Browsing by Subject "Tropical Glaciers"

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    Evaluating the Eco-Hydrochemical Response of Tropical Glacierized Mountainous Watersheds to Climate Change: A Case Study of the Volcán Chimborazo, Ecuador
    (2021-05) Saberi, Leila
    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.
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    Hydrochemical Signatures of Glacial Meltwater on Volcán Chimborazo, Ecuador
    (2017-11) McLaughlin, Rachel
    Glacier recession in the tropical Andes is generating significant concern over future water availability for domestic use, irrigation, and hydropower. Sparse data sets, extreme heterogeneity in climate patterns, and the limited understanding of groundwater and ecohydrological processes in these catchments make predicting the hydrologic response to glacier retreat difficult. Here I examine a glaciated watershed on Volcán Chimborazo, Ecuador. I use geospatial analysis and recent geologic studies to evaluate the vegetation and geologic factors that influence the hydrologic response of the watershed. Additionally, I utilize hydrochemical and stable isotope signatures to investigate how melt and groundwater contributions to streamflow have changed over time along with possible meltwater-groundwater connections. A new landcover map of Volcán Chimborazo, generated using object based image analysis, reveals a significant increase in the upper limit of vegetation on the mountain and expansion of crop and pasture land since 1978. Geologic cross-sections, based on recent studies, show that near surface geology is dominated by glacial deposits and underlain by relatively young volcanic bedrock. Results from a hydrochemical mixing model (HBCM) combined with discharge measurements reveal spatial variability in groundwater discharge and suggest that groundwater discharge during the dry season has decreased from 2012-2017. Short time scale variability is clearly influenced by precipitation, but long-term discharge trends remain uncertain. Lastly, stable isotope and solute concentrations in samples suggest groundwater in the study watershed is recharged by precipitation falling at high elevations where ice and snow may dominate the hydrologic system.