A reference crop evapotranspiration and open water evaporation tool for the southeast US

Evapotranspiration (ET) is a combination of evaporation and plant transpiration processes into a total moisture flux from the ground to the atmosphere. As a result, ET plays an integral role in environmental processes at global, regional, and local scales. It is an important component of the water and energy balance on earth's surface. ET observations are only available for a limited number of locations across the Southeast US. Empirical models are often used to estimate ET at local and regional scales. No estimation technique is universal, but the UN Food and Agriculture Organization (FAO) Penman-Monteith equation is internationally recognized in paper number 56 as a standard method. This method, known as FAO56 Penman-Monteith, estimates ET rates for a reference surface based on physical atmospheric observations. This estimate is also called reference ET. The FAO56 Penman-Monteith technique can be modified to estimate evaporation from an open water surface.

Quality-controlled meteorological observations provided by the State Climate Office of North Carolina are used to model reference ET and open water evaporation using the FAO56 Penman-Monteith methodology. Estimation of these moisture fluxes requires inputs of daily average solar radiation, daily minimum and maximum temperature, daily average wind speed, daily minimum and maximum relative humidity, day of year, latitude, longitude, and elevation for each monitoring location. A daily and monthly short-term climatology of FAO56 Penman-Monteith reference ET and open water evaporation is developed for the Southeast US. To reduce noise, a 7-day moving average is used to generate the daily climatology values.

Using Google technology, map and chart displays of FAO56 Penman-Monteith reference ET are developed for public use to show spatial and temporal variability across the Southeast US region. Two products use Google Maps to display reference ET values estimated using the FAO56 Penman-Monteith method. One product is a daily reference ET estimate, which allows the user to select a date from January 1, 2002 to yesterday. The other product is a historical climate tool which displays the short-term climatology values of daily average and average monthly total reference ET. Users can also choose to display an annual time series for a particular station and for the three different time scales. A similar tool is being developed for open water evaporation estimates.