1~1) - Definition of the representative and experimental basins :

Use of index catchments in evaluation of water balance by J .A. RODIER

First lecture 1). - Definitions :

One index catchment is a particular case of the representative and experimental basins~

1~1)

- Definition of the representative and experimental basins :

In general a rep. or expo Basin is'a basin intensively investigated and is a basin l'lhere coordinated observations, measurements and studies are conducted.

At least the studies are relevant to 1) - precipitations (or snowmelt)

2) - an important part of the various components of runoff at least for instance surface runoff.

In many cases only precipitation and natural discharge of rivers are jointly studied. In other cases all the components 'are studied : evapotranspi-ration, surface runoff, base flow,variations of aquifer and also erosion and qualitr. of runoff~ The area of the basin is small enough to permit a good analyse of the phenomena in all the parts of the basin. vIe must insist about the

character of coordinated studies.

1.2) - Definition of the representative basin and definition of the experimental .basin :'

a) - Representative, basins are basins specially selected as representative of an hydrological region within l'lhich hydrological similarity is presumed.

(All pnysiographical characteristic : slope, drainage net etc. must be considered) The basin remains in relatively stable natural conditions. In many cases

repre-sentative basins are index catchment:

Rep. basins may be cultivated but the essential condition is that the change for a year to another must be unsignificanto

Benchmark basin is a particular case. Benchmark basin remains in natural stage and soll and vegetal cover have not to be c.hang~dfor a very long time.

1.3) - ~er.~~.!1~ bas~?s are basins wher-e experiments are made, one or more of the basins characteristics are deliberately modified and the effects of the modifications on the hydrological characteristics are studied.

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. The size of the representative

ba~s varie~

from 1 Km.2 to 1 000 K1n.2

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The largest basins are generally managed forvstudy of snormelt in plain areas. For Africa the areas vary from 1 to 500 Km.2•

Often the. studies are conducted not on one isolated· basin but on a group of basins.

Such a group may include', :

2 or 3 basins of 10 to 25 Km.2 (one of them is the main basin) 1 basin from 2 to

4

Km2

, 2

1 basin from 200 to 500 Km: • The size of experdmental..basins is smaller.

2

generally less than

4

Km: for 2 reasons :

necessity of very homogeneous conditions and the cost of the treatments.

The research organization shall have the right to manipulate the land at will. Any cultural change is preceded by a calibration period.

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2) - The objectives of Representative and Experimental basins

One representative basin may be used for several purposes but it is necessary ~o specify the primary purpose.

The purposes are following : - ComPletion of general network - Basis',~ of regional synthesis'

- Comprehension of the processes of runoff and water balance at the

sc~e of Representative Basins - study of the influence of man - &~j,s ~ for fundamental research 2.1) - Completion of the general network:

It is often the case in Africa.

In most of the arrd.can countries it is at this time impossible to c.over all the territory by a sufficient network for the small rivers.

In the W.M.O. guf.de the hydrometrrLcal, stations are divided in large rivers and small rivers. SIna1l rivers are much mo:r-e numerous, they are studied by sampling.

For small river~Jwater level recorder are absolutely necessary and generally t,he operationing of these recorders are more difficult than the operationing of staff. For instance a gap of 8 days of observations has might·

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-3-consequenceaon large rivers but heavy consequences on small rivers. And this dii'ficulty is more important in Africa where the density of population is often low.

The idea is to restrain the network to a small number of basins' well representative, ~·iiltensivelystudied, the runoff being tied to precipitations. In such a way it will be possible to ccmpute the runoff data frcm precipitations for various physiographical conditions (soil, slope, etc ••• ). It is the defi-nition of representative or index basins.

Such basins are of'ten used for two particular cases :

a) - study of floods : the joint observations of discharges and. precipitations . '

permit to reduce the statistical sample and make easier the extrapolation• b) - Countries where ther-e are few,big representative rivers

In islands like New-Zea'Iand Around lakes like Victoria Lake.

The representative basins are, not always necessary in many cases for extensive study of water balance because the network of large rivers give good data for this purpose.

2.2) - ~s1~~ of regional synthesis :

Even with not too bad network the best way to proceed to regional synthesis on small rivers is to manage representative basins because the

observations and measurements are more precise and what is more important, the physiographical factors and their influence are studied vdth precision. We shall see later what is the best procedure for planning in this case.

Often purposes 2.1 and 2.3 are tied together.

2.3) - Comprehension of the processes of runoff and water balance on the scale of the Representative Basins :

It was the origin of Representative Basins. The basin is .usedfor detailed study of the water cycle and of its elements : Precipitation,

evaporation, surface runoff, base flow, humidity of soil, underground water• . It is used also for the analysis of erosion, sedimentation, quality of water.

It is not frequent that all these elements be studied in one 'catch-ment, but only the most interesting part of the water cycle. The programn of research varies with circumstances. In such cases the basin is sometimes called research basin.

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-4-2.4) - Study of the infiuence of man :.

