An optimization model has been formulated to maximize the net benefit from a cascade of tank system with conjunctive use of surface water from the tank and ground water from wells under semi-arid hydrological setting in northern Karnataka. The paddy being traditional and priority crop of the tank command area farmers, its optimal water allocation plans have been proposed for paddy along with alternative light irrigated groundnut and sunflower crops. The cropping pattern was derived commensurating with the tank inflow quantity at 10 per cent, 45 per cent, 70 per cent and 90 per cent probability levels in all the four tanks and groundwater that could be pumped from the existing wells to the limit of sustainable draw down (2.18 m and 3.06 m). The minimum water productivity of ` 1.34 m-3 would be assured in the command areas of all the tanks of the cascade system whenever they receive inflows recurrently beyond their capacities (at 10-year or higher return periods) and paddy crop would be cultivated alone. The maximum productivity that could be achieved at very low inflows (at 1 to 1.33 year return period) was in the range of ` 3.77 m-3 (Gunjalli tank) to ` 4.61 m-3 (Devanpalli tank), provided light irrigated crops namely, sunflower and groundnut crops are practiced.