Assessing Water and Nutrient Long-Term Dynamics and Loads in the Enxoé Temporary River Basin (Southeast Portugal)

Abstract

The Enxoé reservoir has been exhibiting frequent high chlorophyll-a concentrations (reaching a geometric mean six times the national limit for eutrophication of 10 μg L−1) since 2000, and represents the reservoir with the highest eutrophic state in Portugal. Toxic algal blooms have also been observed, which pose serious challenges to water managers, as the reservoir is used for potable water production. In an effort to contribute to the reduction of the reservoir trophic state, the watershed inputs (monthly flows, sediment, nitrogen (N) and phosphorus (P) loads) were characterized with the Soil Water Assessment Tool (SWAT). Field data were collected in the ungauged watershed during 2010 and 2011. Model results were then used to characterize the long-term watershed dynamics in terms of water and nutrients. SWAT estimates of the simulated flow, and the sediment and nutrient loads were in good agreement with field data (R2 between 0.42–0.78; Nash-Sutcliffe efficiencies between 0.19–0.75). The Enxoé River was characterized by a temporary flushy regime where high concentrations were transported in short time periods. As a result, nutrient loads delivered to the Enxoé reservoir were estimated to be 18 tonN year−1 and 0.7 tonP year−1 (30 years’ simulation), reaching the reservoir mainly by runoff. These results were consistent with the gentle slopes, extensive agricultural activities, and low urban pressure observed in Enxoé. The magnitude of the nutrient exports suggests that the reservoir eutrophication may also be linked to the reservoir geometry (average depth of 5 m), which provides high light availability to the bottom sediments. Thus, SWAT results were integrated into a reservoir model to depict the origin of the Enxoé trophic state and test management scenarios that may reduce it.