In this case the basin is called experimental basin. As an example we can mention the influence of cutting the bamboo forest and replacing i t by

sort wood plantation on base flow like in Kumakia E.B.. This basin is included in the catchment sup~Nairobi in water.

The Jllicperimental basins can represent the hydrological processes and explain the reasons of the change in the hydrological regimen resulting from the change in natural conditions.

2.5) - J3asis~ for :f:\Lndamental research :

For

many

special researches Representative Basins offer excellent condi.tdons , For exampl.e, the first intensive studies on spatial distribution in lfest Africa have been made on Representative Basins.

In many cases Representative Basins are used for several purposes at the same time: 2.1, 2.2 and 2 •.3 for instance.

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.3) - Planning :

Representative Basins are often operated not isolated but by groups of 2 to

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adjacent basins. The cost in supplement is not too high because what is the most costly is the total bulk of wages and in IBrticular' the wages of hydrologists and of hydrometrists. And. for one group or one basin in Africa it is generaJ.1y necessary

to

have onJy one hydrologist on the field.

In what follows we shall ~,i!e~

to

groups of basins. .3.1) - Planning on a national basia' - homogeneous, re,gions :

The first thing is to determine on map homogeneous areas in respect to the .factors w1p.ch have a significant inf'luen~e.

on

hydrol~gy : natural vegetal cover, land-use,permeability of soil ind subsoil, slope, precipi-tations. F~r instance in Brasil wi.th the same vegetal cover and the same land use we have-coned.dered several categories of precipitation, permeability of soil and slope: defining each C9-tegory by an index for'instance for preci-pitations:

PlC::::::::::: 600 nrm/year'

600

mm

<::P

2.

c::::::

800

mm

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-5-;for pemeabiJity of soU:

x,.

impervious areas: subsoil granite

K2

slightly pemeable

S

pervious areas

(In U.5. department of Agriculture

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categories have been chosen for permea- . . bil:i;ty of soil based on depth of soils). .

For slope :

very slight slope

<

1,25 m/Xin (fiat plains)

reasonably steep slope

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slight slope moderate slope

1,25 to 2,5 m/Fm. 2,5 to 5 m/Fm.

to 12,5

m/llin.

(mountains)

We have to consdderjln the country all existing combinations of P, K and 5. Th.e total number is, fortunately, less than the number of possible canbinations . because sane do not exist, other may be studied by interpolation.

For the example given here the number of combinations has been reduced at 18. It is possible to study together a group of 2 canbinations for instance

P2 ~ 52 and P2 K2 Sl.

" When the available means are very limited, it is possible to study

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a snail number of basins. A choice has to be made. It depends of the nature of the problem to be studied. For a problem of floods, the most dangerous case p

3

K

l 54 is considered in first priority. Th.e least dangerous may be neg'l.ecbed, The area of each homogeneous region, P. K

i S., has to be considered. I f it

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covers only for instance 1% of the area of the country it may be neglected. 3.2) - Fonnulation of a group of basins and the principles of choice of a site

_ Afte~ the choice of the homogeneous hydrological regions to be . studied, the next step is the choice of the group of Representative Basins for each homogeneous region.

Generally, in Africa, a group of basins should consist of : .. - 2 main basins with area fran 10 Km.2 to 30-50 Km2

. 2 2

- 1 or 2 basins with area from. "1 Km to

5

Km: , homogeneous as far as possible (1" (1) - "hcmogeneous" must not be taken in the strict" sense : for-instance, often" a" basin includes the top of the hills with a given type of soil, the slopes with other types of soil, the bottom. of t~ valley ~th a third. type of soU (hydro-morphic generally). The basin of 1 Km: to 5 Kin. considered as homogeneous should

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-- 1 basin with area fraI1100 mito

500

Km.2•

Th basde asan of 500. RiIi2 includes often the snaller basins. Conditions for the choice:

a) - a good representativeness (often in contradiction with hanoge- . neity)

b) - basin divide as distinct as possible : in. some cases for very smaJ.l basin the establishment of artificial divide has to be consi-dered

c) - Access %good-access to the basin, even in ra.iny season and if possible not too bad tracks in the basin for inspection of rain-gauges ; establishment of sane tracks is to be considered.

d) - good quality for gauging station: The first condition is the stability of the bed of the river.

e) - Size not too small : 10

) - water frcm shallow aquifer should feed the river

upstream

ot

the gauging station.

20 _{) - The basin should contain sane of the natural}

channel•.

f) Constancy of conditions : mainJy for vegetal cover, land use, -water work existing before observations in the Representative Basins. This is not easy to obtain, even in Africa.

3.3) -

Choice of exper:i1nental basins

The general organization of the group of the basins depends on the planning of exper:i1nentation.

The number of the basins is in relation with the number of the factors to be considered.

For instance, for study of the effect of cutting the trees, suppo-sing soil slope and precipitation regimen be the same, it is necessary to obser-ve

- 1 basin with forest like it is . - 1 basin with the forest cut

or'

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. The 2 basins have not to be exactly of the same surface. The ratio of the surface should not exceed

5.

The ownership of the basin is to be considered because it has to be

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manipulated at will. Access is more :iJnportantvfor Representative Basins. Provisional observation before treatment may be very useful.

