Technical Abstract: Summer fallow has been practiced to increase the water available for succeeding crops in areas that receive less than 500 mm of annual precipitation. Objectives of summer fallow have been to maximize soil water storage, make plant nutrients available, reduce soil water storage, make plant nutrients available, reduce soil erosion hazards, control weeds during the entire fallow period, and suppress soil water evaporation. From the 1930's to the late 1950's, research indicated fallow principles such as soil water loss due to deep percolation below rooting depth of wheat was negligible. Evaporation losses were great and fallow efficiencies were not likely to exceed about 16%, unless a practical method of reducing evaporation from north to south. Since the late 1950's, better residue management techniques have increased surface residue cover during fallow had increased fallow efficiencies on 16% to over 40%. Residue on the soil surface during fallow reduced evaporative losses, increased soil water storage, and increased deep percolation of soil water below the crop rooting depth. Therefore, the frequency and length of fallow in the future may be reduced by using greater crop intensity and diversity. This would result in cropping systems that are more sustainable.