The author has identified the following significant results. It is proved that FCC's can be used for a simple estimate of the total evaportranspiring area of the Okavango Delta, sufficiently accurate for preliminary inputs for the development of mathematical model of the surface hydrology of the delta. The color coded matrix has shown as interesting inverse correlation with an array on the same grid prepared by ecologists from air photography study, for percent liable to flood

Dickson, W. L.

Wilson, B. H.

English

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Second -1--ditior Re. tisi 4
A)	 BRT ^'^ ."Ol' W-On Program No. 28070^ . . .,. —
An Aid to the De--,;.,- aent of Botswana's Resources
Lluarterly Report. May 1976	
7. 6
SECTION ON HYDROLOGY	 - ^77
Author: Brian Harold Wilson
Senior Hydrological Engineer
FA"	
Department of Water Affairs
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,..i Lr	 Government of Botswana
L) n• , . on,	 i,
Co-ordinated by: William L. Dickson
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"Made 9vailable under NASA sponsors Private Bag 0037,
irector of Surveys and Land-t
in the interest of early and wide dis.
a	
semination of Earth Resources Survey Gaborone, Botswana, Africa.
I.	 Introdu	 information and without liability
11W a" use; made thereot.1 1
The Hydrological work on this program is divided between
two test sites having different functions. The objectives may
be summarised as:- to find out in Test Site I how far Landsat
imagery may be used by simple means to determine the seasonall-
changing pattern of flooding at any period in the Okavango Delt
and, in Test 2, to prepare a base line map of development, ast 
'1 975, in the Okavango catch-ment outside Botswana, to be user'
for co.mparative purposes in the future at five or ten yearly
intervals.	 I
JW 4J
	
W	 Progress may be summarised as follows: two to three Lands7-,
2 coverages of the Delta have been received and one to four
E-4	 04
V) WU coverages of the Okavango catchment. These have been subjecte".
to preliminary scrutiny and comparison. Plans for the more
detailed subsequent investigation are being developed. Quite
	
ow	 extensive ground truth work has been done in the Delta,
-.9
a
0U04= including commercial air photography, observation from low
	
 W	 flying aircraft, field e3,meditions and hydrological measuremer-,sp^ QJ	
.1Mtn	 by ground parties. Unfortunately, owing to the unpredictable
u
.
1	 dates of imagery, much of the ground truth cannot be precisely
	
Cp =
04 
W	 related to it. However, the general pattern of change in the1- W ,
139 4J W Delta is shown by both Landsat and ground truth. It is hoped
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r- £"'.,M 0 that detailed, work will show significant correlations and that
44
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10 in future some expensive field measurements may be eliminated
or reduced by using Landsat instead. Related to the agreed
co-operative Program between NASA and Botswana Government l but
not forming official part of it, is a UN and Botswana Governmen',
programme of relating ground truth of ecological parameters,pi-l.-ii
by/pixel,. to CCT's, for which purpose a Consultant has been engagel.
71-7,II. Technique
1C
(a)
	
