Effect of vegetation cover and transitions on regional wind erosion in drylands

Wind erosion is a critical environmental problem that threatens mainly the arid and semi-arid regions of our planet. Usually this problem is associated with desertification, poverty and other environmental and socioeconomic problems. Wind erosion causes the loss of fertile topsoil, and has a negative effect on agricultural production and on human health. When conditions favorable for wind erosion are present, the process can cause large scale environmental disasters like the Dust Bowl in the USA in the 1930s. This event is considered one of the worst environmental disasters of the 20th century, and was caused by a reduction in vegetation cover due to a change in land use combined with an increased dryness in the region. Wind erosion involves the detachment, transport and deposition of soil particles. Depending on their size, particles can move in three different types of transport: creep, saltation and suspension. Vegetation is one of the key factors in the protection of the soil against erosive winds. Although research on wind erosion has started a few decades ago there is still a big gap between the available knowledge provided by current measurement and modeling tools and the knowledge which is required by policy makers and land managers. This thesis focuses on improving the knowledge of the effects of vegetation cover and land use on regional scale wind erosion. The thesis covers improvement of wind erosion measurement techniques (Chapters 2, 3 and 5) and wind erosion modeling on the regional scale (Chapter 4 and 6). In Chapter 2, the efficiencies of the Vaseline Slide (VS) and Modified Wilson and Cooke (MWAC) catchers were determined with different sand particle sizes (<50, <75, 50–75, 200–400, and 400– 500 μm) at a fixed wind speed (13.3 m s–1) and with different soil textures at different wind velocities (10.3, 12.3, and 14.3 m s–1). The study showed that whereas the VS trap is better for catching fine particles, the MWAC trap is better for trapping coarse particles. In the experiments with different soil textures, the efficiency of each catcher considerably changed with the with wind speed. This also varies importantly between catchers: for instance, for sand the MWAC efficiency was relatively high, whereas the efficiency of VS catcher was relatively low. Results concluded that the efficiency of each catcher varies critically with particle size, soil texture and wind speed. Equipment or measurement techniques for the observation of saltation at the regional scale does not exist although these are essential for improving the understanding of wind erosion problem at that scale. In Chapter 3, the portable plot method for measuring regional scale wind erosion with a specific focus on the saltation process was developed. With this strategy the number of measurement locations is increased with limited budget and time. The portable plot method was applied at agricultural stability zones 4 and 5 in the Khanasser Valley in Syria in 2009 and 2010. During the measurement period, a meteorological station was installed at each plot together with MWAC sediment catchers. Results showed that, with this method, information on the effect of wind regime on the aeolian mass transport for different land uses in the region can be obtained. Also, insights into the interrelation between neighboring land units can be gained and the data for scaling-up a field scale model to the regional scale are obtainable. We concluded that this method provides insight into the wind erosion at regional scale and data collected through it are important for progressing the modeling of wind erosion at a regional scale. The new measurement method enabled the calibration and validation of the field scale model of RWEQ (Revised Wind Erosion Equation) at several land use areas in the Khanasser valley in Syria (Chapter 4). In this chapter, the RWEQ model was modified to estimate mass flux and soil loss at a field scale for different types of land use. We implemented this modified version of RWEQ that represents wind erosion as a transient process, using time steps of 6 hours. Beside this, a number of adaptations including the estimation of mass flux over the field boundaries and the routing of sediment have been added. The results showed that this modified version of RWEQ provided acceptable predictions for the average mass flux from our measurement plots compared with the results of previous tests of the model. While the portable plot method provided insights on the effect of land use and climate on the quantity and intensity of wind erosion in a region, more knowledge was required on the border effect between different land uses as the portable plot method provided only limited knowledge on the effect of vegetation pattern and border effect. To get sufficient knowledge on this subject a simulation of sediment transport in a regional scale environment was designed and tested in wind-tunnel experiments (Chapter 5). This simulation showed the effect of vegetation pattern on sediment transport within a land unit and at the border between land units. Wind tunnel experiments were conducted with artificial shrubs representing Atriplex halimus, a native shrub in Khanassar valley. In the experiments, a wind speed of 11 m s-1 was applied and after each 200-230 second wind run the sediment redistribution was measured using a graph paper. Results showed that: 1) the transport within a land unit is affected by the vegetation density and pattern for the land unit itself and for the neighboring units; 2) plans for re-vegetation of degraded land need to take into account the ‘streets’ effect; 3) the effect of neighboring land units includes a sheltering effect and the regulation of sediment passing from one land unit to the neighboring land units and 4) revegetation projects in regions vulnerable to wind erosion not only need to investigate the effect of vegetation pattern on erosion and deposition within the region in general, but also should consider the redistribution of sediment at smaller scales. In Chapter 6, The Regional Scale Wind Erosion Equation (RS-WEQ) was developed. This model takes the different land uses in a region into account, considers the interrelation between neighboring land units and considers saltation as the main transport mode. Although the RWEQ was the starting point for the development of RS-WEQ, the new model is not restricted in its application to a single field. RS-WEQ predicts mass flux, soil loss and deposition for all land uses in a region. The model considers the erodibility parameters for each land use independently and takes the effect of the borders into account. Its output provides a clear insight on wind erosion processes at the regional scale. RS-WEQ was run using scenarios that represent main land uses in dry regions in general and in Khanasser valley specifically. The model outputs showed that the wind speed, field length and land use patterns affect the quantity and severity of mass flux, soil loss and deposition in a region. Specifically, the results showed that the mean mass flux and its related abrasion risk increased with the increase in wind speed and field length. The model provides further details on the effect of field length and land use patterns on the severity of soil loss and deposition at the regional scale. Therefore the developed model can be considered as a useful tool for land managers and policy makers in regions that are vulnerable to wind erosion. This thesis showed that for comprehensive understanding of the aeolian sediment transport at regional scale a combination of measuring and modeling of wind-blown sediment transport is required. And the intensively calibrated and validated wind erosion models can be used in the framework of wind erosion mitigation.