DRAINAGE BASINS respondin a variety of ways to the

inputs of moisture under differentconditions of climate, and typesof soil, geology and vegetation.The sources of moisture thateventually create the flow ofwater in a river channel may varydepending on the time of theyear. Also, there are factors thatcan cause water to get into riverchannels at different speeds, andsome of these are dependentupon climate. All river basinsmust have inputs, transfers andoutputs of moisture (see Figure1), and it is not always obviouswhere the water that you see in ariver channel comes from.

Water and channel flowStrange as it may seem, very littleof the moisture – known asprecipitation – that falls from thesky actually lands directly in ariver channel. Moisture fallingdirectly into the channel is knownas meteorologic moisture (as in‘meteorology’ – the study ofweather), and in Britain this ismost often in the form of rain,but may also be snow, hail, sleetor drizzle. No matter how heavythis precipitation is, it is veryunlikely to cause a river to flood –you only have to think of hownarrow a river channel is andhow much rain does not falldirectly into the channel, to workout that much precipitation mustget into the channel by othermeans.

Precipitation falling towards theground may not get that far! It isvery likely that it will beintercepted – it will land on plantsand may not reach the ground atall, as it may evaporate from the

surface of the vegetation. If waterdoes land on the ground, it rarelystays where it is. Some of itevaporates, and so is lost back tothe atmosphere. It is also unusualfor water to flow over the surfaceof the earth for any greatdistance. Most precipitation soaksinto the ground – a processknown as infiltration. Once in theground, water percolates (runsdown) through the soil and theunderlying rock. The speed ofinfiltration depends upon manyfactors, including the size of theparticles in the soil. Smallparticles leave small gaps (or porespaces) and so let the water inslowly, while large particles makelarge gaps and allow fastinfiltration. Once the water haspercolated down through the soil,it may be taken up by plants andlost through evapotranspiration.Water in the soil and underlyingrock flows down slopes in aprocess known as throughflow.This is the major source of waterfor rivers in a climate like that ofthe British Isles. However, eventhis water supply is likely to dryup at the end of a long drysummer, and yet the rivers stillkeep flowing – although theirlevels are low. This is becausesome water saturates theunderlying rocks to a great depth.This is known as groundwater,and it provides baseflow – thewater that keeps rivers flowingeven during a drought.

In summary:

• River basins act as systems, withinputs, transfers and outputs.

• Little of the precipitation falling ina river basin lands directly in ariver channel.

• Channel flow is mainly suppliedby water that has passed throughthe soil and rocks around or underthe river channel and in the basin.

How rivers respond toprecipitation inputsThe rate at which a river flowsis known as its discharge, andthis is usually measured in cubicmetres per second (or cumecs).A river’s discharge does notusually increase as soon as itrains. Following a rainstorm,the discharge increases after awhile, and then returns tonormal once the extra water hasflowed away. Several factors candelay the speed at which a riverresponds to a period of rainfallor precipitation. However, theactual speed of response to therainfall can depend on a numberof factors, for example:

• how much interception there is

• the state of the ground (wetness)

• the rate of infiltration

• the speed of throughflow.

Several other factors can alsoinfluence the speed of a river’sresponse.The delay in the time between the

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234

by Steve RichardsonDRAINAGE BASINS IN DIFFERENTCLIMATES

THROUGHFLOW DISCHARGERAIN

Figure 1: Flow diagram showing input into, transfer through and output

from a river basin

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heaviest rainfall and themaximum, or peak, river flow(discharge) is known as the lagtime. This may be quite lengthy inareas where the soil is dry but isable to accept large amounts ofwater by infiltration. However, ifthe surface is impermeable, therewill be no infiltration and thewater will quickly make its way tothe channel by overland flow, andthere will be a rapid rise indischarge. The response ofdischarge to rainfall can be shownon a type of diagram or graphcalled a hydrograph (Figure 2).

If water takes a long time to getinto the channel, then there is along lag time. Similarly, if there isa large amount of water in thebasin, then it will take some timefor the river to return to itsformer level. The length of timewhen a river remains at peakdischarge may also be extendedby repeated falls of rain.

How rivers respond todifferent climatesRivers respond to inputs ofprecipitation, which is an elementof climate. However, rainfall andtemperature combine to meet theneeds of vegetation, andvegetation – as we have seen – canintercept precipitation. So thewetter a climate, the more likely itis that there will be a lot ofinterception. This will have animpact on the shape of that river’shydrograph, and on the pattern of

flow throughout the year.

Case StudiesIn the following case studies welook at the impacts of climate ontwo contrasting drainage basins –one in the Nevada Desert, theother in the south of England.

Carson Desert basin,Nevada, USAThe Carson Desert basin is highin the mountains of western USA(Figure 3). It has an area of 5,610square kilometres and a perimeterof 412 kilometres. The gaugingstation is 1,269 metres above sea

level, and the climate is markedlydifferent from that of southernEngland. As may be seen from thedetails in Figure 4, the rainfalltotals for nearby Las Vegas arelow. Typically such a desert areawould be able to support littlevegetation cover naturally,although irrigation is used tomake the desert bloom. So thereis a shortage of interception, andalso very little rainfall on average.Deserts are peculiar places – thesmall average rainfall totals shouldnot hide the fact that manymonths in a typical year in LasVegas are totally dry, and somehave many dry days, but when itdoes rain several tens ofcentimetres may fall inspectacular thunderstorms. It isalso possible that at such analtitude, in winter severalcentimetres of snow mayaccumulate and this may bereleased by melting to swell therivers. Extensive dry periods arelikely, and irrigation is necessaryfor agriculture to occur at all.Figure 4 shows climate data forLas Vegas, close to the CarsonDesert basin, and Figure 5presents data on discharge for theCarson Desert basin.

