Performance Evaluation of CROPWAT 8.0 Model to Predict and Estimate Water Consumption and Soybean Yield Using Field Data in Karaj

Present limitations on available water resources especially for use in agriculture sector, shows that more attention to irrigation water management is inevitable. Deficit irrigation is a practice to improve use of unit volume of water. In this research, efficacy of CROPWAT 8.0 model was evaluated against field data under deficit irrigation management on soybean in Karaj. In 2008 and 2009 agronomical years, this experiment was conducted in the form of complete randomized blocks design. Furrow surface irrigation treatments consisted of conventional deficit irrigation treatments at 75% and 50% soil moisture deficit compensation (DI75% and DI50%), partial root drying treatment at 50% soil moisture deficit compensation (PRD50%) and full irrigation treatment (FI). The actual amounts of crop coefficient which we got in our research studies were substituted in the model. The results showed that the model evaluation indices RMSE, CRM and SD were equal to 3.47, -0.14 and 15.2 respectively in agronomical year 2008 and were equal to 4.22, -0.11 and 3.77 in agronomical year 2009. Also, the estimated yield reduction by the model was very close to actual results. These conclusions were achieved by replacing some management parameters in the model such as crop coefficients for the recommended values of the model. Positive amounts were obtained for EF index (0.87 and 0.91) which indicates the high efficacy of model in predicting crop yield. This study showed that application of CROPWAT 8.0 model by adjusting crop coefficients in the different growth stages, length of plant growth period during planting, and soil and plant characteristics can lead to acceptable results.