Developing a reference crop evapotranspiration climatology for the southeastern United States using the FAO Penman-Monteith estimation technique

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 part of the water and energy balance on earth's surface. ET observations are only available for a limited number of locations across the southeastern United States. Empirical models are usually used to estimate ET at local and regional scales. No estimation technique is universal, but a standard method is the Penman-Monteith equation as specified by the UN Food and Agriculture Organization (FAO) in 1998. The FAO Penman-Monteith method estimates ET rates for a reference surface based on physical atmospheric observations. This estimate is also called a reference crop ET.

Quality-controlled meteorological observations provided by the State Climate Office of North Carolina were used to model ET using FAO Penman-Monteith. Estimation of ET requires input of daily average solar radiation, daily minimum and maximum temperature, daily average wind speed, daily minimum and maximum relative humidity, Julian day, latitude and longitude, and elevation for each monitoring location. A daily ET climatology for the southeastern United States is developed, and spatial and temporal variability in ET are analyzed for all seasons. To reduce noise, a 7-day moving average is used to provide an estimate of average ET based on all input years. Using Google technology, map and chart displays are developed for public use.