4) - Elements of equipment:

The most. :important measurements are gene~those related to preci-pitation in runoff, but often'it is forgotten that there are oj,!le:r'important measurements

to

be made.

4.1) ~ At first between these,aJ.1 the measurements determirpng the pYsiogra-phical characteristics of the basin :

One must. verify, first of all i f the basin is well in the good place in the classification of hanogeneous regions.

This involves; the availability of aerial photographies which will serve for many purposes :

- topographical surveys to determine the map of the basin, the divide, the area, the contours, profiles :ofthe natural channel, slope of the hills etc•••

:Aerial photographies alone may provide divide and area.

The gecmorphological study shall be made on aerial photographies and not in maps % in Africa, num.ber and total length of streams of first order

can be canputed properly only on aerial photographies. It is stressed that the drainage coefficient ~ is an important factor of runoff.

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A

. Geological study and geological map are useful for mountainous· areas. Not necea-. sary, for very small basinsand very hcmogeneoua,

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fJIy~~giCal

map

may

be very useful.· .

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But the most important map is the pedological map. At first the delimitation

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of various categories of soil is to obtain and after, the stu~ of each soil

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at the hydrological point of view. This is done in specified areas perfectly defined by pedologists % the most important measurements are : penneability

in situ by infiltrometers, profiles of humidity

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times each year during one or tWo years. Nap of vegetation and land use.' of the basin are also necessary•

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-4.2) - Periodic charts of vegetation and land use: .

In order to verifY the permanency of natural conditions, it is often

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necessary to establish on sna.ll basins (A <:::.10 Km:.) a map of vegetation and land use at least one time each year during the period o.f vegetation. For study o.f erosion it is also necessary to specify the conditions (bad or good of the crops) (bad or good cover).

4~3)

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General eguipnent :

The minimum. equipnent consists in :

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- a network of ordinary raingauges with at least one recording

rain-"

gauge

- an hydranetric station ~th water level recorder for almost all cases (only one exception, very slow variations of discharge). - a small cJ..iJna.tological station.

It is ~ossible so shortly to precise all is necessar,y to know about the problems-Jequipnent, but it is necessary to insist about the absolute necessity of a good evaluation of precipitation which is in direct relation with the number of raingauges.

4.3.1) -

Climatological station : The most important purpose of this station is the measurements of evaporation and of its factors. At Least one evaporation pan (national pan or class A pan) with observation of vapour pressure and

temperature of air.

For radiation,the use of apparatus for deter.mination of sunshine duration may be very useful and not too costly. The sunshine duration

may

be put in correlation with global radiation detennined in big climatological station under similar climate.

I f the station is more important, it is possible to use a Gunn Bellani evaporometer for the same purpose.

One recorder of precipitation with possibility of obtaining inten-sity for a quarter of hour is necessary and if possible an ordinary raingauge at the ground level. The choice of the site of the station is very important. Evaporation has to be detennined in the general conditions of the basin•

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The equipnent consists of :

- ordinary raingauges (daily reading~). - recorders

- and in difficult cases totalizers

This last instrument could be very simple (tube '\'Ji.th one or two cm. of oil for preventing evaporation). If possible 2 recorders for one basin.

For the number of instruments the risk of bad readings and of bad recordings shall be taken, into consideration.

The good figur~s are following (n

==

number total of raingauges : ordinary

+

re-

.-corders

+

totalisers) A= .2 YJn.2 n

=

4

. - 10 RiD.2 n =

6 -

8 25

ron

2 n = 10 - 14 100

mn

2 n

~

15 ','

2 . /

.500

mn:

n '" 20 1'. ;-:0 I ~

for A

==

100

Km~

and 500 Km2 , n is the

-miniriLum·.~:·

figure.

These figures are valid for plain conditions. In mountains with very irregular distribution of precipitations the figures for snail Representative Basins have to be majored.

4.3.3) - Runoff :

One hydrometric station equiped with one l'later .level recorder and all what is necessary for the. obtention of a very good rating curve frCllIl

the mirrlmum. low water .up .to the m.a:x:imum. discharge.

Artificial structures like weirs, flumes etc••• are recommended if - there is no risk of destruction

- no risk of overtopping

Often the maximum discharges is underestimated at the beginning of the obser-vations and after destruction of the structure by the flood, it is generally impossible to canpute the discharge for the water level observed before and during desbructdon, (impossible to estimate the peak flood). In:m.any cases it is better to keep the natural conditions in the bed of the river•

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I f a weir

or

a nume has to be established (see the technical note of WMO)a checking of the discharge/level relation is necessary (3 points) •.

For natural conditions it is often possible·

to

avoid the moving· of the bed by careful stabiJization· of the bottan.•

. If it is impossible, check the relation discharge/level after each iJ portan't flood and sanetimes during the recession periods (cross profiles

to

establish after the recession).

