Estimation of Pollutant Loads in Ardila Watershed Using the SWAT Model

Abstract

Excess of organic matter and nutrients in water promotes eutrophication process bserved in the Ardila River. It was classified as much polluted being critical for Alqueva-Pedrogão System. The aim of this study was to estimate the transported nutrients loads in a transboundary watershed using the SWAT (soil and water assessment tool) model and to determine the contribution of nutrients load in the entire watershed. Ardila watershed is about 3,711 km2 extended from Spain (78%) to the eastern part of Portugal (22%). It was discretized into 32 sub-basins using automated delineation routine, and 174 hydrologic response units. Monthly average meteorological data (from 1947 to 1998) were used to generate daily values through the weather generator Model incorporated in SWAT. Real daily precipitation (from 1931 to 2003) was introduced. The model was calibrated and verified for flow (from 1950 to 2000) and nutrients (from 1981 to 1999). Model performance was evaluated using statistical parameters, such as NSE (Nash-Sutcliffe efficiency) and root mean square error (R2). Calibration and verification flow results showed a satisfactory agreement between simulated and measured monthly date from 1962 to 1972 (NSE = 0.8; R2 = 0.9). The results showed that the most important diffuse pollution comes from the two the main tributary (Spain). The estimated nitrogen and phosphorous loads contribution per year was respectively 72% and 59% in Spain and 28% and 41% in Portugal. The SWAT model was revealed to be a useful tool for an integrated water management approach that might be improved taking into count the WFD (water framework directive).