Process-based modelling of the energy and water balance of the Rio Santa Basin, Peruvian Andes

Glaciers in the Peruvian Andes play a crucial role in sustaining regional water resources for downstream populations and ecosystems, have been experiencing rapid mass loss and retreat in recent decades. The region is characterised by a tropical semi-arid climate with minimal seasonal temperature variability, alternating dry and wet seasons, and high-elevation areas frequently experience air temperatures close to 0 °C. These conditions lead to dynamic glacier energy-balance processes, in which intermittent and ephemeral snow strongly controls melt. Furthermore, these glacierized areas provide water resources to fragile downstream ecosystems, as well as subsistence and commercial agricultural systems, which themselves alter the water balance. It is challenging to reproduce the energy and water balance of such a complex environment using simplified, empirically parameterised models, and integrated, process-based modelling approaches might offer a viable way forward under a changing climate. We use a physically-based land surface modelling framework to disentangle the spatio-temporal variability of the energy and water balance of a large catchment in the Peruvian Andes. 

Within this study we focus on the Rio Santa basin (4950 km2), located in the Cordillera Blanca, which contains ~330 km2 of glacier area at elevations of 4300-6300 m a.s.l. We employ the process-based land-surface model Tethys-Chloris to simulate energy and water fluxes over a 9-year period (2010-2018) for the whole catchment. We use downscaled meteorological forcing derived from a WRF climate model simulation forced by ERA5 reanalysis. Meteorological forcings are bias-corrected using observations from multiple automatic weather stations across the catchment. The model is evaluated using in-situ glacier observations, including mass balance, surface albedo, and snow-pit measurements, as well as remote-sensing products covering the catchment. 

We present a comprehensive, process-based simulation of the catchment-scale water balance of the Rio Santa basin. We quantify the altitudinal distribution and the spatial, seasonal, and interannual variability of the blue-green-white water balance and its individual components across the entire catchment. We further estimate the energy- and mass-balance components of all glaciers in the Cordillera Blanca (445 glaciers) to identify hotspots of glacier changes and their controls. This allows us to determine the importance of sublimation for controlling glacier mass balance and the role of ephemeral snow in shaping melt rates. A key step forward is the catchment-wide quantification of catchment losses, where we identify the combined role of sublimation and evapotranspiration in the water balance. These results provide a novel process-based understanding of the energy and water balance of the Rio Santa basin to establish a mechanistic baseline simulations to understand future changes in the system.