Cotton irrigation scheduling in central Asia: model calibration and validation with consideration of groundwater contribution

Abstract

The calibration and validation of the irrigation scheduling simulation model ISAREG for Central Asian conditions were performed using cotton field observations in the Hunger Steppe over the period 1983–87, and in the Fergana Valley for 2001–03. The calibration referred to the crop coefficients and the soil water depletion factor for no stress. Groundwater contribution was considered in computations adopting a set of parametric equations used in ISAREG. Calibration and validation were performed by comparing the observed and simulated soil water content during each crop season. Various indicators of goodness of fit were used to assess model validation. For the Hunger Steppe, the validation also included the comparison of model-computed and field-measured crop evapotranspiration, which was performed with the energy balance method. Results obtained show a good agreement between field observations and model predictions, thus allowing use of the ISAREG model to generate and assess alternative irrigation schedules aimed at improved water use in Central Asia.