Abstract
Abstract. Different methods for assessing evapotranspiration (ET) can significantly affect the performance of land surface models in portraying soil water dynamics and ET partitioning. An accurate understanding of the impact a method has is crucial to determining the effectiveness of an irrigation scheme. Two ET methods are discussed: one is based on reference crop evapotranspiration (ET0) theory, uses leaf area index (LAI) for partitioning into soil evaporation and transpiration, and is denoted as the ETind method; the other is a one-step calculation of actual soil evaporation and potential transpiration by incorporating canopy minimum resistance and actual soil resistance into the Penman–Monteith model, and is denoted as the ETdir method. In this study, a soil water model, considering the coupled transfer of water, vapor, and heat in the soil, was used to investigate how different ET methods could affect the calculation of the soil water dynamics and ET partitioning in a crop field. Results indicate that for two different ET methods this model varied concerning the simulation of soil water content and crop evapotranspiration components, but the simulation of soil temperature agreed well with lysimeter observations, considering aerodynamic and surface resistance terms improved the ETdir method regarding simulating soil evaporation, especially after irrigation. Furthermore, the results of different crop growth scenarios indicate that the uncertainty in LAI played an important role in estimating the relative transpiration and evaporation fraction. The impact of maximum rooting depth and root growth rate on calculating ET components might increase in drying soil. The influence of maximum rooting depth was larger late in the growing season, while the influence of root growth rate dominated early in the growing season.
Highlights
Soil water movement forms the central physical process in the land surface models (LSMs), interacting with surface infiltration, evaporation, root extraction, and underground water recharge
While it has been widely accepted that water vapor and heat transport should be incorporated in a soil water model, especially in arid or semi-arid environments (Bittelli et al, 2008; Saito et al, 2006; Zeng et al, 2009a, b, 2011a, b), it is still not clear how these factors affect soil water dynamics in crop fields
The objectives of this study are twofold: (i) compared with observations obtained through a lysimeter experiment, we investigate how different methods for measuring ET will affect the assessment of soil water dynamics in a crop field located in a semi-arid environment in Northwest China, based on a coupled model considering the transfer of water, vapor, and heat in the soil; (ii) with the calibrated coupled model, a sensitivity analysis is conducted to explore the influence of crop growth parameters on the ET partitioning
Summary
Soil water movement forms the central physical process in the land surface models (LSMs), interacting with surface infiltration, evaporation, root extraction, and underground water recharge. Recent assessment of the HYDRUS-1D model with different ET methods indicated that using the PM equation gave a better model performance in simulating soil water content (Mastrocicco et al, 2010). Previous studies generally based conclusions on the combined analysis of the entire growing season (Padilla et al, 2011) These results could be inappropriate to some extent. The objectives of this study are twofold: (i) compared with observations obtained through a lysimeter experiment, we investigate how different methods for measuring ET will affect the assessment of soil water dynamics in a crop field located in a semi-arid environment in Northwest China, based on a coupled model considering the transfer of water, vapor, and heat in the soil; (ii) with the calibrated coupled model, a sensitivity analysis is conducted to explore the influence of crop growth parameters on the ET partitioning.
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