Precipitation is water in the form of rain or snow falling to the ground. Some water returns to the atmosphere by evaporation from the leaves of plants and from the ground, and by transpiration by plants. Except in the most arid environments, precipitation normally exceeds evapo-transpiration. The surplus water eventually makes its way through the drainage system—although it may be stored first on the surface, in depressions and ponds, or in the soil as soil moisture and groundwater. Overland flow, which occurs when not all the precipitation can infiltrate the soil, moves quickly to streams and rivers. Infiltrated water moves more slowly—as throughflow and interflow in partially saturated soils, and as groundwater flow in saturated soils. Basin channel run-off is the combined result of quickflow (overland flow plus interflow) and baseflow (groundwater flow).

Hydrology of a

Drainage B

asin

Basin or Drainage Basin, area of the Earth’s surface which drains into a single river system. The boundaries of a basin are formed by watersheds that separate it from adjacent areas draining into other rivers. The size and

shape of a basin are usually determined by the underlying geology. The pattern and density of streams and rivers

draining the basin depend not only on geological structure, but also on land surface relief, climate, soil types,

vegetation, and, increasingly, human impacts on the basin environment.

Drainage Basins

Basins are major features of the landscape and over most of the world’s continents, landscape-forming processes are dominated by fluvial erosion, transport, and deposition.

Flood hazards and water resources are also best assessed and managed using a basin-wide approach,

because the input, storage, and output of water within a basin can be measured, budgeted, and modelld

analytically. In addition, piecemeal approaches have been found to solve one problem while inadvertently generating others elsewhere in the drainage system.

Consequently, integrated basin management is recognized as the best practice in water resource

development and river regulation.

Basin Formation

Basins exist over a vast range of scales, from the ocean basins that define the largest drainage units on Earth to field-sized areas feeding

small streams. Many basins are formed by geological processes involving deformation of the Earth’s crust through extension,

downwarping, faulting, folding, or volcanic activity. Others are the result of erosion of the land surface by wind, water, or ice. The structure of the underlying rocks influences the distribution of erosion, with low and high areas developing on erodible and erosion-resistant rocks, respectively. Because rocks stretched upwards in an anticline are weaker than those compressed downwards in a syncline, erosion often leads to inverted

relief, with high areas becoming basins and formerly low areas forming watersheds.

Where the rocks underlying a basin are permeable it is possible for water moving through the ground, or groundwater, to “leak” from one

basin to another. Hence, the boundaries of a groundwater basin do not always coincide with the watersheds of the drainage basin above.

Water in the basin arrives in the form of precipitation as part of the water cycle (hydrological cycle). Some precipitation returns to the

atmosphere, having been intercepted by vegetation and evaporated from the surfaces of leaves and branches. More is lost to evaporation from the ground surface and transpiration by plants. In arid and semi-arid climates all of the precipitation may be consumed in this way for

most of the time; basin run-off occurs only occasionally, following intense storms. Where precipitation exceeds losses to

evapotranspiration, the excess water makes its way through the drainage system. Its rate of progress is not uniform, however; water may be stored in lakes, soils, and as groundwater for considerable

periods before it eventually arrives at the outlet, or basin channel, as basin run-off.

Drainage Hydrology

The major elements of basin hydrology are illustrated in the diagram “The

Hydrology of a Drainage Basin” (SLIDE 1) which shows water following

different pathways to reach the basin channel as run-off. Water which

infiltrates to the permanently saturated groundwater, or phreatic, zone below the water table moves as baseflow; in

the partially saturated aerated, or vadose, zone above, it moves as

interflow and throughflow. Water that is unable to infiltrate the soil becomes

overland flow. The proportion of run-off following the different pathways

depends on a variety of factors, some of which are fixed properties of the basin (geology, structure, and relief). Other

factors can vary with time and in response to human activities (climate,

soils, vegetation), and some depend on the recent weather experienced by the

basin (antecedent conditions). Subsurface drainage by interflow and groundwater seepage occurs much

more slowly than surface drainage by overland flow, a characteristic that is

important in maintaining baseflow in the river system between precipitation

inputs.

