones of corn and other crops can exceed 3 feet, but since the major- ity of the roots and, therefore, the majority of water withdrawal is from 3 feet or less, a 3-foot root zone is commonly used. Any root development