The hidden ice of the Andes ? quantifying rock glacier water storage capacities

LOTHAR SCHROTT; DARIO TROMBOTTO LIAUDAT; CHRISTIAN HALLA; JAN HENRIK BLÖTHE

Wellington

Congreso; SouthCOP, Queenstown, 2019; 2019

University of Waikato

Recent regional assessments suggest that rockglaciers are important water stores in the periglacial zones of arid mountain regions,such as the central Andes of Argentina. However, field based studies quantifyingthe hydrological significance of rock glaciers and their contribution to riverdischarge are still rare in these environments. Here we try to fill this research gap by quantifyingthe water storage capacities of three rock glaciers including talus and morainicderived features as well as one large rock glacier complex in the arid tosemiarid Andes of Argentina. We investigated subsurface material compositionsbased on several kilometres of complementary ERT and RST profiles that werecarried out at different elevations from the root zones towards positions on lowertongues. By means of a petrophysical model, called Four-Phase-Model, theabsolute and relative material compositions of air, ice, rock and water weremodelled. Furthermore, we monitored vertical and horizontal surface changes fora three year period using high resolution digital elevation models (DEMs) derivedfrom structure from motion (SfM) techniques based on aerial pictures taken by adrone. Vertical changes of the rock glacier surface indicate annual changes of subsurfaceice, but can also be caused by the horizontal movement of surface features. Horizontalvelocities were used to check independently potential rock glacier bulk densitiesbased on Glen?s ice flow equation. Thus, we estimated the relative water andice content and the potential ranges of annual water equivalents stored orreleased from different rock glaciers.High water contents and saturatedsubsurface conditions are identified underneath surface depression and furrowsindicating effective water pathways. Increased active layer depths, dissectedpermafrost bodies, and thermokarst show local influences of thermal erosion onthe internal hydrologic structure of all rock glaciers. Ice-oversaturatedpermafrost and massive ground ice with ice contents of 70 up to 90 % couldonly be found in the root zones and the middle part of debris rock glaciers androck glacier complexes. Volumetric ice contents decrease towards the rockglaciers fronts partly to 30-50 %. Talus rock glaciers contain lower mean andmaximum volumetric ice contents compared to debris rock glaciers. However,annual vertical surface changes of talus rock glaciers indicate annual storageand release capacities up to 50 mm/a, which constitutes a significant amount torunoff or ground water recharge in the dry Andes with annual precipitationrates <200mm/a. Thus, rock glaciers constitute an important seasonal andannual hydrological buffer and a significant long-term water store in theCentral and Desert Andes of Argentina. In the light of rising temperatures, thehydrological buffer and storage function of permafrost landforms might gainimportance in the Central Andes, even exceeding glacial water storage in verydry regions.