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Colorado State University (2013)

Erosion mapping and sediment yield of the Kabul river basin, Afghanistan

Sahaar, Ahmad Shukran

Titre : Erosion mapping and sediment yield of the Kabul river basin, Afghanistan

Auteur : Sahaar, Ahmad Shukran

Université de soutenance : Colorado State University

Grade : Master of Science (MS) 2013

Soil erosion by water is a serious issue in Afghanistan. Due to the geographic landscape, soil and climatic conditions, and the latest deforestation activities, there has been intensive soil erosion which has resulted in prolonged and great impact on social and economic development of the region. In fact, recent environmental assessment shows that decades of war and continuous drought have resulted in widespread environmental degradation throughout the country ; therefore, mapping of soil erosion at the basin scale is urgently needed. The Kabul River Basin was selected for the purpose of erosion and sedimentation modeling due to its great socioeconomic impact. The main objectives of this study include : (1) calculations of the annual average soil loss rates at the basin level ; (2) spatial distribution of soil erosion rates at the basin level ; (3) predictions of deforestation effects on sediment losses under different land cover scenarios at the watershed level ; and (4) calculation of sediment delivery ratios based on soil erosion rates, and sediment yields at the sub-watershed levels in the basin. This study uses the Revised Universal Soil Loss Equation (RUSLE) model combined with Geographic Information System (GIS) techniques to analyze the gross soil loss rates and the spatial distribution of soil loss rates under different land uses. Digital elevation model (DEM), average annual precipitation data, land cover map and soil type map were used to define the parameters of the RUSLE model. The annual average soil loss rate of the Kabul River Basin was estimated to be 19 tons/acre/year (4748 tons/km2 /year), and the gross mean annual soil loss rate found to be 47 million tons/year. By producing 57 % of the total annual average soil loss, rangelands were the primary contributor to the basin. In case of the spatial distribution of erosion rates at the Kabul River Basin, the relationship between probability and annual average soil loss rates were analyzed. The analysis indicated that up to sixty percent of the mean annual soil loss rates are in the range of tolerable soil loss rate (0 - 5 tons/acre/year). Moreover, northern part of the basin is prone to more extensive erosion than the southern part. The study predicted that if the forest region of the Kunar watershed is completely reduced to barren lands, the watershed will produce five times more sediment than the estimated soil loss rate from 1993’s UN-FAO land cover map. The annual average soil loss rate in this watershed was about 29 tons/acre/year but it will increase to 149 tons/acre/year as deforestation continues to take place in the watershed. The range of sediment delivery ratios for the basin’s rivers is 2.5 -10.8 %. Based on this evaluation, the sediment delivery ratio for the sediment gauging stations in the basin are in the similar range of predicted values by the methods of Boyce, Renfro, Williams and Maner.


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