Evapotranspiration: Measured with a Lysimeter vs. Calculated with a Recursive Method.

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 in 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 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 in Bushland, TX, for a range of environmental conditions, to those calculated with the RCM. The RCM is based on the same physical principles of the PM but uses iteration to find an accurate answer of ET and can be easily implemented using commercially available mathematical software such as Excel® and Mathcad®.