Snowmelt and sublimation: field experiments and modelling in the High Atlas Mountains of Morocco

Abstract

Snow in the High Atlas Mountains is a major source for freshwater renewal and for water availability in the semi-arid lowlands of southeastern Morocco. Snowfall-and snow-ablation monitoring and modelling is important for estimating potential water delivery from the mountain water towers to the forelands. This study is part of GLOWA-IMPETUS, an integrated management project dealing with scarce water resources in West Africa. The Ameskar study area is located to the south of the High Atlas Mountains, in their rain shadow. As a part of the MGoun river basin within the upper Drâa valley, the study area is characterised by high radiation inputs, low atmospheric humidity and long periods with sub-zero temperatures. Its altitude ranges between 2000 m and 4000 m, with dominant north-and south-facing slopes. Snowfall occurs mainly from November to April but even summit regions can become repeatedly devoid of snow cover. Snow cover maps for the MGoun basin (1240 km²) are derived from calculations of NDSI (Normalized Difference Snow Index) from MODIS satellite images and snow depth is monitored at four automatic weather stations between 20004000 m. Snowfall events are infrequent at lower altitudes. The presence of snow penitentes at altitudes above 3000 m indicates that snow sublimation is an important component of snow ablation. Snow ablation was modelled with the UEB Utah Energy Balance Model (Tarboton and Luce, 1996). This single layer, physically-based, point energy and mass balance model is driven by meteorological variables recorded at the automatic weather stations at Tounza (2960 m) and Tichki (3260 m). Data from snow pillows at Tounza and Tichki are used to validate the models physical performance in terms of energy and water balances for a sequence of two snowfall events in the winter of 2003/4. First UEB modelling results show good overall performance and timing of snowmelt and sublimation compared to field investigations. Up to 44% of snow ablation is attributed to snow sublimation in typical winters with subzero temperatures and low atmospheric humidity at an altitude of 3000 m. At altitudes below 3000 m snowmelt generally dominates over sublimation. Unfortunately, the highest altitude zones suffer long periods with direct water loss into the atmosphere by sublimation in the course of which they cannot contribute to direct runoff or groundwater formation in the southern High Atlas Mountains.

Glacier contribution to streamflow in two headwaters of the Huasco River, Dry Andes of Chile

“…of the seasonal snowpack ablation at 3000 m a.s.l. (Schulz and de Jong, 2004). The same authors noted the wide range of sublimation rates that were observed by other groups in arid mountains, emphasizing the fact that sublimation losses can considerably reduce runoff and groundwater recharge in already dry environments.…”

Evaluation of the Snowmelt Runoff Model in the Moroccan High Atlas Mountains using two snow-cover estimates

“…Fourth, in a semi-arid area, water losses via evapotranspiration for the vegetation and via sublimation for snow are important, but they are perhaps not properly taken into account by a simple conceptual model such as SRM. On the southern slopes of the High Atlas, Schulz & de Jong (2004) found, with an energy balance model, that, on average, 44% of snow ablation was attributed to sublimation, and small pinnacles that could be due to sublimation (penitentes) were observed. Finally, the mechanisms of groundwater redistribution are also unrecognizable and are difficult to model (Duclaux, 2005;Chaponnière et al, 2008).…”

New observations indicate the possible presence of permafrost in North Africa (Djebel Toubkal, High Atlas, Morocco)

“…Snowmelt contributes to 15-50 % of the stream flow in the Tensift catchment, thus playing a significant role for irrigation (Boudhar et al, 2009). The low atmospheric humidity and typically subfreezing temperatures above 3000 m favour losses by sublimation, which can account for up to 44 % of snow ablation, while at lower altitudes melting prevails (Schulz and de Jong, 2004). The only perennial snow patch in North Africa is located below the north-facing cliffs of the Tazaghart plateau (3980 m a.s.l.…”