drainage basins The drainage basin is a unit that integrates the atmosphere, the geosphere, and the hydrosphere. The basin is that area technically delimited on the land (the surface of the geosphere) by the watershed, the imaginary line passing through the highest points that separate land draining to one river from land draining to adjacent rivers. The drainage basin thus delimited is the area that receives, collects, and concentrates water falling from the atmosphere, together with groundwater emerging from springs in the geosphere, to give river discharge (Fig. 1). The drainage basin is therefore a unit or system that is central to the hydrosphere.

The idea that the drainage basin is the unit of the Earth's surface that concentrates run-off to produce river flow is derived from the work of Perrault in the seventeenth century on the basin of the Seine in France. Perrault calculated that approximately one-sixth of the water or precipitation falling over the river basin of the Seine actually flowed down the river. It had previously been thought that rivers emerged on the land surface fed from underground sources; the relationship between the supply of flow from groundwater and rainfall over the basin was not appreciated. It was not until 1945 that the drainage basin was acknowledged to be the fundamental unit in hydrology.

The study of water movements in the hydrosphere is fundamental to hydrology. The water balance equation relates precipitation (P) to discharge or run-off (Q), evapotranspiration (ET), and changes in storage (S). The equation is given as P = Q + ET ± S.

The equation needs to be applied to a standard unit of land area on the surface of the geosphere, and the drainage basin provides that standard unit. In the study of geomorphology, the drainage basin is also acknowledged to be a fundamental unit, central to the way in which the land surface changes in those areas of the world influenced by running water.

In studying a drainage basin unit it is necessary to relate input from precipitation to output in the form of river discharge. The input of rainfall from a particular rainstorm is described by a ‘hyetograph’, which is the graph relating rainfall amount to time. The output of water as river discharge is described by a hydrograph. A hydrograph is simply a graph of the variation in the discharge (volume of water per unit time) over time. For a specific drainage basin it is therefore possible to demonstrate how a particular rain storm is translated into a particular hydrograph of river discharge: the drainage basin is the transfer function responsible for the conversion of rainfall into a particular type and amount of river discharge. The drainage basin also acts rather like a conveyor belt for material in solution (solutes) and for sediment, which is why it is a fundamental geomorphological unit. Water moving through the drainage basin can convey material in solution (as solutes), or mechanical particles in suspension (suspended sediment) or by being rolled or jumped (saltated) along the bed of a river as bedload. The volume of material moved as solutes, suspended sediment, or bedload can be totalled for each year and dividing this total volume by the area of the drainage basin gives an indication of the rate at which erosion is taking place, represented in terms of the depth of erosion from the drainage basin as a whole. In fact, as explained below, the sediment is derived from particular parts of the drainage basin rather than from the entire surface.

Input–output studies simply view the drainage basin as a uniform unit and relate the input from precipitation to the output as river discharge. It is necessary, however, to know how the processes within the drainage basin work and how precipitation is turned into river flow (Fig. 2). Originally it was thought that there were two major types of water flow throughout the drainage basin. First, surface run-off or overland flow is water that flows over the slopes of the drainage basin and is then concentrated into river channels. Secondly, water that infiltrates into the soil, then through the rock and down to the water table leads to discharge of groundwater from springs and seepages producing base flow. In 1945 the view of R. E. Horton, then a leading hydrologist in the United States, was thus that rivers are sustained by two types of river flow: ‘overland flow’ and ‘base flow’. In the 1960s it was realized that there are other types of flow that occur between that flowing over the surface and that derived from below the water table. These include ‘throughflow’, which is the lateral flow of water through the soil, and ‘interflow’ which is the lateral flow of water through the unsaturated zone above the water table. In addition there is ‘pipeflow’, the flow of water through small pipes in the soil, pipes which can vary from several centimetres to several metres in diameter. It is now appreciated that in any one drainage basin there is a range of types of routes that water can follow from the moment that it hits the surface of the drainage basin to the time when it reaches the river channel. It can move over the land surface as overland flow or it can infiltrate down to the water table to take decades or even centuries before it emerges as water from springs. In addition there are a number of other routes through the soil as matrix flow, throughflow, interflow, and pipeflow, as mentioned above. Many of these flow types can either be saturated, when the water table rises locally to higher levels, or unsaturated, when lateral flow is determined by the existence of relatively impermeable layers in the soil or in the unsaturated zone of the weathered bedrock.

