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

Abstract

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.

How Much Water Is Evaporated Across California? A Multiyear Assessment Using a Biophysical Model Forced With Satellite Remote Sensing Data

“…On the other hand, a new and independent study of evaporation across the irrigated crops of the Central Valley, using Morton's complementary evaporation method (Morton, ), estimates that annual evaporation is on the order of 369 ± 62 mm/year (Szilagyi & Jozsa, ), which is close to our calculations. It is also consistent with regional estimates of evaporation across the Central Valley computed with GLDAS (406 ± 50 mm/year) and SSEB (489 ± 51 mm/year; see Supporting Information Figures S10, S11, and S12).…”

“…Typically, they are averaged over many years, to minimize errors due to changes in storage (Dralle et al, 2018;Draper et al, 2003). Others have focused on the evaporation from agricultural regions of the State (Burt et al, 2002;Sorooshian et al, 2011;Szilagyi & Jozsa, 2018).…”

Complementary‐Relationship‐Based Modeling of Terrestrial Evapotranspiration Across China During 1982–2012: Validations and Spatiotemporal Analyses

“…Szilagyi and Schepers () demonstrated that the wet surface temperature is independent of areal extent, thus T wes can be obtained from the Bowen ratio of a small wet patch for which the Penman equation is valid, that is, in which β we is the Bowen ratio of the well‐watered patch (assuming that available energy for the wet patch is close to that of the drying surface), e o, T wes being the saturated vapor pressure at T wes (≈ T wea ). Note that T wes may be larger than T a when the air is close to saturation, and in such cases, T wea should be capped by T a (Szilagyi, ; Szilagyi & Jozsa, ). α in equation is the dimensionless Priestley‐Taylor coefficient, with typical values from the range of [1.1–1.32] (Morton, ).…”

“…in which β we is the Bowen ratio of the well-watered patch (assuming that available energy for the wet patch is close to that of the drying surface), e o,Twes being the saturated vapor pressure at T wes (≈T wea ). Note that T wes may be larger than T a when the air is close to saturation, and in such cases, T wea should be capped by T a (Szilagyi, 2014;Szilagyi & Jozsa, 2018). α in equation (6) is the dimensionless Priestley-Taylor coefficient, with typical values from the range of [1.1-1.32] (Morton, 1983).…”

“…In this context, a recently developed nonlinear CR formulation by Brutsaert () attracts increasing attention (Brutsaert et al, ; Hu et al, ; Liu, Liu, et al, ; Zhang et al, ). Subsequent modifications by Crago et al () and Szilagyi et al (), with a revised physical constraint, further improved the skill of this nonlinear CR model in estimating ET a over different climate conditions (Crago & Qualls, ; Szilagyi, , ; Szilagyi & Jozsa, ). The most prominent advantage in the Szilagyi et al () version of the CR is that it establishes the value of the Priestley‐Taylor coefficient (Priestley & Taylor, ) by spatially integrating temperature and humidity data over wet areas.…”