Brooks,C. E. P.

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THE FLUCTUATIONS OF LAKE VIOTORIA.
By C. E. P. BROOKS, M.SC.
Thevariationsof levelwhichLake Victoriaundergoesfromtime
to timeareof greatimportance,in the economyof Uganda,andsince
1895severalake-levelgaugeshavebeenin useat variouspoints,by
meansof which the level is measureddaily. Thesegaugesconsist
of pillarsof wood,stoneor metalfirmly fixed in the lake; on these
pillarsa scaleis markedin inches. The recordsarenow sufficiently
longfor an attemptto bemadeto analysethem, and the resultsof
suchan analysisarehereset out (1). The lake.leveldata,employed
are those from the gaugemaintainedby the Lake Engineering
Divisionof the UgandaRailway at Kisumu, at the headof the Gulf
of Kavirondoontheeasternsideof thelake. This gulf is asmallinlet
connectedwith the main bodyof waterby a narrowchannel;it has
no largeriverflowinginto it and is in everyrespectsuitablefor the
observationsof level. Theserecords ha,vebeen continuousfrom
August23rd,1899,to the presenttime. From thebeginningof 1896
until July 31st,1899,the recordstakenat Port Victoria havebeen
utilised;this at the south-westernend of BerkeleyBay, an inlet of
moderatesize on the north-eastshoreof the lake. From August,
1897,to September,1898,therewas a gap in the Port Victoria
recordsowingto the Soudanesemutiny; this gap has beenp.artially
filled by meansof observationstakenon the gaugeat Luba's, near
the outflowof the lakeinto the Nile. BetweenAugust1stand22nd,
1899,the Port Victoria gauge was moved to its present site at
Kisumu (formerlytermedPort Ugowe);theidifferencebetweenthe
base levels of the old and new sites was computedfrom the
observationsat Luba's and Port Alice (Entebbe), but there is a
possibleerrorof two or threeinches. Thus from ,January,1896,to
July, 1899,the observationsaremainlythosetakenat Port Victoria.,
pllJs a correctionto makethemrefer to the samezero as the later
observations;from August, 1899, onwardsthe observationswere
t,akenwithouta breakat Kisumu.
At first sightit may seemrashto calculatethe changesof level
of so largea bodyof wateras Lake Victoriafromthe observationsat
a singlegauge,but for the purposeof arrivingat generalconclusions
one gauge is sufficient. The only way in which considerable
differencesof level betweenone part of the lake and anothercould
ariseis throughdisplacementof the waterby considerabledifferences
(1) Th materialson whichthis studyis based'aremainlyderived
from the followingpublicationby the MeteorologicalOffiee,
London: GeophysicalMemoirs,no. 20 Varia,tionssin the
levelsof the CentralAfrican Lakes Victoria and Albert, by
C. E. P. Brooks,M.Sc.
47
of atmospheric pressure over different parts o£ the lake or by
continuous strong winds, neither of which are found in Uganda. In
the Baltic it has been found that the variations of level in the course
ofs few days due to these causesmay reach 'Severalfeet, but in Lake
Victoria the maximum differenceis probably never more thana few
inches, while the averagelevel during one year may differ from that
during another year by nearly four feet, so that the short period and
local changesare negligible,in comparison with the changes from
year to year.
Table 1 shows the mean level of the lake at Kisumu in each year
from 1896 to 1923. The zero of the gaugeis stated to be at a height
of 3,728.15 feet abovemean sea level. The means were obtained by
taking the averageof the highest and lowest gauge readings each
month, as the averagemonthly range is about a foot, this mean is
not likely to differ by more than an inch from the mean of the daily
readings, and will serve for our present purpose. Previous to 1896
we have no gaugereadings, but certain earlier data are available
from the reports of travellers, and these have been summarised by
Col. H. G. Lyons. They are fairly continuous from 1888 onwards:
TABLE I.-LEVEL OFLAKE VICTORIAAT KISUMU,ANDSUNSPOTNUMBERS
LAKE LEVEL. SUNSPOT
NUMBER.
1888-1890General fall on southern shore...
...7
1891
...Low..·
2
Very high, heavy rainfall, tendency to rise73
1895
high...64
YEAR.
LAKE LEVEL.SUNSPOTYEAR.'EL.'P T
(inches)
NUMB . (inches)
1896
...20...4 19101·..19
1897
26
1
- 7·
1898
·2·8 21. 33
900
1914
03
5
4.
2
5
6
55
3
7
10
1 04
8
8
8
5
9
6
9
2
3
89
48
From theseremarksandfrom Table1we seethat the lake was
low in 1890, 1902, 1912, and1922, andhighin 1895, 1906, and1917;
boththe low andhighlevelsrecura,tintervalsof aboutelevenyears.
