Enhancing the Noah‐MP Ecosystem Response to Droughts With an Explicit Representation of Plant Water Storage Supplied by Dynamic Root Water Uptake

Abstract

We developed a model of plant water storage supplied by dynamic root water uptake through hydrotropic growth in Noah-MP.  It enhances plants' efficiency to use antecedent rain water stored in shallow soils and that in aquifers with the aid of capillary rise.  Future plant hydraulic models should consider soil water retention model uncertainties and soil macropore effects on water retention.

Why Is the Terrestrial Water Storage in Dryland Regions Declining? A Perspective Based on Gravity Recovery and Climate Experiment Satellite Observations and Noah Land Surface Model With Multiparameterization Schemes Model Simulations

“…The temporal evolution of canopy water storage is calculated as the sum of snowfall, rainfall, dew, and deposition minus the sum of drip, evaporation, sublimation, and throughfall. The current version of Noah‐MP also explicitly represents the plant water storage as the residual of root water uptake and transpiration (Niu et al, 2020; Wang et al, 2018). Although the plant water storage may be negligible compared to other water storages, it is critical for plants' stomatal opening and transpiration.…”

“…Plant transpiration water loss is controlled by stomatal resistance, which is regulated by the plant water storage, air temperature, humidity, CO 2 concentration, light, and the leaf boundary layer resistance. In the current version, the plant water availability factor β controlling the stomatal resistance is parameterized as a function of water storage in the living plant tissues, M q (Niu et al, 2020). where M q , wilt represents the minimum plant water storage at the wilting point of 30 bars (306 m or 3.0 MPa) and M q , max the maximum plant water storage when the plants are at full hydration.…”

“…A low β indicates that the plants are under water stress due to limited water storage and restricts the opening of stomata, reducing CO 2 uptake and carboxylation and transpiration water loss. While transpiration depletes the plant water storage, M q is replenished by root water uptake driven by the water pressure gradient between the soils and the roots and computed analogous to Ohm's law (see Niu et al, 2020, for details). This new version of Noah‐MP largely enhances the ecosystem resilience to water stress over drylands and during droughts, while its previous version and other LSMs are generally more vulnerable to water stress, producing lower leaf area index or ecosystem productivity during droughts (e.g., Mao et al, 2013; Zhu et al, 2019).…”

“…Uncertainties in model structure and parameters may substantially affect the TWS modeling results as reflected from the model sensitivity experiments ( Figure 6) and thus confound disaggregation of the climatic and anthropogenic drivers. The model version used in the CTRL experiment largely improves the model realism with augmentations in representing plant hydraulics and root dynamics, although it is still uncertain in representing the soil water retention characteristics by different models (Niu et al, 2020). Our conclusion that anthropogenic water withdrawal may have played a more dominant role in the declining TWS in Northern China based on the modeling results is further confirmed by the map of fractional irrigation area ( Figure S20) and consistent with a recent analysis using partial least squares regression (Yuan et al, 2019).…”

Advances in Land Surface Models and Indicators for Drought Monitoring and Prediction

“…These improvements were most notable in agricultural regions. Other physics improvements, such as a dynamic root system and plant water storage, have been made to the dynamic phenology schemes and have shown benefits in drought applications (e.g., Niu et al 2020).…”