Groundwater Doctrine and Water Withdrawals in the United States.

Groundwater is a valuable natural resource which is directly related to food production, human and ecosystem health. In the US, 48% of irrigated agriculture relies on groundwater, but there is no systematic national program responsible for groundwater management. For this paper, each US state was classified based on its most prevalent groundwater doctrine: Absolute Ownership, Prior Appropriation, Reasonable Use, and Correlative Rights. The Köppen climate zone of each state, USGS runoff data and USGS county-level water use data from 1985 to 2015 were used to analyze how groundwater use varies with climate and groundwater doctrine. Semi-arid states, which all follow Prior Appropriation doctrine, have the highest average irrigation rate, while states following Reasonable Use doctrine have the lowest average rate, but the largest variability. Analysis of Covariance shows that in Prior Appropriation states, irrigation volume and area do not increase during warm, dry periods but in Absolute Ownership states irrigation volume does increase. Water use trend analysis shows that irrigated area and groundwater withdrawals have increased over the last 30 years in humid and temperate regions, while irrigated area has decreased in semi-arid regions. At the same time, irrigation rate and the fraction of irrigation coming from groundwater has increased everywhere, suggesting a potential shift in the preferred water source for irrigation. This data analysis will provide insights for future work on how water policy should respond to water scarcity in US. [ABSTRACT FROM AUTHOR]