Towards a better understanding of deep belowground water stores and their influence on land-atmosphere exchange and drought impacts

Abstract

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.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’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.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.