This study was aimed to estimate the spatially distributed mean annual soil loss rate and map the most vulnerable areas in Hangar watershed using Revised Universal Soil Loss Equation

This study was aimed to estimate the spatially distributed mean annual soil loss rate and map the most vulnerable areas in Hangar watershed using Revised Universal Soil Loss Equation (RUSLE) with the aid of Geographical Information System (GIS) techniques. The RUSLE parameters; rainfall erosivity (R-factor), soil erodibility (K-factor), slope steepness and slope length (LS-factor), vegetative cover (C-factor) and conservation practice (P-factor), which consists of a set of logically related geographic features and related attribute data were generated for the analysis. A 30 x 30 DEM was used for catchment delineation and analysis of the LS-factor. The land use/ land cover map of 2013 was used for the analysis of C-factor, Soil map of the study area for the analysis of the K-factor, mean annual rainfall data of the nearby rain gauge stations for analysis of R-factor. By integrating these five map layers in GIS raster calculator, the required spatially distributed annual average soil loss rate was determined. The result of the analysis depicted that the amount of soil loss from the Hanger catchment ranges from 1 to 500 t ha-1 yr-1 with an average annual soil loss rate of 32t ha-1yr-1 from the whole catchment. About 84.2% of the total area experienced soil loss above tolerable limit of 11t ha-1yr-1. The total annual soil loss from the entire watershed was found to be about 24.93 Mtons. This indicates maintaining the sustainability of the soil productivity will be difficult if the specified amount of soil is removed annually. To evaluate the effect of watershed management, contour ploughing with terracing was evaluated in this study and the result indicates if it is fully developed, the average annual soil loss rate will decrease from 32 to 19.2tha-1yr-1. Consequently, applying effective watershed management reduces the vulnerability of the watershed by 40%. Based on the spatial vulnerability of the watershed, most critical soil erosion areas were situated in the steepest upper part of the watershed due to intensive agricultural activities.
Key words: Annual soil loss, Hangar watershed, RUSLE, Soil erosion

1. Introduction
Soil erosion is a natural process resulting from the removal of soil particles from the surface of the earth by water and wind, transporting and depositing elsewhere (Hurni, 1988). And it is one of the reasons of soil degradation which leads to the deteriorations of physical, chemical and biophysical properties of the soil (FAO, 1978).
The action of soil erosion is triggered by a combination of factors such as steeply slopes, heavy rainfall after long dry period, inappropriate use of land cover patterns and ecological disasters (Oldeman, 1998). Moreover, some intrinsic features of a soil can also make it more prone to erosion. such intrinsic features are a thin layer of topsoil being silty textured and low organic matter content (Kosmas, 1997).
Soil erosion is one of the biggest global environmental problems resulting both on-site
effects such as; loss of top fertile soil, minimize water holding capacity of the soil, nutrients and minerals carried off by water and off-site effects such as; silting up of dams, disruption of lake ecosystems, contamination of drinking water and increased downstream flooding (Tamene and Vlek, 2008). Even though these effects have been identified as a global problem in the 20th century, the trend of Soil erosion has continued to increase throughout the whole nation (Adugna et al., 2015). Studies show that in the whole globe, about 80 % of agricultural lands suffered from moderate to severe soil erosion which is a cause of loss of productivity of agricultural lands (Hurni, 1998; Gete, 2000). Pimentel et al. (2009) and Jahun (2015), also reveal a shocking figures about the erosion phenomenon, that is, most of the soil from farmlands is washed away about 10 to 40 times faster than it is being replaced, citing examples that in some parts of United States was losing a soil of 10 times faster than the regular replacement rate. On the other hand, Pimentel et al. (2009) and Jahun (2015) present that, China and India are also said to be losing soil of 30 to 40 times faster than its formation.
In Ethiopia, a number of studies indicate the existence of sever soil erosion in the highland areas and sedimentation in the low land areas of the country (Bewket and Teferi, 2009; Kebede et al., 2015). For instance some of the evidence research shows that an average annual soil loss rate of 35 t ha-1 yr-1 (FAO, 1986); 42 t ha-1 yr-1 (Hurni, 1993) and 57t ha-1 yr-1 (Girmay, 2009) were reported. In addition to this, other researches also show that soil erosion rate ranges from 16 to 300 t ha-1 yr-1 (Hurni, 1986) and 130 to 170 t ha-1 yr-1 (Gete, 2000) in the highland areas of the country. Related study also indicates the existence of sever problems on agricultural lands due to removal of fertile soil and sedimentations on the water bodies and reservoirs in Ethiopia (Kebede, 2012). The study area, Hangar River Watershed, is one of the catchments suffering with this sever soil erosion problem as well (Jemal, 2010).
Studies indicate that splash, sheet and rill erosion by water are the major components of land degradation that affect land productivity in the Ethiopia (Desta et al., 2005; Haregeweyn et al., 2015). In general, Soil erosion and transportation by water due to rain drop impact is the most common erosion agent in the country (Zeleke and Hurni, 2001).
The severity of soil erosion in Ethiopia is due to most part of the country is being steep sloped and mountainous, and the existence of higher and frequent rainfall amount with higher intensities. In addition to this; human activities, rapid population growth, poor cultivation system and poor land use practices, deforestation and overgrazing, has a great contribution to soil degradation in the country (Hurni, 1993; Kebede, 2012). Loss of fertile soil, rapid degradation of natural systems, significant sediment depositions in the lakes and reservoirs and sedimentation of irrigation infrastructures are generally, due to poor watershed management system in the country (Akalu et al., 2009).
