Modeling arid/semi-arid irrigated agricultural watersheds with SWAT: Applications, challenges, and solution strategies

We review over twenty years of publications on Soil and Water Assessment Tool (SWAT) applications in arid/semi-arid irrigated agricultural watersheds. Our review reveals strict dominance of the model's use for better understanding water quantity aspects of water management. While this is to be expected given the reality of water scarcity and associated challenges for agricultural production systems in arid/semi-arid regions, the capabilities of SWAT to model water quality have been underutilized. The main modeling challenges are lack of observational data, poor data quality, concerns about simulation accuracy, and technical limitations of the model despite numerous advancements in the last two decades. To deal with these challenges, modelers (i) combined data from different sources with those in existing SWAT databases, (ii) used supplemental tools to estimate missing data and evaluate model performance, (iii) traded off simulation accuracy when the major aim of the study was not undermined, and (iv) developed modular codes, tools, and algorithms to expand the model's capabilities and improve process representations. A realistic simulation of regional hydrologic fluxes and agricultural water management practices that affect them is essential for meaningful SWAT applications in irrigated dry regions. Modelers are encouraged to apply multi-component calibration (e.g., streamflow, evapotranspiration, crop yield, and groundwater recharge) and report limitations and regional relevance of parameter values to contextualize model performance. We anticipate that model advancements such as conjunctive use of surface water and groundwater, explicit simulation of different irrigation systems, dynamic land use, and impacts of soil and water salinity on crop growth would significantly enhance the utility of SWAT in arid/semi-arid irrigated areas.