Field water supply (FWS) represents the three sources of water that a crop can use for evapotranspiration (ET), and consists of available soil water at planting (ASWP), rainfall, and irrigation. Because it integrates all sources of water available to a crop, it impacts crop water production functions that are used as water management tools, such as water use efficiency (WUE) , which relates yield to ET, and irrigation water use efficiency (IWUE), which relates yield to amount of irrigation applied. The WUE and IWUE of treatments are used to compare the relative efficiencies among treatments, but are limited in their ability to maximize both WUE and IWUE. The objective of this analysis was to compare WUE, IWUE, and FWS of grain sorghum grown under of a range of irrigation treatments (0, 25%, 50%, and 100% replacement of ET), beginning soil water contents, evaporative demands, and different soil types to determine if FWS could be used to enhance the water use efficiency of crop production. By using the yield versus FWS function, along with the WUE and IWUE of individual treatments, the amount of FWS that must be maintained throughout the season to achieve near maximum yield while limiting irrigation water losses was identified for grain sorghum grown in three different soils in 1998 and 1999. This amount can be maintained by utilizing fully the available soil water at planting and precipitation during the season, and irrigation used as a supplement only. This represents an important crop water management tool for increasing both WUE and IWUE.