Suspended sediment transport in a Mediterranean agricultural catchment

Abstract

The aim of this study is to analyze suspended sediment transport in a Mediterranean agricultural catchment under traditional soil and water conservation practices. Field measurements were conducted in Can Revull, a small catchment (1.03 km2) on the island of Mallorca. The mean annual precipitation is 517 mm. The hydrological regime is intermittent, with a mean annual discharge of 4.1 l s-1. This study analyses suspended sediment transport regime, interprets multiple regression models of total suspended sediment concentration (SSC) and of SSC related to stormflow discharge (storm-SSC), and assesses sediment loads and yields based on 3 hydrological years of turbidity records (2004-2005, 2005-2006 and 2006-2007). Annual average SSC was 17.3 mg l-1 with a maximum value of 2,270 mg l-1 (recorded at the middle of the winter period when rainfall intensities are high and headwater slopes are ploughed and thus bare). Strong seasonal contrasts of baseflow dynamics associated with different degrees of dilution provide a large scatter in SSC and in the derived rating curves reflecting that factors other than discharge primarily control the supply of suspended sediment. Multiple regression models identify rainfall intensity as the most significant variable in sediment supply. However, when there is baseflow, physical and biological processes generate sediment in the channel which is easily removed during high flows. In contrast, when baseflow is not present, rainfall intensity is the only variable that supplies sediment to the channel mostly from slopes. Considering study period as average in terms of mean annual rainfall and related intensities, suspended sediment yields were an order of magnitude lower than other catchments in Mediterranean rivers caused by the historical implementation of soil conservation practices.