Globally, maize accounts for more of the world's food and feed, than any other crop. Improvements in genetics and associated agronomy have resulted in a linear increase in yield per acre over the past 60 years, and an expectation that this could continue into the future, given the projected 60% increase in demand for food and feed by 2050. However, crop water use has risen in proportion with yield, and improvements in crop water use efficiency (WUE) has remained stagnant. WUE is expected to worsen in the near future with predicted increases in temperature and water vapor pressure deficit (VPD). The result is that in the near future maize crop cultivation will require considerably more irrigation. Therefore, there is an urgent need to genetically improve WUE, to achieve sustainable yield increases to meet future food demand. Supply function on the ACi curve shows that the balance between stomatal conductance (gs) and photosynthetic capacity appears optimized for the prehistoric atmospheric [CO2], suggesting that in today's elevated [CO2] world gs could be reduced substantially without affecting photosynthesis at the leaf level. However, at the crop level, this is less certain since decreased gs will increase leaf temperature and in turn transpiration, while the benefit in the shaded lower canopy leaves may be minimal. To address this, we couple a steady-state biochemical model of leaf photosynthesis, stomatal conductance, boundary layer conductance, and energy balance with a radiation model of the crop canopy. Applying this crop model on modern maize cultivars, we show that decreasing gs(20%) through known genetic manipulations can improve leaf-level WUE up to (20-25)% at high light levels for no loss in productivity. Over a diurnal course, where leaves within the maize canopy experience a range of light, and temperature conditions, crop water use efficiency would be improved by 59% for less than 1% loss in productivity in a mature crop canopy. Despite higher leaf temperatures and VPDs, experienced when reducing gs(20%), our model simulations show that WUE increases by 23% at the leaf level at high light levels and 59% at the canopy level in 2050, demonstrating an important pathway towards sustainably meeting our future food and water security needs.