Assessing hydrologic alterations due to reservoirs and intensified irrigation in a semi-arid agricultural river basin using SWAT*

Basins with hydrological modifications pose challenges in hydrological modelling, especially when there are limited data about basin characteristics. Models, however, are vital if we are to identify undisturbed hydrological conditions and evaluate the extent of modifications. In this study, we used SWAT to model the hydrological processes in a semi-arid closed basin, where basin hydrology was heavily modified with reservoirs and irrigation canals and drains. Modelling was also challenging due to limited data about basin characteristics and irrigation management. We modelled the hydrological processes using topographic, land use/cover, soil and climate data. The auto-irrigation tool in SWAT, which determines the timing and amount of irrigation based on the plant water stress factor, was used to represent irrigation. We successfully calibrated/validated the model for streamflow and reservoir storage and confirmed it further by analysing simulated water volumes in wetlands collecting drainage flows. We used the model to simulate the undisturbed hydrologic state and the effects of intensified irrigation. This study showed that SWAT can provide reasonable predictions of hydrological processes in agricultural basins where the natural flow regime is altered by man-made structures. Simulations with the model revealed that the construction of reservoirs followed by land use changes and intensified irrigation have affected streamflows and downstream ecosystems significantly.