OF NAMIBIANRAINFALL
THE IDIOSYNCRASIES
A L DU P/SAN/
Namibia
Private
Bag13184,
Windhoek,
WaterandRuralDevelopment,
Ministry
ofAgriculture,
AEZProgramme,
INTRODUCTION
It is probablysafeto say thatthe rainfallof any countryor region
as its own idiosyncrasies.lt is necessarythat all persons
of the character
involvedin agriculture
shouldtakecognizance
so that their actionsare in
of rainfalland its idiosyncrasies
harmonywith these characteristics.Duringthe past year the
(MAWRD)
WaterandRuralDevelopment
MinistryofAgriculture,
in Namibia
has commissioned
a studyof the rainfalldistribution
and the resultsof this study has broughtforwardmany of the
of Namibianrainfall.Thisstudyhasalsoadded
characteristics
greatlyto the databaseof rainfallbeingkeptwithinthe AgroEcologicalZoning (AEZ) programme.This paper looks at
variousinterestingaspectsof annualand monthlyrainfallin
Namibia.
ANNUAL RAINFALL
Namibiais an aridcountry,probablythe driestcountryin Africa
southof the Sahara.lt alsohasdesertson itseastern(Kalahari)
and westernedges(Namib).The Namibis a true desertwith
meanannualrainfalllessthan20mmin placesandmostlybelow
50mm. The Kalaharistretchesover three southernAfrican
countries.i.e. Namibia.Botswanaand South Africa and is
with meanannual
actuallynot a true desertbut a semi-desert
rainfallin the 150-350mmrange.The meanannualrainfallfor
Namibiais about270mmand rangesfrom lessthan 20mm in
the NamibDesertto morethan700mmat KatimaMuliloin the
of land area receivingdifferent
Caprivistrip.The distribution
categoriesof rainfallis as follows:
Rainfall(mm)
<100
100-300
300-500
>500
LandSurface(%)
22
33
37
8
Theseare the figuresthatare usuallyquotedwhendescribing
Namibianrainfallin general.Thesefiguresare notdisputedby
the lateststudy,butthisis too generalto be of any useto anyone
exceptthosethat needonly a cursoryknowledgeof Namibian
rainfall.
Duringthe last five years I have tried to educatefarmersand
otherrole playersin agricultureto use the medianrainfallrather
that the mean as a measureof the rainfallto expectat any
place.(Themedianvalueis thatvaluethatcan be expectedat
least50% of the time.) This is becauserainfallis statistically
skewedand a few large falls tend to inflatethe mean values.
of meanand medianrainfall
Maos 'l to 3 showthe distribution
as well as the amountby which the mean is higherthat the
median(as a percentageof the median).The mean/median
28
increasesfromthe southwestto the northeastwith a secondary
maximumin the so-calledmaizetriangle(Tsumeb-GrootfonteinOtavi).The differencebetweenmeanand medianis negatively
correlatedwith the medianvalueand reachesa maximumat
the coast near Swakopmund.The coefficientof variationof
annualrainfallis usuallymorethan 30% and may even reach
morethan 90%, especiallyin the drierpartsof Namibia.The
of variationfor
distribution
is shownin Map 4. The coefficient
the annual rainfallis much less than that for the individual
months,but the actualvaluesof the standarddeviationare
quite largeand mean that the year-to-year
variationcan be
In a recentpaper(Du Pisani,1999b)| usedthe
considerable.
deviationand variationas an indicatorof droughtvulnerability.
The largertheyear-to-year
variation,the greaterthe vulnerability
The managementof the
of a placeis to droughtoccurrence.
farmer and his bank balancealso determinesvulnerability.
Managementdeterminesthe effectof below normalon the
productivity
and profitof any farmingenterprise.
thevulnerability
is the trend
Anotherfactorthatmightinfluence
of annualrainfall
overtime.BecauseI havebeentoldrepeatedly
during the last five years that Namibias rainfallhas been
decreasing,I used the opportunityof havingthe most data
availableyetto findout if thisis indeedtrue(Du Pisani,1999a).