The main River in the study area (Hangar River) is one of the major tributaries of Didesa River, which finally joins to Blue Nile River. This River has a length of more than 200 km and has its own medium scale tributary rivers, which consists of 11sub-catchments larger than 500 km2 each. The exposure to erosion and Sediment contribution from those tributary Rivers varies depending up on the existed situation of the sub-catchments. Therefore, conducting this research contributes to identify the most sever soil erosion areas in the specified catchment. Knowing and identifying the most prone area, is very important to take interventions measures in line with the identified erosion vulnerable area.
In order to predict and evaluate soil erosion quantitatively, different prediction models have been efficiently developed and employed by different soil scientists in the last few decades (Gelagay and Minale, 2016). Using these models now a day different researches are undertaken in different parts of the world to estimate the rate of soil erosion and mapping of erosion risk areas. One of the most widely used empirical models is universal soil loss equation (RUSLE), with remotely sensed data and GIS software (Renard et al., 1997). The result of this model has been checked by different researcher and showed its efficiency in estimating rate of soil erosion and mapping of erosion risk areas throughout the world. For instance, Millward and Mersey (1999) show the potential of using a combination of remote sensing, GIS, and RUSLE in estimating soil erosion loss on a cell-by-cell basis.
Among the soil loss estimation models, only few are used to measure soil loss in Ethiopian conditions, because of data limitations. One of these few soil erosion prediction models, RUSLE is mostly used model because of its simplicity relative to other conceptual and process based models, relative data availability for this model and integration with GIS. (Temesgen, 2017; Gelagay and Minale, 2016). Even though, this model has been developed after the parameters are tested and validated under diverse soil, climate and management conditions of United State of America, several efforts has been made to calibrate and validate the use of RUSLE model for other countries including Ethiopia. Among those studies for instance (Hurni, 1988; Helden, 1998; in Ethiopia; Angima et al., 2003 in Kenya; Prasannakumar et al., 2012 in India). Specifically as sited by Alemayehu (2012), Mulugeta, 2004 has calibrated RUSLE for Andit Tid watershed while Serkalem (2005) for Mayebar and Mesfin (2008) for Anjeni watersheds in Ethiopia. In all these studies RUSLE was publicized that the model shows satisfactory result. Therefore, the objective of this research is to quantify the amount of annual soil loss rate from Hager River watershed using this most applicable model RUSLE, through the application of GIS technique and to identify the most vulnerable areas of the watershed.
1.1. Statements of the problems
Ethiopia has been described as one of the most seriously affected nation in the world by soil erosion (Hurni, 1988; Mitiku et al., 2002; Gizachew, 2015). Soil erosion and sediment yield from catchments are therefore key limitations to achieve sustainable land use and maintaining water quality in rivers, lakes and other water bodies (Benedict and Andreas, 2006).
Many of Ethiopia’s hydroelectric power and irrigation reservoirs such as Aba-Samuel, Koka, Angerib, Melka Wonka, Borkena, Adarko and Legedadi has been threatened by the heavy sedimentation. Therefore, these dams have been suffered from reduction in their capacity and life span, quality of water and require costly operation for removal and operation and thus these dams loss their intended services (Kebede, 2012; Gelagay, 2016).
The degradation of large part of the Ethiopian highlands has reached a scale where it has become increasingly difficult even to maintain the current level of production of basic food which is already insufficient in many regions of the country (Bekele, 1998). Hence, Soil erosion affects the socio-economic condition of a country directly or indirectly; especially countries like Ethiopia whose economy is extremely dependent on agriculture (Angima et al., 2003; Abate, 2011). Therefore, the economic implication of soil erosion is more series in such countries because of the capacity to cope with it and also to replace the lost nutrients. As sited in Gashaw et al. (2017), Sonneveld and Keyzer (2003) estimates through modeling work and suggests that soil erosion in Ethiopia will reduce the potential production of the land by 10% in 2010 and by 30% in 2030. As a result, the value added per capita per annum in the agricultural sector goes down from US$372 in 2010 to US$162 in 2030.
The top fertile soil which is naturally abundant resource plays a vital role for the agricultural productivity. But, the removal of this top fertile soil leads to reduction in crop production. This reduction of crop production results poverty on the major population in the country. At the same time, sedimentation problems occur in the water bodies and reservoirs and minimize the life span of reservoirs (MoARD, 2010).
Studies conducted by using Water Evaluation and Planning (WEAP) model, and assessed the future potential of irrigation and hydropower in Blue Nile River Basins shows that, Hangar River has a potential of developing more than 14000 ha of irrigation and 1.8 to 9.6 MW of hydroelectric power (Matthew et al, 2005). Accordingly, Federal Government of Ethiopia (FDRE) has a plan of implementing this project. However, the large part of this area is degraded due to deforestation for intensive agricultural activities like; farm expansion, extraction of fuel, constructional wood, overgrazing and for other related purpose which are the consequences of population growth and expansion over the area, as other parts of the country. Now a day agricultural lands in the study area is less productive due to soil degradations. Farmers use different fertilizers for agricultural lands in order to compensate some of the lost nutrients in the soil due to soil erosion, which is costly. This condition was seen during site visit. According to FAO (1986), Rapid population growth, cultivation on steep slopes, clearing of vegetation, and overgrazing are the main factors that accelerate soil erosion in Ethiopia. These, the damages associated with excessive soil erosion problem thought to be severe in this area as there are intensive agricultural activities, rapid population growth, cultivation on steep slope and related activities mentioned by FAO (1986) are common on the study area. Therefore, this study was done to estimate the annual soil erosion rate and add the soil erosion information for the decision makers to plane appropriate soil Conservation practice in the watershed so that reducing fertile soil loss from farm lands and sedimentation in the proposed multi-purpose hydraulic structure on Hangar River.