Understanding the combined impact of vegetation and glacier change on Andean hydrology

The Peruvian Andes have faced substantial glacier loss in recent decades, and as the glaciers have receded, the exposed ground has been gradually occupied by succession vegetation. Previous work assessing the impact of glacier loss on downstream hydrology has tended to assess the cryospheric change in isolation, which may not account for the impact of vegetation changes on the water balance, especially in terms of altering catchment losses. An increasing body of work has demonstrated the importance of snow and glaciers for water resources in this region, especially in the dry season, although continued warming and glacier loss is predicted to decrease these meltwater contributions. Plant growth in deglaciated regions has the potential to compound runoff decreases through increasing evapotranspiration, but few studies have attempted to quantify this. This work aims to provide the first integrated assessment of the combined impact of glacier evolution and post-glacial vegetation succession on water availability in the Peruvian Andes. Here we quantify these changes by modelling the hydrological and ecological functioning of the Shallap catchment (13.6 km2) in the Rio Santa basin of the Peruvian Andes. We apply the mechanistic land surface model Tethys-Chloris which applies a full energy balance approach to resolving the fluxes over clean and debris-covered ice, snow and vegetation surfaces. The model is applied for a present period (2014-2025), forced by measured meteorological data, and using data from vegetation transects to parameterise the succession vegetation cover. The model is validated against ablation stakes, remotely sensed glacier mass balance, ground temperature, soil moisture and river discharge. We then simulate scenarios of climate, vegetation and glacier change to assess the separate and combined impact of glacier change and plant succession on the energy and water balance into the future. We are able to determine the impact of succession vegetation on evapotranspiration rates and water yield compared to bare soil and glacier cover, and determine the overall potential impact of glacier and vegetation change on downstream runoff. This work will provide a basis for understanding the significance of plant succession for the overall water balance in deglaciating catchments, impacting strategies for larger scale catchment and water management modelling throughout the Andes.