Assessment of soil erosion hazard of Victoria catchment area using GIS as a tool

Abstract

Victoria reservoir is located at an elevation of340 m to 440 m with a geographical position of7° 15'to T" 19' Nand 800 39' to 800 48' E which has been constructed by damming the River Mahaweli atVictoria fall. in Sri Lanka in 1983. The reservoir storage capacity is 72 1.2 MCM and upstream damsite comprises 1338 km2 in the districts of Kandy, Nuwara-Eliya and Matale. The elevation of thecatchment ranges from 340 m to 2 I00 m.

Soil erosion is a major water quality issue in the upland reservoirs. The objective of this paper is toanalyze catchment issues contributing to soil erosion in the Victoria reservoir and to evaluate the soilerosion risk areas in the catchment. Study was carried out from 2002 to 2004.

Soil erosion occurs due to natural causes such as rain fall, rainfall runoff and wind, and due to humanactivities. Universal Soil Loss Equation (A = RKLSCP) introduced by Wischmeier and Smith in1965 is a most widely used method for estimating soil erosion. This encounters detachment of soilparticles and its transport by raindrops and surface runoff, which depends on the rainfall erosivity (R),erodibility of soil (K), slope length factor (LS), cover and management factor (C) and the supportpractice factor of the equation.

The data on erosivity points were interpolated with SO rn resolution grid cells. The erodibility valuerelevant to each soil group was entered into the attribute table, which was converted into grid cellswith 50 III resolution, containing soil erodibility values. The Triangulated Irregular Network (TIN) wascreated by contour interpolating with 20 m interval, and grid cell was 50 m. Using TIN, slopepercentages map was derived, which was used to obtain LS factor. The C factor values relevant toeach landuse type were entered into the attribute table. The map was converted into grid cells with SOm resolution, containing C factor values

The soil erosion of the Victoria catchment was categorized into five erosion categories, namely; low,moderate, high, very high and extremely high, which extend within Kandy, Nuwara-Eliya and Mataledistricts. All categories were spread both in the left and right banks.

Extremely high erosion areas extend over 24.34km2 with a percentage of 1.82, Very high erosionareas extend oyer i21 24 km2 with a percentage of 9 06, High erosion areas extend over 302 91 km2 with a percentage of 22.63, Moderate erosion areas extend over 434.01 km2 with a percentage of32.43 and low erosion area extends over 454.97 km with a percentage of 33 .99. The results of mapanalysis were confirmed through field verifications. The soil erosion is high in the high slope regionsand in the areas where soil conservation methods are inadequate or poor.