It is well knownthat the numberof sP.lotson the,surt alsohas an
eleven-yearperiodicity,and it is interestingto comparethe maxima
and minimaof the lake-levelwith the maxima and manimaof
sunspots:
LAKE LEVEL. -,0 SUNSPOTS.
Minima. Maxima. Minima. Maxima.
1890
1902
1895
1912
906
2
17
1889
1901
1913
1923
1893
1905
1917
The annualmeansof the sunspotnumbersareshownin Table1.
The agreementis very close. It is broughtout in figure1, in which
the levelof LakeVictoriais plottedon the samediagramasthemean
sunspotnumbers. The level previousto 1896 has beensketchedin
fromthe datagivenabove. The parallelismbetweenthe two curves
is so remarkablethat it is quite evident there must be some
connectionbetweenthem. Since we cannot supposethat Lake
Victoria influencesthe sunspiOtsthe only inferenceis that in some
way the sunspotsinfluenceLake Victoria.
Let us first of -allstudythevariousfactorswhichaffectthe level
of a lake. In a cisternpartlyfilledwith!waterthe'levelof thewater
is determinedby thll amountof waterput in andthe amountof water
takenout. It is th81samewith a lake-the level is determinedby
the waterput in, by riversflowinginto it andby rain fallingdirectly
on its surface,andby the watertakenout, by riversflowingout of
the lakeandby evaporation.The amountof wa,terbrought.into the
lakebyriversis theamountof rainfallingonthe,lakebasin,lessthat
lost by evaporationfrom v~getationand from the soil. Therefore
49
thechangeof volumeof the lakeis equalto therainfalloverthe lake
and its basinless the sum of the evaporationover the lake and its
basinand the run-offin rivers. Let us try to determinethese
quantities..
Statisticsof therainfalloverthe Lake Plateauarecollectedeach
yearby the PhysicalServiceof Egypt, which takesgreatinterestin
the sourcesof supplyof the Nile, and Mr. P. Phillips, Directorof
the HydrologicaIDepartment,has beengoodenough to send me
(throughNature) a copyof the averagefall over the Plateau each
yearfrom 1904onwards. The annualaveragerainfall is 57 inches.
Overthe lakeitself the annualaverageis probablyabout40 inchHs.
The areaof the lake is about66,000squaremiles. Thus duringan
averageyearsome76 cubicmilesof waterfall over Lake Victoria
and its basin,and haveto be disposedof somehow. Sir William
Garstin'smeasurem,entsshowthat about 41 cubic miles annually
flowout of the lakeinto the upperwatersof the Nile at RiponFalls,
so that thebalance,rathermorethan70cubicmiles,or 94percent.,
mustbe10s1;by eva~oration.
A comparisonbetweenthe changeof level of the lake between
January 1st of oneyearandJanuary 1st of the succeedingyearon
the onehand,andthe rainfalloverthe lake basin during the year
on the otherhand(table2), showsthat whenthe rainfall is above
normalthe lake,rises,andwhenthe rainfall is belownormalthe lake
falls. The rise or fall of the lake is almostexactlyproportionalto
theexcessor deficitof the rainfallcomparedwith the average. Thus
duringthe two years1916and 1917the, total rainfall (both years
together)was 139inches,givinga total excessof 25 inches in the
two years,andthe lakeroseby 33 inches. In 1918the rainfallwas
only 37 inches,givinga deficitof 20 inches,and the lake fell by 28
inches. In this way,takingall the figuresinto account,wefind that
on theaverage,a rainfallof 10inchesabovethe normalcausesa rise
of the lakeof 14 inches;similarlya deficitof 10 inches of rainfall
causesa fall of 14inches. The areaof lakeandbasintogetheris 31
timestheareaof the lakealone,however,sothat if all therain found
its way into the lakean excessof 10inchesshouldcausethe lake to
riseby 35inchesinsteadof only 14. The differenceof 21 inchescan
be accountedfor in two ways, firstly by supposingthat part of the
rainfall goesinto the soil and only reachesthe lake gradually, and
secondlyby supposingthat thereis very greatevaporationfrom the
soil and from the vegetation. The remarkablyclose agreement
betweenthe rainfall in oneyearandthe changeof levelin the same
yearshowsthat the amountof waterheldin the soil cannot differ
greatlyfromyearto year;it alsoshowsthat the evaP.brationfromthe
soil andvegetationmustbeverynearlyproportionalto therainfall.
50
TABLE 2.-CHANGE IN LEVEL OF LAKE VICTORIA AND RAINFALL OVER
LAKE PLATEAU.
Year.
1904
1906
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
Changeof
levelfrom
Jan. toJan.
