Soil Erosion Risk Assessment Using GIS Based USLE Model for Soil and Water Conservation Planning in Somodo Watershed, South West Ethiopia
Abstract
Soil erosion is natural phenomena and is modified by biophysical environment comprising soil, climate, terrain, ground cover and their interactions. Due to different factors, it is difficult to make watershed management successful in all areas atone time. Because of this, prioritization of sub watershed is very important for soil conservation planning and implementation. In Somodo watershed more than five years different soil and water conservation technologies were implemented and satisfactory result was not recorded. In this aspect, it is important to consider further watershed management planning., This study therefore investigated soil erosion risk assessment using GIS and USLE model for soil and water conservation in Somodo watershed southwestern Ethiopia with the aim of estimating soil erosion rate and identify soil erosion hotpot areas through prioritization of sub watershed in Somodo watershed by the help of GIS based USLE model. Both primary and secondary data sources were used for model input. These data were computed at a grid level with 30*30m resolution and then overlaid to generate mean annual soil loss by the help of raster calculator in Arc GIS tool. Results of the study showed that, the mean annual soil loss of the watershed was 18.69 ton ha-1 year-1 ranging from 0 to 131.21. More than 75% of the watershed have soil loss greater than 20 ton ha-1 year-1 and only 25% of the area have soil loss less than 10 ton ha-1 year-1.On the bases of mean annual soil loss SW-4, SW-6 and SW-7 were under slight (0-10 ton ha-1 year-1) erosion severity level, while the remaining SW-2, SW-3 and SW-8 were under moderate (10-20 ton ha-1 year-1) level. And SW-1 was in high (20-30 ton ha-1 year-1) erosion severity level, whereas SW-5 and SW-9 were found in very high (>30 ton ha-1 year-1) erosion severity level. Since large area of the watershed has soil loss more than tolerable level (11 ton ha-1 year-1) attention should be given to identify erosion hotspot areas to minimize the on-site and off-site problems. Therefore, the study suggested that for effective watershed management and soil conservation planning, these sub-watershed priorities should be used in the watershed.
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Introduction
Soil erosion is a major cause of land degradation that affects the physical and chemical properties of soils and resulting in on-site nutrient loss and off-site sedimentation of hydraulic structures in Ethiopia [1]. Lack of effective watershed management system and poor land use practices played significant role inland degradation in Ethiopian highlands [2]. According to the Ethiopian highland reclamation study [3], in the mid 1980‟s, 27 million hectare or almost 50% of the highland area was significantly eroded, 14 million hectare seriously eroded and over 2 million hectare were beyond reclamation. Soil erosion is also a natural phenomena and modified by biophysical environment comprising soil, climate, terrain, ground cover and interactions between them. Important terrain characteristics influencing the mechanism of soil erosion are slope, length, aspect and shape. Impact of slope and aspect would playa major role in runoff mechanism. More the slope, more the runoff and thus infiltration reduces. The runoff generated from slope will find a path nearby and this would lead to erosion of soil as the velocity of the runoff increases [4]
Soil erosion models are useful to estimate soil loss and runoff rates from agricultural land, to plan land use strategies, to provide relative soil loss indices and to guide government policy and strategy on soil and water conservation. The universal soil loss equation (USLE), is one of the most popular empirical models [5] to estimate the long-term average annual rate of soil loss from small field having an average length of 22 m, afield slope of 9% based on rainfall pattern, soil type, topography, cropping system and management practices.
Most of the earlier models, such as the well-known Universal Soil Loss Equation (USLE) [5], were empirically derived. This is relatively simple technique to predict erosion, subsequently led to the application of empirical models in many parts of the world, including Ethiopia. Empirical models could be expected to be used mainly as screening tools in integrated studies, land resource assessments would demand increased accuracy in quantification of erosion rates in a spatial and temporal context when integrated with Geographical Information System (GIS). They all consider slope steepness, slope length, vegetative cover, rainfall, soil properties and erosion control methods as parameters which influence erosion. The efficient and optimum management and conservation of soil, land and water resources is best approached on a watershed basis. Normally, the amelioration processes are developed and applied following prioritization and landscape planning. Prioritization plays a key role in identifying areas that require attention [6]. In a watershed management program due to time and financial limitation, it is difficult to make rehabilitation, and soil and water conservation work atone time in all places. Thus it is important to study the watersheds of the area and make ordering by their risk of erosion [7]. Estimation of soil loss and identification of critical area for implementation of best management practice in watershed is central to success of a soil and water conservation program.
In Somodo watershed soil and water conservation measures was implemented to minimize soil erosion without identification of erosion hotspot areas. However, satisfactory result was not observed in the watershed for past five years through implementation of soil and water conservation technologies to the whole watershed at a time. Because of this the study was aimed to estimate erosion rate of the watershed and identify erosion hotspot areas through prioritization of the sub-watersheds for further soil conservation, and watershed management planning by the help of GIS based USLE model in the study area.
Conclusion
Lack of effective watershed management system and poor land use and land management practices played a significant role inland degradation in Ethiopian highlands. Quantifying the amount of land degradation through soil erosion was difficult at a watershed or basin level for the past many years. Soil erosion models are useful to estimate soil loss and runoff rates at watershed and basin level, to plan land management strategies, to provide relative soil loss indices and guide government policy and strategy on soil and water conservation practices. Estimation of soil loss and identification of critical area for intervention of best management practices in the watershed is central for the success of soil and water conservation program. Somodo watershed also faced similar problems as other watersheds in Nile basin and then this study estimated soil erosion rate of the watershed and prioritized its sub watershed to identify erosion hotspot areas for effective soil and watershed management planning using GIS based USLE model. Accordingly, the watershed mean annual soil loss rate was found 18.69 ton ha-1 year-1 ranging from negligible value to 131.21 ton ha-1 year-1 with a standard deviation of 51.05. SW-9 and SW-5 were felt in very high (>30) soil erosion level and SW-1 was under high (20-30) soil severity level. SW-2, SW-3 and SW-8 were under moderate (10-20), while the remaining SW-4, SW-6 and SW-7 were found under slight (0-10) soil erosion severity level. Therefore, for effective watershed management and soil conservation planning, these sub-watershed priorities should be used in the watershed. Further study, experimental plots or model based, in the study area are also appreciated by this study.