Assessment of Soil Erosion in Uma Oya Catchment, Sri Lanka

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.

GIS and RS Based Soil Erosion Modelling in Sri Lanka: A Review

“…But, the major limitation of the EI 30 index method is that it has been introduced under USA conditions and is not found appropriate for the Asian regions to estimate the R factor (Hudson, 1971). The studies carried out by Dias et al (2019) and Udayakumara and Gunawardena, (2016), have used the Roose equation to calculate the R factor and the remaining studies used the regression correlation developed for Sri Lanka. These two methods may be suitable for the Sri Lankan conditions since the constants used in these two equations have been developed under tropical, sub-tropical, and Sri Lankan conditions.…”

“…Also, another criticism of the manual methods of calculating LS factors is its narrow applicability to complex variable topography (Desmet and Govers, 1996). But, with the improvements in GIS/RS technology, the derivation methods of LS factor have become more significant and this method explicitly accounts for modeling soil erosion over a for more topographically complex landscapes over S/N Authors Method Used to Derive K Factor References 07 (Wijesundara et al, 2018) From available literature and Nomography is used to compute the values of K based on soil properties (Joshua, 1977) (Wijesekara and Samarakoon, 2002) ( Mapa et al, 2010) 08 (Lekamge et al, 2018) From available literature of previous studies Not mentioned 09 (Fayas et al, 2019) From available literature of previous studies (Joshua, 1977) 10 (Thuraisingham and Weerasinghe, 2009) From available literature of previous studies (Joshua, 1977) 11 (Piyathilake et al, 2020) From available literature of previous studies (Fayas et al, 2019) (Hasselo and Sikurajapathy, 1985) (Jayarathne et al, 2010) (Joshua, 1977) (Mapa et al, 2010) (Wijesekara and Samarakoon, 2002) ( Wijesundara et al, 2018) (Zijister, 1989) 12 (Jayasekara and Kadupitiya, 2018) From available literature of previous studies (Joshua, 1977) 13 (Dias et al, 2019) From available literature of previous studies (Weerasinghe et al, 2016) 14 (Udayakumara and Gunawardena, 2016) From available literature of previous studies (Hasselo and Sikurajapathy, 1985) a large extent (Wilson and Gallant, 2000). Thus, ~85% of the cases in Sri Lankan studies have used GIS/RS based derivation methods of LS factor with the disadvantages viz.…”

“…Ultimately, LS map has been prepared by the multiplication of both L and S factors in raster calculator in Where, λ is horizontal projected slope length (m) (λ = flow accumulation *cell size), and m is slope length exponent. (Renard et al, 1997) S = 10.8sinθ + 0.03 for slope percent < 9% S = 10.8sinθ -0.50 for slope percent ≥ 9% θ is slope angle in degrees (Wischmeier and Smith, 1978) The LS factor is derived from the InVEST-SR/SDR model, both of which can be computed using the digital elevation model (DEM) (Sharp et al, 2018) 11 (Jayasekara and Kadupitiya, 2018) 12 (Dias et al, 2019) 13 (Udayakumara and Gunawardena, 2016) 14 (Lekamge et al, 2018) Not mentioned…”

“…As shown in Table 05, out of the above mentioned two methodologies, ~93% of the Sri Lankan studies (Piyathilake et al, 2020;Senanayake et al, 2020;Dias et al, 2019;Fayas et al, 2019;Panditharathne et al, 2019;Jayasekara and Kadupitiya, 2018;Lekamge et al, 2018;Jayarathne et al, 2010;Thuraisingham and Weerasinghe, 2009;Udayakumara and Gunawardena, 2016;Wijesundara et al, 2018;Wijesekara and Samarakoon, 2001) have used previous literature to determine C factor which is the simplest and time-saving method since the NDVI method requires advance RS applications viz. satellite imagery, aerial photography, and image processing.…”

“…Also, it is necessary to have knowledge of terrain, cropping system, and soil management practices (Jahun et al, 2015). Because of that, researchers from all around the world have been carrying out soil erosion related research studies to understand the mechanisms and rates of soil erosion (Onyando et al, 2005;Bhattarai and Dutta, 2007;Jaramillo, 2007;Anejionu et al, 2013;Gunawan et al, 2013;Balasubramani et al, 2015;Biswas and Pani, 2015;Devatha et al, 2015;Ganasri and Ramesh, 2016;Wijesundara et al, 2018;Dias et al, 2019;Fayas et al, 2019).…”