This paper gives an overview of effect of rainfall on crop productivity in Karnataka, based on 40 years data which analyses the shift in cropping pattern and the water used for crop production. Water use estimates showed a spurt in demand and utilization of water, mainly on account of the increase crop area. The analysis of the factors influencing aggregate crop productivity revealed that rainfall had a positive and significant effect on productivity, but its impact was only marginal. The growth rate of rainfall over the reference period was 0.19% per annum which translated into a 3% contribution to the growth in crop productivity in the state. In addition to increased agricultural demand for water, water availability is reduced due to urbanization and industrialization along with poor water management. A decrease in water storage with increased evaporation would further widen the gap between supply and demand for water, adversely affecting crop productivity. Hence, there is an urgent need for water conservation and management technologies, as well as water storage. It is necessary to increase and improve rainwater management along with its equitable and efficient use to serve as a booster to agricultural output in rain fed areas of the state