GIS BASED SOIL LOSS ESTIMATION USING USLE MODEL FOR SOIL CONSERVATION PLANNING: IN KARESA WATERSHED, LOMA WOREDA, SOUTH WEST ETHIO

Abstract

Soil erosion is the most challenging and continuous environmental problems resulting in both on-site and off-site effects in the world particularly in Ethiopia. Karesa watershed is one of the most erosion-prone watersheds which received little attention. Managing the on site erosion is to reduce the negative impacts of downstream water resources and requires an understanding of the rates of soil loss as well as identification of the major controlling factors that enhance or retard these processes. This study was conducted to estimate average annual soil loss rate using Geographic Information System and Universal Soil Loss Equation Model adapted to Ethiopian condition. The following datasets were obtained from different sources for estimating annual soil loss such as 15 years mean annual rainfall data for estimating Erosivity factor, digital soil map for estimating soil Erodibility factor, 30m x 30m resolution Digital Elevation Model for estimating slope length and slope steepness (LS) factor, Landsat6ETM+ images with 30mx30m resolution for detecting Vegetation cover and Conservation practice factor. Raster calculator was used to interactively multiply and produce annual soil loss. The result reveals that 42,413.72 ton per year soil loss from 9939 ha entire watershed and 4.27 tons per hectare per year average annual soil loss rate. The mean annual soil loss rate was classified in to four erosion severity classes as very less, less, moderate and High . The result also implies, two slope classes (0-15% and 15-30%) were categorized under very less to less soil loss (0-6.25 tons ha-1 yr-1 ) which accounts an area of 9383.07 ha (94.4%) of the watershed areas and representing 81.13% of the total soil loss. On the other hand, the watershed slope classes (>30%) fell under moderate to High soil loss (6.25-25 tons ha-1 yr-1 ) together covers 555.93ha (5.6%) of the watershed areas contributing 18.82% of the total soil loss mainly due to cultivation of marginal land,Intensive cultivation, poor vegetation cover during critical rainfall period. Moreover, about 2,184.93 ha of the watershed area was highly affected by erosion which contributes 18,182.25 tones yr-1 (42.87%) total soil loss and requires integrated soil and water conservation measures