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This articleis to introducethe conceptsof aridity and droughf and also to dispelsome misconceptionsabout these
phenomena.lt needsto be stressedthat thereis a hugedifferencebetweendrought andaridity. Namibiais an arid to
yearandeverywhere.
droughtevery
semi-aridcountry,butthisdoesnotmeanthatweexperience
Aridity is a permanent feature of the climateof a certainarea.An arid climateis one that is permanentlydry. In
Namibiaa largeareaof the countrycan be describedas arid.Aridityis thusa characteristic
of the climateof a region-there is thus generallya lack of rainfalland the vegetationand indigenousfauna plus farmingenterpriseshave (or
shouldhave)adaptedto this naturalaridity.Therehave been manyattemptsat climateclassification
in the pastand
therewill probablybe many more in future.One classicexampleof climateclassification
was done by Koppen,who
dividedtheworld'sclimateintofivemaingroups,namely:
A Tropical,rainyclimateswithno coolseason
D Humid,microthermic
climates
B Dry(arid)climates
E Polarclimates
C Humid,mesothermic
climates
The B climatesareseparatedfromthe restas dryor aridclimates.Thecriteriaforthesearea combination
of rainfalland
temperature.
For winterrainfallareasthe criterionis p < 2t, wherep is the meanannualrainfallin cm and t the mean
annualtemperature
in 'C. Forsummerrainfallareasthe criterionis p < 2(t + 14).The B climatesare furthersubdivided
intoBW (desert)andBS (steppe).The cut-offvaluesbetweenBWandBS are p < tforwinterrainfallareas
and p < t t 14
for summerrainfallareas.Thewholeof Namibiafallswithinthe BW and BS climatezones,so thatit is eitherclassified
as desertor steppeclimate.
This is just one of the manyways of classifying
climate;the pointbeingthat aridity is a permanentfeatureof the
climateand the environmenthas adaptedto this dry climate.
The averagerainfallper annumfor GellapOst ResearchStationnear Keetmanshoop
is '150.8mm. A below
averageyear(left)and a wet year(right)with 129,8mm duringthe 1997/8and 318.9mm duringthe 1999/2000
rainyseasonsrespectively,
are indicated.
Drought,on the otherhand,is notan everydayoccurrence,
butan abnormalsituationcausedby a protractedperiod
is normal,droughtdoes not occur,even thoughthis
of deficient precipitation.This meansthat when precipitation
normalprecipitation
maybeverylittle,suchas in the NamibDesert.
r,.."-i
Drought is a temporary aberrationand differsfrom aridity,sinceit can occurin any climatezone,whilearidityis a
permanentfeatureof the climate.Droughtis a non-eventinsofaras it is causedby the absenceof somethingand not
which
tsunamisand volcanoes,
hailstorms,
earthquakes,
likeothernaturaldisasterssuchas tornadoes,hurricanes,
each havea definiteeventwith a startand finish.Droughtis insidiousand may startwiththe firstdry day aftera wet
spellofwetweather.
endwithgoodrains,butonlyafteraprotracted
spellandmay notnecessarily
into
drought(thatcanbe subdivided
drought,agricultural
Therearealsodifferenttypesof drought,likemeteorological
it originates
droughtand others.Althoughit has a scoreof definitions,
cropdrought,pasturedrought),hydrological
from a deficiency of precipitationoveran extendedperiod,usuallya seasonor more.This deficiencyresultsin a
water shortagefor some activity,group or environmentalsector.
precipitation
and
Droughtshouldbe consideredrelatlveto some longterm averageconditionof balancebetween
"normal".lt is alsorelated
+ transpiration)
in particular,
a conditionoftenperceivedas
(evaporation
evapotranspiration
to the timing(principalseasonsof occurrence;delaysin the startof the rainyseason; occurrenceof rainsin relation
of the rains(rainfallintensity; numbersof rainfallevents).
to principalplantgrowthstages)and the effectiveness
with it and
high
wind
and low relativehumidityare oftenassociated
Otherclimaticfactorssuchas hightemperature,
its
severity.
aggravate
cansignificantly
By virtue of these facts, it has been difficultto define and characterizedrought.Worldwide,researchersare
indexesor
of differenttypesof droughtintoclear,comprehensible
and quantification
the classification
investigating
classes.
:
.
i
j=+
The averagerainfallper annumfor OmatjenneResearchStationnearOtjiwarongois 425.4mm. A dry year (left)
rainyseasons
and wet year (right)with 189.0mm duringthe 1998/99and 827.6 mm duringthe 1975176
are indicated.
respectively,
To illustratethe differencebetweendroughtand aridity,the followinggraph has beendrawnfor an averageyear,the
driest El Nifroyear and the wettestLa Nifrayear,for two stationswith widelydifferingrainfallaverages.Bethanieis
definitelymore arid than Katima Mulilo,which is one of the wetteststationsin Namibia.Both can, however,
experiencedry years(drought) orwet years.
AVERAGE,WET ANDDRY SEASOAIS
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Author: Dr AL du Pisani,AEZ Programme,Directorateof Researchand Training,MAWRD,Tel 09264-61-2087062.
Editor: A van Niekerk,PrivateBag 13184,DART,Windhoek,Namibia,[email protected]
€oital ore$ 02-1820. Windh@k . Namibia