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ARS Home » Plains Area » Bushland, Texas » Conservation and Production Research Laboratory » Soil and Water Management Research » Research » Publications at this Location » Publication #210589

Title: Experimental verification of a recursive method to calculate evapotranspiration

item Evett, Steven - Steve

Submitted to: Irrigation Associations Exposition and Technical Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/15/2007
Publication Date: 12/9/2007
Citation: Lascano, R.J., Evett, S.R. 2007. Experimental verification of a recursive method to calculate evapotranspiration. In: Proceedings of the 28th Annual International Irrigation Show, December 9-11, 2007, San Diego, California. p 687-705. 2007 CDROM.

Interpretive Summary: Irrigation of agricultural crops consumes 60% of U.S. fresh water supplies, which is why applying the correct amount of water, not too much nor too little, is important nationally. All farmers understand that crops need water to grow – water is life. And, all farmers know that too much or too little water can decrease crop yields. Too much water can drown plants or leach costly fertilizers to depths below the crop’s roots where both the water and fertilizer are permanently lost – both of which result in decreased yields and profits. Too little water also reduces yields due to plant wilting and drying and sometimes salt accumulation. Irrigators, those farmers who apply water to crops, need to know when and how much to apply, which is why tools are provided by various government and private agencies to help make these irrigation scheduling decisions. Irrigation scheduling tools usually depend on a reference crop water use (evapotranspiration, ET) value that is calculated from weather data (sunshine, wind speed, air temperature and humidity) using the well known Penman-Monteith (P-M) equation. We developed and tested an alternative method that has the potential to be more accurate since it avoids an assumption that was made in formulating the P-M equation. Testing against alfalfa reference ET measured at Bushland, Texas, showed that the new equation was workable and accurate in a preliminary test.

Technical Abstract: Recently, a recursive combination method (RCM) to calculate potential and crop evapotranspiration (ET) was given by Lascano and Van Bavel (Agron. J. 2007, 99:585–590). The RCM differs from the Penman-Monteith (PM) method, the main difference being that the assumptions made regarding the temperature and humidity of the evaporating surface in the PM are not necessary when using the RCM. Rather, the RCM solves for ET by finding the temperature and the humidity by iteration and therefore satisfies the energy balance. We compared values of alfalfa ET, measured with a large lysimeter at Bushland, Texas, for a range of environmental conditions, to those calculated with the RCM. The RCM is based on the same physical principles as the PM except for the assumption that air and canopy temperatures are equal in the calculation of vapor pressure vs. air temperature relation. Unlike the PM, the RCM uses iteration to find an accurate answer for ET. It can be easily implemented using commercially available mathematical software.