Thursday, May 31, 2018

Evapotranspiration and Water Balance Maps Now Available

Atmometer (E-T Gauge)
About six years ago CoCoRaHS added measurement of reference evapotranspiration (ET0) to the phenomena observers can measure. Evapotranspiration is the sum of evaporation from ground surfaces and the transpiration of water to the atmosphere from plant leaves, and is a function of temperature, wind speed, relative humidity, and solar radiation. On average more than half the precipitation that falls is returned to the atmosphere through ET. There are about 120 observers across the the U.S. and in Canada currently measuring E-T using a special E-T gauge called an atmometer. You can read more background on CoCoRaHs E-T measurements in this blog post from 2013.

Recently the Midwestern Regional Climate Center (MRCC) in partnership with the National Integrated Drought Information System (NIDIS) developed are series of maps of E-T and water balance measurements using CoCoRaHS data. These maps can be found on the Midwest Drought Information page in the Midwest Climate Watch section of the MRCC web site.

There are maps of 7-, 14-, 30-, and 60-day accumulations of E-T for the Midwest region and for the continental U.S. and southern Canada. A second set of maps displays water balance calculations for the same periods as E-T. Water balance is the total precipitation minus the total E-T for a designated period. Water balance charts  are available on the CoCoRaHS web site and show the change in water balance over time at a location along with daily E-T and precipitation values. All the maps are updated each day.

Here is today's map of 7-day E-T for the U.S. and Canada. Note that only stations with 100 percent data completion in that period are displayed. E-T occurs every day, so stations with missing observations cannot be used to compare with other stations.

This is a 14-day water balance map. It's pretty clear where it's very dry and has been very wet.

An advantage to having this data plotted spatially is we can see where more E-T observations are needed. ET does not vary to the same extent as precipitation (it's more similar to temperature), so multiple measurements in the same general area are usually not needed, unlike precipitation. ET measurements are only made during the warm season, since freezing temperatures can damage the gauge. There a few stations in the south and west that measure E-T year 'round.

If you are interested in measuring E-T at your location contact CoCoRaHS headquarters for more information.