In the same way that different types of water flow have been recognized, attention has been given to the sources of sediment and solutes. When precipitation hits the surface of the drainage basin it already contains solutes derived from the atmosphere and from vegetation; for example, water flowing over leaves, branches, or trunks of trees collects solutes which are dissolved in the rainwater by the time it reaches the surface of the drainage basin. As water passes over the surface or along the various routes mentioned above, then more solutes are obtained. It is the water flowing over the land surface, through pipes or in open channels, that obtains sediment derived either from the channel banks or from the river bed, and then transports it as suspended sediment. Bedload is the coarser material which is picked up only when the discharges are high and which can be rolled or bounced along the bed of the channel. The relevant significance of bedload, suspended load, and solute load varies significantly from one drainage basin to another according to the characteristics of the drainage basin, including its size.

It is now appreciated that drainage basins are dynamic in that the network of rivers and streams in the basin expands and contracts. This network is analogous to the circulation of blood in the human body, because the network of streams and rivers is equally vital to the dynamics of the basin. When the network of stream and river channels in the drainage basin is viewed in plan form it is possible to characterize the channels into one of three types (Fig. 1). First, there are those channels that carry water at all times throughout the year; these are described as perennial streams. Secondly, there are those channels that have water flowing in them seasonally (for example, during the wet season of a seasonal climate); these are described as intermittent channels. At the extremes of the drainage network of river channels there is a third type, which is described as ephemeral and consists of channels that have flowing water in them only during or immediately after rainstorms. The functions of river channels will also vary as the saturation level rises, so that along some valley floors saturation overland flow may occur when the local water table rises to the surface. It is therefore possible to envisage any drainage basin as possessing a network of channels that expands during and immediately after rainstorms and then contracts rather more slowly after a storm event has ended. There are, of course, considerable variations in the character and density of drainage networks from one area to another; the highest densities of channel that have ever been recorded, expressed as drainage densities in kilometres per square kilometre, have values of over one thousand on gullied landfill sites. Drainage densities in temperate landscapes tend to be between one and two kilometres per square kilometre (km km⁻²), whereas in tropical areas densities can rise to as much as 10 or 20kmkm⁻². It has been found that the density of perennial streams relates directly to the available moisture, which is defined as precipitation minus evapotranspiration. However, the density of the total network, including the perennial, intermittent, and ephemeral channels, reflects the relationship between the intensity of precipitation and the degree of resistance which that precipitation meets when it reaches the surface. The highest densities of ephemeral channels can therefore occur in desert or semi-desert areas where there are very occasional but intense rainstorms combined with a very small amount of vegetation cover to resist the formation of channels.

