Assessment of liquid and solid water storage in rock glaciers versus glacier ice in the Austrian Alps

Rock glaciers are capable of storing water in solid (permafrost ice) as well as in liquid form (groundwater within the unfrozen base layer). The latter is relevant no matter the actual state of the rock glacier (intact, containing ice versus relict, no more ice present). A nation-wide comparison of the water equivalent of glacier ice within the Austrian Alps is conducted to evaluate the role of rock glaciers in the hydrological cycle at its current state and potentially in the future. Estimates of ice volumes and their uncertainties, especially related to thickness estimates of permafrost-ice bodies, are discussed in the light of available data and need to be seen as order-of-magnitude estimates. With intact rock glaciers covering almost 123 km² and an assumed ice content of 40% within the permafrost body, ice volumes are estimated to be 0.93 Gigatons. This corresponds to 8.3% of the ice volume estimated for the most recent glacier inventory of the Austrian Alps. Thus, with the currently available data, a water equivalent ratio of ~1: 12 for rock glacier ice versus glacier ice is estimated. In addition to the solid water storage, the dynamic storage potential within rock glaciers in liquid form needs to be considered. While this volume is relatively small compared to the ice volume in glaciers and rock glaciers (ratio of ~1: 20), the time-scales of hydrological relevance (when they become runoff due to ice melt) are very different. This dynamic liquid water storage is replenishable and therefore available over shorter time scales (seasonal drainage pattern), and moreover relatively stable and may potentially even slightly increase as pore space becomes available due to increased melting of permafrost ice. In the light of climate warming and projected glacier recession, the relative hydrological importance of rock glaciers as water stores (in solid as well as in liquid form) in the European Alps is expected to increase and their storage-discharge patterns need to be accounted for in water management considerations.