Separate Estimation of Maize Evapotranspiration Components by Using Experimental Models under Water Stress Conditions

For irrigation planning, parameters such as actual crop water needs (transpiration) and water losses (evaporation) are considered. In this research, for management of deficit irrigation, the amounts of maize evapotranspiration components were simulated under water stress conditions. Water stress was applied by reducing the soil water, relative to the readily available water. Four treatments were defined as depletion of the available soil water by 40% (I₀), 55% (I₁), 70% (I₂), and 85% (I₃). The amounts of maize evapotranspiration and its components (transpiration and evaporation rates separately) were measured in a mini-lysimeter. The seasonal total values of evapotranspiration and components of transpiration and evaporation were equal to 443, 319 and 124 mm (I₀), 401, 282 and 119 mm (I₁), 303, 211 and 92 mm (I₂), and 201, 127 and 74 mm (I₃), respectively. Soil water deficiency reduced the evapotranspiration and its components relative to the normal conditions (treatment I₀). Reduction of evaporation losses was favorable point in this deficit irrigation method (long irrigation interval). Transpiration and evaporation values were simulated based on the evapotranspiration data (in I₀), evapotranspiration stress coefficient (K_s), and crop growth stage sensitivity (K_pi). For this purpose, we used the linear, exponential, logarithmic, polynomial, and power functions as the regression models. By using the actual data, unknown coefficients in the functions were estimated by SPSS software and regression models were generated. Statistical analyses showed that the linear function (R²= 0.91) and polynomial function (R²= 0.874) were the optimal models for estimation of transpiration and evaporation components (under water stress conditions), respectively. The actual water requirement of crop and evaporation losses can be estimated more accurately by separate estimation of evapotranspiration components. This would provide a suitable criterion for irrigation planning and calculation of water use efficiency.