Assessment of Soil Erosion and Sedimentation Rates in 'My Bouchta' Watershed in North Morocco using Fallout Radionuclides and Stable Isotopes

摘要

Die aim of this work was to combine measurements of fallout radionuclides (FRN's) Caesium-137 (~(137)Cs) and excess Lead-210 (2,0Pbex) with stable isotopes of carbon and nitrogen (6~(13)Cs and 6*'5N) for investigating land degradation and the origin of sediment deposits in the My Bouchta watershed in northern Morocco. FRN's were used as tools for obtaining quantitative information on soil erosion and sedimentation rates over a range of different timescales whilst the stable isotopes enabled the primary sediment source areas to be identified. Using ~(137)Cs the net soil erosion rate for the My Bouchta watershed was estimated to be about 22.1 tonnes (t)-ha~(-1)-year"1 over a period of 50 years with a major contribution from agricultural fields. Net soil erosion rates over a period of 100 years derived from 210Pbex were lower than those estimated by ~(137)Cs. These results indicate that soil erosion has increased significantly during the last 50 years. Sedimentation rates of about 0.50 g-cm"2-year"1 (equivalent to 50t-ha~(-1) -year"1) were obtained for the Talembout water reservoir suggesting additional contributions from gully erosion and mass movement. Similar behaviours of stable isotopes with depth were obtained for forest and shrub fields and agood correlation between 6~(13)Cs and total C was obtained for forest and shrub fields. Using the 5~(13)Cs profile, sediment deposits in the water reservoir seemed to originate mainly from fields under maize culture (C4plant), The sediment profile of 615N indicated an increased use of synthetic fertilizers during the last 15 years.