Temporal upscaling of instantaneous evapotranspiration on clear-sky days using the constant reference evaporative fraction method with fixed or variable surface resistances at two cropland sites
Résumé
Surface evapotranspiration (ET) is an important component of water and energy in land andatmospheric systems. This paper investigated whether using variable surface resistances in the referenceET estimates from the full-form Penman-Monteith (PM) equation could improve the upscaled daily ETestimates in the constant reference evaporative fraction (EFr, the ratio of actual to reference grass/alfalfa ET)method on clear-sky days using ground-based measurements. Half-hourly near-surface meteorologicalvariables and eddy covariance (EC) system-measured latent heatflux data on clear-sky days were collected attwo sites with different climatic conditions, namely, the subhumid Yucheng station in northern China andthe arid Yingke site in northwestern China and were used as the model input and ground-truth, respectively.TheresultsshowedthatusingtheFoodandAgricultureOrganization(FAO)-PMequation,theAmericanSocietyof Civil Engineers-PM equation, and the full-form PM equation to estimate the reference ET in the constantEFr method produced progressively smaller upscaled daily ET at a given time from midmorning tomidafternoon. Using all three PM equations produced the best results at noon at both sites regardless ofwhether the energy imbalance of the EC measurements was closed. When the EC measurements were notcorrected for energy imbalance, using variable surface resistance in the full-form PM equation could improvethe ET upscaling in the midafternoon, but worse results may occur in the midmorning to noon. Site-to-siteand time-to-time variations were found in the performances of a given PM equation (withfixed or variablesurface resistances) before and after the energy imbalance was closed.
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