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

Abstract

Surface evapotranspiration (ET) is an important component of water and energy in land and atmospheric systems. This paper investigated whether using variable surface resistances in the reference ET estimates from the full‐form Penman‐Monteith (PM) equation could improve the upscaled daily ET estimates 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 meteorological variables and eddy covariance (EC) system‐measured latent heat flux data on clear‐sky days were collected at two sites with different climatic conditions, namely, the subhumid Yucheng station in northern China and the arid Yingke site in northwestern China and were used as the model input and ground‐truth, respectively. The results showed that using the Food and Agriculture Organization (FAO)‐PM equation, the American Society of Civil Engineers‐PM equation, and the full‐form PM equation to estimate the reference ET in the constant EFr method produced progressively smaller upscaled daily ET at a given time from midmorning to midafternoon. Using all three PM equations produced the best results at noon at both sites regardless of whether the energy imbalance of the EC measurements was closed. When the EC measurements were not corrected for energy imbalance, using variable surface resistance in the full‐form PM equation could improve the ET upscaling in the midafternoon, but worse results may occur in the midmorning to noon. Site‐to‐site and time‐to‐time variations were found in the performances of a given PM equation (with fixed or variable surface resistances) before and after the energy imbalance was closed.

Estimating Daily Evapotranspiration From Remotely Sensed Instantaneous Observations With Simplified Derivations of a Theoretical Model

“…The constant R c method overestimated the daily LE by 6%–18% with the relative RMSE varying between 16% and 30% for the three groups of the MODIS‐based data sets. The constant Ω method and the constant R c / R a method overestimated the daily LE by 4%–9% and 2%–7% for two (N95‐derived LE data sets from Tang, Li, & Sun, ; Tang et al, and MODIS/TERRA‐based LE data sets from Tang, Li, and Sun () of the three groups of the MODIS‐based data sets, respectively, and underestimated the daily LE by −2% and −6% for the left group of the MODIS‐based data sets (MODIS/AQUA‐based LE data sets from Tang, Li, and Sun (). The relative RMSE varied between 13% and 28% for the constant Ω method and between 15% and 29% for the constant R c / R a method.…”

“…The RE correction method brings all of the imbalanced energy (surface available energy minus the sum of sensible heat flux and LE) to the EC‐measured LE. The ratio of the EC‐measured daytime to daily LEs before and after the correction is assumed constant to derive the corrected daily LE measurement, as done in our previous studies (Tang, Li, & Sun, ; Tang et al, ) to overcome the weakness of the BR or RE method.…”

“…However, due to the concave‐up shape of the daytime EF on clear‐sky or constant‐cloud days, many authors have reported that the constant EF method tends to significantly underestimate the daily ET (Cammalleri et al, ; Chávez et al, ; Delogu et al, ; Tang et al, ; Tang, Li, & Sun, ; Tang & Li, ; Xu et al, ) and the underestimation can even reach as large as 34% (Van Niel et al, ). Similar to the constant EF method that makes use of surface available energy as the conversion variable, some other schemes (Chávez et al, ; Ryu et al, ; Tang et al, ; Tang et al, ; Trezza, ; Van Niel et al, ) have also attempted to assume a temporally stable ratio of ET to surface downward solar radiation, surface net radiation, extraterrestrial solar radiation, or reference grass/alfalfa ET in a diurnal cycle. Except the constant reference EF (the ratio of actual ET to reference grass/alfalfa ET) method, which assumes the constancy of reference EF, the conversion schemes are incapable of accounting for the temporally variable environmental factors (e.g., wind speed, air humidity, air temperature, and air pressure) and the horizontal advection on the ET in a diurnal cycle.…”

Spatial and Temporal Variations of Potential Evapotranspiration in the Loess Plateau of China During 1960–2017

“…Previous research has provided numerous useful methods to evaluate the variation of the regional ET 0 , including the Penman-Monteith (PM) method [10], the Hargreaves method [11], the Priestley-Taylor method [12], and other empirical methods [13]. Due to the accessibility of input data and wide applicability, the PM method is recommended by the Food and Agriculture Organization (FAO) and is widely used for assessing spatial-temporal ET 0 variations during various climate scenarios and the response to variations in environmental factors [14,15].…”

Impact of clouds on the estimation of daily evapotranspiration from MODIS-derived instantaneous evapotranspiration using the constant global shortwave radiation ratio method

“…Evapotranspiration (ET) can be used to express the vegetation water consumption and reflect the surface moisture effectively (Delogu et al, 2012;Anderson et al, 2012;Tang et al, 2017a). change study, compared to instantaneous ET from remote-sensing based models at the time of satellite overpass (Chá vez et al, 2008;Li et al, 2009;Tang et al, 2013Tang et al, , 2017b.…”