UC Davis

Irrigation accounts for the largest consumptive water use in the United States (US), representing 62% of the total water consumption. Water supply managers and government agencies use user-reported water withdrawal data for developing water management programs. However, many users underreport or are not in compliance with the water withdrawal reporting regulations of the state. The regulations in many states also exempt users from reporting irrigation water withdrawals under a specific amount. Such irregularities in non-reported but consumptive agricultural withdrawals result in the underestimation of water use and considerable uncertainty about the impacts of irrigation withdrawals on water supply. Therefore, this study presents a methodology for estimation of non-reported agricultural withdrawals at the county level to understand their potential effects on water supply and enhance management of agricultural water resources. This study developed a data-based method using USDA census irrigation data, USDA Irrigation and Water Management Survey (IWMS) and gridded meteorological data to develop estimates of irrigation water withdrawals from 2002 to 2017. Additionally, user reported withdrawal data was used to validate estimates and quantify the volume of unreported water use for small (below the withdrawal reporting threshold) and large farms at county level. It was observed that small and large farm unreported withdrawals averaged 13% and 110% of reported withdrawals respectively. Notably, 86% of counties in Virginia with irrigation have collective small farm withdrawals of sufficient magnitude that would be subject to reporting requirements if they were from a single user. Small farm unreported withdrawals were consistent through time and large farm unreported withdrawals decreased between 2002 and 2017 due to lower irrigation demand and higher reporting. This approach provides a practical method that can be adopted by water supply managers to better account for agricultural water use in humid climates where supplemental irrigation is not fully captured in existing permitting and reporting regulations.