Coupling SWAT and WET Models for Modeling of Highly Anthropized Coastal Lagoons. Case Study: Mar Menor (SE Spain).

An integration of hydrological and hydrodynamic models can be implemented for the understanding of the hydrology and critical components of the water balance of coastal lagoons. In this research, the SWAT (Soil and Water Assessment Tool) model and the QWET (QGIS Water Ecosystems Tool) model were both applied to the coastal lagoon of the Mar Menor and its drainage basin known as Campo de Cartagena. The SWAT model was first calibrated and validated using remotely sensed evapotranspiration data. The results indicated a satisfactory performance of the SWAT model on a monthly scale in both calibration (R2 = 0.63, NSE = 0.62, PBIAS = 2.91%) and validation periods (R2= 0.68, NSE= 0.68, PBIAS= 2.47%). The flow simulated by SWAT was fed into the QWET model to calculate the water balance of the lagoon. The performance of the hydrodynamic model was assessed from a comparison between simulated and observed water temperatures and also by comparing the evaporation estimated by the model. With an NSE of 0.98 and a BIAS of 2.7%, the simulated daily temperatures indicated a good agreement with the observed data as they captured the temporal and interannual variations. Our water balance estimation, using the reference period 20032019, yields an average annual precipitation of 301 mm and a mean annual evaporation of 1325 mm. The average surface runoff and groundwater discharge to the lagoon are 49 hm3/year and 11 hm3/year, respectively. Extreme storm events cause annual surface runoff to vary between 8 hm3/year and 202 hm3/year. The water balance was closed with the water exchange with the Mediterranean Sea, resulting in an overall positive flow from the Mediterranean Sea of 82 hm3/year. Our study showed that particularly during the summer months, there is a considerable inflow of Mediterranean water into the lagoon, while during autumn and winter months there is a net outflow from the lagoon to the Mediterranean. This novel approach, which combines the SWAT model and the QWET tool, provides a useful framework for understanding the hydrology of the Mar Menor lagoon and can also help decision-makers in developing strategies to mitigate eutrophication. Beyond the Mar Menor study, the methodological approach applied in the present study may also be useful for many other highly anthropized coastal lagoons where observed data are scarce.