Water balance simulation and economic analysis for optimal size of on-farm reservoir

Rainfed rice in eastern India suffers frequent moisture stress leading to severe yield reduction due to uncertainty of rainfall coupled with inadequate field level rainwater conservation structures. Construction of the on-farm reservoir (OFR) is an alternative for the storage of excess rainwater from the diked rice field during monsoon season followed by its reuse as supplemental irrigation to the rice in the same season and pre-sowing irrigation to mustard in the winter season. For determining the optimum size of the OFR, water balance model in the cropped fields (both rice and mustard) as well as in the OFR was developed along with economic analysis. Simulation study revealed that an OFR of 12% of the farm area having 2 m depth of trapezoidal section would be optimal for rice–mustard cropping system. The above-mentioned optimum size of the OFR provides the benefit cost ratio (BCR), internal rate of return (IRR) and pay back period (PBP) as 1.23, 15.5% and 15 yr, respectively for dry year. Values of BCR, IRR and PBP for average year were simulated as 1.22, 15% and 15 yr, respectively, and those for the wet year were 1.21, 14.8% and 16 yr, respectively. To justify the investment in the OFR irrigation system, field experiments were conducted for three years. The observed BCR, IRR and PBP from the experiment were found to be 1.17, 14.8% and 16 yr, respectively.