The relationship between the density of the drainage network, the precipitation input, and the vegetation cover is one example of the way in which the characteristics of a drainage basin control the translation of precipitation input to the output of water and sediment. A drainage basin has four major groups of characteristics: characteristics of rock type, of soil type, of vegetation and land use, and of topography. Each of these has an effect on the way in which the drainage basin acts as a transfer function; for example rocks that are permeable tend to have basins with higher proportions of groundwater flow than of surface run-off or quick flow. Similarly, soils that are permeable tend to facilitate subsurface flow including interflow and throughflow, rather than surface run-off. Vegetation and land use can also encourage infiltration where vegetation cover is dense and continuous. Topographic characteristics include four major categories: the size of the basin, the drainage network (classically expressed as drainage density mentioned above), the relief aspects of the basin, and the shape of the basin. Relief is significant because the greater the relief the higher the slope and the faster the movement of water through the basin. The shape of a drainage basin is significant because the more pear-shaped a basin is, the more efficient it becomes in concentrating precipitation into run-off. Various quantitative indices have, therefore, been invented to express the separate characteristics of the drainage basin to make it possible to construct simple equations relating the precipitation input, the drainage basin characteristics, and the output of discharge. In particular, if data can be obtained from at least ten or twenty different basins it is possible to produce equations giving values of different types of run-off, as the dependent variable, in terms of drainage basin characteristics, precipitation, and other climatic characteristics as independent variables. There are various ways in which discharge can be expressed for such statistical (often multiple-regression) models. It is therefore possible for discharge (as described in the form of a hydrograph) from a single basin to be expressed in terms of instantaneous values of discharge, average values for, say, a period of a year, or to be expressed by an index value that reflects either high or low values of discharge with a particular frequency. A particularly important index in relation to the management of drainage basins is the use of the flood-frequency analysis, which establishes the flood value with a particular recurrence interval for a specified basin.

The development of measures of drainage-basin characteristics for a particular basin might imply that characteristics within the basin are uniform, which is not the case. Along the course of a river channel there are considerable variations in the character of the channel, and a whole range of types of river channel can be established according to the clarity of definition of the channel. A way of characterizing the clarity of channel definition is by its roughness. Various equations developed by hydraulic engineers link the velocity of water flow to the characteristics of the river channel cross-section, usually in terms of the hydraulic radius, R, the cross-sectional area divided by the perimeter, the slope of the channel, s, and the estimated roughness, n. One such equation, proposed by the engineer Robert Manning, has the form: Such flow equations enable estimates of velocity, and therefore of discharge, to be made (by multiplying velocity by cross-sectional area) for locations along rivers for which there are no continuous discharge records. A particular way in which river channels vary in both morphology and process is in the river-channel pattern, which is the pattern of the river channel as seen from the air. Two major types of river-channel pattern have been distinguished. ‘Single-thread’ channels include all unitary river channels that are straight or meandering; ‘multi-thread’ channels are composed of several channels and include braided channels and anastomosing channels.

Using the knowledge of the mechanics of the drainage basin it has been possible to build quantitative models of drainage-basin processes for particular planning purposes such as flood prediction. Models were originally thought of as ‘back-box’ models, whereby links were established between input and output without adequate knowledge of what actually occurred within the drainage basin. It is now more usual to resort to what are described as ‘grey-box’ models, which to some extent reflect a knowledge and understanding of the processes operating in the drainage basin that are responsible for the conversion of precipitation into river flow. It will be some time before we can move to a ‘white-box’ model that is based on a full and complete understanding of all the processes operating within the basin. Although the processes are known and understood in terms of the way in which they operate in different parts of the basin, it is very difficult to link them all together in a quantitative model.

The development of an effective quantitative computer model of the operation of a particular basin is obviously a major way in which management of drainage basins can be informed. The management of drainage basins can focus upon the incidence of flooding, the provision of water supply, the use of the basin for effluent disposal and industrial purposes, the use of the basin for recreational and leisure purposes, and the control of erosion in relation to land-use management and river-channel management. In the case of erosion control, developments occurred in a number of basins in the United States in response to erosion problems: the work of the Tennessee Valley Authority in the 1930s was very prominent in this regard. A number of slogans were developed, including ‘Erosion begins at the top of the hill’ and ‘Stop the little raindrops where they fall’, to press home the message that erosion control in part of the basin was necessary to prevent large-scale erosion taking place elsewhere, perhaps with severe consequences such as the removal of topsoil and extensive gullying, which could not then be reversed. Other ways in which basin management has been applied relate to flood prediction and flood control. Good models of the ways in which floods are generated in a basin make it possible for advance warning to be given of the likelihood of floods occurring and to predict how floods may be generated as flow passes downstream. Flood models and flood-warning systems are in operation in major river basins such as the Mississippi in the United States and the Murrumbidgee in Australia. Four basic strategies are available for flood-prevention measures: doing nothing; providing relief measures, such as tax and financial benefits; introducing land-use zoning (in which the most vulnerable and expensive land uses are kept furthest away from the river); and undertaking structural solutions, which entail either channelization of the river or the institution of small or large dams for flood control. Water supply in drainage basins is provided either by direct abstraction from rivers or by impoundment, which requires the construction of reservoirs. After water has been used for domestic and industrial purposes, it is inevitably returned to the river. To avoid pollution, there must therefore be controls on the condition of the water that is returned. Pollution was certainly a feature of major European rivers before legislation was introduced in the 1960s to regulate the quality of the water returned to them.

