Availability of fresh water for irrigation is decreasing day by day due to increasing demand of water for domestic and industrial use. On the other hand, in Indian sub continent steady increase in population density enhancing the crop production target. Under such situation the main task is to grow more food with less water. Quantification of various water productivity functions through Marginal Water Use Efficiency (MWUE) and Elasticity of Water Production Function (EWPF) may be an effective tool for optimum use of irrigation water. Therefore, a field study was conducted during 2003-04 to workout the MWUE and EWPF for onion crop with different irrigation regimes. Seven irrigation frequencies were: 1.6 ETC – Sprinkler irrigation at 160% of ETcrop; 1.4 ETc - Sprinkler irrigation at 140% of ETcrop; 1.2 ETC– Sprinkler irrigation at 120% of ETcrop; 1.0 ETC – Sprinkler irrigation at 100% of ETcrop; 0.8 ETC – Sprinkler irrigation at 80% of ETcrop; 0.6 ETC – Sprinkler irrigation at 60% of ETcrop and Surface irrigation – where crop was irrigated with 40 mm of water at an IW / CPE ratio of 1.2 through ridge and furrow water application method. Though AET requirement for maximum yield (5409 kg ha-1) was 283.9 mm but the highest (19.48 kg m-3) Water Use Efficiency (WUE) was attained with relatively low AET (268.1 mm). In terms of bulb yield and WUE Micro Sprinkler Irrigation (MSI) found superior over the surface (conventional) one. MSI with 1.4 ETc found most suitable for bulb yield. However MSI with 1.2 ETc resulted in highest level of WUE. In this region highest level of Water Production Function (WPF) can be attained at an AET value of 268.1 mm and AET up to 295.3 mm can be permitted for higher WPF.