Contraryto expectations,the trend over the last half century
has not beenall negative.
Thereare quitelargeareaswhereit
has been positive.Anotherfact, which surfacedduringthis
study,was that the trend mighthave been negativeat many
placesduringthe last50 years,but if you takethe wholecentury,
no trendat all.Thisshowsthatannualrainfall
therewasvirtually
does vary considerablyfrom year{o-year,but over a periodof
a century,what has beenaroundcomesaroundin someform
or the other.
MONTHLYAND SEASONALRAINFALL
a summerrainfallarea(Map5). Only
Namibiais predominantly
a smallportionof the southwesternpartof the countryreceives
a majorityof its rainfallin winter.This area is adjacentto the
winterrainfallarea of the Cape and the few post frontalair
mass surgeswhich reach Namibiaduringwinteralso brings
rainfallto this very dry part of Namibia.In exceptionalyears
theymightalsobringrainto otherpartsof Namibia,but it hasa
negligibleinfluenceon the totalrainfallin the majorportionof
Namibia.The threemonthsof January,Februaryand March
are the 'heart'of the rainyseasonin Namibia.In some years
the contributions
of Decemberand April rainfallcan also be
considerable,
but this is the exceptionratherthan the rule. I
found Namibianstalkingof the short and long rains ('Kleine
Thisis a termusedmostly
Regenzeit'and'Grosse
Regenzeit').
Africa
where
they
have
a
definite
shortrainsin Marchin East
AGRICOLA2OO1
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Map 1. Averageannualrainfall.
Map 2. Medianannualrainfall.
averageandmedianrainfallas
Map3. Difference
between
percentage
of median.
Mao4. Coefficient
of variation
of annualrainfall.
April and then a break in rainfallbefore the long rains from
June to Septemberfall. lmplicitin this conceptis two rainy
seasons with a definite break between them. Looking at
Namibiandata,thereis no definitebreak.lt is simplythe short
rains blendinginto the long rainsof Januaryto March. The
figurefor Windhoekgivesan indication
thatthereis no definite
break betweentwo rainyseasons,but only one seasonwhich
startsratherslowlyin Septemberand peaksin Januaryto March
and thenfallsaway sharplyagain(Figure1).
AGRICOLA2OO1
29
Anotherbeliefthat I pickedup is that the earlyseasons rainfall
can be used as a forecastingtool for the late seasons rainfall.
Correlatingearly (Octoberto December)rainfallwith main
season (Januaryto March) finds correlationgenerallyto be
In one case,
0.1 or less,which meansvirtuallyno correlation.
Grootfontein,Januarywas addedto the early seasonand the
mainseasonwas takenas FebruarytoApril.Thiswas because
personsfromthe area use this methodto predicttotalseasonal
rainfall.The correlationimprovedto 0.278110from 0.110833,
but even that only accountsfor 7.7o/oof the variationin the
mainseasons rainfall.UsingJanuaryin the predictoris bound
to improvethe predictionbecausethe mean ratio(ONDJ:FMA)
is 1.30,so you havealreadyreceivedabout60% of the seasonal
rainfallby the end of Januaryin Grootfontein.So the beliefthat
early season rainfallhas a predictivecapabilityfor the main
season'srainfallis a fallacy.
60.0
= 50.0
.F
E
,F
E 40.0
30.0
20.0
10.0
0.0
0.0
14.0
Figure1.Windhoek
meanmonthly
rainfall.
INTER.ANNUALVARIABILITY
One of the most common observationsthat one hears when
talkingto farmerson the Climateof Namibiais thatthe annual
rainfallhas declinedin the recentpast.The only way to prove
or disprovethis observationis by actuallymaking a study of
the annualrainfallat as many placesas possibleand then to
trend.This
determinewhethertherehas beenany discernible
was done when a data set of monthly rainfallwas recently
acquiredand qualitytested.This gave me the opportunityto
do variousstatisticalanalysesand also to determinewhether
there was any trend to the annual rainfall.