Provisions are to be made for insuring measurements even during very large floods (access to the station, cable way for high water, considere use of floats)~

Problems of ~ll·flood plains: if possible, it is necessary to establish

~c~ss'; the plain a small wall : for instance with earth between bamboos upstream and downstream. leaving the now in the river only in the main bed. I f it is not possible, measurement of discharge in flood plain has to be made after cleaning a band across the valley.

4.3.4) -

Subsurface water: It is strongly recannnended to establish at least 2 or :3 piezometers i f the basin is small (A

<4

Km2) or to follow the varia-tionsof the water level in the wells in order to obtain some qualitative ideas of the variations of groundwater level.

If the basin is to be experimented especia.1JJ" for underground-water it is another problem and the density of piezameters should be enough high. In such case it is necessary to follow at the same time the humidity of soil.

But even in a basin established only for runoff studies it is useful to follow the variations of the humidity of soil in several points of the more common tyPes of soil. I f there are problems of systematical study of humidity of soil, one must use .. a network of neutron scattering tmbes observed weekly in

::.:rainY-:;

season, and not so frequently in dry season.

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4.3.5) -

Sediment transports and erosion: Sediment traps are used on secon-dary catchment (A

<.

0,2 Kin2:.in savannah areas).

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Interception: Measurements are use£u1 for certa.:i.i:t studies using: Dets of 12' to

16

raingauges under the trees and collectors around the stems.

4.3.7) -

Quality of water Samples are taken at the station fran time to time in relation with

et.ry

period and flood period :r611owing a special schedule for each case. But it is often useful

to

have an idea of the natural quality of water in Africa by 5 to 10 samples.

4.4) -

For experimental Basins : It is necessary to make in supplement all measurements related to vegetation and crops •

Use of index catchments in evaluation of water balance by J • A. RODIER

Seconi lecture

5) - Principle of operation of Representative Basins:

" 01.~a,t<y " .

5.1) - General Organization: It is absolutely~oOobtainon a Representative Basin at least coordinated studies in the various field~ of investigations.

The best solution is the managing of aD.. observations and measu-rements by one service only. The efficiency is better and the responsible organization is able to follow perfect:ty well at the same t:ime the" various aspects of hydrological cycle. But it is not always feasible. Good coordination

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in suchYcase ~s absolutely necessary. For the East African basins in the past (Kericho, Kumakia, and so on) it has been possible to' establish very good coordination between meteorological service and Agricultural and Fores-try service of East Africa.. What is :important is a perfect coordination between the operation of raingauges and stream gauging networks.

Another important point for the same reason is to place the

managing of a basin or a group of basinSunder the responsibility of ~

hydro-logist only. This is the case for index catchments. This is not valid in all the partsof the world; For research basin with geological, pedological researches and for experimental basin it is more difficult and often impos-sible to arrive to this solution.

- The importance of the staff on the field differs frem one ba.sin to another in accordance with the prograJIJIneof researches and the general conditions. In desert for instance or where the salaries are very high it is necessary to reduce as far as possible, the number of the personnel.

An important point is that for basins of area snaller than 200

IDn~

the hydrcmetrist responsible must live in the basin or at a distance short enough to join the main etation in half an hour of travel by the worst conditions.

This hydrcmetrist should be of a relative:ty high level, he must lmow perfectly well the observations "and measurements to be made, he must be

able to proceed to the repairs of the recorders for the most current diffi-culties and to take the necessary stepsin case of very large floods which lead to abandon the classical method of measurement; he must understand the genesis of runoff in order to bring good qualitative observations for the most important rainfalls.

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-The hydrologist responsible must know perfectly well the basin

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and not only the main station. He must live on the basin during several perd.ods : of 10 days to 2 weeks in the most humid part of rainy season.

This involves the necessity of the construction of a camp in the basin.

For a representative basin of 25 Kin2 in Africa with one smaller 2 2 ·

of 4 RiD. and a sta.tion for 100 Km: the personnelmay be

1 hydrometrist° "responsible

2

assistants (including 1. driverwho assists for the measurements

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. of discharge)

(. to

6

observers for raingauge and stream gauging stations.

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0' .If it is possible to join all stations by.road these

4 to 6

obser-vers are not necessary and the driver can collect the data each

day.

He pr-eceeds to this operation with the h.ydranetrist fran t:i:m.e to time and. necessarily after the heavy rainfall.

The work in the field shall ~have :a: high priority. It is ge~e­

raJJ...v impossible even with the best canputer in the world to change bad obser-vation and bad measurement into good obserobser-vation ~d·goodmeasurement.

I f there is an important programmeof measurements of h}tlllidity of soil, of level of underground water, of erosion plots and so on, the number of assistants may be more important.

For big basins the number of raingauges and the distance between the raingauges involved an augmentation of the number of observers and even of hydrometrists.

For a basin of

i

000

mn.

2 for research, with very difficult condi-tions of access in Tchad we' had 2 hydrologists,

3

hydrometrists and the rela-.tive number

ot

other personnel with

6

hydrometric stations} 4Oto

50

raingauges.

etc •••

For a basin of about the same area with numerous and good roads, we have at this time 1 hydrologist, 1 h.ydrcmetrist but

¥J.

observers.

