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1. Spatiotemporal Drivers of Hydrochemical Variability in a Tropical Glacierized Watershed in the Andes

   https://doi.org/10.1029/2020WR028722  

   Saberi, Leila; Crystal Ng, G. ‐H.; Nelson, Leah; Zhi, Wei; Li, Li; La Frenierre, Jeff; Johnstone, Morgan (May 2021, Water Resources Research)

   Abstract There is a critical knowledge gap about how glacier retreat in remote and rapidly warming tropical montane watersheds will impact solute export, which has implications for downstream geochemical cycling and ecological function. Because tropical glacierized watersheds are often uniquely characterized by year‐round ablation, upslope vegetation migration, and significant groundwater flow, baseline understanding is needed of how spatiotemporal variables within these watersheds control outlet hydrochemistry. We implemented a recently developed reactive transport watershed model, BioRT‐Flux‐PIHM, for a sub‐humid glacierized watershed in the Ecuadorian Andes with young volcanic soils and fractured bedrock. We found a unique simulated concentration and discharge (C‐Q) pattern that was mostly chemostatic but superimposed by dilution episodes. The chemostatic background was attributed to large simulated contributions of groundwater (subsurface lateral flow) to streamflow, of which a notable fraction (37%) comprised infiltrated ice‐melt. Relatively constant concentrations were further maintained in the model because times and locations of lower mineral surface wetting and dissolution were offset by concentrating effects of greater evapotranspiration. Ice‐melt did not all infiltrate in simulations, especially during large precipitation events, when high surface runoff contributions to discharge triggered dilution episodes. In a model scenario without ice‐melt, major ion concentrations, including Na⁺, Ca²⁺, and Mg²⁺, became more strongly chemostatic and higher, but weathering rates decreased, attenuating export by 23%. We expect this reduction to be exacerbated by higher evapotranspiration and drier conditions with expanded vegetation. This work brings to light the importance of subsurface meltwater flow, ecohydrological variability, and interactions between melt and precipitation for controlling hydrochemical processes in tropical watersheds with rapidly retreating glaciers. 

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2. Increased {DOC} concentrations and earlier stream flow generation in response to warming in a high elevation mountain watershed in Colorado

   Kerins, Devon; Zhi, Wei; Sullivan, Pamela; Williams, Kenneth; Brown, Wendy; Dong, Wenming; Carroll, Rosemary; Kirchner, James; Li, Li (December 2021, American Geophysical Union annual meeting)

   High elevation mountain watersheds are undergoing rapid warming and declining snow fractions worldwide, causing earlier and quicker snowmelt. Understanding how this hydrologic shift affects subsurface flow paths, biogeochemical reactions, and solute export has been challenging due to the entanglement of hydrological and biogeochemical processes. Coal Creek, a high-elevation catchment (2,700 3,700 m, 53 km2) in Colorado, is experiencing a higher rate of warming than surrounding low-lying areas. This warming corresponds with dynamic and increased responses from biogenic solutes and dissolved organic carbon (DOC), whereas the behavior of geogenic solutes and dissolved inorganic carbon (DIC) has remained relatively unchanged. DOC has experienced the largest concentration increase (>3x), with annual average flow weighted concentrations positively correlated to average annual temperature. This suggests temperature is the main driver of increasing DOC levels. Although DOC and DIC response to warming is influenced by many drivers, the relative contribution of each remains unknown. DOC and DIC were analyzed to incorporate both carbon component products of soil respiration (DOC and CO2) and to represent high solute concentrations transported by shallow (DOC) versus deep (DIC) subsurface flow. The contrasting behavior of these carbon solutes indicates climate change and warming are driving changes in organic matter decomposition and soil respiration. Modeling results from the process-based model HBV-BioRT show increased temperatures cause earlier snowmelt and streamflow generation and lower peak discharge. As stream flow generation occurs earlier, so do DOC flushing and DIC dilution events. Additionally, post-snowmelt periods show greater DOC production and concentrations under warming scenarios. Results indicated increased production of DOC in post-snowmelt periods. DOC is then flushed out by earlier snowmelt partitioned through the shallow soil zone. Most process-based studies lack a watershed-scale understanding of carbon transformation and flow path alterations. This work demonstrates complex hydrologic and biogeochemical coupling at the watershed scale to illustrate how water flow paths and chemistry are responding to a changing climate in highelevation mountain watersheds. 

