Assessing soil redistribution in a complex karst catchment using fallout 137Cs and GIS

Abstract

Caesium-137 derived from nuclear testing in the past century has been widely used as a sediment tracer of soil redistribution, providing information on medium term (40-50years) erosion rates. To date, most studies have focused on individual fields of limited extent, but estimated rates and patterns of soil redistribution require upscaling to catchment level. An attempt to assess soil redistribution processes with strong geomorphic control on complex terrain, such as an internally drained karst catchment, is presented in this work. A comprehensive geomorphological survey of the Estanya Lake catchment (Spanish Pyrenees) enabled the main geomorphic elements to be identified with a grid resolution of 5×5m, which was the basis for the terrain analyses. A detailed DEM was constructed and improved after field identification of the drainage system and point measures on gentle slopes with Total Topographic Station. A 100×100m sampling grid was established to derive point data of 137Cs inventories across the catchment. Geostatistical interpolation of point samples of 137Cs inventories in soil was carried out to identify and assess the areas of sediment gain and loss by comparing with 137Cs reference inventories. Profile activity models were used to calibrate 137Cs data and to derive estimates of soil redistribution. GIS was used to provide spatially distributed erosion and deposition rates in the landscape for the whole catchment. The median values of soil erosion and deposition of 4.7 and 3.1Mgha-1year-1, respectively, showed a large variability and reveal coupling between erosional and depositional forms in the catchment. The highest erosion and deposition rates were found in cultivated fields at the southern sector of the catchment. The spatial analysis of the results supports the importance of taking into account geomorphological features in karst catchments as relevant controlling factors in soil redistribution processes.