5.2) - Operation of the network of raingauge and climatological station The climatological station is similar to these of the general network in many cases. But, observation of precipitation depths, intensity of precipitation and evaporation have the first priority•

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-3-It is often necessary that the drum. of the raingauge recorder be provided for a tour per day. Checking and settling of this apparatus must be frequent and perfect.'

For sma1J. basins it is necessary to check frequently the time .for all observations in order to avoid that on' the charts floods begin before

rains. Pro:Qlems of watches are very important. For collection of data of raingauges, we have given scme indications.

5.3) -

Stream gauging measurements : They shaD. be made fran the minimum discharge to the maxiJnum discharge if possible, mainly where the study of

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floods is provided. All shall be foreseen fran. the beginning of the study

ot

the basin, it is too late when the large floods arrive for bringing modifi':' cations to the structures. In case of large floods, the measurements are made at the station of the smallest basin.at first. After that, the hydrometrist follows the flood for various stations from upstream to downstream.

The charts of water level recorders should not present any gap .. If the large flood destroys the recorder, water levels should be recorded'

manually.

Good instruction should be given for the files of charts and the tables of primary infonnation•

5.4) -

Case of experimental basins

Two sorts of. operations have to be made :

- nonnaJ. observations and measurements for hydrological and meteorological factors as for Representative Basins

- agricultural practices and researches for relations between hydrology and vegetation (influence of roots and so on)

The present lecture has to deaLmainly with Representative Basins, it:"is not ·.the ..place here-to diBCU~S'in..detail· the: probkems 'ofExperimental

Basins.:.'.. ·· --'--"'-

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5.5) -

Other measurements :

Sys'\;ematical studie's' cif humidity of soil and o:f~rosio;n'invPlve'8 great

amount of work on the field. The hydrologist shall check and follow the measu-rements on the field. very carefu1J.y in order to avoid systematical errors.

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-4-For instance bad rating of the ~n'scattering instruments, bad practices for measurements of secliment discharges in sediJllent trap etc ••• could lead to the loss ,of considerable amount, of money without result.

In conclusion, all the operations in the field shall be oriented . to the obtention of good data for the interpretation.

The use of canputer can change nothing. With good food put into the ccmputer one can obtain good results. With bad food, it is :impossible to obtain sanething •

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6) - Data processing

What follows is adapted to the conditions of the work in A£ri.ca to now or for the very near future ; for instance, we do not present anything about the use of direct recording of water-level on magnetic or punched tape, nothing about the central telem.etering of observations somet:imes used in

developped countries. /

These last practices are not :impossible to use in Africa.

For instance, we have used central telemetering in Guinea and in Ivory Coast 10 or 15 years ago. But the cost of equi.pnent., the necessity of high trained hyd.rcmetrist are such that we have to W'ait

5

to 10 years for the

generali-zation of this in Africa. But already for a big mmber- of the Representative and Exper:imental Basins in Africa, it is possible to use computers for the processing without too big difficulties.

In order to make understand the method we begin by the processing without canputers.

6.1) - Precipitations :

The processing is time-consuming. An :important operation is the checking of the data before use, and eventual corrections. Processing shall follow the reading of the raingauge as near as possible.

Fifteen years ago the objective of the processing of precipitation was the establishment for almost

all

precipitations producing runoff and for the most :important precipitation without runoff of a set of. diagrams and maps

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3

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-- The map of pr~cipit.ationswith contour" of . isohyets (1)

- The diagram. of int.ensity of precipitation for each recorder

(intensity versus t.:bne) (2)

- The mean diagram of diagrams nO 2 (3) Fbr the isobyetal map each ponctual precipitation for

24

hours or for a specific storm is plotted on a map of the basin. The drawing" of isohyets takes into account, not only the ponctual precipitations but the relief and the exposition of various parts of the basins.

;rf the pattern of isobyets preseIIlis the same shape for the same general situation, this fact. can be used in the case of gap in the observations of raingauges. The mean precipitation depth on the basin is computed from

iso,hYet. The maximum observed and the minimum observed are plotted (a technical report of WO will" be pUblished about this subject).

The diagram of intensities or the table of intensities are esta-blished after correction of time and of cumulated depth (checkl.ng of the rough data of the chart from the water in the bucket).

For computation of intensities we consider onJJr the point corres-ponding to the change of slope in the registered curve.

We establish a table with 6 colums :

le) - time of each breakage poiIIli of the curve on the registered chart 2°) - accumulated precipitation depth from the beginning of the storm up

to the breakage point

3

0) - accumulated precipitation depth after corrections above

4°) -

partial precipitation between consecutive points

5°) - corresponding intensities

Before the establishnent of the mean intensity diagram, one has to verify at first if it is necessary to establish it (for instance no necessity if the spatial distribution of precipitation depth is too irregular).

For isolated peaks of storms, the maximum intensities' are put in coincidence if the difference in t:ime is small (o~ for the study of runoff/ depth/intensities relationship but not for detennination of 1ag time).

Ai'ter that the mean intensity in the basin is computed by intervals of

5

minutef or 10 mimtes •

,

."

