Water is an important natural resource and is vital for all life on the earth. The water balance over a place on earth provides in format ion about the surface and ground water. The water balance consists of the components of hydrological cycle like precipitation, evaporation, evapotranspiration, soil moisture, soil storage and runoff, but the major input is precipitation and output are evapotranspiration and runoff. In the current study a preliminary investigation is conducted for available surface water over a flood and drought prone Bihar state of India. For the purpose, monthly precipitation and monthly temperature are obtained from the Climate Research Unit (CRU), University of East Anglia, U.K. These reanalysed data at surface resolution of 0.50 x 0.50 degree (50x50km 2) is used for the period of 1901 to 2000 for each district of agro climatic zones of Bihar state of India. For estimating available surface water, a simple water balance equation is used. The empirical relations are used to compute surface runoff and water losses. It is found that frequencies of water deficit years are more in agro-climatic zone-III in compare to the agro-climatic zone I and II. There is sharp increase in frequency of water deficit years during 1981-2000 in all three agro climatic zones of Bihar state of India.