Evapotranspiration is the transfer of water from the earth's surface to the atmosphere, and comprises the sum of the processes of evaporation of moisture from soil, water bodies, and wet vegetated canopies and the transpiration of moisture from plants. Forecasts of this crucial link in the hydrologic cycle can be of great use to irrigators, municipal water managers, reservoir operators for their planning and scheduling purposes, and to operational and academic hydrologists across the western US.
Briefly, the evapotranspiration rate is driven by various meteorological factors, and crucially, by the presence of moisture to evaporate. Holding all else equal, evapotranspiration increases with increasing sunshine, wind speed and temperature. Conversely, evapotranspiration decreases with increasing humidity, but is always limited below the prevailing moisture availability. Because this latter quantity is basically unknowable over areas useful in size to water managers and agricultural users, it is impossible for these vital interests to directly estimate, observe, or predict evapotranspiration. Instead, we are limited to asking a simpler question, "given well-defined, ideal surface conditions, how much water would evaporate from the land surface to the atmosphere?" To answer this question, we fix the water availability by assuming that the evaporating surface is a well-watered "reference" crop, and then estimate the amount of evapotranspiration that would occur under a given set of meteorological conditions. This estimate, called "Reference Crop Evapotranspiration", can then be used by irrigators or reservoir operators as a starting point from which to determine the actual evapotranspiration under actual field conditions.
The forecasts of reference crop evapotranspiration shown here are called FRET (Forecast Reference crop EvapoTranspiration), and are presented as the most likely depth of water (in inches) that would evaporate and transpire from a reference crop under the forecast weather conditions on a daily and weekly basis.
In addition, departures from normal are presented to provide a historical context for FRET. Bimonthly FRET normals are calculated using climate data from 1980 to 2009. The departures from normal are equal to the current FRET forecast minus the bimonthly normal. Therefore, a positive (negative) departure from normal indicates above (below) average FRET.
Information for the FRET from this National Weather Service Office:
The Forecast Reference Evapotranspiration (FRET) is for a short canopy (or 12cm grasses). The short canopy ET values are calculated using the Penman-Monteith Reference Evapotranspiration Equations, adopted by the Environmental Water Resources Institute - American Society of Civil engineers (ASCE-EWRI, 2004), and uses the National Weather Service forecast of temperatures, relative humidity, wind, and cloud cover. This product will be issued daily by 5 am Pacific time, year round.