Since it would be unrealisticto use data for this trend analysis
from largelydifferingtime periods,it was decidedto use only
the data after 1950, so that comparisonscould be made.All
analyseswere made usingthe MicrosoftExcel97 spreadsheet
softwarepackage,whichincludesmanystatisticalanalysesand
also has the abilityto do trend analyses.
Trend analysiswas done on the march of annual rainfallthat
was plotted by the softwareand a linear trend analysiswas
alsodone.Thisfirstanalysisshowedatwhichstationsthe trend
was positive(increasingwith the marchof time) or negativeor
where there was no trend at all.
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fg:1ry"'l|t liis*{ t1s*ii*t
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Map 5. October-Aprilas percentageof totalannualrainfall.
30
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1
Map 6. Trendin annualrainfall(% of medianper decade)
AGRICOLA2OOl
Table1. Trendsof annualrainfallfor NamibianRainfallStations
NumberStationswith Negativetrend
Numberof Stationswith Positivetrend
Numberof Stationswith Zero trend
80
201
These positiveand negativevalueswere plottedon a map of
Namibiathatshowedwhethertherewas any spatialcoherence
to the trend.This showedsome coherencebut did not give a
good idea of the steepnessof the trend and did not lend itself
to spatialanalysis.Furtheranalysiswas done on the plotted
graphswherebythe values of annual rainfallwere read off at
relevantdecadalvalues(say 1960and 1990)andthenthe slope
of the linewas calculatedin mm of rainfallper decade(positive
or negative).Expressingthis slope as a percentageof the
medianrainfallfor the stationfollowed. This was done to be
able to make valid comparisonsbetweenrainfallstations. A
trendof 10.0mmper decademightnot meanmuchat a place
like Katima Mulilowith a median annual rainfallof 625.2mm
but might be considerablefor Swakopmundwith a median
annualrainfallof 7.6mm.
Table1 showsthat in the majority of casesthe annual rainfall
did show a negativetrend, i.e.the perceptionthat the rainfall
has decreased is true. lt is impossibleto reproducethe whole
tableof all the rainfallstationsand the trendsdetectedbecause
of its sheer size.A map has been prepared,which shows the
areas with positiveand negativetrend (Map 6). Anotherfact
that arosefrom this study is that the periodused for the study
has a profoundeffecton the result.Not manyplaceshavemore
than half a century'sdata, but Windhoekhas more than a
century'srecord.The trendover the lastfifty years is definitely
negative,but usingthe total lengthof recordfound zero trend.
Anotheraspectof rainfallis an area'svulnerabilityto drought.
Drought vulnerabilityis caused by many factors such as
factors.The inter-annual
and othersocioeconomic
management
variabilityof rainfallas well as the intra-seasonalvariability
to droughtdue to the rainfallregime.
usuallycausesvulnerability
The variabilityfrom year to year effectsall farmingenterprises,
while the within seasonvariabilityusuallyplays havoc with
drylandcropproductionsincea relativelyshortdry periodduring
a particularlyvulnerablegrowthperiodmightwipe out an entire
crop while the total seasonalrainfallmight be above-normal.
To illustratethe range in inter-annualvariability,and thus
vulnerability,let us look at the actual variabilityfor selected
rainfallstationsin Namibia(Table2). These were selectedto
be examplesfor differentrainfallregimesin Namibia.Namibia's
r a i n f a l l g e n e r a l l yd e c r e a s e sf r o m t h e N o r t h e a s tt o t h e
Southwest.
The standarddeviationsteadilyincreaseswith increasingmean
annual rainfall,while the coefficientof variation(CV) in turn
steadilydecreases.The CV values stay high (30% or more)
exceptfor Outjoand Rundu.A mapof the coefficientof variation
can indicatethe spatialvariationin this parameter.According
to this map the western and southernportion of Namibia is
much more vulnerableto droughtsthan the Northeast.On
g e n e r a l , t h e l o w e r t h e a n n u a l r a i n f a l l ,t h e g r e a t e r t h e
Of course,it is not only the coefficientof variation
vulnerability.
