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Ministryof Agriculture,Water and Rural Development. Directorateof AgriculturalResearch and Training. PrivateBag 13184 . Windhoek .Illlay!002..r ffiffiffiffi..PffiffiW. "wltrffitrsiffiffi WK.
This articleis to introducethe conceptsof aridity and droughf and also to dispelsome misconceptionsabout these phenomena.lt needsto be stressedthat there is a hugedifference between drought andaridity. Namibiais an aridto semi-aridcountry, butthis does not mean thatwe experience droughtevery yearand everywhere.
Aridity is a permanentfeature of the climateof a certainarea. An arid climateis one that is permanentlydry. In Namibiaa largearea of the countrycan be describedas arid.Aridity is thusa characteristicof the climateof a region-- there is thus generallya lackof rainfalland the vegetationand indigenousfauna plus farming enterprises have (or shouldhave) adapted to this naturalaridity. There have been many attempts at climateclassification in the pastand therewill probablybe manymore in future.One classicexample of climateclassification was done by Koppen,who dividedtheworld's climate intofive main groups, namely: A Tropical,rainy climates with no cool season D Humid,microthermic climates B Dry(arid)climates E Polarclimates C Humid,mesothermic climates
TheB climatesare separated from the rest as dryor aridclimates. The criteria for these are a combinationof rainfalland temperature.For winter rainfall areas the criterionis p < 2t,where p is the meanannual rainfall in cm andt the mean annualtemperature in 'C. Forsummer rainfall areas the criterion is p < 2(t+ 14).The B climatesare further subdivided intoBW (desert)andBS (steppe).The cut-off values between BWand BS are p < tforwinterrainfallareas and p < t t 14 for summerrainfall areas. The whole of Namibiafalls within the BW and BS climatezones, so thatit is eitherclassified asdesert or steppeclimate.
This is just one of the manyways of classifyingclimate; the pointbeing that aridity is a permanentfeature of the climateand the environmenthas adaptedto thisdry climate.
The averagerainfall per annumfor GellapOst ResearchStation near Keetmanshoop is '150.8mm. A below averageyear (left) and a wetyear (right) with 129,8 mm duringthe 1997/8and 318.9 mm duringthe 1999/2000 rainyseasons respectively, are indicated.
Drought,on the otherhand, is notan everydayoccurrence, but an abnormalsituation caused by a protractedperiod of deficient precipitation.This meansthat when precipitationis normal,drought does not occur,even though this normalprecipitation may be very little, such as inthe Namib Desert. r,.."-i Drought is a temporaryaberration and differsfrom aridity, since it canoccur in anyclimate zone, while aridity is a permanentfeature of the climate.Drought is a non-eventinsofar as it is causedby the absenceof somethingand not likeother natural disasters such as tornadoes,hurricanes, hailstorms, earthquakes, tsunamis and volcanoes, which eachhave a definiteevent with a startand finish. Drought is insidiousand maystart with the firstdry day aftera wet spellandmay not necessarily end with good rains, butonly aftera protracted spellof wetweather.
Thereare also different types of drought,like meteorological drought, agricultural drought (that can be subdivided into cropdrought, pasture drought), hydrological drought and others.Although it has a scoreof definitions,it originates froma deficiencyof precipitationover an extendedperiod, usually a seasonor more.This deficiency results in a watershortage for some activity,group or environmentalsector.
relatlveto some longtermaverage condition of balancebetween precipitation and Droughtshould be considered "normal". evapotranspiration(evaporation +transpiration) in particular,a condition often perceived as lt isalso related to thetiming (principal seasons of occurrence;delays in the startof the rainyseason; occurrenceof rainsin relation to principalplant growth stages) and the effectivenessof the rains(rainfall intensity; numbers of rainfallevents). Otherclimatic factors such as hightemperature, high wind and low relativehumidity are often associated with it and cansignificantly aggravate its severity.
By virtue of these facts, it has been difficultto define and characterizedrought. Worldwide, researchers are investigatingthe classificationand quantificationof differenttypes of droughtinto clear, comprehensible indexes or classes. : .i j=+
The averagerainfall per annumfor OmatjenneResearch Station near Otjiwarongo is 425.4mm. A dry year (left) and wet year (right)with 189.0mm duringthe 1998/99and 827.6 mm duringthe 1975176rainy seasons respectively,are indicated.
To illustratethe differencebetween drought and aridity,the followinggraph has beendrawn for an averageyear, the driest El Nifroyear and the wettestLa Nifrayear, for two stationswith widelydiffering rainfall averages. Bethanie is definitelymore arid than Katima Mulilo,which is one of the wetteststations in Namibia.Both can, however, experiencedry years(drought) orwet years. AVERAGE,WET ANDDRY SEASOAIS
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Author: Dr AL du Pisani,AEZ Programme,Directorate of Researchand Training,MAWRD, Tel 09264-61-2087062. Editor: A van Niekerk,Private Bag 13184,DART, Windhoek, Namibia, [email protected]
€oital ore$ 02-1820. Windh@k . Namibia