Measurements of Standard Maize Evapotranspiration Using Water Balance Method and Variable Root Depth in an Arid and Semi-Arid Region

In measurements of crop evapotranspiration by lysimeter, climate of cultivated field may be different from lysimeter point, also a lysimeter cannot be utilized for measurements of several crops during a limited time. For measurements of the standard maize evapotranspiration by the water balance method, soil water content was measured in a field experiment under furrow irrigation system during 2003 and 2004. Maize root depth was varied in respect to time and for reducing the error of measured soil moisture and calculating the stored water in soil profile, soil water content was only measured in the root zone until the root reached to its maximum length (1.8 m). In this study, the measured standard evapotranspiration of maize via water balance equation in the field, was compared with the estimated values using the methods of Penman-Monteith, HargreavesSamani and MSM2 model (Maize Simulation Model). The results showed that applying the water balance method for measuring maize standard evapotranspiration was simple and favorable in field conditions. The investigations confirmed that by use of MSM2 model, modified Hargreaves and Penman-Monteith’s equations, the standard maize evapotranspiration values could be estimated, with the errors of -2% , 5% and -25%, respectively. The results showed that with limited data condition, if only, the minimum and maximum air temperature were available, the maize standard evapotranspiration value could be estimated favorably, using the modified Hargreaves-Samani’s equation and crop coefficient.