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Introduction
By the middle of the 21st century, global population is projected to be 50% larger than the beginning of the century and global grain demand is projected to increase at least by 50% and likely to double (Alexandratos, 1999; Alexandratos & Bruinsma, 2012; Valin et al., 2014). The income effect from economic growth will lead to a significant increase in per capita food consumption in the world, from a current (2005–2007) level around 2,700 Kcal/day to more than 3,000 Kcal/day by 2050 (Alexandratos & Bruinsma, 2012). Given the limited quantity of arable land on Earth and the need to preserve natural ecosystems for a sustainable future, the most feasible solution to accommodate the projected growth is to promote the productivity of the agricultural system. In US agriculture, according to the 2007 Census of Agriculture, irrigated farms account for 55% of the total crop sales value on only 57 million acres of land (23 million hectares, 7.5% of the total US farmland). Irrigation can substantially improve land productivity. In general, irrigated land has a higher yield than rainfed ground due to stable soil moisture and longer growing season (Mendelsohn & Dinar, 2003; Khan et al., 2006). Therefore, in the coming decades of growing food consumption, irrigated agriculture will play a critical role to meet the demand.
In the USA, irrigation in agriculture accounts for a large proportion of total water withdrawal. In 2010, according to the US Geological Survey (USGS), total irrigation withdrawal accounts for 38% of the total US freshwater withdrawals. In western states, irrigation use makes up as high as 80% of the total withdrawal. Given the increasing population pressure, growing food demand will need more water to expand irrigated agriculture unless there is a significant improvement in water use efficiency (Tilman et al., 2002). The current situation of usable water resources, however, is not optimistic. Many irrigated regions in the USA are supplied by groundwater pumped in excess of aquifer recharge. According to Konikow (2013), the rate of groundwater depletion has been consistently increasing since WWII, and the depletion rate has reached almost 25 km3/year in a recent decade (2000–2008). Competition from other civil uses (e.g., growing urban water use, sustainability requirements of natural ecosystems) also reduces...