Runoff, erosion, and water quality of agricultural watersheds in central Navarre (Spain)

Two experimental watersheds, La Tejeria (1.69 km(2)) and Latxaga (2.07 km(2)), appointed by the Government of Navarre (Spain) for assessing the effect of agricultural activities on the environment, were monitored during 10 years (1996-2005). Both watersheds are roughly similar with regard to soils, climate (humid sub Mediterranean) and land use (almost completely cultivated with winter grain crops). The first results for both sites on runoff, exported sediment, nitrate and phosphate are presented. Most runoff, sediment, nitrate and phosphate yields were generated during winter, when variability was also the highest of the whole year. La Tejeria had much higher sediment concentrations and sediment yield than Latxaga. Nitrate concentrations were also significantly higher at La Tejeria, with values constantly over the critical threshold (>50 mg NO3 1(-1)). However, phosphate concentrations were similar in both watersheds and corresponded to water with a significant risk of eutrophication. Differences in watershed behaviour could be mainly due to differences in morphology, topography, and amount of stream channel vegetation between both sites. This is an unprecedented research for the region and the generated dataset is of paramount importance for research issues such as hydrology, erosion and water quality. The results highlight the complexity of Mediterranean agricultural landscapes and the need for further analyses to better ascertain the processes behind them. (C) 2008 Elsevier B.V. All rights reserved.