On the importance of the cryosphere in a tropical Andean basin: the past, present and future of the glaciers and runoff in the Rio Santa

The Peruvian Andes contains the largest mass of glaciers in the tropics and these glaciers have shown considerable decay over the past 4 decades and into the present day. The historic and future runoff is tied to the cryospheric changes in the region and this could have important consequences for water resources, given the importance of snow and ice melt for dry season runoff. To disentangle the role of the cryosphere in the water cycle in the tropical Andes we run the fully distributed, hourly glacier-hydrological model TOPKAPI-ETH, both in the past (from 1987) and into the future over the upper Rio Santa catchment in the Cordillera Blanca. Meteorological forcing is provided by bias-corrected WRF simulations, which are also used for statistical downscaling of CMIP5 model projections to provide the future climatology. Calibration of model parameters is conducted using a step-wise approach using a wealth of ground-based data and model outputs are evaluated against gauged runoff data and remote sensing estimates of snow cover, glacier cover and glacier mass balance. 

We find that under present conditions (2008-2018) snowmelt is an important contributor to runoff, comprising 16% to 47% of inputs (the range in weekly average as a proportion of all snow melt (on and off-glacier), ice melt and rain contributions) into the catchment with its proportional contribution largest at the beginning of the dry season (early June). Off-glacier snowmelt is important in the wet season, but snow is confined to on-glacier areas by the mid-dry season. Snow cover <5000 m is ephemeral, lasting hours to days, with correspondingly thin average snow depths and rapid fluctuations in the wet season snowline. Meanwhile, ice melt is an important contributor to runoff in the dry season (up to 54% of the inputs in early August) in all glacierised catchments, even those with a small glacier cover, but the wet season contribution is small. We also explore the long term evolution of glaciers and snow cover in the catchment and its implications for catchment runoff. Through the long term modelling we investigate the timing of peak water in the catchment and the key drivers of runoff change in the past. Our future projections will allow us to examine the impact of future climate changes on the glaciers and snow dynamics. A key vulnerability is the impact of temperature increases on the ephemeral snowpack and the consequences for glacier mass balance. We will also investigate the potential implications for catchment runoff and dry season water availability.