Water productivity is defined as the ratio of the net benefits from crop, forestry, fishery, livestock, and mixed agricultural systems to the amount of water required to produce those benefits. In its broadest sense it reflects the objectives of producing more food, income, livelihoods, and ecological benefits at less social and environmental cost per unit of water used, where water use means either water delivered to a use or depleted by a use. Put simply, it means growing more food or gaining more benefits with less water. Physical water productivity is defined as the ratio of the mass of agricultural output to the amount of water used, and economic productivity is defined as the value derived per unit of water used. Water productivity is also sometimes measured specifically for crops (crop water productivity) and livestock (livestock water productivity). To feed a growing and wealthier population with more diversified diets will require more water for agriculture on an average annual basis [well established]. Evapotranspiration from agricultural land is estimated at 7,130 cubic kilometers and without increases in water productivity could increase by 60%–90% by 2050 (see chapter 3 on scenarios). Agricultural water withdrawals from natural systems are estimated at 2,664 cubic kilometers, or about 70% of water withdrawn for human purposes. Additional water for agriculture will strain terrestrial and aquatic ecosystems and intensify competition for water resources. Improving physical water productivity in agriculture reduces the need for additional water and land in irrigated and rainfed systems and is thus a critical response to increasing water scarcity, including …