Evapotranspiration describes water from the Earth’s surface that is moved to the atmosphere as water vapor, with the transpiration component representing water used by plants. The rate of evapotranspiration depends on 1) how much energy is absorbed by the Earth’s surface, 2) the water available at the surface, and 3) the capacity of the atmosphere to hold more water vapor (or its humidity).
More than half the water that falls on land surfaces as rainfall is returned to the atmosphere via evapotranspiration. In regions where there is insufficient rainfall to support agriculture, evapotranspiration may be supplemented by groundwater pumped to the surface and used to irrigate crops.
A well developed, but underused, modeling approach is called the complementary relationship of evaporation; it predicts evapotranspiration without needing detailed information on the Earth’s surface by instead using observations of the humidity of the lower atmosphere. Szilagyi and Jozsa  used the complementary relationship approach in a well-conceived study of evapotranspiration trends for California.
They demonstrate that irrigated regions had sustained or increasing evapotranspiration during a period when rainfall was declining. Their analysis thus demonstrates a way to quantify the effects of irrigation on a massive scale needed to inform sustainable groundwater management.
Citation: Szilagyi, J., & Jozsa, J. . Evapotranspiration trends (1979–2015) in the Central Valley of California, USA: Contrasting tendencies during 1981–2007. Water Resources Research, 54. https://doi.org/10.1029/2018WR022704
—D. Scott Mackay, Editor, Water Resources Research