Linking the field to the stream: Soil erosion and sediment yield in a rural catchment, NW Spain

Abstract

Abstract This paper presents the results of an integrated study that considered the relationship between erosion and stream sediment yield in an agroforestry catchment (16 km2) in NW Spain. The study combined measurements of soil erosion by concentrated flow at field scale with sediment yield measured at the catchment outlet during winter 2007/08. The results indicate that concentrated flow erosion, i.e., rills and ephemeral gullies, occurred on unprotected crop fields. Erosion by concentrated flow was highly discontinuous within the catchment, both spatially and temporally. Soil loss differed significantly from event to event. About 68% of soil loss took place in a single event with a sediment yield of 8.5 Mg, i.e., about 30% of the winter sediment load. Soil losses measured at the Corbeira catchment outlet (28.53 Mg) were indeed more than 5 times lower than those measured on the fields (140.5 Mg of rills and ephemeral gullies and 15.6 Mg for estimated interrill erosion) due to sediment deposition along the route from field to catchment outlet. The sediment delivery ratio was 22%. This value is relatively low due to the presence of abundant vegetation-induced sediment deposition. Sediment delivery to the catchment outlet was determined to a great extent by the spatial distribution of land use, as well as by the poor connectivity between erosion areas and the stream. This catchment is characterized by mosaic vegetation with the grasslands located at the bottom of cropped fields, making sediment delivery to the stream difficult. The results of this study suggest that in the Corbeira catchment soil loss cannot be estimated from the sum of individual field soil losses. Our observations reveal that soil erosion rates measured at one scale are not representative of sediment yield at another scale level.