Surface Water

Surface water hydrology includes the study of water movement on the surface of the earth and its distribution in space and time. Surface water is found in networks of channels (streams and rivers), water bodies (lakes and reservoirs), and as runoff (water flowing on the surface outside defined channels, usually after a rainfall event). Streams flow from a source, such as a seep or a spring, down-slope to join other streams, eventually becoming a river. Excessive flow due to increased rainfall, snowmelt or dam releases (planned or otherwise) that cannot be absorbed by the vegetation or infiltrate into the soil, can lead to river exceeding its banks and flowing out on to the floodplain. This is known as flooding.

In arid and semi-arid climates, the variability in water quantity and flow (within each year and between years) greatly influences the availability of water. This variability is largely determined by climate (precipitation and temperature). Together with geo­graphical characteristics (topography, soils, land use) hydrologic variability affects the development and character of surface water systems such as lakes and rivers.

Streamflow is the water flowing along a river channel. Discharge is the volume of water passing a certain point per unit time. Streamflow and discharge are usually measured by a hydrometric gauging station. Discharge can be calculated using the following equation:

Discharge (Q) = Channel Width (W) x Channel Depth (D) x Velocity of Water (V)

As discharge is dynamic, changing over time depending on the variety of factors that determine how much water is in a stream at a particular time, it is necessary to visualise this information in a chart, called a Hydrograph.

An example of what a hydrograph may show:

• The initial small rise in discharge from normal flow conditions (base flow), when the water falling on or close to the river channel itself increases the average flow slightly
• Then, after a short lag, the main discharge event, characterised by the steep increase in discharge as the run-off from closer parts of the basin passes through the point of measurement
• Then the falling discharge response, as water from further away in the basin takes longer to pass through
• Finally, the return to normal flows
• The shape of a hydrograph is determined by two types of factors:
  • Permanent factors – physical characteristics of the river basin, such as river channel morphology, slope, soils, vegetation cover
  • Transient factors – the variable elements of the rainfall event, such as the size, intensity and duration of the rainfall event

Source: Pidwirny 2006

The diagram below is intended to be a typical hydrograph, illustrating the points listed above.

Run-off and stream/river flow can be reduced by water abstraction/diversion for irrigation, implementation of infrastructure such as dams, and modification of land use. Such changes in the upper catchments of the basin can have significant impacts in the lower basin, reducing the availability of water for all uses, including environmental requirements. These reductions in flow can negatively affect socio-economic development and also result in environmental degradation. Therefore, it is important, particularly in a transboundary context, to maintain flows through cooperative management of water resources.

Next: Surface Water: Streams and Rivers >