Change of land use
We depend on land to provide many essential life-supporting systems.
Think back through the previous study sessions and consider the different ways that we all use land. What different types of land use can you think of?
We use the land to provide food from agricultural activities; to supply wood from trees for construction and fuel; for water which is extracted from rivers and lakes on the land’s surface or from underground, and to provide rocks and building materials.
You may also have thought of the land that is used when we build houses, shops, factories, roads and other components that make up the urban environment. This is the aspect of land use that we will be focusing on in this study session.
Land use can be defined as arrangements, activities and inputs by people to produce, change or maintain a certain land cover type (FAO/UNEP, 1999). This definition makes it clear that there is a link between land use and land cover. Land cover is the observed biophysical cover on the Earth's surface. In non-urban areas land cover is usually described by the dominant vegetation type, such as forest, grassland or cropland. Changing the way the land is used (for example by building towns and cities on it) changes the land cover and has many direct and indirect effects.
Most consequences of changing land use through urbanisation can be grouped into two main categories: the decrease in natural and agricultural land, and the increase in hard surfaces of built-up areas.
Decrease in natural and agricultural land
Change from non-urban to urban land use causes the loss of many different types of vegetated land cover. This may be grassland used for grazing animals, cultivated fields that produce food and other crops, uncultivated areas of river banks and hillsides, and wooded areas covered with trees. Deforestation was discussed in Study Session 1 as one of the negative impacts of human use of resources.
What are the main negative effects of deforestation?
Deforestation can reduce the infiltration of water into the soil and groundwater, which can lower the water table and increase the volume and speed of surface run-off; if this happens it can increase soil erosion because the land surface is exposed. Deforestation also results in a loss of wildlife habitat and a reduction in biodiversity; and the loss of ecological services provided by trees (such as converting atmospheric carbon dioxide to oxygen by photosynthesis). You may also have mentioned the loss of the aesthetic value of trees, which are attractive elements in our biophysical environment.
The reduction in agricultural land also shifts the balance in the land area available for food production. The most productive land for agriculture tends to be near to towns. When towns expand this land gets covered with buildings and so it is no longer available for food production. This means there is less productive land available to meet the increasing demands for food from a growing urban population. The additional demands for food production in the areas around towns and cities can encourage the increased use of pesticides and fertilisers to improve productivity, which can have negative environmental impacts. It also encourages the cultivation of previously unused land such as sloping hillsides which, when ploughed, are extremely vulnerable to soil erosion when it rains.
Increased area of hard surfaces
The construction of urban areas increases the area of hard surfaces such as roofs, roads, and pavements. Unlike the natural land cover they replace, these hard surfaces are impermeable, meaning water cannot pass through them. When rain falls it does not infiltrate into soil and groundwater but instead pours off the surface very quickly. Water collects in gutters and drains and flows directly into rivers and ditches, which rapidly fill up and can overflow. The volume and speed of the flow of run-off leads to frequent flooding in many city areas (Figure 6.1). These problems are made even worse if there is no drainage system or if drainage is inadequate or becomes blocked with rubbish.
Figure 6.1 Flooding in an urban area.
In addition to the effect on the water cycle, the increase in hard surface area also influences the exchange of energy with the atmosphere, which can lead to localised changes to the weather and climate. In large cities the temperature can be a few degrees warmer than in surrounding rural areas, an effect known as an urban heat island. This is caused by the hard surfaces of roads and buildings, which absorb energy from the sun and radiate heat into the surrounding air to a much greater extent than natural vegetated surfaces, especially at night. The raised temperature can increase the impact of poor air quality on people’s health.
Extraction of building materials
A third category of changed land use is the extraction of rocks and minerals for the construction industry (Figure 6.2). This process results in the loss of vegetated land cover where the rocks are extracted. Most of this resource extraction takes place in peri-urban areas because they are located conveniently close to the construction sites to minimise transport costs and time. Many small-scale, unregulated and low-technology extraction activities can be seen on sites around towns in Ethiopia.
Figure 6.2 Construction of new buildings uses many different resources.
These impacts of change in land use from rural to urban, combined with the effects that you read about in Study Session 5, add up to a lengthy list of negative consequences from urbanisation. Managing and minimising these effects is one of the main purposes of urban planning.