Evapotranspiration (ET) is an important hydrological process that determines North Carolina's water availability but has been poorly characterized. Evaluation and calibration of the available estimating methods are necessary to achieve a better understanding of ET. Investigation of reference evapotranspiration (RET), the rate of ET occurred from a reference crop surface that is only influenced by atmospheric conditions, allowed the first characterization of the spatio-temporal variations of the evaporative demand for the state.

RET estimates by the ETgage atmometers at the Environment and Climate Observing Network (ECONet) stations were compared with the benchmark Penman-Monteith RET equation established by the American Society of Civil Engineers. The analysis suggested a good correlation; however the observations were only 80% of the Penman-Monteith values on average. The ETgages were insensitive to wind, causing even lower readings than the Penman-Monteith values in windy conditions. On rainy days water retention on the evaporating surface also led to greatly reduced readings. Linear regression functions were developed for adjusting daily ETgage observations to Penman-Monteith equivalent values based on different rainfall amount.

Investigating the performance of empirical RET equations against the Penman-Monteith equation has allowed the development of operational RET equations. The Hargreaves-Samani and solar radiation-based regression equations were calibrated to obtain Penman-Monteith equivalent RET estimation based on different data availability, land regions and time-steps. The operational equations could produce mean absolute error (MAE) of about 20% and 12% in daily and weekly time-steps and less than 10% in the monthly time-step. Monthly coefficients for transferring reference to potential evapotranspiration were developed for six major land cover types, which showed varying regional variations for different surfaces.

Analysis of statewide RET indicated no secular trend and only weak interannual fluctuations in the past half century. RET peaked in June and July and January and December were the least evaporative months. Highest RET occurred in the Sandhills region due to its high radiation, temperature and dry atmosphere. Lowest RET rates occurred in the Mountains. A statewide southward increasing pattern dominated the winters while an inland-coast decreasing trend was most pronounced in the summers in the Coastal Plain.