Modeling irrigation scheduling under shallow groundwater conditions as a tool for an integrated management of surface and groundwater resources

Abstract

To restructure land- and irrigation-water use in Khorezm towards sustainability and economical feasibility, the current water use demands improvement. This requires increasing water use efficiency as much as possible, while at the same time minimizing negative impacts on the production system. These objectives can be reached with an integrated management of the irrigation and drainage system. To develop optimal management strategies, models describing the water distribution (irrigation scheduling model) and analyzing the impact on the groundwater (groundwater models) will be very helpful. In the Water Users Association (WUA) Shomakhulum, located in the southwest of Khorezm and with an irrigated area of approximately 2,000 ha, current irrigation strategies were monitored. Overall irrigation efficiency of the sub-unit representing the WUA is rather low (33%). Besides the poor state of the irrigation infrastructure, major reasons for the low efficiency are on the one hand a lack of detailed and up-to-date information on the system and its temporal behavior, and on the other hand missing options to consider detailed information in the procedures to establish water distribution plans. To tackle these issues, the irrigation scheduling model FAO CROPWAT was applied as an alternative to the current rather rigid water distribution planning. Feeding the model with detailed information on the irrigation system and its behavior (application efficiency by field-water balancing, network efficiency based on ponding experiments) provided a powerful tool to improve water use. As the groundwater in Shomakhulum is shallow, the model was further developed in order to assess the importance of the capillary rise. Therefore, the soil-water model Hydrus-1D was applied. The results of the study show that capillary rise is an important factor in water balancing and can contribute a maximum of 28% of crop-specific evapotranspiration in cotton, 12% in vegetables and 9% in winter wheat. In-practice irrigation scheduling, when simulated and assessed with the CROPWAT model, showed a 7-42% reduction in cotton yield. If the overall irrigation efficiency is improved to 56%, water saving of 41% can be achieved. Introducing alternative crops to cotton can result in 6% water saving. About 15-20% of the water can be saved by leaving marginal lands, i.e., land of low quality, out of the production.