, ,.

}~j

..

..

-6-More recently we tried to avoid the establishment. of the isob..yetal pattern for most of the st.crms, The procedure used for this is' following : to realize on the field a regular spatial distribution of raingauges with relatively high density. In such conditionsthe mean depth of precipitation can be detennined with good precision by the method of arithmetical mean. But note is taken of maximum and m;nimum punctual depth for each stom or an index is choosen for the representation of the homogeneity of precipitation in the space for each sborm,

This procedure was not used for research basin if the spatial distribution of precipitation has an important influence on the runoff.

''We abandonned the establishment of the mean diagram of intensity for most of the storms. But mean diclgram of intensity and pattern of isOhyets are kept for very interesting stonns (relatively to dept h, intensity and for possible use as unit storms).

At this time, for basins in plains, the mean depth is computed by computer u.sing the method of THIESSEN polygons. The isoh.yetal pattern is esta-blished for the more interesting sborma,

A programme-has:-r'ecerrt.1y..~been established for the processing of

inten-<:lJ

sities" but it is rather complex. What is important; in suchYcase is to avoid the loss of any element of information collected in the field. Blt with the use of computer the primary checking of rough data is more important. as before the use of computer. It is absolutely necessary to establish with great care the table o£ rough data used for punching of the cards.

6.2) - Processing of discharge data: I f possible we avoid for Representative Basins to use computer for processing' of discharges measuremenbs • These measu-rements must be very precise, it is necessary to know perfectly well the hydrau-lic phenomena in the cross section and. for this reason it is a good thing to proceed by the isotachs method.

For similar reasons, it is necessary to draw the rating curve

manually. After this operation, it is possible to divide this curve in several portions of parabola or of logarithmic curves for the transformation of water levels into discharges by' computer-,

For this transfonnation in small basin, it is not wise to use a constant interval of time, in many cases it would be necessary to choose an interval of 10 minutes 144 intervals for one day 1 It is impossible •.

t

r

."

• I

-7

A programmetransfonns water levels into discharges only for the points of breakage or change of direction of the curve of the water 1eveJ. rela-ted to time.

The use of reader of curve, like BENSON apparatus is very usefuk for this, but this operation involves an important work of preparation of the chart before.

6.3) - Processing of ground-water and humidity of soil data:(and. other elements For the level of ground-water the work of processing is not. very important but for humidity of soil it imro1vesnumerous operations if the basd.n is specia1.1Y instrumented for systematical study of this e1emeIIt. It is not the general case. For evaporation the problem of processing has been dealt with in other conferences.

7) - Some concepts on interpretation and analysis :

For .i l l purposes mentioned in·the first lecture it is necessary to transform. the set of data obtained during a short; period: 2 to

6-7

years into data reJ.ating to a long period and to look for the permanent characteristics of behaviour indepenciantly of the variations of precipitations and other clima-to10gical factors in tiJD.e.

It is necessary for this reason to represent the behaviour of the basin by a model. This model will Use the data of one or several raingauge stations operated during a long period,

30

to

40

years (stations of general network) and will transform these precipitations data into runoff for instance and by this way it is possible to obtain Qischarges for

30

to J.i) years.

It is the reason why the elements of the model must -be based on pby-siographical characteristics of the basin and on precipitation and not on

discharge. For instance it is possible to obtain good correlations between precipitation and runoff using Q , base flow before each flood as secondary

o

factor. lht this practice is not. to be recommended because_. Q₀ is not given in long term. series of precipitations. Nobody has to be afraid by the word model. A model could consist in the 3 following curves : uni..t graph repreaem.ed by a triangle, runoff coefficient

BR

= Runoff mm versus Precipitation

·

. 8 -CorreCt'ion curve

D..RR

'versus anliecedenli precipitation index AI. P I.

ror our simple purpose we can define 2 sorts of models : global modeJ and matricial models.

For global models the' characteristics of the basin and of the storms are supposed homogeneous: P uniform in space" runoff coefficient uniform in space etc ••• The unit graph is a part of a glob~ model. I f the precipitation depth is too heterogeneous this method gives bad results.

In the matricial models we divide the basin into small parts and

for each part" the pbysiographical characteristics" precipitations and runoff are supposed homogeneous. For instance, the isochrones method corresponds, to a matriciaJ. model with double division by Lsochrons.lines and by the THJESSEN poly·

,.

gons ,

Tb

I

I

~I

K

%

d$

One element like dS is an element of matriciaJ.·model.

Before the use of model we have not to forget the statistical basis of the interpretation : ~ood statis-tical analysis of data of precipitation station

observed during a long period shall preceed the use of model. Quality of data of these stations has to be checked before statistical analysis.

''1

_{I .}

7.1) - Global models ': For most of the Representative and ExperiJn.entaJ:, Basins the area is small and we use genera.l1y globaJ. modal.s , It is not too difficult to Use global model t-rithout computer. But with computer~tricia1models are not more difficult to manipulate and in the future the use of global model wi1J. be reduced.

