Automatic extraction of tank outlets in a sub-watershed using digital elevation models

Watersheds contain many sub-watersheds (SWS) and the existence of a number of "small reservoirs" also called "tanks", located near the outlet points of the SWS, that serve as water-harvesting and storage structures, is common today. Estimation of the volume of water collected in these tanks, and determining the spatial and temporal distribution of the available water at various locations in the watershed are the two main aspects in agricultural landuse management. While, geographic information systems (GIS)-based approaches are used in estimating the volumes of water, routing is carried out in passing these volumes through the channels to the outlet of the main watershed. Extraction or determination of the location of the outlets of these tanks therefore, becomes essential to know the points where the water is available and wherefrom it could be distributed appropriately. An algorithm was developed for automatic identification or location of tank outlets as well as tank boundaries, wherein the advantages of digital elevation models (DEM), that have become an essential and integral part of distributed, i.e. GIS-based approaches in hydrological modelling, are used beneficially and to overcome the difficulty in locating these tank outlets manually. The details of the algorithm are reported in this paper. A program in ‘C’ has been developed and the algorithm is tested in the study area of Kalyanakere SWS, Karnataka, India and the results of the algorithm match excellently with the field data.