# Sand and Dust Storms: Impact Mitigation
## 4 SDS Hazards and Their Mitigation
### SDS Source Mitigation
#### 4.1.1 Controlling Wind Erosion on Cropland  
When toposoil is entrained from agricultural fields it represents a severe form of accelerated soil erosion with deleterious effects on crop yields [71] so numerous techniques to control wind erosion on cropland have been developed. They are frequently classified into three categories:  
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* Agronomic measures (crop management practices)
* Soil management techniques
* Protective barriers  
All these measures aim to reduce erosion by decreasing the wind speed at the soil surface. This is achieved by increasing surface roughness and/or by increasing the threshold velocity that is required to initiate soil particle movement by the wind. Figure 3: Two-fold approach to mitigate SDS hazards for disaster risk reduction. Agronomic measures for controlling soil erosion use living vegetation or the residues from harvested crops to protect soil from the wind. Maintaining a sufficient vegetative cover is often referred to as the \"cardinal rule\" for controlling wind erosion [66]. One widely-used agronomic measure involves rotating crops grown in rows with cover crops such as grasses or legumes grown on the same field every other year. Mulching--the practice of leaving some residual crop material, such as leaves, stalks and roots, on or near the surface--is another commonly used agronomic technique. It is successful in reducing erosion and in reducing the loss of water from fields by decreasing evaporation. The relationship between soil loss and vegetation cover (live or dead) is generally exponential: the soil loss ratio is at a maximum of 1 on a bare unprotected surface but decreases rapidly to a value of approximately 0.2 with 40% soil cover [72]. However, work in Sahelian Africa has shown that maintaining a crop residue cover of just 2% on a field reduces the potential wind erosion by at least a factor of three [73]. Most soil management techniques are concerned with different methods of soil tillage, an essential management practice that provides a suitable seed bed for plant growth and helps to control weeds. Strip or zone tillage leaves protective strips of unfilled land between seed rows, requiring weed control on the protective strips. Other tillage methods leave varying degrees of vegetative matter from the previous crop on the soil surface to provide protection against erosive winds. Minimum tillage incorporates the idea of stubble mulching, while zero-tillage (or no-tillage) leaves most of the soil covered with plant residues. Zero-till technology allows the farmer to lay seed in the ground at the required depth with minimal disturbance to soil structure, the specially designed machinery eliminating the need for plowing and minimizing the tillage required for planting. In addition to being very effective at controlling erosion, these forms of tillage have effectively revolutionized agriculture in many parts of the world by allowing individual farmers to manage greater areas of land with reduced energy, labor and machinery inputs [74]. Their adoption is widespread in Australia, Canada, the USA and parts of South America. In the pampa region of Argentina, zero-tillage practices are credited with improving soil fertility by reversing decades of degradation [75]. Windbreaks and shelterbetts have been used to reduce the erosive force of the wind in many agricultural areas [76; 77]. Fences or walls placed at right angles to erosive winds serve this purpose, or windbreaks may be created from living plants such as trees or bushes, in which case they are known as shelterbetts. Reductions in wind velocity are achieved both upwind, for a distance of 2-5 times the height of the windbreak, and downwind, extending 10-30 times windbreak height [78]. Numerous other benefits to crops can also often be associated with the establishment of windbreaks. These include increased soil and air temperatures, reduced pest and disease problems, and an extended growing season in sheltered areas.