Abstract
Abstract. Soil erosion is one of the major factors affecting sustainability of agricultural production in Ethiopia. The objective of this paper is to estimate soil erosion using the universal soil loss equation (RUSLE) model and to evaluate soil conservation practices in a data-scarce watershed region. For this purpose, soil data, rainfall, erosion control practices, satellite images and topographic maps were collected to determine the RUSLE factors. In addition, measurements of randomly selected soil and water conservation structures were done at three sub-watersheds (Asanat, Debreyakob and Rim). This study was conducted in Koga watershed at upper part of the Blue Nile basin which is affected by high soil erosion rates. The area is characterized by undulating topography caused by intensive agricultural practices with poor soil conservation practices. The soil loss rates were determined and conservation strategies have been evaluated under different slope classes and land uses. The results showed that the watershed is affected by high soil erosion rates (on average 42 t ha−1 yr−1), greater than the maximum tolerable soil loss (18 t ha−1 yr−1). The highest soil loss (456 t ha−1 yr−1) estimated from the upper watershed occurred on cultivated lands of steep slopes. As a result, soil erosion is mainly aggravated by land-use conflicts and topographic factors and the rugged topographic land forms of the area. The study also demonstrated that the contribution of existing soil conservation structures to erosion control is very small due to incorrect design and poor management. About 35 % out of the existing structures can reduce soil loss significantly since they were constructed correctly. Most of the existing structures were demolished due to the sediment overload, vulnerability to livestock damage and intense rainfall. Therefore, appropriate and standardized soil and water conservation measures for different erosion-prone land uses and land forms need to be implemented in Koga watershed.
Highlights
The livelihoods of human kind are closely linked to soil resources
In different parts of the watershed, 29 auger-holes to a depth of 3 m and 128 test pits to a depth between 2 and 4.5 m were carried out in order to determine the soil properties. Based on these data, which is supported by the field soil survey, K factor was determined by giving it the value according to the soil type map of the watershed based on the Kaltenrieder (2007) and Andersson (2010) studies for Ethiopian conditions
The spatial distribution of annual soil loss calculated using the revised universal soil loss equation (RUSLE) model ranges from 12 t ha−1 at the outlet to 456 t ha−1 at the upper part of the study area, which is above the tolerable soil loss (2 to 18 t ha−1 yr−1) determined by Hurni (1985) for Ethiopian highland conditions
Summary
The livelihoods of human kind are closely linked to soil resources. Soil erosion is a worldwide environmental problem that reduces the productivity of all natural ecosystems and agriculture, which threatens the lives of most smallholder farmers (Dai et al, 2015; Erkossa et al, 2015; Gessesse et al, 2015; Ochoa-Cueva et al, 2015; Taguas et al, 2015; Prosdocimi et al, 2016). Soil erosion by water is the greatest factor limiting agricultural productivity in the humid tropical regions (Sunday et al, 2012). Soil erosion is further aggravated by environmental land-use conflicts (ELUCs), as recently recognized by Pacheco et al (2014) and Valle et al (2014). ELUCs in developing countries have been reported to cause a decline in soil fertility (Valera et al, 2016).
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