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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.

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.