International Symposium on
Drylands Ecology and Human Security
Empirical Models for the Assessment of Specific Sediment Yields
Sahira Joundi1 & Donald Gabriels2
1 UNESCO-Division of Water Resources, Paris, France
Management of dams and reservoirs is of important public concern regarding the water supply in Morocco. A call for action is launched because of natural constraints linked to irregular scarce precipitation, especially in drylands, and also because of human constraints linked to demography and to the development and well-being of the population. However the increasing need for the efficient and sustainable use of natural resources is not followed by a similar input of efforts in watershed management and in conservation of soil and water. The situation and problems with regard to protection of the watersheds supplying water in reservoirs and dams is considered as highly critical in Morocco. Morocco has 48 large dams and reservoirs and an important number of checkdams (more than 100) with an annual capacity of 14.109 m³ of water for irrigating 1 million hectares. This capacity can however not be maintained because of deposition of 50 Mm³ sediment per year resulting in silting up of the reservoirs. This sediment deposition corresponds to an annual decrease in capacity with 0.5%, equivalent to a loss of a large reservoir every two years. The loss of capacity was already estimated in the 1990’s as being more than 800 Mm³. Most of the reservoirs constructed during the last twenty years were already silted up to more than 10% of their capacity after the first two years after their construction. The costs for the recuperation of this capacity loss can be related to and expressed in terms of off-site damage caused by erosion in the watershed. However the mechanisms and processes of silting up of reservoirs and the off-site consequences of watershed erosion are not fully understood.
In view of this, the mechanisms and processes of the silting up of 24 large dams of Morocco and the delivery of sediments at the exit of the watershed are studied in detail in this work, following two methodologies.
The methods are either based on a direct ‘batimetric’ measurement of the volume of sediment deposited in the reservoir, and/or on assessing the specific degradation SEa (ton.ha-1yr-1) of the watershed (an indicator of the rate and amount of erosion and sedimentation per unit surface of the watershed and contributing to the silting up of the reservoir) as a function of the parameters governing the processes of water erosion and sediment transport. In a first attempt the selected parameters are the characteristics of precipitation and the morphology of the watershed with the assessment based on an empirical approach and analysis of the processes on a watershed basis. In view of the availability of date on the Modified Fournier Index (IFM) and the surface (A) regressions were retained to assess the specific degradation SEa for humid and semi arid zones and in 24 watersheds of Morocco.
In a later stage of the study a factor combining soil and vegetation is introduced with reference to a combined action of soil erodibility and vegetation cover as factors in the Universal Soil Loss Equation (USLE).
Assessing or estimating the soil loss due to erosion in the watershed is based on an empirical approach as well. This soil loss, as source of the sediment entering the reservoir, results also in a decline of productivity of the agricultural lands. Hence the economic value of loss in soil productivity and the preservation of the natural resources should come in balance with the damages and costs caused by erosion in the watershed.
For a number of watersheds the erosion has empirically been estimated applying the USLE, based on an assessment of soil loss in the watershed of the Nakhla dam, in the humid zone in North Morocco, taking into account changes in vegetation from 1993 to 2003. This methodology has not only leaded to identify erosion risks zones in the Nakhla watershed but also to predict the ‘life time’ of a number of reservoirs.
This empirical approach in assessing the sediment in reservoirs, caused by erosion in the contributing watershed, will enable experts, managers and decision makers to work out soil and water conservation strategies, giving priority to the highest risk zones and watersheds and affected dams and reservoirs.