With global climate change, some of the highest rates of warming are occurring at high elevations in low latitudes, making tropical glacierized mountains some of the most vulnerable hydrological systems in the world. In the Andes, which hold 99% of all tropical glaciers, observations reveal that streamflow in many watersheds is already decreasing due to the retreat of glaciers. With the water security threat this presents to populations who rely on stream discharge from these glacierized mountains, understanding the hydrological impacts of climate change in these systems is critical. Recent studies have begun to investigate the response of streamflow to fast-retreating glaciers. However, many important knowledge gaps remain. For example, the contribution of meltwater to streamflow through subsurface flow has been largely overlooked, and this may be biasing estimates of how much groundwater may buffer glacier retreat. Further, in addition to causing glacier retreat, warming temperatures are also driving upslope vegetation migration, yet little is known about how this will further affect stream discharge in tropical glacierized watersheds. Finally, climate-driven changes in hydrology can alter solute weathering and transport on glacierized mountain slopes, but the effect on solute export from these watersheds has not been investigated, even though this could have implications for geochemical cycling and ecological function downstream in the Amazon Basin. Data sparsity in these remote, tropical glacierized mountainous watersheds, as well as unique characteristics such as their year-round glacier ablation and endemic páramo ecosystem, present major uncertainties associated with predicting hydrogeological, hydrochemical, and ecohydrological responses to climate change. We address these challenges by implementing a recently developed watershed model with reactive transport, BioRT-Flux-PIHM, for a sub-humid glacierized watershed on Volcán Chimborazo in the Ecuadorian Andes. BioRT-Flux-PIHM integrates multicomponent reactive transport with hydrological processes and land surface interactions, and thus has the potential to capture spatiotemporally distributed watershed surface and subsurface flows and pathways as well as hydrochemical processes. Implementing BioRT-Flux-PIHM with available field observations makes it possible extend sparse measurements over space and time, and to uncover unobserved processes. Our model results indicate that glacier melt contributes a broad range of 10-90\% of weekly discharge (~50% on average) over the course of one study year, mostly through fast surface runoff, but also through infiltration that increases groundwater flow by nearly 37%. Combined removal of glacier melt, upslope migration of vegetation, and a 4.5 °C increase in temperature results in substantial reduction of streamflow by 74% from current conditions, primarily due to an increase in evapotranspiration. Under this scenario, the model shows that near no-flow conditions can occur in the stream, which has crucial implications for local communities who rely on this water for irrigation. The model further predicted a unique relationship between the concentration (C) of weathering solutes in the stream and discharge (Q) that was mostly chemostatic (constant C with varying Q) because of large melt-supported groundwater inputs, but superimposed by melt event-driven dilution episodes. In a model scenario with no glacier melt, major ion concentrations, including Na+, Ca2+, and Mg2+, became higher and much more stable, but weathering rates decreased, which ultimately attenuated solute export by 23% compared to current-day estimates. We expect this reduction to be exacerbated by higher evapotranspiration and drier conditions with expanded vegetation. This work brings to light the importance of understanding interactions among warming temperatures, mineral weathering, subsurface meltwater flow, and vegetation changes to predict hydrological and hydrochemical processes in tropical watersheds with rapidly retreating glaciers.
University of Minnesota Ph.D. dissertation. 2021. Major: Earth Sciences. Advisor: Gene-Hua Crystal Ng. 1 computer file (PDF); 184 pages.
Evaluating the Eco-Hydrochemical Response of Tropical Glacierized Mountainous Watersheds to Climate Change: A Case Study of the Volcán Chimborazo, Ecuador.
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