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Home The River Basin People and the River Governance Resource Management
The River Basin
 Introduction
Geography
Climate and Weather
Hydrology
 Principles of Hydrology
 Water Cycle
Surface Water
 Stream Order
 Lakes and Reservoirs
 Flooding
 Groundwater
 SW/GW Interactions
 Water Balance
 Hydrology of Southern Africa
 Hydrology of the Kunene Basin
Water Quality
Ecology & Biodiversity
Watersheds
 References

 



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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 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. Drainage basins, also called catchments or watersheds, include all the land that drains into a river, from its source to its mouth (Dunne and Leopold 1978). Excessive flow due to increased rainfall, snowmelt or dam releases (planned or otherwise), can provoke that the river exceeds 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) this variability affects the development and character of surface water systems such as lakes and rivers.

Discharge is the volume of water passing a certain point per unit time. Flow velocity and water level 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 a variety of factors that determine how much water is in a stream at a particular time, this information can best be presented in a chart, called a hydrograph.

Demonstration Hydrograph.
Source: Pidwirny 2006
( click to enlarge )

From the demonstration hydrograph shown above the discharge can be described as follows:

  1. Rising limb: an initial small rise in discharge from normal flow conditions (base flow), when the rainfall close to the river channel itself increases the average flow slightly.
  2. Crest: after a short lag, the main discharge event takes place, characterised by the steep increase in discharge (up to the maximum), as the run-off from closer parts of the basin passes through the point of measurement.
  3. Falling limb: Discharge decreases again when drainage water of distant parts of the basin become effective at the observation point.
  4. Return to base flow level: finally, the flow returns to normal conditions.

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 intensity and duration of the rainfall event.
The Kunene River in normal flow conditions.
Source: Stieglitz 2000
( click to enlarge )
The Kunene River in flood.
Source: Stieglitz 2000
( click to enlarge )

 

 



Interactive

Explore the sub-basins of the Kunene River


Video Interviews about the integrated and transboundary management of the Kunene River basin


Explore the interactions of living organisms in aquatic environments


Examine how the hydrologic cycle moves water through and around the earth