Watershed management for soil and water conservation in Amban Ganga sub-basin of Mahaweli Basin, Sri Lanka: Morphometric analysis based approach under GIS environment

Watershed prioritization using the morphometric analysis method is one of the best ways to ensure proper planning and soil and water resources management in a watershed. Amban Ganga, an 81 km-long tributary of the Mahaweli River, plays a crucial role in nourishing agricultural fields and forest areas in the Central and North Central parts of Sri Lanka. The study aims to prioritize watersheds of this river basin using Geographic Information System (GIS) techniques. By analysing spatial data obtained using a Digital Elevation Model (DEM) with a 20 m contour interval, the research assesses the hydrological characteristics and soil erosion potential of this river basin. A study found that the Amban Ganga stream network exhibits a dendritic drainage pattern and extends to Strahler's 7th order, comprising 8,581 streams over 1,555.98 km². Further, 18 micro-watersheds were identified within the sub-basin, and morphometric analysis (considering linear, areal, and relief parameters) was conducted for micro-watersheds. Findings indicated that permeable sub-soil, dense vegetation, and a smoother peak runoff have led to somewhat reduced flood risks within the area. Elongated basin shape and a gradual slope further help to mitigate flooding and soil erosion risks. Additionally, the relief analysis confirmed that lower flood risk is due to the gradual slope of this sub-basin. According to morphometric analysis, micro-watersheds were prioritized into high (4), medium (6), and low (8) categories. Prioritizing watershed areas and understanding their unique features enables stakeholders to develop effective conservation strategies to protect soil and water resources and promote sustainable development within the studied river basin.