Assessment of ET estimation by ECOWAT

ECOWAT is a relatively simple model for estimating ecosystem evapotranspiration using the agricultural crop coefficient approach. The model estimates evapotranspiration as ETw = ETo×Km×Kv×Kd >= ETo×Ke, where ETo is the standardized reference evapotranspiration, Km adjusts for microclimate, Kv corrects for differences between ETm and ET of well-watered, dense vegetation, Kd adjusts for plant density, Ks regulates ET for plant water stress, and Ke is a base-line soil evaporation coefficient that is used to estimate the evaporation of bare soil. The theory behind the ECOWAT model is described in a related paper “ECOWAT Theory and Calibration”. In this paper, we present some examples where the ECOWAT model was evaluated against measured evapotranspiration. The study sites include two CarboEurope forest sites in Italy, one CarboEurope shrubland site in Italy, one Ameriflux grass savannah site in California, and one wetland site in California. The ECOWAT evapotranspiration estimates for the wetland site were good in three different years. There was no water stress, the canopy was dense, and the ETo station was within 1 km of the ecosystem, so only the vegetation factor was changed to refine the vegetation coefficients. The grass savannah site is characterized by cold wet winters and hot, dry summers without rainfall. ECOWAT provided an excellent prediction of the evapotranspiration at this site, where the water stress function was especially important. At the Mediterranean shrubland site, the vegetation was sparse so a density coefficient was used. In addition, the water stress coefficient was important to correct for the long summer drought. The site is, however, adjacent to the Mediterranean sea, and it is characterized by considerable morning dew deposition and the site has an extensive shallow water table. By adding contribution of water from these sources, the ECOWAT model predicted evapotranspiration well. Two deciduous forest sites near the coast and north of Rome, Italy were also studied, and the ECOWAT model was able to accurately predict evapotranspiration after accounting for fog and dew contributions. In these comparisons, the climate and vegetation varied greatly, yet the ECOWAT model consistently gave good ET estimates.