Abstract
The dual-source Shuttleworth-Wallace model has been widely used to estimate and partition crop evapotranspiration (λET). Canopy stomatal conductance (Gsc), an essential parameter of the model, is often calculated by scaling up leaf stomatal conductance, considering the canopy as one single leaf in a so-called “big-leaf” model. However, Gsc can be overestimated or underestimated depending on leaf area index level in the big-leaf model, due to a non-linear stomatal response to light. A dual-leaf model, scaling up Gsc from leaf to canopy, was developed in this study. The non-linear stomata-light relationship was incorporated by dividing the canopy into sunlit and shaded fractions and calculating each fraction separately according to absorbed irradiances. The model includes: (1) the absorbed irradiance, determined by separately integrating the sunlit and shaded leaves with consideration of both beam and diffuse radiation; (2) leaf area for the sunlit and shaded fractions; and (3) a leaf conductance model that accounts for the response of stomata to PAR, vapor pressure deficit and available soil water. In contrast to the significant errors of Gsc in the big-leaf model, the predicted Gsc using the dual-leaf model had a high degree of data-model agreement; the slope of the linear regression between daytime predictions and measurements was 1.01 (R2 = 0.98), with RMSE of 0.6120 mm s−1 for four clear-sky days in different growth stages. The estimates of half-hourly λET using the dual-source dual-leaf model (DSDL) agreed well with measurements and the error was within 5% during two growing seasons of maize with differing hydrometeorological and management strategies. Moreover, the estimates of soil evaporation using the DSDL model closely matched actual measurements. Our results indicate that the DSDL model can produce more accurate estimation of Gsc and λET, compared to the big-leaf model, and thus is an effective alternative approach for estimating and partitioning λET.
Highlights
Accurate estimation of evapotranspiration is important in understanding terrestrial hydrological cycles because lET is the largest component in the terrestrial water balance after precipitation [1]
The penetration of beam radiation, its variation and the dynamics of sunlit and shaded leaf area index (LAI) throughout the day all affect the ability of the big-leaf model to simulate diurnal changes in Gsc [18,29]
It is more complex than the big-leaf model, but the dynamic partitioning of LAI and irradiance between sunlit and shaded leaves has further reduced the errors associated with simplifying the leaves to only a big-leaf using either the total or empirically effective LAI
Summary
Accurate estimation of evapotranspiration (lET) is important in understanding terrestrial hydrological cycles because lET is the largest component in the terrestrial water balance after precipitation [1]. In agricultural production, improved estimation of crop lET is needed to develop precise irrigation scheduling and enhance water use efficiency, as soil water depletion is mostly determined by the rate of lET [2,3,4]. Mathematical models are needed to estimate lET using readily measurable meteorological and environmental variables. Vegetation transpiration (Tr) and soil evaporation (Es), which are controlled by different biotic and physical processes, are the two major components of lET. Transpiration is strongly linked to crop productivity since it occurs concurrently with photosynthetic gas exchange [7]. Because the two separate processes occur simultaneously, there is no simple way to distinguish between them [9,10]
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