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

Abstract

Abstract. Climate models predict amplified warming at high elevations in low latitudes, making tropical glacierized regions some of the most vulnerable hydrological systems in the world. Observations reveal decreasing streamflow due to retreating glaciers in the Andes, which hold 99% of all tropical glaciers. However, the timescales over which meltwater contributes to streamflow and the pathways it takes -surface and subsurface -remain uncertain, hindering our ability to predict how shrinking glaciers will impact water resources. Two major contributors to this uncertainty are the sparsity of hydrologic mea-5 surements in tropical glacierized watersheds and the complication of hydrograph separation where there is year-round glacier melt. We address these challenges using a multi-method approach that employs repeat hydrochemical mixing model analysis, hydroclimatic time series analysis, and integrated watershed modeling. Each of these approaches interrogates distinct timescale relationships among meltwater, groundwater, and stream discharge. Our results challenge the commonly held conceptual model that glaciers buffer discharge variability. Instead, in a sub-humid watershed on Volcán Chimborazo, Ecuador, meltwater drives 10 nearly all the variability in discharge (Pearson correlation coefficient of 0.89 in simulations), with glaciers contributing a broad range of 20-60% or wider of discharge, mostly (86%) through surface runoff on hourly timescales, but also through infiltration that increases annual groundwater contributions by nearly 20%. We further found that rainfall may enhance melt contributions to discharge at timescales that complement melt generation, possibly explaining why minimum discharge occurred at the study site during warm but dry El Niño conditions, which typically heighten melt in the Andes. Our findings caution against extrap-15 olations from isolated measurements: stream discharge and meltwater contributions in tropical glacierized systems can change substantially at hourly to interannual timescales, due to climatic variability and surface to subsurface flow processes.

Groundwater Buffers Decreasing Glacier Melt in an Andean Watershed—But Not Forever

“…During the rainy season, interflow (shallow, preferential flow through the unsaturated zone) dominates (60%) streamflow. These results complement work by others (e.g., Andermann et al, ; Baraer et al, ; Hood & Hayashi, ; Pohl et al, ; Saberi et al, ; Somers et al, ) to demonstrate that groundwater is an important contributor to streamflow in glacierized mountain catchments.…”

“…The low contribution of glacier melt to groundwater suggests that overall, groundwater recharge is resilient to a reduction in glacier meltwater input. To our knowledge, the only other quantitative estimate of glacier melt contribution to groundwater comes from a headwater catchment on Volcán Chimborazo in Ecuador, where 15% of groundwater discharge was found to originate from glaciers that cover 34% of the watershed area (Saberi et al, ). These results suggest a useful relationship between glacier coverage as a proportion of watershed area, and glacier‐sourced groundwater discharge, similar to the relationship proposed by Baraer et al () between glacierized area and glacier melt contribution to streamflow.…”

“…These results suggest a useful relationship between glacier coverage as a proportion of watershed area, and glacier‐sourced groundwater discharge, similar to the relationship proposed by Baraer et al () between glacierized area and glacier melt contribution to streamflow. This relationship likely varies with climate, bedrock lithology, and topography (Markovich et al, ; Saberi et al, ; Tague et al, ), and requires more estimates in different basins to expound.…”

Open-source Arduino-derived data loggers designed for field research

“…ALog development evolved iteratively over more than seven years of lab design and field deployments (Figure 4). ALog data loggers have been deployed in the high alpine (Niwot Ridge, Colorado, USA), the high desert (Quebrada del Toro, Argentina), coastal wetlands (Wax Lake Delta, Louisiana, USA), subalpine valleys (Gordon Gulch, Colorado, USA), tropical mountains (Chimborazo, Ecuador), continental lacustrine regions (Minnesota, USA, and Ontario, Canada), and on large glaciers (Kennicott Glacier, Alaska, USA) (Wickert, 2014;Armstrong et al, 2016;Tauro et al, 2018;Saberi et al, 2018). During these deployments, the ALog recorded data from weather stations, glacier ablation monitoring stations, thermistors, stream gauges, soil moisture probes, pressure transducers for water levels in wells, subsurface temperature profilers, and frost-heave gauges; a full list of sensors for which firmware has been developed is in Table 2.…”

“…This station, here programmed to record data every five minutes, dramatically increases ablation data density beyond traditional methods, which incorporate daily to weekly field surveys of snow and/or ice surface elevation change around ablation stakes. Furthermore, by including on-stake temperature measurements, we are able to compute at-stake melt factors for degree-day melt models, which are significant for both glaciological and water resources research (Saberi et al, 2018).…”