Sizing of the lined on-farm pond (OFP) for various crop substitution ratios (CSR) in rainfed uplands of eastern India was addressed in the present study. Cross laminated polyethylene tarpaulin of 120 g m(-2) was used for lining of the OFP. Root-zone water balance model was used to simulate the water harvesting potential of maize and rice fields under various CSR (ratio of maize and rice areas) such as 70:30, 60:40, 50:50, 40:60, and 30:70. Daily variations in soil moisture storage in crop root zone, supplemental irrigation (SI) requirement during critical growth stages, and water balance in the lined OFP (using volume balance approach) were simulated for 30 years using soil, crop and meteorological data of the study area. Optimal size of the lined OFP for 60:40 CSR was found to be 8% of the farm area to meet the SI requirement of rice (Oryza sativa L.) in rainy season and black gram (Vigna mungo L.) and mustard (Brassica campestris) in winter. Among the inflow components to the lined OFP, the direct rainfall contribution was 67.72 and 73.90% (major contributor) and surface runoff from the field was 32.16 and 26.17% during the year 2007 and 2008, respectively. Among the outflow components from the lined OFP, the SI demand of the crops was found to be 78.84 and 46.73% of the storage during 2008 and 2009, respectively. The optimal size of the lined OFP incur initial investment of Rs. 13742 (1 US Dollar = 53 Indian rupees, Rs.); net profit of Rs. 14012; the benefit-cost ratio as 1.54; the internal rate of return as 11.06%; and the payback period as 4 years.