SOIL EROSION HAZARD MODELING OF THE NAMGNEN WATERSHED, LAO.P.D.R

Abstract

Assessment and inventory on soil erosion hazard are essential for formulation of effective soil conservation plans of a watershed for sustainable development. In terms of the natural environment, Lao PDR is arguably the keystone of the region. This is particularly true with regards to biological diversity, forest cover and water resources. Laos PDR's environmental endowments represent a heritage of critical natural, cultural and economic values at the regional and global level. The natural resource base will continue to be the backbone of the Lao economy for the integrity of Lao PDR's ecosystem. Soil losses in the Country is one of main causes of decreasing fertility of lands, sedimentation in canals and rivers, decreasing of storage capacity in the dams or reservoir, increasing of floods frequency , environmental pollution and it consequences. The major factors responsible for soil erosion include rainfall, soil type and vegetation, topographic and morphological characteristics of the basin. Surface erosion and sediment yield quantities are found to have large variability due to the spatial variation of rainfall and catchment heterogeneity. This study is, therefore, undertaken for use empirical Universal Soil Loss Equation (USLE) with transport limiting sediment delivery (TLSD) concept to compute soil and sediment outflow in GIS environment utilizing remotely sensed data and other data for assessing the vulnerable soil erosion area within a watershed. The Arc GIS package has been used for carrying out geographic analysis and Earth resource data analysis system The ERDAS imagine image processor has been used for the digital analysis of the satellite data for deriving the land use cover characteristics of the catchment. To compute soil erosion and sediment outflow in GIS using USLE with TLSD concept, the catchment was divided into smaller grid cells of 50m x 50m to account for catchment heterogeneity by considering smaller grid cell as hydrologically homogeneous area. Grid thus formed was categorized as cells lying on overland areas and lying on channel areas based on channel initiation threshold in order to differentiate the processes of sediment erosion and delivery in them. GIS is used for Page: 4 generating representative raster layers based on rainfall erosivity, slope length/gradient, soil erodibility and conservation practices factor for estimation of spatial distribution soil erosion. Landsat TM imagery is utilised to produce a land use/cover map of the study area based on the Maximum Likelihood Classification method. This map was then, used to generate the conservation practice factor in the USLE. The empirical USLE model calculates the soil loss on each cell as a function of the rainfall — runoff erosivity and the soil erodibility factors. This is then modified with the factors of topography, cover management and the support practices. The rate of sediment transport from each of the discretized cell depends upon the transport capacity of the flowing water. The eroded sediment was routed from each cell following the defined drainage path to the catchment outlet. The concept of transport limiting sediment delivery (TLSD) was used for determination of spatial distribution of transport capacity of flow within the watershed and the total sediment yield at the watershed outlet. The Normalized Difference Vegetation Index (NDVI) is used for determination of spatial distribution of transport capacity factor used in TLSD equation. Application of the proposed method resulted in generation of maps for spatial distribution of soil erosion/deposition from the watershed. Such maps are helpful in identification of critical areas to be taken up on priority basis for planning and implementation of watershed conservation measures.