Crucial water resources in a region of water stress

The mountains of the Cordillera Blanca, in the Peruvian Andes are home to high-elevation glaciers and snow cover which provide vital water resources to the communities downstream. This water is used by small scale subsistence farmers, hydropower companies and large irrigated agriculture schemes situated along the arid Pacific coast. This area has a distinct seasonality in precipitation, with summers (October to March) defined by wet conditions where snow falls high in the mountains but in the valley bottoms, still at over 3000 m above sea level, precipitation falls as rain. Meanwhile, winters (April to September) are very dry, making meltwater particularly important at this time of year. Quantifying the importance of meltwater from glaciers and snow in this region is therefore essential.

New modelling sheds light on the importance of snow

Despite this crucial importance of melt for water resources in this region there has been no clear view of just how much runoff in rivers comes from glacier ice, how much from snow, and how much from rain. Previous studies either applied computer modelling simulations which neglected important cryospheric processes or relied on analysis of the water chemistry of streams which cannot easily distinguish between snow and ice melt sources. Our new study applied a more sophisticated glacio-hydrological model to the Rio Santa catchment in the Cordillera Blanca, which was able to give us an understanding of the ice and snow melt every hour across the whole catchment. We also validated the model against a collection of field and satellite data to confirm that glacier melt, snow characteristics and river runoff were correctly represented. 

Surprisingly, our new simulations revealed that snow melt is a significant contributor to runoff all year, with it being particularly important in the early dry season (38% of the dry season water inputs into the catchment are composed of snow melt). It is so important because the elevations at which snow cover occurs comprise a large part of the total catchment area. The high temporal resolution of our model simulations allowed us to understand the interesting dynamics of the snow: it falls and then melts quickly, over hours or a few days, with snow depths remaining thin at elevations between 4000 and 5000 m above sea level. Only at the highest elevations does the snow cover last all year and provide meltwater throughout the dry season. We were also able to determine that ice melt was a large proportion of the available water in the dry season - with it composing up to 44% of water inputs into the catchment. Even in parts of the catchment where the glaciers were small, ice melt was still substantial during the driest months of the year. 

So what are the implications for the population and ecosystems of the Rio Santa?

The new insights from our modelling have some important implications. For instance if rain falls instead of snow then it impacts on the rest of the hydrological cycle, since it could potentially modify how much water reaches groundwater or is lost through evapotranspiration to the atmosphere. The results of our simulations could also help guide adaptation strategies for improved water management. Both the snow and glaciers are also vulnerable to climate change: a small increase in air temperature could cause precipitation to fall as rain instead of snow, and glacier shrinkage would clearly impact on river flow in the midst of the dry season, potentially impacting water availability for municipal use or agriculture. The team are planning to conduct future simulations to quantify how changes in glaciers and snow might impact on water availability, since this is crucial for long term planning of water resources in the region.