Summertime Potential Evapotranspiration in Eastern Washington State

The demands for water in agricultural regions depend on the rate of evapotranspiration (ET). Daily records of potential ET (pET) are available from the late 1980s through the present for five stations in eastern Washington State (George, Harrah, LeGrow, Lind, and Odessa) through the Pacific Northwest Cooperative Agricultural Weather Network (AgriMet) under the auspices of the Bureau of Reclamation. These records reveal a secular increase in the summer (June–August) mean pET over the period 1987–2014. This increase can be attributed largely to an increase in solar irradiance of 20–30 W m −2 over the same period. The seasonal mean solar irradiance accounts for approximately 35%–50% of the variance in the interannual variations in seasonal mean pET at the individual stations and for approximately 60% of the variance from a five-station average perspective. The period of analysis includes a mean increase of temperature of about 0.3°C (10 yr) −1 , and the variability in temperature relates more to the year-to-year fluctuations in pET than to the overall increase in pET. The time series of surface relative humidity and wind speed exhibit only minor trends. Daily and seasonal mean data for 500-hPa geopotential height and other variables are used to determine aspects of the regional atmosphere associated with periods of high pET. Anomalous ridging aloft and negative anomalies in 925-hPa relative humidity tend to occur over the study area during the summers with the greatest pET. The relationships that are emerging may provide a basis for empirical downscaling of pET from global climate model projections.