Drainage Basins and Rivers

What Is A Drainage Basin?

An area of land drained by a river and its tributaries. Its boundary, known as the watershed, is s ridge of high land, beyond which precipitation will drain into adjacent basins.

It can be described as an open system, forming part of the water cycle, if it is viewed as a system, its characteristics are:-

The System

Inputs - being precipitation

Outputs - being where the system looses water. By either

The river carrying the water out to sea,

Through evapotranspiration.

Within the system, some water:-

Is stored in lakes and/or the soil

passes through a series of transfers or flows. Eg infiltration, percolation, throughflow.

Elements Of The Drainage Basin System

*See Waugh p58 fig 3.1

Key:Inputs, Storage, Transfers, Outputs

Precipitation

Forms the major input into the system.

Amounts vary over time and space.

Greater the intensity of the storm, the shorter its duration.

e.g. Convectional thunderstorms are short, heavy, and maybe confined to small areas,

but the passing of a warm front of a depression will give a longer period of more steady rainfall extending all over the basin.

Evapotranspiration

Two Components:

Evaporation - Physical process by which moisture is lost directly into the atmosphere from water surfaces (including vegetation and the soil,) due to the effects of air movement and the sun’s heat.

Rates affected by - temperature, wind speed, humidity, hours of sunshine, and other climatic factors.

Transpiration - Biological process by which water is lost through stomata in       leaves.

Rates affected by - time of year, type and amount of vegetation, availability of moisture, and length of the growing season.

It is possible to distinguish between potential and actual evapotranspiration of an area.

E.g. in deserts there is a high potential evapotranspiration, because the amount of water which could be lost is greater than the amount of water actually available.

+ In Britain, the amount of water available for evapotranspiration nearly always exceeds the amount which actually takes place.

So evapotranspiration is limited by the availability of water in the soil.

Interception

interception storage - when the first raindrops of a rainfall event fall on vegetation which shelters the underlying ground. (It is greater in a woodland area or where tree crops are grown than on grass or arable land.)

If the precipitation is light and of short duration much of the water may never reach the ground, and may be quickly lost from the system by evaporation.

It is estimated that in a woodland area, up to 30% of the precipitation may be lost through interception, which helps to explain the limited soil erosion in forests.

Newson 1975 “Interception is a dynamic process of filling and emptying a shallow store (about 2mm in UK trees.) The emptying occurs because evaporation is very efficient for small rain drops held on tree surfaces.”

In an area of deciduous trees, interception and evaporation rates are higher in summer, although the 2 processes do not occur at the same time.

If rainfall carries on then water reaches the ground in 1 of 3 ways:-

Throughfall - where water drops off the leaves

Stemflow - where water flows down the trunk.

by undergoing Secondary Interception - by undergrowth.

Surface Storage

After a warm, dry spell in summer the ground may be hard. So at the start of a rainfall event water will lie on the surface until the upper layers because moistened sufficiently to allow the water to soak downwards.

Surface Run Off / Overland Flow

If the ground is hard, and precipitation is heavy to begin with, or if the soil has become saturated then excess water will flow away over the surface.

In most environments this is relatively rare, except during exceptionally heavy storms or urban areas, which have impermeable coverings of tarmac and concrete.

Infiltration

Soil will gradually admit water from the surface, if the supply rates are moderate, allowing the water to infiltrate downwards vertically through pores in the soil.

Infiltration Capacity - is the maximum rate at which water can pass through the soil, and is measured in mm/hr.

The amount of infiltration depends on:-

antecedent precipitation. (precipitation which occurred in the past.) - the amount of water already in the soil

the porosity

structure of the soil

nature of the soil surface (e.g. crusted, cracked, ploughed)

type, amount, and seasonal changes in vegetation cover.

Some water will flow laterally - as throughflow.

During dry periods some water may be drawn up towards the surface by capillary action.

Percolation

The slowing of the downward progress of water as it reaches underling soil or rock layers (which tend to be more compact.)

It is a constant movement, and causes groundwater storage.

Zone of saturation - when water collects above an impermeable rock layer, or fills all pore spaces.

Water Table - the upper boundary of the saturated material, i.e. the upper surface of the groundwater layer.

Water may then be transferred laterally as groundwater flow or baseflow.

In areas of Carboniferous limestone, groundwater levels are usually slow to respond to surface storms, or short periods of drought.

During a lengthy dry period, some of the groundwater store will be used as river levels fall.

In a wetter period, groundwater must be replaced before the water level can rise appreciably.

If the water table reaches the surface, it means the ground is saturated ;

excess water will then form:-

a marsh on flat land,

or become surface run off on sloping land.

Channel Flow

Although some water falls directly into a river channel - Channel Precipitation, most water reaches the channel by a combination of 3 transfer processes:-

Surface run-off (overland flow)

Throughflow

Groundwater flow (baseflow)

Once the water is in the river as channel storage, water flows towards the sea and is lost from the drainage basin system.