The limitation of use of globaJ. model is the dimension of the catch-ment and. the heterogeneity of this basin and. of the precipitations~ ,

It will be spoken here only of the transformation of precipitat~ons into runoff.

" ,

.,.

i~

,

.

-~

9

-Tb'O important elements are to be considered: . .

1°) - the transformation of a given volume or depth of precipitation into a vo~ume or depth of runoff : for instance

60

mm. of precipitation into

6

mm. of runoff

2°) - the distribution in time of this. runoff or more precisely the shape of hydrograph.

7.1.1) -

Determination of the element of the hydrograph :

There are several methods. Here only the unit· hydrographs method . will be presenbed, The general. principles are recalled here:

- For a storm homogeneous of very short duration, the time of surface runoff is constant for aJ.1 depths and aJ.1 intensities of precipitation

- The ordinates of the hydrographs for two differenl; storms are deduced from

one

to another by using a coefficienl; equal. to the ratio of ra:inf'aJJ. excesses (or ratio of volumes of surface runoff)

- One storm. of too long duration can be divided into short storms and the resul-ting bydrograph can be obtained by addition of hydrographs correspoUding

to various partial stonns with lags corresponding to the durations of these successive partial storms (property of additivity).

In fact this is not so simple : a catchment may present different forms of unit bydrographs : one for the srn.a.ll and moderate floods and another one for the large floods. Strickly speaking the unit hydrograph sh6ul.d be used only for surface runoff. In fact. it is used sometimes for intennediate f'10'"1'l.

The bydrographTIJ.8Y be defined by

3

elements .;.. the rise time

- the base time or surface l"UIlOff time

- the ratio

01...

= ~ between QM : discharge of the peak of surface .Qn

I'UDOff and Qn average discharge during the base t:im.e or sometimes

c:Z

for a triangle 0( == 2

for small basino!.... =

3

inmany cases

for smaJJ. basin with intense runoff

4

~o<.

<

5

for sm.a1l basin in tropical fo rest 1;

7<;

c>«

-e;

2

;

..

.

(,

-

10-How the elements of the unit hydrographs can be detennined for an index cat chnent .? :

10) - Find the unit storms : they present. 2 conditions :' to be homogeneous and short enough.

The simultaneouB examination of the isuhyetal pattern and of the in-intensities diagramr. permits a first selection. For all the rough hydrographs .the ris.e time shall be noted :. there is a certain dis-persion. The small values are related to storm for which there is surface runoff only on a part of the basin. The biggest values are related to sto~s with too long duration (for the part of storm with high and moderate intensities). By this indirect way it is possible to obtain an idea of the limit time of duration.

No~this limit,. is less than a third of the rise time. This offers a second elimination. In many cases~ after. this double selec-tion the number of remaining sborms is very low and sometimes it is necessary to consider storms with duration of half of the rise time • . It is better to eliminate sborm with too small surface rumff than a

too long sborm with intense runoff.

2°) - mer thissele~ionone has to proceed to the separation of surface runoff from the other forms of flow (intennediate flow;underground flo1

i:.. :1. -:: :;. _':". : ':; ..,I.n order to obtain comparable results for estima-tionof the

3

_{parameters TmJ Tb,o<:..,reconnnandations have been pUblished by} ~'lMO

in a technical note. The simplest method consists to drive a straight line between the beginning of the rise of the flood rot too difficult to determine on the rough bydrograph and the first change of slope on the recession curve; . more easy. to see with logarithmic' ordinates. This simple procedure is

recommen-ded when the runoff is very important in regard to intermediate flow.

After this. operation the bydrographs corresponding' to unit stOI'DlS are established only for surface runoff (the other forms of flow being elimina-ted). They are 1:ra.PsfQrmed by multiplications of the ordinates in such a way that each of them corresponds to a surface runoff of 100 000 m

3

or 1 000 m

3

or'l mm on all the

area.

The hydrographs for aD. unit .stom.s may be represented t

- on a common graph

- on a table with in the central column the various ordinates of the' peak discharge and in the other columns the discharges 10~

zo,

30 mi-nutes etc... before and ai'ter the maximum

- on a table with the..various values for the various unit aborms of the

3

parameters T ; Tb

arid

•

.

.

..

11

-Following the purpose the hydrologist chooses the mean value or the most dangerous value for ordinate of maximum'and other ordinates or for T_{. m}I Tb

7.1.2) - Determination of the relationships rainfall/runoff :

There are many approaches. One is the infUtration approach. For each storm the rainfall excess isdetennined on each intensity diagramm giving a value of infiltration (or infiltration

+

interception) losses ; and an idea of the variations of these losses with the duration of rainfall and the ante-cedent conditions.

tOff

..

~.~.)'~" .._: ~. ," .'.:;,

. ; .' M• • • • . " - : . " • • • •' "

.'

•

lime

'r

irn e

Another approach uses directly the relation rainfall/i-tmoff •

.The runoff. R in mm i~ compared with depth of precipitation P in mm or the runoff coeffi-cient

KR

=

~ is compared with P in mm.

P mm ' .

