Evapotranspiration of applied water as an indicator of irrigation demand and groundwater response in southwestern U.S. vineyards

Authors: ZHANG, NING - University Of California, Davis; PARKER, LAUREN - University Of California; Ostoja, Steven; KISEKKA, ISAYA - University Of California, Davis

Submitted to: Journal of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2026
Publication Date: 7/8/2026
Citation: Zhang, N., Parker, L., Ostoja, S.M., Kisekka, I. 2026. Evapotranspiration of applied water as an indicator of irrigation demand and groundwater response in southwestern U.S. vineyards. Journal of the ASABE. 69(4):545-558. https://doi.org/10.13031/ja.16597.
DOI: https://doi.org/10.13031/ja.16597

Interpretive Summary: Water is a precious resource, especially in agriculture. With climate change affecting water availability, farmers face pressure to use water more wisely. This study looks at ways to measure and manage irrigation water use effectively. One important measure we used is evapotranspiration of applied water (ETaw), which basically tracks how much water crops use. By understanding this, farmers can figure out the best ways to use water, manage their crops, and ensure they're not harming the environment. The study looked at various factors that affect ETaw, like how much rain falls (effective precipitation), changes in soil moisture, and the overall water needs of the crops (ETc). These factors impact water use differently in dry and wet regions. What we found is that in irrigated areas, the biggest factor affecting water use is how much the crops actually need (ETc), but in rainfed regions, natural rainfall plays a bigger role. Interestingly, we found that changes in soil moisture have a limited impact on overall water use, which means simpler methods for estimating soil moisture might be okay to use. The study also looked at how all these factors change over the year, especially during the growing season and the water year. This helps us understand how to use groundwater sustainably and adapt to climate change. Overall, this research gives us important insights into how to manage water in agriculture better, which is crucial for dealing with climate change and ensuring we can keep growing food sustainably.

Technical Abstract: Groundwater sustainability planning in irrigated agricultural regions requires spatially explicit metrics that capture the magnitude and variability of irrigation demand. Evapotranspiration of applied water (ET?w), estimated as the residual of crop evapotranspiration (ETc) after accounting for effective precipitation (Peff) and soil moisture change (?SM), provides a planning-level estimate of irrigation demand, yet the factors controlling its interannual variability and its relationship to groundwater response remain insufficiently characterized across hydroclimatic gradients. This study quantifies the relative contributions of ETc, Peff, and ?SM to interannual ET?w variability and evaluates the relationship between ET?w and groundwater-level change across five irrigated American Viticultural Areas (AVAs) in the southwestern United States. ET?w was estimated using a soil water balance framework consistent with Cal-SIMETAW over 1991–2020, and a variance–covariance–based variability attribution was applied over both growing-season and water-year periods. Results reveal a clear hydroclimatic gradient: ETc and Peff contributions are relatively balanced in Mediterranean climates, whereas Peff dominates variability in arid, monsoon-influenced regions. ?SM functions primarily as a seasonal buffer and contributes minimally at the annual scale. ET?w exhibits statistically significant but moderate correlations with groundwater-level changes (growing season: r ˜ 0.06–0.51; water year: r ˜ 0.36–0.65), indicating that it captures irrigation demand pressure but does not fully explain groundwater system response. Long-term ET?w exceeds basin-scale water-availability constraints at most AVAs, providing context for irrigation demand pressure relative to available resources, although such comparisons involve differences in spatial scale and accounting frameworks. These findings demonstrate that ET?w is a scalable, climate-responsive metric for regional irrigation demand assessment and can help contextualize irrigation demand pressure within broader groundwater system dynamics when interpreted alongside complementary hydrologic information