Surface water resources is one of the two major sources of water supply on earth. It is also a major component of the hydrological cycle.
So far, we have examined how water moves below the land surface. Now, we turn to tracing the flow paths of surplus water that runs off the land surface and ultimately reaches the sea.
Here, we will be concerned primarily with rivers and streams. In general usage, we speak of “river” as large watercourses and “stream” as smaller ones.
However, the word “stream” is also used as a scientific term designating the channeled flow of surface water of any amount. This article will focus on the definition of surface water, overland and stream flows, and surface water pollution.
Definition of Surface Water
Surface water is water in a river, lake or fresh water wet land. Surface water is naturally replenished by precipitation and naturally lost through discharge to the oceans, evaporation and unrecovered infiltration.
Surface water can be withdrawn from the stream, lakes and reservoirs for human activities but only part of the total annual runoff is available for use. Some of it flows into the sea two rapidly to be captured and some must be left in the stream for wildlife. In some years the amount of runoff is reduced by below average precipitation (Miller, 1996).
Although the only natural input to any surface water system is precipitation within its watershed, the total quantity of water in that system at any given time is also dependent on many other factors – landscape, temporal variation, geographic factors and human activities. Hence, surface water is non-uniformly distributed over the earth surface (Viessman and Hammer, 1998).
Overland Flow and Stream Flow
Runoff that flows down the slopes of the land in broadly distributed sheets is overland flow. We can distinguish overland flow from stream flow, in which the water occupies a narrow channel confined by lateral banks. Overland flow can take several forms.
Where the soil or rock surface is smooth, the flow may be a continuous thin film, called sheet flow. Where the ground is rough or pitted, flow may take the form of a series of tiny rivulets connecting one water-filled hollow with another.
On a grass-covered slope, overland flow is sub-divided into countless tiny threads of water, passing around the stems. Even in a heavy and prolonged rain, you might not notice overland flow in progress on a sloping lawn. On heavily forested slopes, overland flow may pass entirely concealed benefit a thick mat of decaying leaves.
Overland flow eventually contributes to a stream, which is a much deeper, more concentrated form of runoff. We can define a stream as a long, narrow body of flowing water occupying a trench like depression, or channel, and moving to lower levels under the force of gravity. The channel of a stream is a narrow trough. The forces of flowing water shape the trough to its most effective form for moving the quantities of water and sediment supplied to the stream.
As a stream flows under the influence of gravity, the water encounters resistance – a form of friction – with the channel walls. As a result, water close to the bed and banks moves slowly, while water in the central part of the flow moves faster. If the channel is straight and symmetrical, the single line of maximum velocity shifts toward the bank on the outside of the curve (Strahler and Strahler, 2006).
Streams may be classified as influent or influence. In an effluent stream, flow is maintained during the dry season by groundwater seepage into the stream channel from the subsurface. An influent stream is entirely above the groundwater table, and flows only on direct response to precipitation. Water from an influent stream seeps down into the subsurface.
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Surface Water Pollution
Pollution of surface water occurs when too much of undesirable or harmful substance flows into a body of water exceeding the natural ability of that water body to remove the undesirable material, dilute it to a harmless concentration, or convert it to a harmless form.
Water pollutants, like pollutants in general are categorized as emitted from point or nonpoint sources. Point sources are distinct and confined, such as pipes from industrial or municipal sites that empty into streams or rivers.
Nonpoint sources, such as runoff, are diffused and intermitted and are influenced by factors such as land use, climate, hydrology, topography, native vegetation, and geology. Common urban nonpoint sources include urban runoff from streets or fields; such runoff contains all sorts of pollutants, from heavy metals to chemicals and sediments.
Rural sources of nonpoint pollution are generally associated with agriculture, mining, or forestry. Nonpoint sources are difficult to monitor and control (Botkin and Keller, 1998).
From an environmental view point, two approaches to deal with water pollution are- reduced the source or treat the water to remove or convert the pollutant to a harmless form. Which of the options is used depends on the specific circumstances of the pollution problem. However, reduction at the source is the most environmentally preferable way of dealing with pollutants.
In summary, it is evident that surface water constitutes an important source of water supply for our everyday usage. However, because of the peculiar nature of surface water, it is easily abused leading to pollution.
The pollution problem has created a great burden for those who use this source of water supply. To enhance the quality of surface water supply, it has to be treated to remove the harmful substances in it.
Surface water is one of the major sources of water supply; it is also an important component of the hydrological cycle.
Surface water is water in a river, lake or fresh water wetland. Overland flow is runoff that flows down the slopes of the land in broadly distributed sheets; while stream flow is when the water occupies a narrow channel confined by lateral banks.
Surface water pollution occurs when the water quality is degraded. Two of the approaches to dealing with water pollution are: reduce the source or treat the water to remove or convert the pollutant to a harmless form.