Quantification of Unreported Water Use for Crop Irrigation using Publicly Available Agricultural Data and Reported Irrigation Withdrawals

Irrigation accounts for the largest consumptive water use in the United States (US), representing 62% of the total water consumption (water not returned to the source after being withdrawn). Areas with a relatively smaller percentage of irrigation withdrawals and moderate irrigation volumes may significantly impact water supply, especially at the time of low flows. Water supply managers and government agencies use user-reported water withdrawal data for developing water management programs. Limited data availability is one of the key challenges in quantifying and understanding irrigation withdrawals. Additionally, 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 (e.g., Virginia only requires reporting of withdrawals over a million gallons in any month for crop irrigation). 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. Therefore, this study aims at developing a methodology for estimation of non-reported agricultural withdrawals at the county level to understand the potential effects on water supply and enhance water management. This study developed a data-based method using USDA census irrigation data (reported every five years), USDA Irrigation and Water Management Survey (IWMS), Virginia Department of Environmental Quality (VDEQ) reported withdrawal data, and meteorological data to generate estimates of unreported water use across Virginia. Furthermore, the meteorological analysis was conducted to evaluate the variation of unreported and total irrigation withdrawals under different weather conditions, i.e., an average year, moderate drought, and drought of record.