This paper introduces the concepts of marginal water use efficiency (MWUE) and elasticity of water production (EWP),and reveals dynamic interrelations of crop production and water consumption and water use efficiency (WUE) based on the functional relation between water consumption and crop yield ( or photosynthate). Providing the relation of seasonal evapotranspiration (ET) and crop yield (Y) is as Y=f(ET),then WUE=Y/ET,MWUE=dY/dET,and EWP=MWUE/WUE.WUE increases with the rise of ET if EWP>1 and decreases with the rise of ET if EWP<1. Proper construction of the function of crop water production,Y=f(ET),serves as a basis for correct analysis of WUE. If the Y=f(ET) function is linear,the changing trend of WUE with ET is directly affected by the constant term. If the Y=f(ET) function is quadratic parabolic,the maximum WUE comes earlier than the maximum yield with the rise of ET. The ET that makes WUE reach to its maximum value equals the arithmetic square root of a ratio of the constant term to the coefficient of quadratic term(). WUE increases with the rise of ET when ,conversely WUE decreases with the rise of ET when . Field experiment on spring corn yield response to water and fertilizer in the loess tableland of China showed that the form of Y=f(ET) appeared parabolic relationships at both high and low fertility levels,and high fertility plots had higher WUE and ET than low fertility ones under the same level of water supply. At high fertility plots in 1987,WUE increased with the rise of ET when ET< 472mm,reached to the maximum value of 16 3kg/ha.mm when ET =472mm,and decreased with the rise of ET when ET<472mm. The yield reached the maximum of 8 111kg/ha when ET = 524mm. MWUE always decreased with the rise of ET within the experimental range and dropped to zero when ET = 524mm. At low fertility plots,WUE reached the maximum value of 11 6kg/ha.mm when ET =436mm,and the maximum yield of 5 298kg/ha was gained when ET = 485mm; the WUE and the MWUE were similar to the ones of high fertility plots in the aspect of the changing trend.