Effect of Digital Elevation Model Resolution on the Simulation of the Snow Cover Evolution in the High Atlas

International audienceThe snow melt from the High Atlas represents a crucial water resource for crop irrigation in the semiarid regions of Morocco. Recent studies have used assimilation of snow cover area data from high-resolution optical sensors to compute the snow water equivalent and snow melt in other mountain regions. These techniques however require large model ensembles, and therefore it is a challenge to determine the adequate model resolution that yields accurate results with reasonable computation time. Here we study the sensitivity of an energy balance model to the resolution of the model grid for a pilot catchment in the High Atlas. We used a time series of 8-m resolution snow cover area maps with an average revisit time of 7.5 days to evaluate the model results. The digital elevation model was generated from Pléiades stereo images and resampled from 8 to 30, 90, 250, 500, and 1,000 m. The results indicate that the model performs well from 8 to 250 m but the agreement with observations drops at 500 m. This is because significant features of the topography were too smoothed out to properly characterize the spatial variability of meteorological forcing, including solar radiation. We conclude that a resolution of 250 m might be sufficient in this area. This result is consistent with the shape of the semivariogram of the topographic slope, suggesting that this semivariogram analysis could be used to transpose our conclusion to other study regions

Linkages between snow cover, temperature and rainfall and the North Atlantic Oscillation over Morocco

Snowpack significantly contributes to water resources in North Africa. The objective of this work was to analyze the impact of the North Atlantic Oscillation (NAO) on rainfall and temperature in Morocco at a sub-seasonal scale and to evaluate, for the first time, linkages between the NAO and snow cover area (SCA) derived from remote sensing observations. For this purpose, we analyzed daily SCA products (2000-2013) from the MODIS sensor, meteorological data from 37 stations (1993-2011), NCEP re-analysis of surface air temperature and rainfall rate, and a monthly NAO index. The linear linkages sought using simple correlation analysis demonstrated that negative (positive) NAO (1) is associated with enhanced (reduced) rainfall in December and February for the northern part of the country; (2) occurs with above-normal (below-normal) temperature and, in contrast with rainfall, the correlation persists far inland and late in the season. These results highlight a possible competing influence of NAO on the snowpack dynamic through rainfall and temperature. Indeed, negative (positive) NAO tends to favor earlier (later) melting and lesser (greater) SCA in spring (April-May) through milder temperatures. Results have direct implications for seasonal forecast of SCA in Morocco

Projected changes on the surface water resources of the Rherhaya basin (High Atlas, Morocco) by a set of Med-CORDEX models.

International audienceTo anticipate the potential changes in water quantity available within the Rherhaya mountainous watershed (near to Marrakech), it’s important to know the evolution of this resource in relation with climate changes. In this study we use the GR4J model with a snow module with time series of precipitations and discharge (1989 - 2009). The model was calibrated and validated successfully over various periods. Then we used an ensemble of 5 regional climate models (RCM) provided by the Med-CORDEX program with a method of perturbation by quantiles to simulate future scenarios of flow predictions.The evaluation of the precipitations simulated by the RCMs models (RCM) shows a strong underestimation of ~50% but a good reproduction of the cycle for the temperatures. The future changes according to two scenarios RCP4.5 and RCP8.5 show a rise of the temperatures (+1.4°, +2.6° respectively) in conjunction with a decrease in total precipitation (-19%,- 31%). Concerning the hydrological modeling with GR4J, stable results are obtained for calibration and validation whatever the chosen period, with maximum bias of 15% in validation on the monthly flows. Flow forecasts (2049-2065) present a strong projected decrease in surface runoff (-30%, -60%) and significant drops of the snow-covered reservoir levels, related to the precipitation decrease and the temperature increase

Assessment of daily MODIS snow cover products to monitor snow cover dynamics over the Moroccan Atlas mountain range

