

Yan, H, Shugart, HH (2010). An air relativehumiditybased evapotranspiration model from eddy covariance data. JOURNAL OF GEOPHYSICAL RESEARCHATMOSPHERES, 115, D16106. Abstract We present an air relativehumiditybased model of evapotranspiration (ARMET) using relative humidity (RH) as a water stress factor to modify the Priestley and Taylor (1972) model potential evapotranspiration (PET) to actual evapotranspiration (ET). This model estimates ET using RH, air temperature (Ta), surface net radiation (Rn), and ground heat flux (G). The ARMET model was calibrated with 8 day averaged ET observations at a flux site and validated with independent ET observations at 14 flux sites from a wide range of climates and vegetation types. Estimates of ET(RnG) with available ;energy defined as (Rn  G) had an R2 = 0.71 with a root mean square error = 23.62 W m(2) and a bias = 8.02 W m(2). As the eddy covariance method for ET measurements can suffer from an energy imbalance problem, we quantitatively analyzed the impact of such energy imbalance by replacing the available energy of RnG with the sum of latent heat flux (lambda E) and sensible heat flux (H) in the modified ARMET model, denoted ARMET(lambda E+ H). Further validation at 16 flux sites indicates that ARMET(lambda E+ H) had a more accurate estimate (R2 = 0.83, RMSE = 16.98 W m(2), and bias = 5.42 W m(2)) compared with ARMET(RnG), which shows that the energy imbalance problem did affect model accuracy. This evaluation also showed RH was more closely related to the evaporation fraction than to the vapor pressure deficit (VPD), soil water content, and RHVPD indicating that RH was superior to other water stress metrics in ET estimation. As a pure meteorological model with a simple parameterization, the ARMET model is applicable over large spatial scales under a variety of climates and land cover types. DOI: 10.1029/2009JD013598 ISSN: 01480227 