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

Plants are able to adapt to changing environments and thus survive droughts. However, most land surface models produce unrealistically low ecosystem resiliency to droughts, degrading the credibility of the model‐predicted ecohydrological responses to climate change. We aim to enhance the Noah‐MP modeled ecosystem resilience to droughts with an explicit representation of plant water storage supplied by dynamic root water uptake through hydrotropic root growth to meet the transpiration demand. The new model represents plant stomatal water stress factor as a function of the plant water storage and relates the rate of root water uptake to the profile of model‐predicted root surface area. Through optimization of major leaf, root, and soil parameters, the new model improves the prediction of leaf area index, ecosystem productivity, evapotranspiration, and terrestrial water storage variations over most basins in the contiguous United States. Sensitivity experiments suggest that both dynamic root water uptake and groundwater capillary rise extend the plants' "memory" of antecedent rainfall. The modeled plants enhance their efficiency to use antecedent rain water stored in shallow soils mainly through more efficient root water uptake over the U.S. Southwest drylands while use that stored in deep soils and aquifers with the aid of groundwater capillary rise in the Central United States. Future plant hydraulic models should not ignore soil water retention model uncertainties and the soil macropore effects on soil water potential and infiltration. Plain Language Summary: Plants are able to adapt to changing environments and thus survive droughts. However, the plants represented in current computer models do not well survive droughts for lacking a representation of adaptation mechanisms. This study develops explicit representations of plant water storage and plant water availability, which are enhanced by root water uptake that is linked to the predicted vertical distribution of fine root biomass in response to soil water content. The new model enhances ecosystem productivity and transpiration under droughts in most large river basins in the contiguous United States. Virtual experiments reveal two "pumping" mechanisms for plants under droughts to use antecedent rain water. The plants tend to more efficiently use antecedent rain water stored in shallow soils through more efficient root water uptake over the U.S. Southwest drylands and that stored in deeper soils or aquifers with the aid of groundwater capillary rise in the Central U.S. basins. Soil water pressure becomes critically important for pushing the soil water into plant tissues and up to the leaves in the new model. Therefore, uncertainties in soil water retention models and the effects of soil macropores on soil water potential and infiltration should be well treated in future models. Key Points: We developed a model of plant water storage supplied by dynamic root water uptake through hydrotropic growth in Noah‐MPIt enhances plants' efficiency to use antecedent rain water stored in shallow soils and that in aquifers with the aid of capillary riseFuture plant hydraulic models should consider soil water retention model uncertainties and soil macropore effects on water retention [ABSTRACT FROM AUTHOR]