Understanding the shift in drivers of terrestrial water storage decline in the central Inner Mongolian steppe over the past two decades

Abstract

Terrestrial water storage (TWS) in mid-latitude arid and semiarid Eurasia steppe is under threat from climate change and human activities. The central Inner Mongolian steppe is a hotspot of this change, and in addition serves as an important ecological barrier in North China. However, the limitations of scarce observations make the variations of TWS in this study area have not been well tracked and quantified yet, and the drivers of TWS remain unclear. Here we use the gravity recovery and climate experiment (GRACE) satellites and its follow-on (GRACE-FO) missions, combined with data-driven models to fill the data gaps between GRACE and GRACE-FO. A significant TWS decrease (−2.7 ± 0.2 mm/yr, p < 0.05) was captured over the past two decades (2002–2020), which is consistent with land surface models (LSMs) that assimilated with GRACE data (R = 0.81, p < 0.01). By integrating GRACE satellites and open-loop LSMs, the contrary trends of TWS were identified over the period, and GRACE contrasts with the increase in TWS simulated by LSMs that excludes human activities. We observe a shift in the dominant factor driving the TWS decline from climatic (2002–2010) to anthropogenic factors (2011–2020). This shift accounts for the anthropogenic-driven increase in TWS decline from −2.0 mm/yr to −4.4 mm/yr, indicating the increasingly significant impact of human activities on TWS. We further reveal the climate-driven mechanisms of TWS using wavelet analysis and multiple drought indicators. The results showed that the TWS decline in the first period was mainly caused by continuous droughts, which were driven by the deficit in precipitation-minus-evapotranspiration. This study highlights the risk of TWS depletion in the central Inner Mongolian steppe that is eco-fragile, providing insights into the water resources sustainability and ecosystem conservation in vast dryland ecosystems of the world.