Evaluation of percolation tanks in a semitropical watershed in India using SOFTANK model

Watershed development projects in India have proved their utility in making more water available for drinking and crop production and conserving the soil resources in the fragile rural ecosystems. Water harvesting structures viz. nala bunds, percolation tanks, farm ponds are the main structures which create surface water and groundwater storages in watershed. These structures are effective in creating more surface and groundwater resources but there is lack of an analytical tool for designing these structures optimally. This often results in non optimal rainwater harvesting through these structures. The combination of one or more harvesting structures in watershed is referred to as watershed based tank system in this paper. These systems have some unique characteristics and requirements. While designing the tank system one has to decide the number of tanks, their locations and type. Tank type depends on the orientation of command area around the tank. The in situ treatments like CCT in the catchments of these tanks affect the inflow to the tank. To avoid upstream-downstream conflict some portion of runoff needs to be allowed to go downstream of the watershed. SOFTANK (Simulation Optimization For Tanks) model has been developed which takes into account all these characteristics. This paper discusses the SOFTANK model and its application for evaluation of percolation tanks in the Pimpalgaon Ujjaini watershed in the scarcity region of the Maharashtra state of India. In Pimpalgaon Ujjaini watershed there are two tanks at stream point No 3 and 6 with capacities of 695877 and 216937 m3 (total storage capacity 912814 m3). These tanks are used for groundwater recharge only. The water balance analysis of two percolation tanks of Pimpalgaon Ujjaini watershed was carried out with the SOFTANK model for 29 years (1975 to 2003). Model showed that 42% runoff is harvested by the tanks and 58% goes out of the watershed. Seepage was the major (84%) outflow component from the tanks. The tanks are overdesigned and therefore any in situ soil water conservation works in the catchments of these tanks should be discouraged.