Many woody ecosystems tend to have deep root systems. During droughts, they often access substantial volumes of water beyond the groundwater table. Therefore, to understand ecosystems’ drought response, information is needed about the moisture availability in deep soil. However, existing observations of underground water supply over large areas are limited to near surface depths (<5 cm). We addressed this gap by demonstrating that time-variable gravity measurements from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) are reliable proxies for total plant-available water (Zhao et al., 2017a;b). The primary process driving the gravitational variations that GRACE and GRACE-FO measure is the redistribution of water. To better characterize drought, we developed a drought severity index based on GRACE and GRACE-FO (above figure) to provide information about both root zone soil moisture and groundwater changes. This new index complements other atmospheric and shallow surface drought indicators such as precipitation, temperature, and surface soil moisture. We are now using this approach to delineate the time evolution of droughts so that we can study how vegetation respond at different drought stages.

Evapotranspiration (ET) of terrestrial ecosystems is closely linked with photosynthesis and constitutes a major flux in the water cycle. Thus, it is critical to characterize ET behaviors and their connections with other eco-hydrological processes. However, current large-scale ET datasets (such as remote sensing-based) often implicitly assume a vegetation response to water stress, preventing them from providing direct observational constraint for ET behaviors. To avoid this problem, we developed new estimates of ET based on GRACE/GRACE-FO TWS and a water balance approach (above figure), which are free of assumptions about plant-water relations. We am currently using these data to characterize the prevalence of ET increase during drought, which is a concerning phenomenon that quickly depletes water resources, may cause flash drought, and exacerbate ecosystem stress. This phenomenon has previously been observed in specific regions, but our findings show that this phenomenon is globally widespread and systematically underestimated by Earth system models. These findings suggest models may underestimate the role of evapotranspiration in exacerbating droughts, and their potential impacts on the water-energy-food nexus.

Human-induced land use change, such as afforestation and reforestation, can significantly impact the regional water cycle. Until recently, assessing this impact could only use data from a few water components, such as precipitation, runoff, and streamflow. It is much more challenging to document changes in total water resources. By combining GRACE with eco-hydrological modeling, we concluded that policy-driven revegetation projects were the primary cause of the sustained total water storage (TWS) depletion in the Mu Us Sandyland and its surrounding areas in northern China, a region of similar size to the State of Indiana (above figure). This illustrates that revegetation activities should be evaluated based on not just their value for erosion control and carbon storage, but also in the context of the water resources depletion they cause. This study is the first of its kind to take into account all water resources. We are currently using the approach we demonstrated in China to assess ecosystem restoration impacts on water resources in other regions and other land use change scenarios.