ER' J"..'F, I
It had been hoped that ERTS I would enable 10
to 20 hopeful small areas ofthe Okavango Delta in WTest Site I to be pinpointed in 1973 for aetailc-,dinvestigation. Unfor.tunately ERTS I imagery ceased	 >
after only six months operation, which did not
include the period of rising flood. Also the ERTS 1
negatives at 70 mm received here were of a quality
which could not be used by us. Although negatives were ilt
made here from the I in I million positives,. resolution,--,.--^'.
was lost and the density scale became too distorted for
meaningful comparisons, which were therefore postponed
a.	 2.---	 -	 -- ----	 -- -, - -- -1--------- -- . ....	 J,until Lands. ,t.
-4
- 2 -
(b) Comparison of 1/500 000 prints
	 I
After various different preliminary approaches,
paper prints of MSS 7 at I in 500 000 were prepared
here from 70 mm negatives. Four frames, dates
Aug+ist and September 1975, were assembled into a
mo;;aice A 5 km square grid was made as a transparent
overlay, corresponding to the 5 km grid used for
low flying ecological studies by colleagues in a
UN/Botswana Government study of the Okavango Delta.
The-Aug/Sep '75 mosaic: was then compared on
the grad, square by square, with imagery for November
1975 (this being the greatest contrasting, but comparable,
imagery available up to the present). Each square
was rated subjectively on a scale O to 5 for detectable
contrast between the two dates. Differences possibly
wholly attributable to fire were marked "F". Otherwise
an attempt was made subjectively to discount the component-
of change due to fire.
The marked ratings were then made readily
visible by shading in colour on a code: red, half red,
yellow, half yellow, white.
The results are shoi^i and discussed in Section
III below.
The next stage of the investigation, now beginning, 	 qis to use the results of the grid rating to select
significant places where the degree of flooding can
be quantified in one of the following ways:-
O Flooded/Not flo oded.
(ii) mean width of ^hannel etc.
(iii) visually planimetered area of flooding
as percentage of the 5 km square	 f
(iv) visually planimetered area of flooding
in the 5 km square. (This last, though
as percentage of the area liable to flood
involving more preliminary work . witp air
photos, can probably give a much more
accurate and significant total, capable of
extrapolation to the other areas.)
A. "significant_ place" in the above paragraph
means either a place which seems capable of relating
to known, or easily determinable, ground truth, or
one which would appear to be indicative of 'flooding
over a large area,
(c)	 Consideration of Other Methods	
-
U) Planimetry
An attempt has been made to .esttimate the
tota'..awamp area (liable to uninhibited
evapotranspiration) by planimetering the
areas appearing to be flooded as revealed
on false colour composites of sRTS 'I at I
in 500 0000
_3.
This has been used as an input for the
development of a simple mathematical model
of the Delta for desk computer (Olivetti
Programma 101). FCC's are better for this
;purpose than MSS 7 since the permanent swamp
area is largely covered by aquatic vegetation,
much of which gives a high reflectance on MSS 7.
As FCC's are expensive and subject to
delays and the planimetry rather inaccurate with
eyeball methods, this procedure for assessing
seasonal flood change is not at present regarded
to be as practicable as MSS 7.	 But FCC's should
be used for occasional checks.
(ii)	 Contrasts and Seasonal Chang, by Colour
Additive Viewer
After seeing demonstrations in Nairobi and
Pretoria we regard this method as too expensive in
hardware for immediate application here.
(iii)	 Contrasts and Seasonal Change by Density
Slicing and Electronic Planimetering
This method is also too expensive in hardware.
Also the advantages of the method would be somewhat
nullified by the difficulty of distinguishing
between fire and flood except by human judgement	 7
(iv)	 Contrasts and Seasonal Change by Stereoscope 	 "{
An ordinary mirror stereoscope has been	 -
tried to compare MSS 7 at different dates.	 This
proves to be a quick method of dete,dting change
by winking the eyes.
	
Possibly this, in the long
run, using 1 in 1 million prints or transparencies,
may prove to be better than the method of comparing
1 in 500 000 prints.
	 It will, however, be necessary
to use a completely transparent graticule in order
to tabulate the information.
(v)	 Diazo Methods
_s These have not yet been tried but, on
recommendation, materials have been ordered for
some experiments, including, it is hoped, positive-
negative masking.
(vi)	 Use of Bands Other than MSS 7
Apart from occasional use to prepare FCC's
to reveal the total evapotranspiration area, use
of the other bands seems to be a complication
	 -^
not at present applicable to our 'limited time
and limited hydrological objectives.
	 However
the hydrologists engaged on this program have
fruitful contacts with botanical and ecological
colleagues and it may be that multiband work in
co-operation with them may be revealing.
.- 4-
(d) Technigye in Test Site 2
The objectives are being simply achieved by making
a I in 'I million mosaic from selected MSS prints, using
the other bands visually (or perhaps later with Diazo
methods) to pick out agricultural development.
(e) Ground Truth
Water..levels, discharges and, to a lesser extent,
flooded areas have been recorded for a number of parts
of the Delta during the whole of the period of ERTS 'I
and Landsat 2.
Air photographs covering the whole flooded area
were taken at a low water period in 1973 and at a high
waver period in 1974 at a cost of some Z 50 000.
Unfortunately Landsat was not operative during those
two periods, and direct comparison of detail is not
possible, though indirect comparisons are being made.
From mid 1975, low level flights on a 5 kin gridhave been flown at four - monthly intervals with
detailed observations of ecological parameters including
some of hydrological importance. A large mass of data
has been accumulated. An attempt has been made to
a	 display the hydrological content visually in a simplified
manner in 5 km squares for comparison with Landsat.;
f k
In the first two weeks of October. 1975 some six	 .`
field parties went into the remoteparts of the Delta
to determine water levels and extent of flooding in rather
unknown area„ The dates were chosen because it was
expected that Landsat would be operative over the area.
Unfortunately- Landsat was not switched on for the two
successive passes nearest to the ground truth dates,
nullifying the detailed tests of Landsat's ability to
distinguish just-flooded and just-not- flooded land.'
During 1975 seven special water level gauges were
established to enable water levels to be determined from
low flying air-craft. These have proved quite successful
and it is easy to determine water levels to better than
1 cm accuracy. A modified gauge has now been designed,
and it is hoped to establish more of these gauges. The
modified gauge is quite cheap to manufacture, but
establishment in remote parts of the Delta is expensive
in time and transport, requiring long journeys by rough
track, boat, dugout canoe and on foot. The Department
hoped to obtain a hovercraft early in 1 975 to assist
in this work but the manufacturer went into liquidation.
III & IV Accomplishments and Significant Interim Results
The interim accomplishments of this Program are two:-
(a) It is proved that FCC's can be used for a
simple-estimate of the total evapotranspiring area
of the Okavango Delta, sufficiently accurate for
preliminary inputs for the development of a mathematical
model of the surface hydrology of the delta. With
the limited equipment available to us here, we do not
j	 see any very great accuracy obtainable by this method
alone. 	 _
5-
(b) The subjective assess-ent of 'I in 500 000 prints
MSS: 7 at different dates shows the following results:-
Out of a total of 1102 matrix elements.
(each one 5 km square):
Contrast rating oa 0	 345	 elements
1	 127
2	 76
3	 90
4	 123
5	 197
F	 24
Not yet analysed due to
absence of suitable imagery 	 120
Although the intended purpose of the above preliminar
procedure was simply to pick out useful test areas to
relate to ground truth and to eliminate those of no use,.
the coloured matrix has in fact proved a stimulus to
thinking about the hydrological mechanism of the delta,
as it highlights the areas of seasonal change, leaving
the permenantly flooded and the permenantly dry areas
uncoloured. The colour coded matrix hat also shown an
interesting inverse correlation with an array on the san..
grid prepared by ecologists from air photograph study;
assigning the elements as "percent liable to flood".
Hydro/ecological information obtained from low flying
aircraft on the same grid also shows promise of useful
correlation.
Publications
Nil
Problems i
(a) Really effective use of Landsat, and in particular
the accurate ana sure correlation with ground truth in
remote areas is hardly possible unless one ]mows in-adva^	 y
on which days the satellite is going to be switched on oi..
Botswana. Much ground truth obtained in October 1975
was wasted when the satellite missed two passes. We
cannot afford to repeat such a ground truth exercise. Zc
there any way we can be better co-ordinated with NASA
in this matter?
(b) Although we are impressed by the speed and efficienc
of the EROS Data Center's computerised service, some
omissions have been detected, in particular the computer
did not list the following FCC's although our Geologica?, 	 a
Survey Department were 'supplied with them by NASA in
1972/3:
16 SEP 72	 08023
I5 SEP. 72	 07565
26 NOV. 72	 07574
06 'FEB 73
	
