<p>Water availability controls vegetation activity and the carbon balance of terrestrial ecosystems across a large portion of the global land surface. Although the influence of terrestrial water storage (TWS) on the land carbon balance is evident in globally aggregated measures, it remains unknown whether the large annual amplitudes in TWS are causally linked to water availability in the rooting zone of vegetation, or whether they reflect a correlation of plant water stress with water stored in other landscape elements that may not directly be connected to vegetation functioning (lakes, rivers, groundwater). Global models of the land surface typically ignore hillslope-scale variations in plant water availability, and water stores that are located beyond the soil, and beyond prescribed plant rooting depths. This simplification is partly owed to a lack of empirical information.</p> <p>Here, we approach this gap from two angles: from the site scale using eddy covariance observations, and from the global scale using earth observations. Water mass balance constraints derived from thermal infrared-based evapotranspiration (ET) estimates and precipitation reanalysis data indicate plant-available water stores that exceed the storage capacity of 2 m deep soils across 37% of the Earth&#8217;s vegetated surface. Large spatial variations of the rooting zone water storage capacity across topographic and hydro-climatic gradients are tightly linked to the sensitivity of vegetation activity (measured by sun-induced fluorescence and by the evaporative fraction) to water deficits. Similar patterns between ET and cumulative water deficits emerge from site-level flux measurements. We found large variations of the vegetation sensitivity to dry conditions across sites and at several sites a muted response of ET to dry conditions in spite of large (>300 mm) seasonal water deficits at some sites.</p> <p>Taken together, results we show here hint at a critical role of plant access to deep water stores and the need to extend the focus beyond moisture in the top 1-2 m of soil for understanding and simulating land-atmosphere exchange. Our results add to the emerging evidence that water stored in the weathered bedrock and plant access to groundwater may have a more important role in regulating land-atmosphere exchange and the carbon cycle than previously appreciated.</p>
<p>Terrestrial evapotranspiration (ET) is a key factor in the global energy and water cycles. It is constrained by the transport of moisture from the soil and from vegetation to the atmosphere. The water storage capacity in the root zone (S<sub>r</sub>) is an important parameter in land-atmosphere water exchanges, defining how long vegetation is able to transpire during drought. However, S<sub>r</sub> is hard to measure directly, being associated with the depth of plant roots actively involved in water uptake and the potential of tap roots accessing deep water and enabling sustained transpiration during drought.</p><p>&#160;</p><p>In this study, we present a method to estimate S<sub>r</sub> from flux measurements, based on a deep neural network approach trained on eddy covariance (EC) data, multiple soil moisture datasets and a remotely sensed index of vegetation greenness. We derive a soil moisture stress function (fET) that isolates the control of soil moisture on ET. We then use EC data to estimate S<sub>r</sub> by investigating how it relates to the climatology of the maximum cumulative water deficit (CWD, defined as the cumulative difference between actual ET and precipitation) experienced by the vegetation across different sites. We hypothesize that plants exposed to high CWD develop higher S<sub>r </sub>(acclimation to water stress) and that maximum CWD is thus a good estimator of S<sub>r</sub>. To identify root zone water storage from flux measurements, we regress the output of fET against CWD and estimate the maximum CWD for stress by calculating the intersection of the regression line with the x-axis. The apparent sensitivity of fET to CWD and its correlation with the maximum CWD across sites are indicative of adaptation to the prevailing climate and drought regime.</p><p>&#160;</p><p>We find that for many sites, particularly in seasonally dry climates, fET does not exhibit a continuous decline with increasing CWD, but follows a step-change and levelling off. That is, at the high end of the CWD spectrum, fET no longer appears sensitive to further increases in CWD. This suggests that plants may have access to deep water reservoirs during the unfolding of a drought event, indicating that plant access to water becomes decoupled from water use.</p><p>&#160;</p><p>This study highlights the need to investigate the representation of plant access to deep water reservoirs during drought in terrestrial ecosystem models. These findings could improve our understanding of land-climate interactions, particularly under water-limited conditions.</p>
<p>Energy partitioning between surface latent (LE) and sensible (H) heat fluxes is a key factor in the development of the boundary layer and the regulation of the hydrological cycle. Climate factors and surface cover are commonly considered the major controlling effects on energy partitioning. However, the influence of other drivers such as water table depth and groundwater convergence has rarely been considered.</p> <p>Here, we use an extensive dataset of eddy covariance and global remote-sensing data to show that not only climate, but also water table depth and plant functional type (PFT) play an important role in energy partitioning across different biomes. Our findings illuminate the understanding of plant water stress in terrestrial ecosystems.</p>
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