Abstract:
Sri Lanka's dry zone is studded with nearly 12,500 small tanks built by ancient kings to store rain water for irrigation purposes. Most of them are called minor tanks which cater to land extents less than 80 ha.. Farmers--^nder these reservoirs request assistance to rehabilitate .Aese reservoirs to increase storage for increased water security. Though the common method of increasing the capacity is to increase the spill level, it is often not possible to carryout such raising because of the threat of upstream inundation. Most of the farmers are of. the belief that the bed excavation or desilting is the best solution for the need to increase water storage. Though there are some who favour the desilting/bed excavation, some are for the spill level raising. A literature survey identified the need of a comparative study of these options to determine which could facilitate the safe cultivation of more land under the reservoir. The present work carried out a comparative monthly water balance on two already desiited/excavated tanks from the dry zone of Sri Lanka. The study revealed that the advantages of spill raising or bed excavation depends on the rainfall received by each catchment, the topography, location and extent of earth removal from the tank bed ,etc. In the studied reservoirs it was felt that the increase If capacity contributed to the irrigation water release was negligible. The water balance provided comparative results that could be used as an eye opener for any attempts of reservoir storage enhancements.