P is the ~ precipitation on the basin. The Use of

BR

has the advantage of more easy comparrsons . from' one basin into arobher-, The definitive curves are similar to the following figures.

\<R

%

Prnm

R

rnrn

/.

/

r

..

.

.

:;

(",

'.

_~

..

J

12

-The use of logarithmic values permits to obtain straight lines. Genera1 formulas of these curves have been estabJished. But other factors than depth of precipitation are important such as : the anbecedenf conditions of humidity of the soil,the total amount of: precipitations from the beginning of raiD\1 season, the intensity pattern of precipitations, the distribution in space of precipitations, the season etc ••• and the representative points in the dia-grams above

are

scattered.

. .

In order to obtain a better representation of the relations rainfaJJ./ runoff it is possible to u~e the residual. method, taking into account the most important of the factors mentionned above. P being gener~the main factor,

. " ' . t i

after P the second is often the humidity index like KOHIER index I=~P.k

Oc<

or another illdex I =

~

Pi • The, difference between IL of each point

o~

the

(ti)Il . " .

-a

' ,

diagram

ER

related to Pm and the ordinate of the mean curve.6. ~ is correlated with I corresponding to the representative poinli and we obtain a mean curve like presented below. :'

The points of the first diagrarrs: are corrected using the conrectdon: curve(.6

BR

vs I) and suppo-sing an uniform value of I

==

I •

o

Wecbtain amther cloud of points with less dis-persion than in the first one

(KR

vs p).

This second curve

ina.y

be differeIIt from the first one. In such a case we correct, the curve and we reconmence. If the scattering is too important we look for another parameter,for instance intenaity,we establish. " a second curve of correction.

GraphicaJ.J.y the problem is simplified by the coaxial method. ID these operations may be realized by computer.

fut some precautions have to be taken:, for instance not to use too many parameters, without that the model rtill fit perfectly well for the index catchment stadi.ed and be of poor value for the comparison~with the models of other index catchments.

"

.

I

..

.13

-The model consists in the

3

parameters T

m, Tb,o( and 2 or

3

curves defining

Ira

or R mm from P mm.

. P is defined by the depth';; the intensity pattern, the distribution in space and by the anbecedenb moisture conditions.

7.2) - Summary of concept of matricial model: The basin is divided into element~ of small area. For each element there is a pattern of variations of precipi-tations in time,; a curve of variation of runoff' coefficient with precipitation depth and .duration of the storm, a curve of variation of discharge for a preci- . pitation of short duratzlon

~

even for an area of 10 m2 and a precipitation of 2 minutes; 'there is an bydrograph, with smooth variations of discharge.

The elements of the basin are defined bytheisochrone lines and the side of THIESSEN polygone. The runoff is routed up to the station. ID. the parameters' are adjusted to fit with observed floods. \'lith computer, methods of optimization are used for this purpose. And the results are checked with observed floods but not with the floods already used for stablishing the model. It is in almost aD. cases necessary to use computer for matricial models.

The g10bal method is often usedfor representative and experimental basins because it Ls simple and it does not need of a computer, but in the future its use will be not so important.

7.3) -

Utilization of results : \'lith the global model it is possible to compute

the flood for each storm during 30 years from data of precipitation of a long term station after reduction;' of precipitation taking into account the area. We have considered above, the case of storm or of da.ily rainfall. I f \'Te consi-der only the volume of runoff and not the shape of bydrograph and the peak. value, it is possible to proceed to similar analysis with runoff of 1 1oIeek, 2 weeks or one month with the same multiple regression. This permits' the compu-tation of mean weekly or monthly mean discharge.

7.4) -

Generalization of results : First of all, it is necessary for the various catchmerrts to use homogeneous results determined in such a way that they are comparable. At the same time it is necessary to detennine good quantitative estimations of various physiographical parameters (slope, permeability of Boil, density of drainage etc ••• ).

.

I

14

-After that it is possible to compare the results (for instance ave-rage annual, now in l/s x ffin.2 or the 10 years nood) checking the in.f1.uence on these results of the physiographicaJ. parameters. At the same time we may use together the results of general network. It would be better to compare the models of various catchments. But at this· time, it is difficult to obtain .

comparable forms· of models even if the modeJs are established in the same service; because often, the particu.la.r features of basins lead to simplifications which· result in different types of models. Much is to do for uniformization of models.

At this time, analysis and interpretation are evo1uting in relation with the generalization of use of computers. Simp1B forms of modelswith use

.of fictive~eservoirs are elaborated and it is possible that in the future,

we

.r will abandorr.c-; the use of residual method but for the t:bn.e being it m>uld be premature in many cases to use this sort of model in Africa.

8) - Publications of results :

What is important is to try to speak the same language in the publi-cations. For instance,use the symbols prepared by WO completed by those include<

in generaJ. publications of results in USA (Department of lfIgricultureh New-Zealand (Public Works)" France (ORSTOM).

Rodier Jean.

Use of index catchments in evaluation of water balance. In :

Conférence d'Entebbe.

Paris : ORSTOM, 1972, 25 p. multigr.