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examBoard: Cambridge
examType: IGCSE
lessonTitle: Bunds and Wind Breaks
Soil erosion is the wearing away of topsoil by natural forces like water, wind and gravity. It's a serious environmental problem because topsoil contains most of the nutrients plants need to grow. When this layer disappears, land becomes less fertile and harder to farm.
Key Definitions:
Every year, the world loses about 24 billion tonnes of fertile soil to erosion. That's equivalent to removing a football field of soil every 5 seconds! Without intervention, we could lose all farmable topsoil within 60 years, threatening our ability to grow food.
Bunds are raised barriers built across slopes to slow down water flow and trap soil. They're one of the most effective methods for preventing soil erosion, especially in hilly or sloped areas where water runoff is a major problem.
Bunds work by intercepting water flowing down a slope, reducing its speed and allowing it to spread out or be channelled safely. This prevents the water from gaining enough force to carry away soil particles. The trapped water also has more time to soak into the ground, improving soil moisture and reducing water loss.
Bunds not only prevent soil erosion but also help conserve water, increase groundwater recharge, improve soil moisture and can create terraces for farming on slopes. They're relatively low-cost and can be built using local materials and labour, making them accessible for farmers in developing regions.
Different types of bunds are used depending on the terrain, climate and specific erosion challenges:
Built along the contour lines of a slope (at the same elevation). They're most effective on gentle slopes and moderate rainfall areas. Contour bunds are typically spaced 10-30 metres apart depending on the steepness of the slope.
Similar to contour bunds but built with a slight gradient to safely drain excess water. They're better for areas with heavy rainfall where water accumulation behind contour bunds might cause them to break.
Made from stones arranged in a line across a slope. They're permeable, allowing water to pass through slowly while trapping soil. Stone bunds are ideal for rocky areas where earth bunds might be difficult to construct.
Building bunds requires careful planning and consideration of local conditions:
In the drought-prone region of Maharashtra, continuous contour trenches (a type of bund system) have transformed the landscape. In the village of Hiware Bazar, implementing bunds and other water conservation measures helped raise the water table by 12 metres and increased irrigated land from 120 hectares to 260 hectares. Crop yields increased by 400% and poverty rates dropped dramatically.
Windbreaks are barriers established to reduce wind speed and prevent wind erosion. They're particularly important in flat, open areas with dry, loose soils that are vulnerable to being blown away.
Windbreaks work by creating a barrier that forces wind to go over or around it, reducing wind speed on the leeward (downwind) side. This reduction in wind speed prevents soil particles from being picked up and carried away. Effective windbreaks can reduce wind speed by up to 80% immediately behind the barrier, with protection extending to a distance of about 10-30 times the height of the windbreak.
Beyond preventing soil erosion, windbreaks provide numerous benefits: they create microclimates that protect crops from extreme temperatures, reduce evaporation and water loss, provide habitat for wildlife and beneficial insects, can produce timber or fruit if appropriate species are used and can act as snow traps in colder regions, increasing soil moisture.
Windbreaks come in various forms, each suited to different conditions and needs:
The most common type, consisting of rows of trees planted perpendicular to the prevailing wind direction. They provide long-term protection and multiple benefits but take several years to become fully effective.
Lower than tree windbreaks but quicker to establish. They're often used in combination with trees to create a more complete barrier from ground level upwards.
Made from materials like fencing, netting, or straw bales. They provide immediate protection and are useful as temporary measures or in areas where growing vegetation is difficult.
The effectiveness of a windbreak depends on its design:
The Great Green Wall is an ambitious project to combat desertification in the Sahel region of Africa by creating a 8,000 km long, 15 km wide belt of trees and other vegetation across the width of Africa. Started in 2007, the project aims to restore 100 million hectares of degraded land, sequester 250 million tonnes of carbon and create 10 million jobs by 2030. In Senegal, over 11 million trees have been planted, helping to stabilise soils and create new farming opportunities.
In many situations, the most effective soil conservation strategy involves combining different methods:
When deciding whether to use bunds, windbreaks, or a combination of both, consider:
Soil erosion isn't just a problem for farmers. Everyone can help by supporting sustainable land management practices, participating in tree planting initiatives, reducing water runoff from gardens and paved areas and raising awareness about the importance of soil conservation. Remember: it takes up to 1,000 years to form just 1 cm of topsoil, but it can be washed or blown away in a single storm if not protected.
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