Research published in the journal Hydrology and Earth System Sciences highlights the importance of liquid water percolation for modelling snowpack evolution in Mediterranean mountain regions.

In many Mediterranean mountain regions, seasonal snowpack is an essential yet poorly understood water resource. To help tackle this knowledge gap and support improved water resource evaluation and management, researchers have, for the first time, examined the spatial distribution and evolution of snow water equivalent (SWE) during three snow seasons (from 2013 to 2016) in the coastal mountains of Lebanon. The results were published in the journal Hydrology and Earth System Sciences earlier this year.

“Snow melt is a critical water resource in Lebanon,” says paper co-author Simon Gascoin, Research Scientist at the Centre d'Etudes Spatiales de la Biosphère at CNRS, France. “In particular, few people know that the capital Beirut heavily depends on snow melt for urban water supply.”

Simulating snow evolution

For this study, the researchers ran SnowModel (Liston and Elder, 2006a) – a spatially distributed, process-based snow model – at 100 metre resolution forced by new automatic weather station (AWS) data in three snow-dominated basins of Mount Lebanon. In doing so, they evaluated a recent upgrade of the liquid water percolation scheme in SnowModel, which was introduced to improve the simulation of the SWE and runoff in warm maritime regions. The model was then evaluated against continuous snow depth and snow albedo observations at the AWS, manual SWE measurements, and MODIS snow cover area between 1200 and 3000 metres above sea level.

The researchers found that the new percolation scheme yields better performance – particularly in terms of SWE, but also in terms of snow depth and snow cover area. Over the simulation period between 2013 and 2016, the maximum snow mass was reached between December and March. Peak mean SWE (above 1200 metres above sea level) changed significantly from year to year in the three study catchments, with values ranging between 73 and 286 millimetres water equivalent (RMSE between 160 and 260 millimetres water equivalent). In their paper, the researchers suggest that the major sources of uncertainty in simulating the SWE in this warm Mediterranean climate can be attributed to forcing error, and also to limited understanding of the separation between rain and snow at lower-elevations, the transient snowmelt events during the accumulation season, and the high variability of snow depth patterns at the subpixel scale due to wind-driven blown-snow redistribution into karstic features and sinkholes.

A much-needed estimate

However, in spite of these uncertainties, the researchers stress that use of a process-based snow model with minimal requirements for parameter estimation provides a basis for simulating snow mass SWE in non-monitored catchments and characterizing the contribution of snowmelt to the karstic groundwater recharge in Lebanon. While this research focused on three basins in Mount Lebanon, they add, it also serves as a case study to highlight the importance of wet snow processes in estimating SWE in Mediterranean mountain regions more broadly.

“Our study highlights the importance of liquid water percolation for modelling snowpack evolution in Mediterranean mountainous regions, where the winters are mild,” says Gascoin.

“From a water resource perspective our model simulation provides a much-needed estimate of the snow water equivalent distribution in Lebanon mountains, where there is no operational snow monitoring network – despite the importance of snowmelt for urban and agricultural water uses.”

The findings of this research also have implications for future water security in the region, Gascoin adds. “A specific feature of Mount Lebanon is that most of the snow accumulates on a high-elevation plateau. This morphology implies that the seasonal snow cover could be highly impacted by a rise in the mean zero-degree isotherm altitude in winter, which determines the partition of precipitation into rain or snow.”

Given the importance of snowmelt for groundwater recharge and water supply in Lebanon, the researchers conclude their paper with an urgent call to evaluate the vulnerability of Mount Lebanon’s snowfall regime to climate change in future.

Read more: Fayad, A. and Gascoin, S. ‘The role of liquid water percolation representation in estimating snow water equivalent in a Mediterranean mountain region (Mount Lebanon)’, Hydrol. Earth Syst. Sci (2020): https://doi.org/10.5194/hess-24-1527-2020.

Cover image: Snow on the mountains in Bsharri, Lebanon, captured by Raimund Andree.