In semi-arid Mediterranean areas, the snow in the mountains represents an important source of water supply for many people living downstream. This study assessed the daily MODIS fractional snow-covered area (FSC) products over seven catchments with a mixed snow-rain hydrological regime, covering the Atlas chain in Morocco. For this purpose, more than 4760 daily MODIS tiles (MOD10A1 version 5) from September 2000 to June 2013 were processed, based on a spatio-temporal filtering algorithm aiming at reducing cloud coverage and the problem of discrimination between snow and cloud. The number of pixels identified as cloudy was reduced by 96% from 22.6% to 0.8%. In situ data from five snow stations were used to investigate the relative accuracy of MODE snow products. The overall accuracy is equal to 89% (with a 0.1 m. threshold for snow depth). The timing of the seasonal snow was also correctly detected with 11.4 days and 9.4 days of average errors with almost no bias for onset and ablation dates, respectively. The comparison of the FSC products to a series of 15 clear sky FORMOSAT-2 images at 8 m resolution in the Rheraya sub-basin near to Marrakech showed a good correlation of the two datasets (r = 0.97) and a reasonable negative bias of -27 km(2). Finally, the FSC products were analyzed through seasonal indicators including onset and melt-out dates, the Snow Cover Duration (SCD) and the maximum snow cover extent (SCAmax) at the catchment level: (1) the dynamic of the snow cover area is characterized by a very strong inter-annual signal with a variation coefficient of the SCAmax reaching 77%; (2) there is no evidence of a statistically significant long-term trend although results have pointed out that the SCD increased in February-March and, to a lesser extent, decreased in April-May for the 2000-2013 period. The study concludes that the daily MODIS product can be used with reasonable confidence to map snow cover in the South Mediterranean area despite difficult detection conditions

Remote Sensing of Water Resources in Semi-Arid Mediterranean Areas : the joint international laboratory TREMA

Monitoring of water resources and a better understanding of the eco-hydrological processes governing their dynamics are necessary to anticipate and develop measures to adapt to climate and water-use changes. Focusing on this aim, a research project carried out within the framework of French-Moroccan cooperation demonstrated how remote sensing can help improve the monitoring and modelling of water resources in semi-arid Mediterranean regions. The study area is the Tensift Basin located near Marrakech (Morocco) - a typical Southern Mediterranean catchment with water production in the mountains and downstream consumption mainly driven by agriculture. Following a description of the institutional context and the experimental network, the main recent research results are presented: (1) methodological development for the retrieval of key components of the water cycle in a snow-covered area from remote-sensing imagery (disaggregated soil moisture from soil moisture and ocean salinity) at the kilometre scale, based on the Moderate Resolution Imaging Spectroradiometer (MODIS); (2) the use of remote-sensing products together with land-surface modelling for the monitoring of evapotranspiration; and (3) phenomenological modelling based only on time series of remote-sensing data with application to forecasting of cereal yields. Finally, the issue of transfer of research results is also addressed through two remote sensing-based tools developed together with the project partners involved in water management and irrigation planning

Isotopic signatures for the assessment of snow water resources in the Moroccan high Atlas mountains: contribution to surface and groundwater recharge

International audienceTo characterize snow isotopic signatures, monitoring of snowmelt was carried out at two sites (Oukaimden and Ifni) in the Moroccan High Atlas Mountains. For the Oukaimden site, samples of snow were taken by two methods to compare sampling techniques: (1) coring with a polyvinyl chloride (PVC) tube and (2) passive capillary sampling (PCS) installed at the snow/soil interface. The analyses show variable isotope contents, ranging from −14.7 to −2.5 ‰ for oxygen-18 and from −116 to −28.2 ‰ for deuterium. The most depleted values are observed in March 2013 at high elevation (3229 m asl). The majority of snow core samples display fractionation by sublimation, whereas those collected by the PCS sampling method are close to the Global Meteoric Water Line. The isotopic signature is comparable for snow, surface water and groundwater samples, indicating that snowmelt plays an important role in recharging aquifers, lakes, and rivers on the southern and northern sides of the Atlas Mountains. Recharge by snowmelt allows the dilution of salinity in adjacent aquifers. Characterization of the stable isotopic composition of snow obtained from snow cores is limited in comparison with the PCS method, which provides realistic compositions of the melt water contribution to water resources in this semi-arid area