Abstract
Soil erosion is one of the major threats which affects farm productivity and water quality in river catchments. Quantification of human-induced soil loss is an imperative measure in catchment management. Among the catchments in Sri Lanka, the highest sediment yield has been reported from the Upper Uma Oya catchment (UOC) which is a sub-catchment of the Upper Mahaweli catchment. The Mahaweli watershed is the most important watershed in Sri Lanka since the size and the economic importance of it. Hence, this study was conducted considering the ten selected sub-watersheds of Uma Oya Watershed (UOW) in Sri Lanka to quantify soil erosion. Soil erosion of UOW was calculated using Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) Sediment Delivery Ratio (SDR) model. It was found that the current soil erosion rates in the watershed is varied between 14.0–648.6 t/ha/yr. Results of the study disclose that the soil erosion rates of UOW are 3 to 130 times faster than the soil loss tolerance in Sri Lanka. Findings of the study suggest that the implementation of sustainable watershed management options are crucial in order to reduce soil erosion and to enhance the quality of the watershed.
Highlights
Watersheds are important in supplying precious water resource for humans and for ecosystem functions (Alam et al 2006; Jayawardana et al 2016) but are significantly affected by human induced soil erosion (Al-Kaisi et al 2009)
Soil erosion is a process of soil loosening, transport and deposition. This process generally results in relocating the top soil that is rich in nutrients, organics and life of soil elsewhere on-site or is transported offsite where the eroded matter accumulates in drainage channels and this occurs severely on poorly managed sloppy areas (Issaka and Ashraf 2017; Shi et al 2012)
The Uma Oya Watershed (UOW) can be categorized into 13 Land Use Land Cover (LULC) classes (Figure 2)
Summary
Watersheds are important in supplying precious water resource for humans and for ecosystem functions (Alam et al 2006; Jayawardana et al 2016) but are significantly affected by human induced soil erosion (Al-Kaisi et al 2009). Agricultural practices, clearance of natural forest cover, surface mining, urbanization and construction activities are causative land use practices that accelerate the natural soil erosion rates (Cheng et al 2017; Diyabalanage et al 2017; Pimentel 1993). The resultant impacts of soil erosion include water quality degradation in watersheds, (Al-Kaisi et al 2009) increasing siltation in the inland reservoirs, decreasing soil fertility on arable lands (Diyabalanage et al 2017; Lal 2003; Pimentel 1993) and increasing the risk of low land flash flooding (Al-Wadaey and Ziadat, 2014; Issaka and Ashraf 2017). Severe soil erosion that results in excessive silt export to waters or reservoirs causing disturbances on aquatic life as well as reduced quality of the eco systems (Issaka and Ashraf 2017)
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