(inches)
- 9
+ 7
+ 5
=-14
+ 2
- 7
-15
- 5
+ 8
+ 5
+ 1
+ 8
+12
+21
-28
- 5
-10
-12
- 7
Rainfall
aboveor
belownonnaI.
(inches)
...-
- 2
+11
+ 7
9
+ 2
- 9
- 7
- 3
+ 6
- 2
+ 9
+ 6
+14
+11
-20
+ 1
- 2
-11
- 5
We cannowmakethe followingtableto accountfor the rainfall
fallingoverthe lakeandits basin:
Rainfall overlakebasin(excludingthe lake itself) ... 60cubicmiles
Loss by evaporationbeforereachinglake ... (about) 35cubicmiles
Amount reachinglake
Rainfall on surfaceof lake
Total waterreachinglake
Run-oft
Loss by evaporationfrom lake surface
(about) 25cubicmiles
(about) 16cubicmiles
41cubicmiles
5 cubicmiles
36cubicmiles
The latteramount,36cubicmiles,is sequivalento the removal
eachyearof a layerof water63inchesdeepoverthe wholelake.
How do sunspotscomeinto this balance-sheet?Sincethe level
of the lakeshowssoclosean agreementwith thenumberof sunspots,
the latter must havea dominatinginfluenceon one or both of the
primefadorswhich influencethe lake-level,namely rainfall and
evaporation.The averagerainfall over the lake plateauaccording
to Mr. Phillips is setout againin table3 as a differencefromnormal.
The secondcolumnshowsthe sunspotn"umbers;it is seenthat the
rainfall is generallyhigh whensunspotsare rising and low when
sunspotsare falling. The third columnshowsthe change in the
averagesunspotnumberfrom oneperiodof twelvemonths(July to
June) to the succeedingtwelvemonths,and this columnshowsgood
agreementwith the rainfall amounts. This agreementis expressed
numericallyby what is knownas a "correlation coefficient." A
correlationcoefficientof +1 indicatescompleteharmonybetweenthe
variationsof the two elementswhich are being compared,a co-
efficientof -1 indicatescompleteop~ositionbetweenthem, and a
coefficientof 0.0 indicatescompleteindependence.
TABLE3.-RELATIONOFRAINFALLTOSUNSPOTS.
Plateau
SunspotCha gein
Year.
Rainfall.n mber.sunspotnumber.
inches.
1904
·..'- 2.42·..+17
1905
163. 0
0 6
54 3
7
·..98
8
9
9
17
10
1 0
1
6·..
2
,6.
19 3 4
.
026734105. 58 6
2
3882 4
52
Now the correlationcoefficientbetweenplateaurainfall and the
changeof sunspotsfromyearto yearis +0.64,whichindicatesgood
but by no meansremarkableagreement.The correlationcoefficient
betweenplateaurainfallandthe changein the levelof Lake Victoria
(table2) is +0.91,indicatinga very close agreement. Since the
levelof the lakedependson the rainfall and the rainfall dep;endson
the sunspots,it is'evidentthat the level of the lake would show
agreementwith sunspotsevenif there were no other factor. To
measurethis agreementbetweenlake level and sunspots through
rainfall, we multiply togetherthe two correlationcoefficientsgiven
above,i.e,.,0.64 x 0.91 = +0.58,andthis wouldbe the correlation
coefficientbetweenlakelevelandsunspotsif no other factor than
rainfallhad to be takeninto account.
But the connectionbetweenlake level and sunspots is much
closerthan this; it givesa correlationcoefficientof +0.87 (table1).
Thereforesomeotherfactorin the lakelevelbesidesrainfallmust be
closelyconnectedwith sunspots,and from whathas previouslybeen
said it will readilybe seenthat this factor must be evaporation.
Evaporationmust be much less in the yearsimmediatelypreceding
sunspotmaximathanin thoseimmediatelyprecedingsunspotminima.
This conclusionis quite unquestionable,and throws interesting
light on a well-knownmeteorologicalparadox. It is knownthat the
sunis hottestat sunspotmaximaandcoolestat sunspotminima,but
W. Koppenand othershaveshownthat temperaturein the tropics
is on the averageabouta degree(Fahrenheit)lower nea,r sunspot
maximumthan it is nearsunspotminimum. That is, the hotterthe
sunthecoolerthe earth. This unexpectedresulthasbeenattributed
to greaterevaporationfrom the oceansat sunspotmaximumcausing
greatercloudinessand thereforea lowertemperature. But we see
that over Lake Victoria which is a very large sheet of water,
evaporationis least,not greatest,at sunspotmaximum;thereseems
no reasonwhy this conclusionshouldnot be extendedto the oceans
also. Hencethe low temperatureat sunspotmaximacannotbe due
to increasedevaporationat thosetimes but must find some other
explanation.
