Observations and Modeling of Evapotranspiration across North Carolina

Abstract

Evapotranspiration (ET) is measured at 14 stations in North Carolina using an ET gage, which allows distilled water to evaporate through a waterproof surface acting as the reference crop alfalfa. These stations are part of the State Climate Office of North Carolina's Environment and Climate Observing Network (ECONet) and are distributed throughout the state's three broad climate regions: mountains, Piedmont and coastal plain.

Observations from the ET gage at Lake Wheeler were compared with the gradient method to show that the ET gage observations are accurate and reliable. Averages of reference evapotranspiration, precipitation, soil moisture and temperature are analyzed for spatial trends. Daily reference evapotranspiration values are also compared with precipitation, soil moisture, temperature and humidity at one location in each of the three climate regions. Evapotranspiration decreases initially when precipitation occurs. This is due to the increase in atmospheric humidity and decrease in temperature. After precipitation ends, evapotranspiration increases due to the increase in the water supply. Daily pan evaporation values at two sites from years 2003 and 2004 were compared with evapotranspiration at the same locations. Effective precipitation was calculated for 2004 at each of the 14 evapotranspiration measuring stations.

Two different empirical methods for estimating evapotranspiration were used for comparison with observations: Priestley-Taylor and Penman-Monteith. An appropriate value for the Priestley-Taylor parameter was estimated. Estimates of ET from these empirical methods indicated that the Penman-Monteith method overestimated ET as compared to observations and the Priestley-Taylor method gave better results.

A mesoscale numerical simulation was performed for the period, 00Z July 18, 2004 to 00Z July 24, 2004 using the MM5 model with a domain centered over North Carolina. Spatial distribution of evapotranspiration was obtained and comparisons made between model predictions and observations of latent heat flux, soil moisture, soil temperature, wind speed, and air temperature. Latent heat flux was estimated using model parameters and empirical methods and compared with observations. A special focus on the simulations of days July 21 through July 24 included one day prior to a precipitation event, two days during the event, and one day after the precipitation ended. The model has a tendency to overestimate the latent heat flux in the coastal plain and eastern Piedmont, due to the overestimation of precipitation in this area. The mountains had the highest latent heat flux predicted by the model. Across the state, the Priestley-Taylor method using simulated parameters performed better than the Penman-Monteith method.