The impact of agriculture on solute and suspended sediment load on a Mediterranean watershed after intense rainstorms

The solute and suspended-sediment load following five rainstorms (2005-2007) with varied intensities were studied at the Vernega experimental watershed, north-western Spain. Two land-use areas are located within this watershed, the upstream one (forest) with 160 ha a 100% forested area, and the downstream one (agricultural) with 97 ha being 9 ha conventional agricultural field and 88 ha forest. This study investigates the capacity of each land-use to yield water, suspended sediment concentration (SSC) and dissolved solid concentration (DSC). The hypothesis is that DSC and SSC from the agricultural area are greater than DSC and SSC of the forest area. Results showed that the agriculture area produced significantly greater mean DSC than in the forest area, the main contribution was the Ca2+ (24.68 +/- 46.52 mg l(-1)) ion at the agricultural area. A long-term sediment production rate at the agricultural outlet was calculated (69.1 tonnes per 100 years) based on the total sediment discharge (TSD) and the recurrence interval of the largest event of the five rainstorms (October 2005). Geographic information system (GIS) spatial data layers of the watershed were produced to determine the relation of tracks, landforms, slopes and forest management to SSC yield in the forest outlet (133.89 +/- 308.14 mg l(-1)) during the five rainstorms. Agriculture practices are the main cause of soil erosion at the study area. Copyright (C) 2010 John Wiley & Sons, Ltd.