Evapotranspiration Trends (1979–2015) in the Central Valley of California, USA: Contrasting Tendencies During 1981–2007

Trends in monthly evapotranspiration (ET) rates across three watersheds covering the Central Valley in California were calculated by the latest calibration-free version of the complementary relationship of evaporation for 1979–2015. While a recent study concluded that ET rates of the irrigated fields in the Central Valley were declining in 1981–2007, here an ET trend of about 2.6 ± 12 mm per decade was found over the same period in spite of a drop in precipitation (−22 ± 30 mm per decade) and ET rates (−9.5 ± 10 mm per decade) for the rest of the watersheds, none of them statistically significant. After 2007, the precipitation decline accelerated causing a sharp drop in both irrigated and nonirrigated ET rates across the watersheds. Observations from the California Irrigation Management Information System support the present findings: Under increasing air temperatures, both dew point temperature and relative humidity values increased (at a statistically significant rate) during 1983–2007, while they reversed afterwards, in agreement with the estimated sharp irrigation ET trend decline for the remainder of the study period. Actual (in this case over irrigated fields) and reference ET rates complement each other, that is, they express opposite tendencies, as was demonstrated with California Irrigation Management Information System data, yielding a statistically significant plot-scale irrigation ET rate increase of 31 to 41 (±17) millimeters per decade for 1983–2007 in accordance with a similar drop in reference ET rates of −28 to −50 (±16) millimeters per decade, depending on whether published monthly or daily values (aggregated to monthly ones after leaving out spurious measurements) were employed.