Active Rock Glaciers as Dynamic Water Storage: The Case Study of Rock Glacier Lazaun (South Tyrol, Italy)

Abstract

<p>Rock glaciers have been proved to be able of storing water both as permafrost ice (solid form) and as groundwater (liquid form). The aim of the present study is the characterization of the runoff pattern of the active rock glacier Lazaun, its seasonal variations and its relationship to permafrost ice in the rock glacier. For this purpose, a recession analysis and a lumped-parameter rainfall-runoff model have been implemented on a multi-year dataset, supported by stable isotope and electrical conductivity analyses. Stable isotope and electrical conductivity values as natural tracers allow the separation of flow components. Therefore, reference samples for snowmelt, ice melt and rainwater were collected and analyzed, and electrical conductivity in combination with discharge data was applied for a two-component mixing model. The recession analysis also supports the flow component separation. Preliminary results show a typical discharge pattern for active rock glaciers, with maximum discharge during the snowmelt period (June-July) and a gradual decrease towards the fall season, interrupted by rainfall-induced runoff peaks. Discharge and water temperatures around 1.2-1.4 °C are recorded during winter, supporting the hypothesis of a groundwater dominated base flow. Isotope data in combination with electrical conductivity indicate at least 3 sources of the discharge components: i) low electrical conductivity, low δ<sup>18</sup>O values during spring and early summer indicate melt water of the winter snow cover, ii) a baseflow component with high electrical conductivity values and enriched in δ<sup>18</sup>O mainly during autumn and winter indicates most probably groundwater, and iii) rainwater of summer precipitation as the event water component with lower electrical conductivity and slightly higher δ<sup>18</sup>O values. A fourth source (melt water of the permafrost or glacial ice) is assumed. In general, electrical conductivity increases from the beginning of the melt season until autumn/winter and δ<sup>18</sup>O of the stream water becomes more and more positive. This correlates with the results of the two-component mixing model, which shows an increasing percentage of event water versus total discharge in spring, and its decrease towards the summer (mean 24%; min 0%; max 70%). Recession analysis also shows 2 to 3 components (“fast flow” and “baseflow”, plus an intermediate flow component), characterized by different recession coefficients. These indicate also at least two different storage units. The baseflow storage unit might be ascribed to an unfrozen sediment layer detected by core drilling at the base of the frozen core of the Lazaun rock glacier. Thickness and storage capacity of this layer are, however, still unknown. The designated results of the rainfall-runoff model are expected to provide a better understanding of the future development of active rock glacier discharge.</p>