Water availability for irrigated agriculture is declining in both China and the United States due to increased use for power generation, municipalities, industries and environmental protection. Persistent droughts have exacerbated the situation, leading to increases in irrigated area as farmers attempt to maintain profitable production. With world population expected to increase by 50% by 2050, the increased need for food, fiber, feed and fuel will rely on irrigated agriculture, which produces about 40% of total production on only 20% of farmed land. Faced with declining water availability, stakeholder interest in producing more crop per drop is greatly heightened and focuses on both the efficiency of irrigation water application and the water use efficiency (WUE) of crops as influenced by irrigation management and systems. Crop WUE is the economic yield divided by the crop water use (evapotranspiration, ET). ET flux is measured indirectly using the soil water balance, in which all other fluxes into or out of the crop root zone (irrigation, precipitation, deep soil water fluxes, runon and runoff) are quantified and totalized over a time period and then ET is determined as the residual plus the change in stored soil water in the soil profile to well below the root zone over the same time period. Both soil profile water content measurements and weighing lysimeters can be used to determine change in storage; and new and current technologies for these approaches will be discussed. Methods of both irrigation water application and management can affect the efficiency of water use through their effects on plant vigor and yield formation. Application methods will be compared, and the effects of new management methods for regulated deficit irrigation will be examined in light of yield and water use efficiency.