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Forest–savanna boundaries are ecotones that support complex ecosystem functions and are sensitive to biotic/abiotic perturbations. What drives their distribution today and how it may shift in the future are open questions. Feedbacks among climate, fire, herbivory, and land use are known drivers. Here, we show that alternating seasonal drought and waterlogging stress favors the dominance of savanna-like ecosystems over forests. We track the seasonal water-table depth as an indicator of water stress when too deep and oxygen stress when too shallow and map forest/savanna occurrence within this double-stress space in the neotropics. We find that under a given annual precipitation, savannas are favored in landscape positions experiencing double stress, which is more common as the dry season strengthens (climate driver) but only found in waterlogged lowlands (terrain driver). We further show that hydrological changes at the end of the century may expose some flooded forests to savanna expansion, affecting biodiversity and soil carbon storage. Our results highlight the importance of land hydrology in understanding/predicting forest–savanna transitions in a changing world. 

Abstract. We investigate the interannual and interdecadalhydrological changes in the Amazon River basin and its sub-basins duringthe 1980–2015 period using GRACE satellite data and a physically based, 2 kmgrid continental-scale hydrological model (LEAF-Hydro-Flood) that includes aprognostic groundwater scheme and accounts for the effects of land use–landcover (LULC) change. The analyses focus on the dominant mechanisms thatmodulate terrestrial water storage (TWS) variations and droughts. We findthat (1) the model simulates the basin-averaged TWS variations remarkablywell; however, disagreements are observed in spatial patterns of temporaltrends, especially for the post-2008 period. (2) The 2010s is the driestperiod since 1980, characterized by a major shift in the decadal mean comparedto the 2000s caused by increased drought frequency. (3) Long-term trends in TWSsuggest that the Amazon overall is getting wetter (1.13 mm yr−1), but itssouthern and southeastern sub-basins are undergoing significant negative TWSchanges, caused primarily by intensified LULC changes. (4) Increasingdivergence between dry-season total water deficit and TWS release suggests astrengthening dry season, especially in the southern and southeasternsub-basins. (5) The sub-surface storage regulates the propagation ofmeteorological droughts into hydrological droughts by strongly modulatingTWS release with respect to its storage preceding the drought condition. Oursimulations provide crucial insight into the importance of sub-surface storagein alleviating surface water deficit across Amazon and open pathways forimproving prediction and mitigation of extreme droughts under changingclimate and increasing hydrologic alterations due to human activities (e.g.,LULC change). 

Abstract This study explores the impacts of groundwater processes on the simulated land‐surface water balance and hydrometeorology. Observations are compared to multiscale Weather Research and Forecasting (WRF) simulations of three summer seasons: 2012, 2013, and 2014. Results show that a grid spacing of 3 km or smaller is necessary to capture small‐scale river and stream networks and associated shallow water tables, which supplies additional root‐zone water double that of simulations with 9‐km and 27‐km grid spacing and is critical to replenishing the depleted vegetation root zones and leads to 150 mm more evapotranspiration. Including groundwater processes in convection‐permitting models is effective to reduce: (1) 2‐m temperature warm biases from 5–6 to 2–3 °C and (2) the low precipitation bias by half. The additional groundwater supply to active soil flux in convection‐permitting simulations with groundwater for June‐August is nearly translated into the same amount of increased precipitation in the domain investigated. 

Abstract Manmade reservoirs are important components of the terrestrial water balance. Thus, considering the hydro‐climatic effects of reservoirs is important in water cycle studies at a river basin to global scales; yet, reservoirs are represented poorly in large‐scale hydrological and climate models. Here we present a high‐resolution (5 km) continental‐scale reservoir storage dynamics and release scheme by enhancing existing schemes and adding critical novel parameterizations to improve reservoir storage and release simulations. The new scheme simulates river‐floodplain‐reservoir storages in an integrated manner considering their spatial and temporal variations. A new calibration scheme is also incorporated to better simulate reservoir dynamics considering cascade‐reservoir effects. Further, since no reservoir bathymetry data are available over large domains, we use a state‐of‐the‐art digital elevation model and reservoir extent data to derive reservoir bed elevation. The new scheme is integrated within the river‐floodplain routing scheme of a continental hydrological model LEAF‐Hydro‐Flood. Results from the simulation of ~1,900 reservoirs within the contiguous United States suggest that the model well captures the observed reservoir storage‐release dynamics. Comparison of our results with those from the existing schemes suggest a significant improvement; importantly, the new scheme reduces the excessive and frequent reservoir overfilling and underfilling. Comparison of results with satellite‐based surface water data shows that the model accurately reproduces the large‐scale patterns of reservoir‐floodplain inundation extents. It is expected that the results of this study will inform the incorporation of reservoirs in hyper‐resolution models to improve simulations of terrestrial water storage and flow and examine reservoir‐atmosphere interactions over large domains.