Improved representation of groundwater-dominated catchment using SWAT+gwflow and modifications to the gwflow module
Abstract. Recent water availability and scarcity problems have highlighted the importance of surface water-groundwater interactions. Thus, groundwater models such as Modular ground-water Flow (MODFLOW) were coupled to the Soil and Water Assessment Tool (SWAT (+)). However, this solution is complex, needing code modifications, complex coupling, and high computation time. Lately, a new groundwater module (gwflow) was developed directly inside the SWAT+ code to tackle those issues. This research assesses gwflow’s capabilities in representing surface – groundwater systems interactions in the Dijle catchment, Belgium. A hydrological model was set up using the standalone SWAT+ and SWAT+gwflow. In addition, the interaction between the soil and the groundwater is not represented in the new module, hence, it was modified to account for such exchanges. Finally, pumping is also included in the module to enable the modeling of transient state conditions. Model comparison is made using Nash-Sutcliffe efficiency (NSE) for the calibration period (1986 to 1996) and two validation periods (1975 to 1983 and 1997 to 2002).
It is found that the SWAT+gwflow model is better representative (NSE of 0.6) than the standalone SWAT+ (NSE of 0.4). This is signified during two validation periods where the standalone scored negative NSE while the new model’s NSE was 0.7 and 0.5. This shows that, in a highly groundwater-driven catchment of this type, the simplistic representation of groundwater systems by the standalone SWAT+ model has pitfalls. In addition, the modification we made on the gwflow module has improved the model performance as groundwater-soil interaction is inevitable whenever the water table reaches the soil profile. In conclusion, groundwater-surface water processes need to be appropriately designated in hydrological models; hence, the modification we made to the gwflow module (groundwater–soil interaction) is found to be critical. This novel modification can also have an implication for other distributed hydrological models to consider such exchanges in their modeling scheme. This paves a road towards refining coupled ground-surface water hydrogeological models. Finally, the modified SWAT+gwflow model is more appropriate for assessing the Dijle catchment hydrology than the standalone model.
Estifanos Addisu Yimer et al.