in Namibia
rainfallstations
for selected
Table2.Actualvariability
Station
Latitude
Longitude
DiazPoint
26'38' S
Aus
Keetmanshoop
2641'S
26'32'5
Aroab
26'47'Sl
15'06' E
16.19'E
1807'E
19'39'E
Mariental
2437'5
17.58' E
Aranos
24'08' S
19"07'E
Rohrbeck
24"08'S
23'43'S
2328'5
2239'5
2256'5
22'34'5
21"39'S
20'24'S
1828'E
Tsumis
Beenbreck
Ahnchaihis
Dordabis
Windhoek
Hochfeld
Omatienne
Outjo
Otjiwarongo
Otavi
Grootfontein
Rundu
KatimaMulilo
AGRICOLA2OOl
20"07's
17'12' E
MeanAnnual
Rain(mm)
18.1
91.1
159.6
167.8
202.0
205.3
206.1
217.6
17"56' E
255.7
16'18'E
17"41' E
17"06'E
279.0
310.9
353.5
405.8
17"52'E
1629'E
16'09' E
SD
(mm)
15.6
54.2
97.7
99.2
113.4
125.5
113.0
188.9
141.6
243.9
265.0
300.2
325.6
395.0
125.4
429.7
399.9
427.2
424.2
120.1
125.0
143.4
153.4
154.9
127.4
457.1
446.9
188.7
501.6
515.0
625.2
164.3
175.9
163.1
220.7
19'38' S
'19"36'S
16'40'E
17'20' E
18'08' E
17'55'S
19"46'E
528.1
542.5
573.7
17"28'S
241s'E
669.1
2027'5
MedianAnnual
Rain(mm)
14.0
85.5
150.8
47.5
184.5
198.5
166.3
594.6
cv
(ol
85.9
59.5
61.2
59.'l
55.2
62.1
54.8
65.1
49.2
43.0
40.2
40.6
37.8
36.0
29.8
41.3
31.1
32.4
28.4
33.0
31
1
that effectsvulnerability,but also the absolutevalues of the
deviations,which tend to grow largeras the rainfallincreases.
This makeseven the rainierpartsof Namibiastillvulnerableto
droughts, especiallysince the crop growing areas, both
commercialand subsistence,is situatedin these parts of
Namibia.The monthlyvariationsare much largerthan the
annualvariationsand duringthe rainiestmonthsof Januaryto
March, Rundu,which has an annualcoefficientof variation
below30%. has coefficientsof variationof 55.7.59.2 and 68.8
percent,respectively.
Anotherway of illustratingthe vulnerability(variation)is to plot
the annualrainfallagainsttime.The annualrainfallfor Rundu
againsttime is plottedin Figure2. This figure shows us how
havevariedduringthe past.During
the year-to-yearfluctuations
1960's
the variationwas quitesmallbut
the 1950'sto the early
quite
looks
alarming.Duringthe 1970's,
after1963the varying
good
years,
the annual variationwas
which are reckonedas
quite large and only damped duringthe 1980'swith larger
variationsagain duringthe 1990's. The trend line was also
drawn in and it is obviousthat the high rainfallduringthe late
1960'sand 1970'stends to make it negativefor the whole
oeriod.
--i-
-
Linear(AI.]NLAL
RA[9
1970
YEAR
Figure2. Annualrainfall(mm) for Runduplottedover time.
CONCLUSION
REFERENCES
This paper has illustratedthat Namibianrainfallmight be low,
that shouldbe taken into
but that it has definiteidiosyncrasies
accountwhen planningor managingany agriculturalactivity.I
hope that certainbeliefsor myths have been demystifiedand
put into perspective.This papershowsthat the studyof rainfall
has many facets and should be activelypursuedto make
agriculturalactivitiesmore scientific,effectiveand efficient.
DU P|SANl,A.L. 1999a.Has Namibia'srainfallincreasedor declined
duringthe recentpast?Agri-lnfo.Windhoek,Namibia.
32
DU P|SANl,A.L.1999b.UsingNamibianDatato lllustrateVulnerability
of an Area to Drought.Workshopon Drought Planning,16/17
Seotember1999.Pretoria.RSA.
AGRICOLA2OOl