340 – Snow cover monitoring in the Moroccan High-Atlas for better water resources management - Institut de recherche pour le développement (IRD)

Snow is a a significant water resource for the mountainous countries of the Mediterranean Rim. In Morocco, the High Atlas concentrate a large proportion of the precipitations, notably in the form of snow, and play the role of water tower for the agricultural plains. Water resource managers are faced with major challenges. The High Atlas is a region of irregular precipitation and this variablity combined with a high evaporation rate makes management of these water resources difficult. In the short term, the agricultural and socio-economic development are exerting a strong pressure which demands the development of rules for rational use and improved seasonal forecasting. Improved assessment of the contribution dams make in the course of the snow-melt season and a reasoned approach to water distribution require understanding of the processes and variables which govern the hydrological cycle. In the longer term, the changes in climate create the fear of a decrease, not only in the total precipitations, but especially of the solid precipitations, which represent a reserve placed in storage during winter which becomes available during summer in the period of lowest-water level.

Monitoring snow cover in the High-Atlas

Observation of the snow cover, accumulation and melt, assessment of the impact of snowmelt on wadi stream flow: research conducted in the context of the SudMed project by IRD scientists and their Moroccan research partners ¹ have the aim of assessing and forecasting water resources in the Tensift Al Haouz region, not far from Marrakech, whose economy is essentially based on agriculture requiring intensive exploitation of water resources. The team recently used a seven-year time series of SPOT VEGETATION satellite images for mapping all the snow-covered areas along Morocco’s mountain chain. This enabled them to analyse the spatial and temporal variations of the snow cover on the whole of the High Atlas from 1998 to 2005.

The spatial distribution of snow cover, and the fact that it is not permanent, or is even ephemeral, quite logically depends on the elevation. Very wide fluctuations with time were found: the extent of the snow-covered surfaces, the number of snowfall episodes and the duration of snow-cover vary enormously from year to year. Also suggested was a possible influence of the North Atlantic’s climate variability, in particular the North Atlantic oscillation, a winter event which has a climate impact on the climate all around the Atlantic Basin, on snowfalls.

Remote sensing ideal for observations of mountain regions

This research also showed that observation of snow-covered surfaces by the satellite SPOT-VEGETATION can produce improved streamflow simulations of the wadis in the Moroccan High Atlas. The data acquired effectively help correct certain elements of bias occurring in snowfall observations, especially when high-altitude clouds generate precipitations which are not detected by the weather station network, mostly located in the valley floors.

Given the rapidity of the snowmelt in Mediterranean climates, current satellites do not always have the necessary revisit time and resolution characteristics for following up the trends in snow cover. New space missions, and especially the Taiwanese satellite FORMOSAT2 or the Venµs project, allow monitoring of the snow-covered areas with a resolution of around 10 m and a daily revisit time, but over small areas. They are complementary with low-resolution satellites which makes possible the overall surveillance of the mountain ranges with a resolution of about one kilometre and a daily revisit time.

The research team is now extending its investigations ² to the north and east of the Mediterranean in order to produce nival indices for assessing the resource by means of satellite remote sensing, and analyse the spatial and temporal variability of these indicators in relation to the climatic processes of the Mediterranean. And as human habitation is sparse in the high mountains, the snow is a good indicator of climate change.