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United States Department of Agriculture

Agricultural Research Service

Research Project: DEVELOPMENT OF WATER MANAGEMENT TECHNOLOGICES FOR THE MID SOUTH Title: Effect of reference ET method on irrigation scheduling model

Authors
item Fisher, Daniel
item Pringle Iii, H -

Submitted to: National Decennial Irrigation Conference
Publication Type: Proceedings
Publication Acceptance Date: October 5, 2010
Publication Date: December 5, 2010
Citation: Fisher, D.K., Pringle III, H.C. 2010. Effect of reference ET method on irrigation scheduling model. National Decennial Irrigation Conference. Paper Number: IRR10-9814.

Interpretive Summary: A common used method of scheduling irrigations involves maintaining a soil-water deficit. A critical component, evapotranspiration, is estimated using a weather-based model, but a variety of models are available, ranging from simple empirical equations to highly sophisticated, physically based models. The various methods produce a range of estimates which can vary considerably, affecting daily evapotranspiration estimates, water- balance calculations, and irrigation schedules. A variety of evapotranspiration methods were used in two water-balance scheduling models to examine the variability in the resulting irrigation schedules. The resulting schedules were greatly influenced by the evapotranspiration method used, and the number of irrigations required varied considerably. Selection of the appropriate method for estimating evapotranspiration is necessary for properly scheduling irrigations. If the standard FAO-56 method cannot be used and an alternate method is required, care must be taken to ensure that the alternate method provides accurate estimates under local environmental conditions. Water-balance irrigation scheduling models will then provide better guidance for proper timing of water applications, resulting in yield, quality, and water-use improvements.

Technical Abstract: A commonly used method of scheduling irrigations involves maintaining a soil-water balance and estimating a daily soil-water deficit. The water balance accounts for water moving into the soil (as irrigation or rainfall) and out (as crop evapotranspiration or runoff). A critical component, evapotranspiration, is often estimated by calculating a reference evapotranspiration (ETo) and adjusting it with a crop coefficient function. Many ETo models are available which use various types of weather data and range from simple empirical equations to highly sophisticated, physically based models. The various methods produce a range of estimates which can vary considerably, affecting daily crop evapotranspiration and water-balance and soil-water deficit calculations. In the humid Mid-South region, rainfall can be frequent during the growing season, replenishing the soil-water reserves and resetting the deficit in the soil-water balance. A question arose regarding the required accuracy of ETo estimates: under conditions of frequent rainfall, was it necessary to use a sophisticated, data- and calculation-intensive ETo model or could a simpler model be used without greatly affecting the irrigation schedule? This paper discusses an irrigation scheduling simulation for two crops during 13 growing seasons using two scheduling models, a spreadsheet water-balance model and a computer program (the Arkansas Irrigation Scheduler). A variety of ETo methods were used, and the impact of the ETo estimates on daily seasonal irrigation schedules was examined.

Last Modified: 12/17/2014
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