Soil Erosion Hazard Modeling Using Remote Sensing and GIS Tool: A Case Study Namgnen Watershed in Phongsaly Province of Laos

Abstract

The major factors responsible for soil erosion include factors such as rainfall, soil type, vegetation of the area, topographic and morphological characteristics. Due to the spatial variation of rainfall and catchment heterogeneity, surface erosion and sediment yield are much variable. This study is undertaken the use of empirical Universal Soil Loss Equation (USLE) with transport limiting sediment delivery (TLSD) concept to compute soil and sediment outflow in GIS environment. This involves remotely sensed and other related data for assessing the vulnerable soil erosion area within the watershed.

To compute soil erosion and sediment outflow in GIS using USLE with TLSD concept, the catchment was divided into smaller grid cells of 50m x50m to account for catchment heterogeneity by considering smaller grid cell as hydrologically homogeneous area. Grid thus formed was categorized as cells lying on overland and channel areas based on channel initiation threshold in order to differentiate the processes of sediment erosion and delivery in them. In the study, GIS is used for generating representative raster layers based on various factors such as rainfall erosivity, slope length/gradient, soil erodibility and conservation practices for estimation of spatial distribution of soil erosion.

In addition to this, Landsat TM imagery is utilized to produce a land use/cover map of the study area. The land use/cover map was then used in USLE model. 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 discritized 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. Thus the total amount of sediment coming out to the outlet is the sediment yield of the catchment.

The Namgnen watershed with a hydrological perspective is very significant with dense channel network of rill and gullies and significant alluvial. Further, results indicate that areas within a watersheds having high topographic factor with waste land and agricultural land and areas near first order stream produce more erosion. However, spatially computed soil removal from most of the catchment area is limited to 0-5 tons/hectare/year except few pockets which produce more sediment yield, indicating most of the areas in the catchment fall within tolerable limits of soil erosion.