Antecedent conditions are particularly important in determining the amount of overland flow. Where the soil is saturated rainfall cannot infiltrate. The resulting overland flow drains rapidly to the channel

network. Following a series of closely spaced storms, or a prolonged period of precipitation, the area of saturated soil

expands, increasing the amount of overland flow. This results in the rapid delivery of a large volume of water to the channel system, which may overwhelm its capacity and cause a flood. In basins which receive substantial precipitation in the form of snow, large volumes of water may be stored on the surface during the winter

months. Basin run-off is often dominated by high flows during spring melting. Flooding is a risk if high temperatures or heavy rainfall

induce rapid melting.

“Flooding is a risk if high temperatures or heavy rainfall induces rapid melting”

Streams tend to form five different kinds of drainage patterns: dendritic, rectangular, radial, centripetal, and trellis. The patterns result from the type of soil in the area of drainage and the erosion of the soil by flowing water. Dendritic, branching patterns form in areas of flat sedimentary rock, while areas with high central peaks, such as volcanoes, exhibit radial drainage patterns. Sometimes, water flows into a bowl-shaped valley by centripetal drainage and creates a lake, or erodes areas between ridges to create deep valleys, as seen in trellis drainage.

Drainage Patterns

Where run-off concentrates, the land surface is eroded to form a channel. Drainage channels form a network that collects water from all parts of the basin and carries it to a single river at the

basin outlet. The pattern formed by the network is influenced by climate and land relief, but the underlying geological structure is usually the most important determining factor. In fact, drainage

patterns are generally so closely related to geology that they are widely used by geophysicists to identify rock faults and interpret

structure. The major pattern classifications include dendritic (tree-like), trellis, parallel, rectangular, radial, and annular

networks. They are illustrated and described in relation to their structural controls in the chart “Drainage System Networks”.

Drainage Patterns

The concept of stream order, the first quantitative method of analysing drainage networks, was developed in the 1940s by American hydraulic engineer and hydrologist Robert Horton. Streams are ranked hierarchically: headwater streams with no tributaries belong to the first, and lowest, order; two first-order streams unite to form a second-order stream; two second-order streams form a third-order stream, and so on. Horton subsequently developed a number of laws of drainage network compositon, which relate stream order and a number of associated indices, such as stream length and number. However, Horton’s laws have been criticized in recent years because they use a statistical approach that has no basis in the physics of water flow and channel formation.

Stream O

rder

The characteristics of a basin and of the streams making up the drainage system can be represented quantitatively using indices of

basin shape and relief, and of linkage in the channel network.

The American hydraulic engineer and hydrologist Robert E. Horton was the first to establish a quantitative method for analysing

drainage networks. Stream order, developed in the early 1940s, ranks streams hierarchically. In Horton’s original system, a

headwater stream with no tributaries belongs to the first, and lowest, order. Two first-order streams unite to form a second-order stream; two second-order streams form a third-order stream, and so on (for

details see the diagram “Stream Order”). This ranking system shows how each stream is related to the network and how the overall

network is linked together.

Associated indices include stream number and length, the bifurcation ratio (see below), and drainage area. Stream number is the number of streams in each order for a particular drainage basin; the number in

each order decreases exponentially, or geometrically, with increasing stream order. Stream length measures the average (or mean) length of a stream in each order, and is calculated by dividing the total length of all streams in a particular order by the number of streams in that order.

The stream length in each order increases exponentially with increasing stream order. The bifurcation ratio is the ratio between the

number of streams in one order and in the next. It is calculated by dividing the number of streams in the lower by the number in the higher

of the two orders; the bifurcation ratio in most networks is approximately constant, varying between 3 and 5. Drainage area measures the average drainage area of streams in each order; it

increases exponentially with increasing order.

Drainage morphometry today tends to focus upon basin area, length, shape, and relief attributes, and on drainage density. The

main indices used to analyse basin shape and relief are the elongation and relief ratios. The elongation ratio is calculated by dividing the diameter of a circle of the same area as the drainage

basin by the maximum length of the basin, measured from its outlet to its boundary. Knowledge of the elongation ratio is important in understanding basin hydrology and in estimating flood hazards. This is because, for a given rainfall event, the less elongated the

basin the greater will be the peak run-off and the faster it will reach the outlet. The relief ratio is defined as the difference in elevation between the highest and lowest points in the basin divided by the

maximum length of the basin. Relief controls the rate of conversion of potential to kinetic energy of water draining through the basin.

