Satellite Observations Constrain Plant-accessible Water Storage Influencing Global Grassland Response to Drought

Land water supply for plant growth directly links the water and carbon cycles. The amount of plant-accessible water storage influences how plant production responds to climate extremes including drought, especially for the grassland ecosystem where water is the primary factor limiting plant production. However, plant-accessible water is rarely quantified due to the lack of regional to global scale observations of water storage at different soil depths, and the influence of deeper water supply on plant-water relation remains unknown. In this study, we combine total terrestrial water storage (TWS) estimates from GRACE/GRACE-FO and surface soil moisture (SM) estimates form ESA CCI to represent deep and shallow water storage, respectively. We delineate water-limitation regime based on the inter-annual correspondence of MODIS EVI, a proxy for grassland productivity, against both TWS and SM over 83 GRACE mascons covering the majority of the global grassland areas. We find that the change of TWS during water limited period constrains plant-accessible water storage and show strong correspondence with plant rooting depth derived from in-situ measurements, especially for regions featuring deep roots. Our results further show that land water storage buffers drought impact on plant growth during dry episodes with prolonged rainfall deficit. These results are instrumental in improving Earth system modeling.