Abstract
Transpiration represents more than 30% of the global land–atmosphere water exchange but is highly uncertain. Plant hydraulics was ignored in traditional land surface modeling, but recently plant hydraulics has been found to play an essential role in transpiration simulation. A new physical-based representation of plant hydraulic schemes (PHS) was recently developed and implemented in the Common Land Model (CoLM). However, it is unclear to what extent PHS can reduce these uncertainties. Here, we evaluated the PHS against measurements obtained at 81 FLUXNET sites. The transpiration of each site was estimated using an empirical evapotranspiration partitioning approach. The metric scores defined by the International Land Model Benchmarking Project (ILAMB) were used to evaluate the model performance and compare it with that of the CoLM default scheme (soil moisture stress (SMS)). The bias score of transpiration in PHS was higher than SMS for most sites, and more significant improvements were found in semi-arid and arid sites where transpiration was limited by soil moisture. The hydraulic redistribution in PHS optimized the soil water supply and thus improved the transpiration estimates. In humid sites, no significant improvement in seasonal or interannual variability of transpiration was simulated by PHS, which can be explained by the insensitivity of transpiration demand coupled to the photosynthesis response to precipitation. In arid and semi-arid sites, seasonal or interannual variability of transpiration was better captured by PHS than SMS, which was interpreted by the improved drought sensitivity for transpiration. Arid land is widespread and is expected to expand due to climate change, thus there is an urgent need to couple PHS in land surface models.
Highlights
Transpiration accounts for 30% of the land–atmosphere water exchange, or60% (mean ± 1 standard deviation (s.d.)) of continental evapotranspiration [1,2]
In plant hydraulic schemes (PHS), a higher fraction of water was taken from the deep soil layer in dry sites than in wet sites, whereas in soil moisture stress (SMS) we found no significant differences in the proportion between deep and shallow soil water uptake (Figure 7A)
Recent benchmarking analysis on Community land Model Version 5 (CLM5) was consistent with our conclusions that the plant hydraulic scheme significantly increased the fraction of transpiration in evapotranspiration in semi-arid regions, especially in the southern hemisphere [14]
Summary
Transpiration accounts for 30% of the land–atmosphere water exchange, or60% (mean ± 1 standard deviation (s.d.)) of continental evapotranspiration [1,2]. Transpiration accounts for 30% of the land–atmosphere water exchange, or. The transpiration proportion of evapotranspiration is related to an important scientific question of how to dominate the biotic contribution in the land–atmosphere water exchange [3]. Several approaches have been developed to estimate evapotranspiration partitioning at large spatial scales. Wei et al [2] have built up empirical relationships between leaf area index (LAI) and transpiration for different vegetation types using satellite data and model estimation results. Both approaches have represented large-scale transpiration partitioning well with reliable technical theories
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