Suspended sediment delivery from small catchments to the Bay of Biscay. What are the controlling factors?

Abstract

The transport and yield of suspended sediment (SS) in catchments all over the world have long been topics of great interest. This paper addresses the scarcity of information on SS delivery and its environmental controls in small catchments, especially in the Atlantic region. Five steep catchments in Gipuzkoa (Basque Country) with areas between 56 and 796 km2 that drain into the Bay of Biscay were continuously monitored for precipitation, discharge and suspended sediment concentration (SSC) in their outlets from 2006 to 2013. Environmental characteristics such as elevation, slope, land-use, soil depth and erodibility of the lithology were also calculated. The analysis included consideration of uncertainties in the SSC calibration models in the final suspended sediment yield (SSY) estimations. The total delivery of sediments from the catchments into the Bay of Biscay and its standard deviation was 272 200 ± 38 107 t yr.−1, or 151 ± 21 t km−2 yr.−1, and the SSYs ranged from 46 ± 0.48 to 217 ± 106 t km−2 yr.−1. Hydroclimatic variables and catchment areas do not explain the spatial variability found in SSY, whereas land-use (especially non-native plantations) and management (human impacts) appear to be the main factors that control this variability. Obtaining long-term measurements on sediment delivery would allow for the effects of environmental and human induced changes on SS fluxes to be better detected. However, the data provided in this paper offer valuable and quantitative information that will enable decision-makers to make more informed decisions on land management while considering the effects of the delivery of SS.