GIS-based modelling for soil erosion in lowland areas : case study : Lambidaro sub-watershed, South Sumatra

Soil erosion is one of the major environmentally problems in tropical and semi-arid area both in lowland moreover in upland areas. Its long-term effects will cause soil degradation, lowering soil productivity and off course sustainability of the land as a whole. Erosion is a three-step process: detachment followed by transport and deposition. Recently, Revised Universal Soil Loss Equation (RUSLE) is becoming widely implemented in predicting annual average soil erosion. By considering five factors influenced soil erosion by water; rainfall erosivity (R), soil erodibility (K), slope-length (LS), cropping management (C) and erosion control practices (P) factor and also combined with Geographic Information System (GIS)techniques, the prediction results of potential annual soil erosion will be more comprehensive. This study was conducted in Lambidaro Sub-Watershed, which is located in the upper part of Musi River that passing through the city of Palembang, South Sumatra Province, Indonesia. The objectives are: to predict the level of erosion level using GIS-RUSLE method, to determine the different potential soil erosion (A) due to the changes of C factor and P factor value, to analyze the characteristics of the sedimentation process and to recommend somemeasures to reduce soil erosion. Five scenarios have been developed related to the implementation of erosion controlpractices and the possibility of land use change. P factor value will be adjusted based on the method applied, while C factor value will change due to the possibility of land use change inLambidaro sub-watershed. Scenario I is defined as the existing condition for R, K, LS, and C factor value, while P factor value is defined as 1 for the entire area. It is assumed that there is no erosion control practices implemented. For Scenario II, III and IV, three mechanical erosion control measures were applied. Contouring method is implemented in Scenario IIwith P factor values varied from 0.55 - 0.85 based on the slope value. Scenario III is applying level retention bench terrace with P factor value equal to 0.01 for the entire area. In addition, in Scenario IV contour bunds method is applied with P factor value ranging from 0.275-0.425. Furthermore, other factors such as R, K, LS, and C factor values for those three scenarios are based on the existing conditions. Dissimilar assumption is applied in ScenarioV. The value of R, K, LS and P factor are the same as the existing condition in the Scenario I, however there is a change in C factor value due to the changing of swampy area and paddy field (C factor = 0.01) into settlements (C factor = 0.5). From the computation results of the first scenario, the Annual average soil erosion (A)values are dominated by 0-0.5 t/ha/yr (50% of total area), 0.5-1 (8%) and 1-10 (36%). In addition, other values are ranging from 10-20 (7%) and 20-250 (<1%). Assuming that theaverage soil depth in Lambidaro sub-watershed is 60-90 cm from the surface (moderate), the level of erosion can be predicted as follows: low level (0-15 t/ha/yr) is 97% of area, moderate level (15-60 t/ha/yr) is