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Figure 2: A flood hydrograph

Figure 3: Carson Desert basin in Nevada, USA

Month J F M A M J J A S O N DMm 18 13 8 8 5 5 13 13 8 8 5 10°C 7 10 13 17 21 25 29 29 24 18 12 7

Figure 4: Climate details for Las Vegas, Nevada

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Stour basin, Dorset, UKThis is a very different kind ofbasin. The Dorset Stour has abasin area of some 1,300 squarekilometres (see Figure 8 on page4). The south of England istypical of a mild maritime climate.There are no dry months in theyear, but there is a differencebetween summer and winter interms of temperature,precipitation and vegetationcover. It is seldom too cold forsnow to last long, and thesummers are mild, sometimeswarm, but never too hot (Figure6). However, droughts do occur,and in the recent past some ofthese have been extensive – therehave been periods of as long asthree years when rainfall has beenbelow average in the basin of theStour, for example in the late1980s, and again in the mid-1990s. The lower part of theStour basin is on the coast, andthe land surrounding the Stourbasin does not in any way matchthe elevation of the Carson Desert– amounts of precipitation are notsignificantly increased by theeffects of altitude.

The Stour is a typical British river.It floods from time to time, butwhen it does, there is usuallyplenty of warning that it is goingto do so. In any case there wouldhave to be an extended period ofvery wet weather in order tomake the river flood.

The evenly distributed rainfall inthe Stour basin ensures that undernormal conditions there is nomonth in the year when the riverdries up (Figure 7). Similarly, thepresence of vegetation cover forat least half of the year means thatmuch moisture is intercepted andso rapid rises in discharge areunusual. The winter maximum ofrainfall ensures that winterdischarge is high, and the lack ofvegetation cover in the fields andloss of leaves at that time meansthat there is more likely to be aflood in the winter. Sometimeswinter months are dry – Figure 6was drawn from average figurestaken over a number of years –and the flow of the Stour mayvary from the pattern shown inFigure 7. Summer monthsgenerally have lower rainfall,although sometimes

thunderstorms may beaccompanied by heavy rain. Thisseldom makes any impact on theflow of the Stour because much ofthe basin lies over chalk, which isporous (water flows rapidlythrough it). Also, the fields in thisrural area are filled with growingand ripening crops in the summer,which intercept the rainfall.

Month J F M A M J J A S O N D

Cumecs 0.73 0.45 0.11 0.05 0.06 0.03 0.08 0.07 0.06 0.08 5.44 0.04

Figure 5: Mean monthly discharge, Carson Desert basin, 1997Source: US Geological Survey

°C

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Figure 6: Southampton’s climate

3028262422201816141210

86420 J F M A M J J A S O N D

Cumecs

Figure 7: Mean monthly flow in the Stour basin, 1997Source: Environment Agency

1 Use the terms ‘Inputs’,‘Transfers’ and ‘Outputs’ asheadings for columns in a table.List as many of these as you canfor moisture in a river basin. Giveyour table the title: ‘Inputs,transfers and outputs in adrainage basin’. This unit willhelp you (see Figure 1, forexample), but you should alsorefer to a geography textbook.

2 Make a large copy of Figure 2,and complete your copy. Checkthe definitions of any of the termsyou write on your graph, andmake a list of these. Shade in thearea that represents the extrarunoff during a flood period.

3 Why may hydrographs for thesame river vary from one seasonto another? Try to account forchanges that are the result ofclimate and vegetation cover.

4 (a) On an outline map of theUSA, mark the location of Nevada.(b) How far is the westernmostpart of Nevada from the ocean?Indicate this on your map.(c) Give your map a title.(d) In general terms, how doesNevada’s location differ from thatof the Stour basin in Dorset?

5 (a) Using the data for Las Vegas(Figure 4), plot a climate graphsimilar to the one forSouthampton (Figure 6), showingbars for rainfall and a line fortemperature. Make sure that yougive your graph a title, and labelthe axes. (b) Describe your graph –remember to quote figures.(c) How does it differ from thatshowing the climate ofSouthampton? Quote figures!

6 (a) Using the data in Figure 5,draw a labelled graph to show themean monthly flow of the CarsonDesert basin in 1997. Give yourgraph a title.

(b) How does this graph differfrom the one for the River Stour(Figure 7)? Quote figures!

7 Using a copy of Figure 8, andother information in this unit,prepare a display explaining whythe pattern of flow in the RiverStour is so different from thepattern of flow in the CarsonDesert basin. You may have toguess at some parts of the answer,but there is enough material inthis unit for you to make a broadassessment of why the two riversflow as they do.

8 In groups, put yourself in therole of the water authorityresponsible for managing theRiver Stour drainage basin. Whatstrategies would you introduce to:(a) reduce the risk of flooding inwinter(b) reduce the risk of watershortage in the summer?

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Activities

Figure 8: The River Stour basin

0 10 km

Extent of drainage basin

Dorset

Poole

Bournemouth

ThroopChristchurch