Water Resource Management
Both soil and water are crucial natural resources which are essential for life on earth. Because of the position soils occupy at the surface of the earth and the fact that they receive huge quantities of water which they then help to re-distribute, there is an important relationship between soils and water. The total land surface of the world is about 150 million sq. km (58 million sq. miles). Precipitation falling on this large surface can take a number of routes. It can flow over the surface, especially on the steep slopes or on soils with compacted surfaces, down to reach the nearest water courses. It is important that as little of the water falling to the soil takes this route because it can lead to flash floods and deprives the soil of water which is important for the growing vegetation and for the many soil organisms. It can also cause soil erosion by which the soil particles and attached nutrients are picked up and transported overland to other sites including streams and rivers. Such surface water can also pick up freshly applied fertilisers and manure from fields and deposit them in the water courses where they lead to problems of eutrophication.
Most water falling to the land surface enters the soil where it spends an amount of time depending on the nature of the soil. In sandy soils the water passes through the soil quickly, in loamy soils water will spend longer being held in pores and around particles. In clay soils the water tends to be held longest because clay soils contain many fine pores in which the water can be stored and held for some time. The soil acts as a very good filter as it can absorb some of the impurities that the rainfall may contain. The water that eventually moves on through the soil can, however, pick up excess nutrients from the soil and carry them in solution to the watertable below, from which they can pass into streams, rivers and onto the sea. This was the case in the decades following World War II when fertiliser use increased substantially. As a result many streams and rivers became polluted or too nutrient-rich.
Thus there is a very important relationship between water reaching the soil, its residence in the soil, the watertables in the soil and in the rocks below, the streams and rivers and ultimately the sea. The soil has an important role in rainwater and snow melt distribution. It can also influence the nutrient content of the water that leaves the soil for the water courses. This latter has become one of the most important factors in the transfer of potential pollutants from the land to the water courses and ultimately to the oceans. An increasing problem is also that of urban development which, as it increases at a pace, is covering and sealing more and more soil. When this happens the soil can no longer act as a filter. Instead, there can be direct and often rapid run-off of water and various polluting substances from urban areas directly into water courses without any filtering. This has already caused many instances of water courses becoming polluted and the impact of this is very evident in some countries.