Evidence and implications of the highly-variable tracer signature of water sources in a glacierized catchment.

Glacierized catchments are essential sources of fresh water in Alpine areas especially duringwarm and dry periods. Therefore, the quantification of glacier melt contribution to streamrunoff represents a crucial issue in Alpine catchments affected by rapid glacier shrinking. Theglacier melt water contribution to stream runoff can be estimated by different approaches, oneof which is represented by the application of geochemical tracers. However, the spatial andtemporal characterization of geochemical tracers in water sources in glacierized catchments isparticularly difficult. In this work, we aim to characterize the tracer signature (δ2H and electrical conductivity(EC)) of the main water sources and study the hydrological response of a glacierizedcatchment during three consecutive summer seasons. The specific objectives are to: i) analyzethe spatial and temporal variability of the tracer signature of snow, glacier ice and glacier meltwater, in relation to the fractional snow cover of the catchment, ii) analyze the inter-annualand seasonal dynamics of streamflow, stream water EC and isotopic composition in relationto the hydro-meteorological conditions, and iii) identify the main end-members to streamrunoff. The study area is an 8.4-km2 catchment (42% glacierized) in the Eastern Italian Alps.Meteorological and streamflow data were collected continuously during the 2013-2015ablation seasons. Samples for EC and isotopic analyses were taken from rainfall at the outletof the catchment and from fresh and residual winter snow, firn, glacier ice and melt water atdifferent elevations on the glacier. Stream water samples were taken manually andby an automatic sampler at the outlet, and from three sections close to the glacierfronts. Results showed that variations in the daily streamflow range due to melt-induced runoffevents were controlled by maximum daily air temperature and snow covered area in thecatchment. Maximum daily streamflow decreased with increasing snow cover and a thresholdrelation was found between maximum daily temperature and daily streamflow range. Therewas a high spatial and temporal variability in the isotopic composition of snow, glacierice and glacier melt water. The isotopic composition of snow, glacier ice and meltwater was not significantly correlated with the sampling point elevation and thespatial variability was more likely affected by post-depositional processes. Duringmelt-induced runoff events, stream water EC decreased due to the contribution ofglacier melt water to stream runoff. In this catchment, EC or more conservativetracers could be used to distinguish the contribution of subglacial flow (enriched inEC) from glacier melt water to stream runoff at the seasonal and daily timescale.The high spatial and temporal variability in the tracer signature of the identifiedend members (subglacial flow, rain water, glacier melt water and residual wintersnow), together with small daily variability in stream water δ2H dynamics, areproblematic for the quantification of the contribution of the end members to streamrunoff, and call for further research, possibly integrated with other natural or artificialtracers. Keywords: runoff generation; stable water isotopes; electrical conductivity; end members;glacier; snow; Alpine catchment. [ABSTRACT FROM AUTHOR]