07573
15 SEP. 72 07571
26 NOV. 72 07580
19 JAN. 73 07574
r.II
^G
Y rµ
a
V„
VI
- F -
Miis waz in answer to our enquiry your ref BOX 8
53/s 18D S20D 30M E22L` E24D date 02/27/7E Report DI002
Timm 09;26).
(c) We do not quite understand how the standing order
system works as no imagery was supplied for the perioi
Feb, 1975 through June 1975 although listed by the ERCS
Co,-puter as being available.
(0 There is still some delay in the receipt imagery.,
(e) On our side we have had problems in making staff
:available for Landsat analysis (as distinct from r.roa.::'_
truth) until recently.
(f) Our enlarger will enlarge 7 times only, i.e. up
,o 1 in 500 000 from 70 mm negatives. For greater
mag"ification purposes we propose to ask for 1 in 1 nil -"...
_:^gatives, also useful for Diazo eomparions.
(g) Bush fires complicit, interpretation. We; have not
disco-itred any non subjcctivt• way of distinguishing °i r,
from flood as the spect-PI signatures appear similar.
Corr.-ion sense enables the larger fires to be: immeaint'-'
eli.::i nated and some of the: smaller ones. The: difficii.1.
arises with small fires adjacent to channels, which
-..ght, on the face of it, be either fire or flood.
Successive Landsat passes, when available:, often in0i.c-.
which i_s which, though the: procedure is tiresome:. `.'c
u orking on methods of analysis which will ^-nablc sure,
fire areas to be ignored without loss of significanc-
Data rality and D(Aivury
Qi ality and resolution excellent.
Tuna site coverage:
Test Site 2 more than adequate,
Test Site 1 nothing received before July 1 975 or
after December 1975. This is inadequate for
seasonal comparisons.
Two critical passes missed in Sept/Oct 1 975 (see
!'rob-',---.ms above).
',lo knowledge of datt: of imagery until w,- . ks afterwardo.
Rcccipt of data somewhat delayed and nuvtr quick enouc'i
for feed-back to ground truth teams.
II. Recommendations
11i general proee;ed as plannt.d.
It is essential to extend the period of imagery obta;_-i:,_
th<: Program, backwards to September, 19`J2 and forward,,
i Dece=mber, 1976 or thtruabouts.
Conclusions
The '_nterim conclusions are in general optimistic.
^^	 r


An aid to the development of Botswana's resources: Section on hydrology

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