It is clear that there are many potential uses for the water in a drainage-basin system. Leisure use of waterways and river systems, for example, includes water sports, bathing, and access to rivers and streams; various uses related to agriculture and irrigation require specialized types of water supply. All these uses can affect the hydrological cycle and the way in which the drainage basin operates, particularly the amount of, and the rate at which, water and sediment are conveyed through the drainage basin. It has therefore been necessary to devise ways in which the management of the drainage basin reflects the fact that all these components are interrelated: integrated basin management was developed as a way of signifying the need to manage the basin with an awareness of the impact of the range of activities operating within a specific basin. As a number of approaches to integrated basin management have been devised, they have increasingly tended to reflect the fact that the activities that are dominant and produce problems in one basin are not those that are dominant in another, and also the fact that particular disciplines tend to be associated with their own particular emphasis in drainage-basin management. Thus, an engineering approach may tend to focus on a structural solution to river problems, and an ecological approach may emphasize the impact upon the river ecology, whereas a geomorphological approach may reflect an awareness of the impact upon the flood plains and on the drainage system as a whole. In the light of the differences from one basin to another and from one disciplineto another, there have now been many calls for a holistic approach that seeks to acknowledge all the ways in which the drainage basin and its processes are affected by human activity and proposing a method of management that is most similar to natural processes and involves the least amount of disturbance. Whereas initial approaches to a particular problem within the drainage basin were perhaps driven by the idea that ‘technology can fix it’, it has subsequently transpired that such technological fixes could have significant implications for other parts of the drainage basin; for example, immediately downstream. It is therefore being realized that it is better to work with the river rather then against it and wherever possible to imitate nature rather than to superimpose a system unlike that which occurs naturally in the drainage basin. It is now generally accepted that a knowledge of the drainage basin and how it works is important, not only to understand this aspect of the operation of nature, but also to underpin ways in which management of drainage basins is constructed.

Kenneth J. Gregory

Bibliography

Downs, P. W.,, Gregory, K. J.,, and and Brookes, A. (1991) How integrated is river basin management? Environment Management, 15, 299–309.
Gregory, K. J. and and Walling, D. E. (1973) Drainage basin form and process. Edward Arnold, London.
Newson, M. (1992) Land, water and development. Routledge, London.
Walling, D. E. (1987) Hydrological processes. In Clark M. J., Gregory K. J., and Gurnell A. M. (eds) Horizons in physical geography. Macmillan, London.

Drainage basins and drainage patterns

A drainage basin is the area that encompasses all the land from which water flows into a particular stream or river. Stream is a synonym of river, and although typically something called a stream is smaller than a river, here, any flowing body of water in a clearly defined channel will be called a stream. The size of a drainage basin can vary from being as small as a few square miles or kilometers to as large as part of a continent. An example of a divide is the continental divide of North America , which separates streams that ultimately empty into one ocean (the Pacific Ocean) from those that ultimately empty into another (the Gulf of Mexico ). The smallest streams in any particular area are called first order streams, and the land from which water flows into a particular first order stream is called a first order drainage basin. First order streams flow into second order streams, and each second order stream has its own second order drainage basin. There is no limit to how high an order a stream may be.