Run-off is generally faster in steeper basins, producing more peaked basin discharges and greater erosive power.

Drainage density is considered to be an important index; it is calculated by dividing total channel length by total basin area. It is a

measure of the texture of the network, and indicates the balance between the erosive power of overland flow and the resistance of

surface soils and rocks. Run-off production and peak flows increase markedly with drainage density.

Depending on their relation to regional structure, streams may be classified into five main types. Consequent streams are

those whose courses follow the initial, geologically controlled slope of the land. In folded rocks, longitudinal consequents flow along depressions aligned to the axis of folding; lateral

consequents flow down the sides of these depressions. Subsequent streams develop by headward erosion along lines

of weakness presented by the underlying structure, such as fault lines or weak strata. Resequent streams, which are also termed secondary consequents, flow in the same direction as consequent streams, but are younger. Obsequent streams are

those which flow in the opposite direction to consequent streams. Finally, insequent streams are those which bear no

obvious relation to structure and have no determinable control.

5 Main Types of Streams

Once established, the channel pattern may persist even when the initial geology and structure changes due to

tectonic activity, or is removed by erosion. There are two general classes of persistent drainage pattern. Antecedent

drainage occurs when a channel system maintains its original direction by cutting through uplifting tectonic blocks or folds

rising across its path. Superimposed drainage occurs when a drainage pattern persists after the land surface on which it

originally formed has been eroded away completely to reveal underlying rocks with a different dip and structure.

Channel Patterns

An antecedent stream is a stream that was established before the land beneath it was uplifted through geologic processes such as crustal compression. An antecedent stream will maintain its course in spite of crustal compression, and the stream will continue to erode the land at almost the same rate as the crustal compression uplifts it.

Antecedent

Drainage

Drainage patterns are themselves dynamic. They evolve as a result of the action of flowing water in eroding, transporting, and

depositing rock and sediment from the channel boundaries. With time, these fluvial processes can themselves modify the drainage pattern. A good example of this phenomenon is river

capture, which occurs when headward erosion by a stream leads to it capturing the upper course of another. The abrupt

change of direction that usually occurs at the point of capture is termed the elbow of capture. The beheaded river is termed a misfit, or underfit, stream because without the contribution of

run-off from its headwaters it is too small to have cut the valley containing it.

Fluvial processes modify drainage patterns

                                                              

Over time, a stream on the steep side of a mountain slope will erode the slope faster than a stream on the less steep slope, and may erode the drainage divide that separates them. When the fast-eroding stream erodes a notch in the drainage divide, it eventually takes over the headwaters of the slow-eroding stream on the other side and captures it.

Stream

Capture

Basins have provided the cradles of human development since the earliest known civilizations in Mesopotamia (the Tigris and Euphrates basins), Egypt (the Nile basin), India (the Indus and Ganges basins), and China (the Huang He, or Yellow River, and Yangzi basins). Early scientists and engineers recognized the need to study basin run-off

and its characteristics.

For most of human history, basins have been managed almost exclusively to increase their economic usefulness and to reduce the most serious threats to their human inhabitants: drought and flood. The focus has been on improved land drainage and control of river flow to ensure a reliable supply of water for irrigation and industry, to increase the area available for crop farming, and to reduce the risks posed by floods. Rivers have also been used to carry away

domestic and industrial waste (effluent).

Basin management

This approach to management reached its peak during the mid-20th century in developed nations, when very large-scale engineering projects became possible through advances in engineering and

technology. Projects characteristically involved the construction of pumping works for draining wetlands; dams for water supply,

hydroelectric power generation, river regulation, and flood control; river barrages and canals for irrigation; and levees for flood control.

Schemes of this type may be found in all developed nations, but perhaps the best examples are the Tennessee Valley Authority (TVA) in the southern United States, and the Snowy Mountains Scheme in New South Wales, Australia, which involves water transfers between

the basins of the Murray and Darling rivers.

Large-scale Engineering Projects

The Tennessee Valley Authority constructed Douglas Dam as part of a massive project to harness the power produced by the Tennessee River system. The United States Congress created the TVA, as the Authority is known, in 1933. Today, the TVA directs operations of more than 50 dams in parts of Alabama, Virginia, Kentucky, Georgia, North Carolina, Mississippi, and Tennessee.

