This dissertation investigates the intricate dynamics of hydrologic systems in the Amazon River basin (ARB) in the face of evolving climate patterns and human interventions. The ARB – a pivotal element of the global climate, hydrological, and biogeochemical systems – holds immense biodiversity and profoundly influences global water, energy, and carbon cycles. Climate variations and human activities, especially deforestation in the southern subbasins, have considerably altered the basin's functioning. Despite extensive research, critical scientific gaps remain regarding key processes that govern hydrologic dynamics and the resilience of the rainforest. This research disentangles the impacts of climate and land use/land cover (LULC) changes toward devising robust resource management strategies. Recent acceleration of the hydrological cycle of the ARB and the increase in the frequency of extreme events could be early indicators of the change in hydrological cycle in the region surpassing some irreversible thresholds. While some systematic tipping points are inferred over the ARB, no tipping points associated with dominant hydrological processes over the ARB are investigated. This inhibits the understanding of hydrological considerations needed for sustainable forest management under climatic change and growing human stressors. The dissertation employs high resolution (~2km), long-term simulations from a process-based hydrological model (LEAF-Hydro-Flood) to investigate the dominant hydrological processes across the ARB, their key roles in shaping basin functions, and the decadal evolutions therein. Further, by developing static and dynamics LULC scenarios, the impact of climate variability and LULC change are isolated. Finally, through a comprehensive area fraction analysis and using a corresponding tree cover dataset, the tipping points associated with dominants hydrological processes in the ARB are assessed. Results indicate that shallow groundwater (<5m deep) strongly modulates the seasonality of the surface fluxes across the ARB and at least 34% of the Amazonian Forest is supported by groundwater during the dry season. This study reveals a two-month lag between seasonal peak evapotranspiration (ET) and river discharge as a crucial mechanism in preventing rainforest tipping into savanna. The ARB is dominantly energy limited; however, the results suggest that in the absence of groundwater support, and with less than ~125 mm/month of precipitation, the ARB could have become water-limited over some regions. The long-term basin-averaged ET—dominated by transpiration—changed with a split pattern of ±9% in the past three decades. Similarly, water table depth (±19%) and runoff (±29%) changed with a heterogenous patterns across the ARB. The contribution of canopy interception loss and ground evaporation changed heterogeneously across the ARB in response to deforestation. River discharge did not change substantially due to the crucial buffering role of groundwater, but terrestrial water storage (TWS) decreased (increased) in the 2000s (2010s) compared to that in the 1990s. Although groundwater is the dominant contributor to total TWS, the dynamics of TWS over the major river channels are controlled by flood water, given relatively shallow groundwater. Despite extensive deforestation, climate variability remains the dominant influence on WTD dynamics; however, the impacts on ET varied across the basin. Runoff patterns were intricately tied to precipitation and water table dynamics, demonstrating regional variations influenced by both climate variability and LULC changes. The area fraction analysis of WTD seasonality confirms the existence of tipping points associated with groundwater dynamics in the ARB. This study provides crucial insights on (i) the dominant hydrological processes, (ii) isolated impacts of climate variability and LULC change on the water cycle of the ARB, and (iii) tipping points in the ARB that are associated with groundwater dynamics. These findings could be used to inform effective water resource management and sustainable environmental practices in this ecologically significant region.
Read