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3. Multi-scale temporal variability in meltwater contributions in a tropical glacierized watershed

   https://doi.org/10.5194/hess-23-405-2019  

   Saberi, Leila; McLaughlin, Rachel T.; Ng, G.-H. Crystal; La Frenierre, Jeff; Wickert, Andrew D.; Baraer, Michel; Zhi, Wei; Li, Li; Mark, Bryan G. (January 2019, Hydrology and Earth System Sciences)

   Abstract. Climate models predict amplified warming at high elevations in low latitudes,making tropical glacierized regions some of the most vulnerable hydrologicalsystems in the world. Observations reveal decreasing streamflow due toretreating glaciers in the Andes, which hold 99% of all tropicalglaciers. However, the timescales over which meltwater contributes tostreamflow and the pathways it takes – surface and subsurface – remainuncertain, hindering our ability to predict how shrinking glaciers willimpact water resources. Two major contributors to this uncertainty are thesparsity of hydrologic measurements in tropical glacierized watersheds andthe complication of hydrograph separation where there is year-round glaciermelt. We address these challenges using a multi-method approach that employsrepeat hydrochemical mixing model analysis, hydroclimatic time seriesanalysis, and integrated watershed modeling. Each of these approachesinterrogates distinct timescale relationships among meltwater, groundwater,and stream discharge. Our results challenge the commonly held conceptualmodel that glaciers buffer discharge variability. Instead, in a subhumidwatershed on Volcán Chimborazo, Ecuador, glacier melt drives nearly allthe variability in discharge (Pearson correlation coefficient of 0.89 insimulations), with glaciers contributing a broad range of 20%–60%or wider of discharge, mostly (86%) through surface runoff on hourlytimescales, but also through infiltration that increases annual groundwatercontributions by nearly 20%. We further found that rainfall may enhanceglacier melt contributions to discharge at timescales that complement glaciermelt production, possibly explaining why minimum discharge occurred at thestudy site during warm but dry El Niño conditions, which typicallyheighten melt in the Andes. Our findings caution against extrapolations fromisolated measurements: stream discharge and glacier melt contributions intropical glacierized systems can change substantially at hourly tointerannual timescales, due to climatic variability and surface to subsurfaceflow processes. 

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4. Concentration‐Discharge Patterns Across the Gulf of Alaska Reveal Geomorphological and Glacierization Controls on Stream Water Solute Generation and Export

   https://doi.org/10.1029/2021GL095152  

   Jenckes, Jordan; Ibarra, Daniel E.; Munk, Lee Ann (December 2021, Geophysical Research Letters)

   Abstract High‐latitude glacierized coastal catchments of the Gulf of Alaska (GoA) are undergoing rapid hydrologic changes in response to climate change and glacial recession. These catchments deliver important nutrients in the form of both inorganic and organic matter to the nearshore marine environment, yet are relatively understudied with respect to characterization of the sediment and solute generation processes and total yields. Using multiple linear regression informed by Bayesian Information Criterion analysis we empirically demonstrate how watershed characteristics affect suspended sediment and solute generation as represented by concentration‐discharge relationships. We find that watershed mean slope and relief control solute generation and that solute yields are influenced most by glacier coverage. We contribute a new flux and concentration‐discharge based conceptualization for understanding solute cycles across a hydroclimatic gradient of GoA watersheds that can be used to better understand future watershed responses to rapid hydrologic change. 

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5. Solute export patterns across the contiguous USA

   https://doi.org/10.1002/hyp.15197  

   Kincaid, Dustin W; Underwood, Kristen L; Hamshaw, Scott D; Li, Li; Seybold, Erin C; Stewart, Bryn; Rizzo, Donna M; Haq, Ijaz Ul; Perdrial, Julia N (June 2024, Hydrological Processes)

   Abstract Understanding controls on solute export to streams is challenging because heterogeneous catchments can respond uniquely to drivers of environmental change. To understand general solute export patterns, we used a large‐scale inductive approach to evaluate concentration–discharge (C–Q) metrics across catchments spanning a broad range of catchment attributes and hydroclimatic drivers. We leveraged paired C–Q data for 11 solutes from CAMELS‐Chem, a database built upon an existing dataset of catchment and hydroclimatic attributes from relatively undisturbed catchments across the contiguous USA. Because C–Q relationships with Q thresholds reflect a shift in solute export dynamics and are poorly characterized across solutes and diverse catchments, we analysed C–Q relationships using Bayesian segmented regression to quantify Q thresholds in the C–Q relationship. Threshold responses were rare, representing only 12% of C–Q relationships, 56% of which occurred for solutes predominantly sourced from bedrock. Further, solutes were dominated by one or two C–Q patterns that reflected vertical solute–source distributions. Specifically, solutes predominantly sourced from bedrock had diluting C–Q responses in 43%–70% of catchments, and solutes predominantly sourced from soils had more enrichment responses in 35%–51% of catchments. We also linked C–Q relationships to catchment and hydroclimatic attributes to understand controls on export patterns. The relationships were generally weak despite the diversity of solutes and attribute types considered. However, catchment and hydroclimatic attributes in the central USA typically drove the most divergent export behaviour for solutes. Further, we illustrate how our inductive approach generated new hypotheses that can be tested at discrete, representative catchments using deductive approaches to better understand the processes underlying solute export patterns. Finally, given these long‐term C–Q relationships are from minimally disturbed catchments, our findings can be used as benchmarks for change in more disturbed catchments. 

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