Douglas Dam

                          Dry Basin in Nevada

Most of Nevada lies within the Great Basin, an arid desert region

made up of small basins with no direct drainage to an ocean. Many of

these basins are slowly filling with soil and sand

washed down from higher elevations.

Nevada, the driest of the 50 states in the United States, averages only

about 20 centimetres (8 inches) of rain a year.

During the 1950s and 1960s western nations began exporting these approaches to basin management to the developing world. While the

developed nations were well intentioned and benefits have undoubtedly accrued from many schemes, it soon became clear that there were

fundamental problems with the transfer of this form of basin management. For example, fledgling economies could be crippled by the huge costs of running and maintaining schemes, and repayment of loans taken out to

finance such schemes could also create problems, both for the lender and borrower. In addition, the complex design of many schemes, combined with a shortage of engineering and technological expertise in many developing nations has often led to dependency on technical aid from abroad. Finally, many schemes have had unacceptable environmental, health, social, and

cultural impacts that were unforeseen by water resource engineers responsible for their design and implementation. The World Bank will no

longer back the construction of large dams in developing nations and has recently withdrawn from the controversial Narmada River Project in India.

Drainage management schemes in developing countries include the Aswān High Dam on the River Nile in Egypt, and the multiple dams and barrages

on the River Indus in Pakistan.

The Aswān High Dam, formally inaugurated in January 1971, holds back Lake Nasser and controls the flow of the River Nile in Egypt. It was built

to support a basin management scheme which had the aims of: controlling flooding in August and September; ensuring a reliable supply

of water for irrigation, thereby offsetting the effects of the severe droughts that periodically afflict Egypt and allowing crops to be grown year-round; increasing the area of cultivated land under irrigation; and

generating large amounts of hydroelectricity to support industrial development and improved living standards.

The Aswān High Dam

These aims have been achieved and, measured by its original technical-design goals, the scheme has been hugely successful. For example, the Aswān High

Dam has enabled agricultural production to be maintained through some of the most severe and prolonged droughts in Egypt’s history. However, there have been negative impacts too, many of them unforeseen. The many thousands of

Nubians who were displaced by the creation of Lake Nasser were provided with alternative land and housing which have proved unsuitable to their society and culture. Agricultural irrigation channels and the margins of Lake Nasser provide

perfect breeding grounds for disease vectors such as malaria-carrying mosquitoes and the snails which carry the bilharzia (schistosomiasis) parasite.In addition, flood prevention has stopped the annual deposition of fertile silt on the Nile flood plain, leading farmers to use more fertilizer. The reduction in silt has had other detrimental effects. As a result, the delta is eroding, and coastal

villages, farms, and valuable wetlands are being destroyed. Fisheries in the delta are being badly affected not only by the loss of silt, but also by the pollution of

the Nile by fertilizers, herbicides, and pesticides.

Drainage, removal of surface or subsurface water from a given area by natural or artificial means. The term is commonly applied to the removal of excess water by canals, drains, ditches, culverts, and other structures designed to collect and transport water either by gravity or by pumping. A drainage project may involve large-scale reclamation and protection of marshes, underwater lands, or lands

subject to frequent flooding. Such a project usually involves a system of drainage ditches and dykes; often pumps are required to raise the

water into the drainage network.In cases of large-scale drainage where improvement of outlet facilities is essential to the protection of adjacent property, it is

customary to improve natural stream channels to provide required discharge capacity and to excavate main and lateral drains as open ditches or canals to convey the effluent from farm drainage systems

to these improved channels. Such connecting drains commonly follow the natural surface drainage pattern of the area, intercepting

the normal surface run-off that takes place during periods of excessive rainfall.

Drainage – the removal of surface or subsurface water

Drainage CanalThe removal of excess water

by canals is used in large-scale land-reclamation

projects and in the protection of marshes,

underwater lands, or lands subject to frequent flooding.

Thorne, Colin Reginald. "Basin", Microsoft® Encarta® 2007 [CD]. Microsoft Corporation, 2006.

Microsoft ® Encarta ® 2007. © 1993-2006 Microsoft Corporation. All rights reserved.

BIBLIOGRAPHY