Based on surface energy balance and the assumption of fairly invariant evaporative fraction (EF) during daytime, this study proposes a new parameterization scheme of directly estimating daily EF. Daily EF is parameterized as a function of temporal variations in surface temperature, air temperature, and net radiation. The proposed EF parameterization scheme can well reproduce daily EF estimates from a soil-vegetation-atmosphere transfer (SVAT) model with a root mean square error (RMSE) of 0.13 and a coefficient of determination (R²) of 0.719. When input variables from in situ measurements at the Yucheng station in North China are used, daily EF estimated by the proposed method is in good agreement with measurements from the eddy covariance system corrected by the residual energy method with an R² of 0.857 and an RMSE of 0.119. MODIS/Aqua remotely sensed data were also applied to estimate daily EF. Though there are some inconsistencies between the remotely sensed daily EF estimates and in situ measurements due to errors in input variables and measurements, the result from the proposed parameterization scheme shows a slight improvement to SEBS-estimated EF with remotely sensed instantaneous inputs.

This study develops a method for estimating surface energy fluxes (surface sensible heat flux (H), latent heat flux (LE), and soil heat flux (G)) simultaneously from continuous observations of surface temperature (T_s), air temperature (T_a), and net radiation (R_n) without calculating various resistances. First, H, LE, and G are parameterized by some constant parameters that remain fairly invariant during a given day and some known functions related to T_s and T_a. Second, these constant parameters are solved by a minimization technique based on surface energy balance. Data from ground-based measurements at the Yucheng station were used to evaluate the performance of the developed method. Results show that the simplified parameterization schemes well reproduce H, LE, and G with a root mean square error (RMSE) of ∼20 W/m² at the instantaneous time scale, and perform better at the daily scale. For the estimates of H, LE, and G using the known T_s, T_a, and R_n measured at the Yucheng station as inputs, the RMSE is ∼60 W/m² at the instantaneous time scale and ∼20 W/m² at the daily scale. The requirement of continuous observations throughout a day in the developed method could be met by remotely sensed data from geostationary meteorological satellites. Fewer input variables and the obviation of calculating various resistances give the method the potential to generate surface fluxes over a large area.

In a previous study, a daily evaporative fraction (EF) parameterization scheme was derived based on day–night differences in surface temperature, air temperature, and net radiation. Considering the advantage that incoming solar radiation can be readily retrieved from remotely sensed data in comparison with surface net radiation, this study simplified the daily EF parameterization scheme using incoming solar radiation as an input. Daily EF estimates from the simplified scheme were nearly equivalent to the results from the original scheme. In situ measurements from six Ameriflux sites with different land covers were used to validate the new simplified EF parameterization scheme. Results showed that daily EF estimates for clear skies were consistent with the in situ EF corrected by the residual energy method, showing a coefficient of determination of 0.586 and a root mean square error of 0.152. Similar results were also obtained for partly clear sky conditions. The non-closure of the measured energy and heat fluxes and the uncertainty in determining fractional vegetation cover were likely to cause discrepancies in estimated daily EF and measured counterparts. The daily EF estimates of different land covers indicate that the constant coefficients in the simplified EF parameterization scheme are not strongly site-specific.