Identification of potential runoff harvesting sites in a water scarce rural watershed using GIS approach

Abstract

Water scarce regions in western parts of India are subject to various hydrological constraints. The resource poor agricultural communities that depend on rainfed agriculture are the hardest hit. Rainfall patterns in these arid and semi-arid areas are unpredictable. Frequent occurrence of mid-season dry spells or late monsoon result in poor soil water availability during growing season, reduced crop yield and increased risk of crop failure in severe cases. Therefore, in such circumstances, the ability to efficiently harvest the rainfall runoff is of critical importance to maintain agricultural production in an economically and environmentally sustainable manner. However, implementation of runoff harvesting in an extensive way, without any adverse impacts on downstream hydrological systems, requires a better understanding of the hydrological processes. The representation of spatial variation in land characteristics such as slope, soil, and land use as well as rainfall is important in identifying potential runoff harvesting sites. Thereafter, modeling of the hydrological response in watersheds where runoff harvesting is being considered can be performed and likely impacts can be assessed. At present, there is a growing need for cost effective and time saving methods for identifying areas that are suitable for implementing runoff harvesting technologies particularly in rainfed rural agro-ecosystems as a means to supplement water availability. Advances in computer technology and Geographic Information System (GIS) provide useful tools that allow efficient integration of spatial characteristics of watersheds leading to improved representation of hydrological processes in the landscape. In this study, a small rural semi-arid watershed, located in the Jaisamand lake Catchment in Aravalli Hills in Udaipur district of Rajasthan, is considered. The watershed is predominantly inhabited by tribal communities. Scarcity of water together with lack of proper natural resource management have resulted in degradation of land and water resources, and, poor social and economic conditions in the region. To cope with water scarcity, there is a need to implement rainfall runoff harvesting to develop the water resources in a sustainable manner and mitigate the adverse impacts of droughts, stabilize agricultural production as well as reduce the community’s vulnerability to water shortage. The study identifies priority areas for runoff harvesting in the rural watershed with the use of GIS. For small watersheds, the Natural Resources Conservation Service (NRCS) method is the most widely used technique for estimating surface runoff for a given rainfall event. It considers the relationship between landcover and hydrologic soil group, which together make up the curve number. A curve number is used to ascertain how much rainfall infiltrates into soil and how much rainfall becomes surface runoff. Traditionally, an area weighted average curve number for the entire watershed is used to study the runoff of a watershed. The detail of spatial variation is often lost. This study takes help from GIS to produce curve number and runoff maps with the help of ArcCN-Runoff tool available in ArcGIS. Slope, derived from the Digital Elevation Model (DEM) of the watershed, along with the NRCS curve number were used to determine the runoff potential. Locating optimal sites for runoff harvesting was based on the physically derived potential runoff map as well as the socio-economic factors such as distance to croplands and rural settlements. Thus, through GIS analysis, potential runoff harvesting sites are identified in relation to the areas that concentrate runoff and where the stored water will be appropriately distributed. Output of this study, based on integrated GIS modeling system, is presented using ‘suitability maps’ developed for potential runoff harvesting sites for the rural watershed. It is concluded that providing an accurate spatial representation of the runoff generation potential within a watershed is an important factor in developing a strategic runoff harvesting plan for any watershed. The use of GIS approach to facilitate this process improves the accuracy level for locating suitable areas for runoff harvesting in view of the GIS capability to utilize spatial information in an integrative manner and spatially displaying it through maps.