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Accueil du site → Doctorat → Allemagne → 2009 → Soil characteristics and soil erosion by water in a semi-arid catchment (Wadi Drâa, South Morocco) under the pressure of global change

Rheinischen Friedrich-Wilhelms-Universität (2009)

Soil characteristics and soil erosion by water in a semi-arid catchment (Wadi Drâa, South Morocco) under the pressure of global change

Klose Anna

Titre : Soil characteristics and soil erosion by water in a semi-arid catchment (Wadi Drâa, South Morocco) under the pressure of global change

Auteur : Klose Anna

Université de soutenance : Rheinischen Friedrich-Wilhelms-Universität zu Bonn

Grade : Doktorgrades (Dr. rer. nat.) 2009

Soil resources are crucial for the well-being of man and the environment. The results of the first Global Assessment of Human-induced Soil Degradation (GLASOD) indicate that 13% of the world’s soils are degraded ; thereof 55% suffer from soil erosion by water. Drylands are especially vulnerable due to the sparse protecting vegetation cover, soils that feature a low organic matter content and rare but intense rainfall events. Soil erosion in drylands is likely to intensify as a result of climate change and human activities, such as forest clearing or overstocking. This study aims at understanding and describing the spatial distribution of soil characteristics as well as the current extent and distribution of soil erosion by water. Based on these findings, the impact of global change on soil erosion risk is assessed.
Soil characteristics in the semi-arid upper and middle Drâa catchment (30 000 km2, South Morocco) are examined by investigating soil profiles that are arranged along toposequences that cover the main geological units. Soil properties are regionalised based on their relationship to environmental factors by using multiple linear regression including dummy variables. The physically-based, distributed soil erosion model, PESERA (Pan European Soil Erosion Risk Assessment), is used to assess the current and future soil erosion risk for five periods between 1980 and 2050. Climate change scenarios that are simulated with the regional climate model REMO are applied together with the scenarios of socio-economic change, which have been defined in the IMPETUS project.
Typical semi-arid soil properties are found : high skeleton content, high CaCO3 content, high pH values, low organic matter content and partially strong salinity. The most common soil types are Calcisols, Regosols and Leptosols. Between 22 and 89% of the variance of the soil characteristics can be explained depending on the parameter. The resulting maps reflect the identified relationships to the environmental factors well and provide a reasonable view of the distribution of soil properties in the Drâa catchment.
A mean erosion rate of 19.2 t/ha/a is simulated under the current conditions. Erosion hotspots are identified in the high mountain zones, more precisely in the western (Tizi-n-Tichka), central (Skoura Mole) and eastern (M’Goun chain) part of the Central High Atlas. Rainfall reduction and higher temperatures that are expected following the climate change scenarios lead to a decrease in vegetation cover. Together with more intense precipitation events, this will cause an increase in soil erosion by up to 31%. Due to further marginalisation, people are forced to satisfy their energy demand by enhanced extraction of firewood that further degrades vegetation cover. This results in an increase in the erosion rate of 27%. In contrast, rural development brings about a loss of the nomadic lifestyle and, consequently, a reduction in the animal numbers and grazing pressure. Thus, the soil loss is reduced by 54%. Combining the impact of climate and socio-economic changes shows that human activity can aggravate (+64%) or mitigate (-25%) soil erosion risk. The “Mansour Eddahbi” reservoir that is located at the outlet of the upper catchment is endangered by upstream soil loss. Its simulated capacity in 2050 varies between 0 and 46% of the initial storage volume, depending on the scenario. The efficiency of anti-erosive measures is analysed by simulating two intervention scenarios that consider afforestation (6300 ha) and grazing exclusion (75 000 ha). Efficiency depends on the spatial scale that is under consideration. At the local scale (i.e., the intervention zone), soil loss is reduced by 36-99% up to 2050 ; afforestation is more efficient. At the scale of the upper Drâa catchment, i.e., the relevant scale for reservoir siltation, erosion is reduced by 1 to 13%. Pasture exclusion is more efficient due to the larger intervention zone. This work presents a comprehensive study on the risk of soil erosion by water in the Drâa catchment and can serve as a scientific basis for local decision making processes.


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