When is Plant Hydraulics Necessary for Predicting Soil Water Stress in Land Surface Models?
Abstract
For decades, most land surface models (LSMs) have used a normalized ratio of the ambient to maximum transpirable soil moisture, called a β factor, to downregulate plant photosynthesis and transpiration due to water stress. Recent studies have indicated that the specification of the β factor as a function of soil moisture is a major source of uncertainty in modeling global carbon and water cycles. An alternative option is to use physically-based plant hydraulic models (PHMs) to describe physiological responses to water stress. Here, we present results showing that although β factors can adequately approximate canopy-scale predictions of PHMs under some conditions, that certain combinations of plant traits, soil types, and environmental forcings cause β factor and PHM based predictions to diverge dramatically. In the latter cases, LSM predictions based on β factors could become significantly erroneous.
We compared β factor and PHM predictions within a calibrated LSM we developed based on the Community Land Model v5, which can be run in either a standard PHM or modified β factor "mode". In β mode, the model uses effective β factors derived from the outputs of the PHM mode under dynamical environmental conditions. The model was applied across FLUXNET 2015 sites selected to span representative combinations of vegetation, soil, and climate types. LSM results were compared in terms of predicted surface fluxes, including latent and sensible heat fluxes and gross primary productivity, when run in each mode. The differences between model outputs under PHM and β modes reveals model structural limitations that fail to account for nonlinear interactions between plant physiology and environmental conditions. The differences were sensitive to variations in environmental conditions and plasticity in plant hydraulic traits. This research provides a framework for specifying when the added complexity of PHMs is necessary, quantifies the errors incurred by using β factors, and illuminates the relationship between plant hydraulics and the empirical β factor.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H51M1669S
- Keywords:
-
- 0476 Plant ecology;
- BIOGEOSCIENCES;
- 1812 Drought;
- HYDROLOGY;
- 1813 Eco-hydrology;
- HYDROLOGY;
- 1818 Evapotranspiration;
- HYDROLOGY