WATER-SUPPLY IN MONSOONAL ASIA - MODELING AND PREDICTING SMALL TANK STORAGE

Traditionally, small dams (tanks) have been both centres of village se ttlements and the source of irrigation in Asia. Being solely dependent on local rainfall characteristics and other climatic factors, the wat er storage behaviour closely follows the rainfall pattern. Carry-over of storage from year to year is rare because of natural evaporation an d withdrawals for irrigation. The efficient allocation of storage, wit hin any year, is analogous to the optimization of a time-varying exhau stible resource stock. This study attempts to model water storage of d ams using weekly data with a view to intrayear optimization of storage . The behaviour of storage is approximated by a dynamic system driven by rainfall distribution and temperature, and characterized by a linea r transfer function model following modification of rainfall to allow for losses. The order of the linear part of the model is identified an d parameters estimated for three tanks in Sri Lanka using a recursive estimation procedure. This model can be used for the prediction of sto rage for a given rainfall scenario. This is demonstrated for past real istic rainfall experiences' using stochastic simulation of storage.