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.

Most land surface models (LSMs) used in Earth System Models produce a lower ratio of transpiration (T) to evapotranspiration (ET) than field observations, degrading the credibility of Earth System Model‐projected ecosystem responses and feedbacks to climate change. To interpret this model deficiency, we conducted a pair of model experiments using a three‐dimensional, process‐based ecohydrological model in a subhumid, mountainous catchment. One experiment (CTRL) describes lateral water flow, topographic shading, leaf dynamics, and water vapor diffusion in the soil, while the other (LSM like) does not explicitly describe these processes to mimic a conventional LSM using artificially flattened terrain. Averaged over the catchment, CTRL produced a higher T/ET ratio (72%) than LSM like (55%) and agreed better with an independent estimate (79.79 ± 27%) based on rainfall and stream water isotopes. To discern the exact causes, we conducted additional model experiments, each reverting only one process described in CTRL to that of LSM like. These experiments revealed that the enhanced T/ET ratio was mostly caused by lateral water flow and water vapor diffusion within the soil. In particular, terrain‐driven lateral water flows spread out soil moisture to a wider range along hillslopes with an optimum subrange from the middle to upper slopes, where evaporation (E) was more suppressed by the drier surface than T due to plant uptake of deep soil water, thereby enhancing T/ET. A more elaborate representation of water vapor diffusion from a dynamically changing evaporating surface to the height of the surface roughness length reduced E and increased the T/ET ratio.

Drylands cover over 40% of the global land area and are home to more than 2 billion humans. Here, we use the terrestrial water storage (TWS) anomaly data derived from GRACE satellites to assess water storage changes globally and find that drylands lost~15.9 ± 9.1 mm of water between April 2002 and January 2017. The TWS trends are more significant and apparent in dry regions than in humid regions. The decrease in TWS occurred mainly in hyperarid and arid regions. Exact causes to the observed declines in TWS remain elusive due to anthropogenic water withdrawals, atmospheric demand (potential evapotranspiration, PET) in contrast to supply (precipitation, P) caused by the warming, and terrestrial ecohydrological responses. Therefore, we use a process-based model forced by climate data to interpret the causes over three selected dryland regions showing the strongest drying trends. We find that the modeled TWS without considering anthropogenic water withdrawals explains most of the declining GRACE TWS over the southwestern North America (NA) and Middle East but underestimates the drying trend over North China. This suggests that TWS declines in the southwestern NA and the Middle East were primarily driven by the contrast between atmospheric demand and supply, whereas anthropogenic water withdrawals may have played a relatively more dominant role in TWS declines over North China. Additional model experiments indicate that terrestrial ecohydrological processes that help extract deep substrate water are critical for providing water supply additional to precipitation to sustain ET in the drying drylands at decadal scales.

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