Comparing SWAT with SWAT-MODFLOW hydrological simulations when assessing the impacts of groundwater abstractions for irrigation and drinking water

Abstract. Being able to account for temporal patterns of streamflow, the distribution of groundwater resources, as well as the interactions between surface water and groundwater is imperative for informed water resources management. We hypothesize that, when assessing the impacts of water abstractions on streamflow patterns, the benefits of applying a coupled catchment model relative to a lumped semi-distributed catchment model outweigh the costs of additional data requirement and computational resources. We applied the widely used semi-distributed SWAT model and the recently developed SWAT-MODFLOW model, which allows full distribution of the groundwater domain, to a Danish, lowland, groundwater-dominated catchment, the Uggerby River Catchment. We compared the performance of the two models based on the observed streamflow and assessed the simulated streamflow signals of each model when running four groundwater abstraction scenarios with real wells and abstraction rates. The SWAT-MODFLOW model complex was further developed to enable the application of the Drain Package of MODFLOW and to allow auto-irrigation on agricultural fields and pastures. Both models were calibrated and validated, and an approach based on PEST was developed and utilized to enable simultaneous calibration of SWAT and MODFLOW parameters. Both models demonstrated generally good performance for the temporal pattern of streamflow, albeit SWAT-MODFLOW performed somewhat better. In addition, SWAT-MODFLOW generates spatially explicit groundwater-related outputs, such as spatial-temporal patterns of water table elevation. In the abstraction scenarios analysis, both models indicated that abstraction for drinking water caused some degree of streamflow depletion, while abstraction for auto-irrigation led to a slight total flow increase (but a decrease of soil or aquifer water storages, which may influence the hydrology outside the catchment). In general, the simulated signals of SWAT-MODFLOW appeared more plausible than those of SWAT, and the SWAT-MODFLOW decrease in streamflow was much closer to the actual volume abstracted. The impact of drinking water abstraction on streamflow depletion simulated by SWAT was unrealistically low, and the streamflow increase caused by irrigation abstraction was exaggerated compared with SWAT-MODFLOW. We conclude that the further developed SWAT-MODFLOW model calibrated by PEST had a better hydrological simulation performance, wider possibilities for groundwater analysis, and much more realistic signals relative to the semi-distributed SWAT model when assessing the impacts of groundwater abstractions for either irrigation or drinking water on streamflow; hence, it has the potential to be a useful tool in the management of water resources in groundwater-affected catchments. However, this comes at the expense of higher computational demand and more time consumption.