| Summary: | Watershed development and management has been adopted as a new approach for land and water resources management in India. Water resources are created (in the form of small tanks) in these watersheds along with other development activities. These tanks are primarily used for irrigation or groundwater recharge or for both purposes. Since gaps were found in the literature on the optimum design of watershed based tank Irrigation systems this research was carried out with the aim to "Design an optimum tank irrigation system for the watershed". The philosophy of watershed management and the nature of tank system in the watershed required a new approach for their optimum design. Therefore a comprehensive methodology has been developed in this research for design of optimum tank system in the watersheds of semi and and sub humid tropics. A new classification of tank system is proposed. The concept of tank strategy is introduced and used in the methodology of optimum tank system design. The methodology takes into account the effect of in situ rainwater harvesting practices on the tank system, inflow coming to the watershed from upstream watersheds and downstream release from the candidate watershed. The methodology is based on the concept of Integrated Water Storage System (IWSS) in which three storage media in the watershed i. e. soil, tank and aquifer are integrated to derive the optimum tank system. Field, tank and aquifer water balances are simulated for deriving optimum tank system. The methodology has been converted into computer code which resulted into a computer model -SOFTANK (Simulation Optimization For TANKS). The SOFTANK model was applied to two case study watersheds - Akola and Pimpalgaon Ujjaini. Both these watersheds come under semiarid region of Maharashtra state of India. When optimum tank strategies were derived for these watersheds, it was found that tank system was not economical for Akola watershed whereas it was economical for Pimpalgaon Ujjaini watershed. Accordingly the optimum tank system for the Pimpalgaon Ujjaini watershed was derived. This research is expected to make an innovative and practical contribution to the literature on the design of optimum tank systems for watersheds in semiarid and sub humid tropics.
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