It maybe of interestto refer briefly to two other periodical
variationsshownby the level of Lake Victoria, namelythe annual
and diurnal changes. Superposedon the marked eleven year
periodicityis a d~stinctannualwavewith a range of ten inches.
This annualwaveis sufficientlydistinctto be seen as a series of
peaksand valleysdiversifyingthe greatsweepsof the eleven-year
158
periodicityin figure1. Overthe twenty-sixyears1896,1899to 1903
the averagelevelin eachmonthwasas follows(inchesabovezero):
<
Jan. Feb. Mar. Apr. May. June. July. Aug. Sep. Oct. Nov. Dec.
5 5 5 8 13 13 10 75 3 3 5
The lakeis normallylowestin OctoberandNovember,risesvery
slowly until the end of March, and rapidlyduringApril and May,
remainssteadyduringJune and then falls moderatelyrapidlyuntil
September.
The otherperiodicvariationis completedin the spaceof twenty-
four hours. For the past few yearsthe lake gaugeat Kisumu has
beenreadtwicedaily, in the morningand evening. Thesereadings
showthat the level duringthe eveningis persistentlyhigher by a
few inchesthan that duringthe morning. During the five years
1917to 1921the averagedifferencewas 4 inches;it was greatestin
February(5 inches)and leastin July and October(3 inches). This
diurnalvariationis probablydue to the influenceof land and lake-
breezes,whichowingto the greatareaof Lake Victoria are well
develop!ed. During the night and early morningthe land breezes
tend to blow the water away from the shores on all sides,
accumulatingit in the openpart of t,helake. During the day, on
the otherhand,the lakebreezetendsto drivethe water before it,
andit collectsagainstthe shore,raisingthe levelthereat the expense
of the centreof the lake. Such an effectwouldbe well developed
in an inlet like KavirondoGulf. The monthswhenthe effectis best
shown-Decemberto April-are in generalthosewith least rain on
the shoresof the lake, andalsomainlythosein which the diurnal
rangeof temperatureis greatest;hencein thesemonthsthe landand
lake-breezeswouldbe expectedto be mostvigorous.
Anotherlake-gaugefrom whichthe recordshavebeenstudiedis
that at Butiaba in Lake Albert. This givessimilarresultsto those
obtainedfrom the gaugeat Kisumu, showing that the eleven-year
cycleis developedin Lake Albertaswell asin Lake Victoria. Owing
perhap;sto its smallerareathe fluctuationsare in fact more than
twiceas greatin Lake Albert, the absoluterangebeingfrom6 inches
belowzeroin July, 1908to 156inchesabovezeroin November,1917,
a differenceof nearly fourteen feet. The annual means are as
follows(inchesabovezero):
1904 29...1910-... 648
1905
1 1... 71 0
1906
621· 85
7
234953
8
423
9
57
54
TABLE 4.-VARIATIO~S IN THE LEVEL OF LAKE ALBER'!'.
In thelatterpartof the recordthe agreementwith sunspot
numbersis remarkablyclose.
ThustheCen.tralAfricanL~es VictonaandAlbertpresentus
withoneof themostremarkableknoWllMsociationsof cosm.icaJnd
terrestrialphenomena.·TheagreE!inentbetweenthesunspotcurve
andthelake-levelsinth!}pastthirtyyearshasbeen80 cloaethatone
canhavelittlehesitationis prophesyingthatit will bemaintainedin
thefuture,andthathavingreacheatl1eirlowestlevelfor the time
beingin 1922thelakeswill riseagain~s1Qwlyatmat, andthenmorerapidly,toanotherm~imumabout1927or1928..
IVVV vV' crr..:..-'-~-7~' v-r \:7U ~!l) I"VV vc.. v-r
INS. ':;~1.
+40 100 175
+20 60
o 20
o
-20
/I
(" I II, I, I
" / II
, '
125 "'" //
" ./" ,.--..-
.".,.-----
,; .•..••,. "" '/ "-I '
I~ 'f~ .~ \ \\
LEVEL OF LAKE VICTORIA [Black] SUNSPOT NUMBERS [Red]
..
INS. eM.
+20 175
/"". "-,~I
0150 r
-20 125
LAKE VICTORIA -CHANGE
RAINFAll OVER LAKE PLATEAU [Green]
to
,-,-, Iv'-V
"-
.")
\NGEOF LEVEL FROM YEARro YEAR [R.JrpleJ RAINFALL O\/ER LAKE PLATEAU [Green]
t


The Fluctuations of lake Victoria

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The Fluctuations of lake Victoria

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