The drainage pattern that streams in a drainage basin trace out, visible in aerial photographs or even from the window of an airliner, can provide a lot of information about the type of terrain that the streams flow over. The dendritic drainage pattern of streams resembles the veins of a leaf, or the structure of a tree. It typically develops in areas with homogenous or flat-lying rocks that provide no preferred direction to the development of stream channels. Streams that flow over the flat-lying rock units of the American Midwest often display this type of drainage pattern. An annular drainage pattern forms when layers of rock are uplifted into a dome or down-warped into a basin, and the stream channels preferentially follow the weakest concentric beds of rock. A radial drainage pattern develops where there is a central highpoint, such as an isolated volcanic peak. The streams all flow away from the highest point. Fractures in massive rock such as granite can produce a drainage pattern in which the streams have many right-angle turns, and this is called rectangular drainage. When layered rock units are folded or tilted up, lower-order streams that flow into larger streams tend to be straight and follow weaker beds of rock. This trellis drainage pattern is common in the Appalachian Mountains of the United States. Centripetal drainage is found where streams flow into the center of a depression such as a basin or crater. Deranged drainage forms on terrain that is freshly exposed, and where the streams have not had a chance to develop in response to underlying geologic structure or bedrock . Finally, parallel drainage tends to develop in areas of massive rock with a uniform slope, where all the streams tend to flow in the same direction.

See also Avalanche; Delta; Drainage calculations and engineering; Hydrogeology; Runoff

drainage basin shape index A measure of the shape of a drainage basin (catchment), normally expressed as the ratio between two dimensions of the basin being considered. One such measure is the circularity index (or ratio), C, expressed as C = A_b/A_c, where A_b is the area of the basin and A_c is the area of a circle with the same length of perimeter as the basin. Another index is the form factor, F, expressed as F = A/L, where A is the area of the basin and L is its length. Such indices may help in forecasting the flood potential of a basin.

drainage basin shape index A measure of the shape of a drainage basin, normally expressed as the ratio between two dimensions of the basin being considered. One such measure is the circularity index (or ratio), C, expressed as C = A_b/A_c, where A_b is the area of the basin and A_c is the area of a circle with the same length of perimeter as the basin. Another index is the form factor, F, expressed as F = A/L², where A is the area of the basin and L is its length. Such indices may help in forecasting the flood potential of a basin.

drainage basin morphometry The measurement of the characteristics of the surface form of a drainage basin (catchment), and of the arrangement and organization of the associated river network. Properties such as area, shape, gradient, and relief are important elements of form (see drainage basin shape index and drainage basin relief ratio), while the stream network is investigated through a study of its components and of the ways in which they are related. See drainage network analysis.

drainage basin morphometry The measurement of the surface form of a drainage basin, and of the arrangement and organization of the associated river network. Properties such as area, shape, gradient, and relief are important elements of form (see DRAINAGE BASIN SHAPE INDEX; and DRAINAGE BASIN RELIEF RATIO), while the stream network is investigated through a study of its components and of the ways in which they are related. See DRAINAGE NETWORK ANALYSIS.

drainage basin relief ratio An index (Rh) of the relief characteristics of a drainage basin. It is expressed as Rh = H/L, where H is the difference in height between the highest and lowest points in the basin and L is the horizontal distance along the longest dimension of the basin parallel to the main stream line. The ratio can provide a measure of the rate of sediment loss from a basin, with which it tends to be positively correlated.

drainage basin relief ratio An index (Rh) of the relief characteristics of a drainage basin. It is expressed as Rh = H/L, where H is the difference in height between the highest and lowest points in the basin and L is the horizontal distance along the longest dimension of the basin parallel to the main stream line. The ratio can be positively correlated with the rate of sediment loss from a basin.

drainage basin see catchment area .

basins, drainage see drainage basins

drainage basin See catchment.