8/3/2019 Science 789

1/17

E L S E V I E R Journal of Hydrology195 (1997) 335-351

JournalofH y d r o l o g y

Mo n i to r in g s m a l l d a m s in s e m i -a d d r e g io n s u s in g r e m o tes e n s in g a n d G I S

J.W. FinchInstitute of Hydrology, Wallinfford OXIO 8BB, UKReceived 6 September 1995; accepted 26 June 1996

AbstractThe analysis of data from high spatial resolution satellite-mounted infrared sensors has the poten-tial to monitor the water stored by small dams in semi-arid areas. A simple method of analysis isdescribed and particular attention is given to selecting an appropriate threshold level to discriminatewater from non-water land cover. The use of a Geographic Information System (GIS) allows furtherdiscrimination of the areas of water from other areas, suchas deep shadow, which have beenclassified incorrectly and facilitates the automatic calculation of dam capacities. The success ofthe technique is demonstrated on dst~ from two regions in Botswana.

1. IntroductionThe major ity of the semi-arid regions of the world occur in developing countries and are

distinguished by distinct wet and dry seasons. After the rainy season has finished, watersupplies for industry, agriculture and the human population are sustained from ground-water and surface water sources. In the case o f the latter, large impoundments may supplythe bigger population centres but the rural areas are often dependant on much smaller damswhich are not monitored. Monitoring these resources can give advance warning ofimpending water shortages, allowing actions towards ameliorating the situation to betaken in good time. Remotely sensed data has the potential to monitor the volumes ofwater stored by these dams by measuring the surface areas of the lakes if a simple andpractical method can be developed that can be used be used within the countries.

Sensors with a high spatial resolution are carried on the Landsat, IRS and SPOTsatellites. Dam from these sensors are capable of discriminating areas as small as0.05 ha and so are suitable for monitoring the surface areas of small lakes. In addition,the satellites pass over the same area about once every two weeks and so have thepotential, under cloud-free conditions, to monitor the decline in the volumes of water0022-1694/97/$17.00 O 1997- Elsevier Science B.V. All fights reservedPII S0022-1694(96)03228-3

8/3/2019 Science 789

2/17

336 J.W. Finch/Journal of Hydrology 195 (1997) 335-351s t o re d b y t h e d a m s t h ro u g h th e d r y s e a s o n. D u r i n g t h e s e p e r i o d s c l o u d c o v e r i s a l s o a t am i n i m u m . Th i s p a p e r d e s c r i b e s t h e r e s u l t s o f a p ro j e c t t o d e m o n s t r a t e t h e f e a s i b i l i t y o fs u c h a s y s t em w h i c h h a s b e e n a p p l i e d t o d a t a f o r t w o r e g i o n s , c o v e r i n g 7 9 4 0 k m 2, i nB o t s wa n a , S o u t h e rn Af r i c a .

2. BackgroundO n e o f t h e e a rl ie s t s tu d ie s t o d e t e c t w a t e r b o d i e s u s i n g r e m o t e l y s e n s e d d a t a w a s c a r r ie do u t b y W o r k a n d G i h n e r ( 1 9 7 6 ) w h o u s e d d a t a f r o m t h e M u l t i - S p e c t r a l S c a n n e r ( M S S )

c a r d e d o n L a n d sa t -1 t o m a k e a n i n v e n t o r y o f p r a i ri e p o n d s a n d l a k e s in N o r t h D a k o t a ,U S A . T h e f ir st m e t h o d o l o g y u s e d w a s a s i m p l e t h r e s h o l d i n g t e c h n i q u e o n d a t a f r o m t h en e a r - in f r a re d b a n d t o d i s ti n gu i sh b e t w e e n b o d i e s o f w a t e r a n d o t h e r l a n d c o v e r t y p e s a n dt h e n t o e s ti m a te t h e a r e a o f e a c h w a t e r b o d y . T h e t h r e s h o ld i n g t e c h n i q u e a s s u m e s t h atwa t e r r e f l e c t s s o l i t t l e i n f r a r e d r a d i a t i o n t h a t a l e v e l c a n b e s e l e c t e d , a b o v e wh i c h t h er a d i a n c e d e te c t e d b y t h e s e n s o r i s d u e t o o t h e r l a n d c o v e r t y p e s . W o r k a n d G i l m e r c o n -e l u d e d th a t, u s in g t h e M S S d a t a w h i c h h a s a n o m i n a l g r o u n d r e s o l u t io n o f 8 0 m b y 8 0 m ,p o n d s a b o v e 1 .6 h a w e r e r e c o g n i z e d c o n s i s t e n t l y b u t t h e r e c o g n i t i o n o f p o n d s w i t h a r e a sb e t w e e n 0 . 4 a n d 1 . 6 h a w a s d e p e n d e n t o n w h e t h e r th e p o n d w a s w h o l l y i n c l u d e d i n o n eg r o u n d e l e m e n t o f t h e r e m o t e l y s e n s e d d a t a o r f r a c t i o n a l ly d i s tr i b u te d o v e r s e v e r a l. T h es e c o n d m e t h o d o l o g y u se d a n a l g o r i th m t o e s t im a t e t h e p r o p o r t io n s o f l a n d c o v e r t y p e sw i t h in e a c h g r o u n d e l e m e n t f r o m i n f o r m a t i o n i n s e v e r a l s p e c tr a l b a nd s . A l t h o u g h t h i sg a v e i m p r o v e d e s ti m a t e s o f t h e a r e a i t w a s a t t h e c o s t o f c o n s i d e r a b l y in c r e a s e d c o m p u t i n gt i m e a n d t h e n e e d t o i d e n ti f y m a n u a l l y t h e s p e c tr a l c h a r a c te r i st i cs o f e a c h t y p e o f l a n dc o v e r . T h is t y p e o f m e t h o d o l o g y h a s b e e n d e s c r i b e d b y m a n y a u t h o r s b u t t h e w o r k o fS e t t l e a n d Dra k e (1 9 9 3 ) g i v e s a p a r t i c u l a r l y c l e a r d e s c r i p t io n . A l t h o u g h i t i s p o t e n t i a l l y ap o w e r fu l t e c h n i q u e , i n p r a c t ic e i t i s d i f f ic u l t t o u s e . Th e m a i n p ro b l e m i s i n d e f in i n g t h es p e c t r a l c h a ra c t e r i s t i c s o f a l l t h e l a n d c o v e r t y p e s r e p re s e n t e d i n t h e s t u d y a r e a a s i t i se s s e n t i a l th a t th i s i s d o n e a c c u ra t e l y i n o rd e r t o o b t a i n r e l i a b l e e s t im a t e s o f l a n d c o v e rp ro p o r t i o n s . I f t h e s p e c t r a l c h a ra c t e r i s t i c s a r e d e r i v e d f ro m t h e r e m o t e l y s e n s e d d a t a i t s e l ft h e n i t m u s t b e p o s s i b l e t o l o c a t e v a l u e s w h e re o n l y a s i n g l e l a n d c o v e r t y p e i s p r e s e n t ; ad i f f i c u l t p ro p o s i t i o n in m o s t s e m i -a r i d l a n d s c a p e s . An a l t e rn a t iv e i s t o d e f i n e th e s p e c t r a lc h a ra c t e r i s ti c s f ro m g ro u n d m e a s u re m e n t s b u t , i n o rd e r t o u s e th i s i n fo rm a t i o n , i t b e c o m e sn e c e s s a r y t o f u ll y c o r r e c t t h e r e m o t e l y s e n s e d d a t a f o r t h e e f f e c t o f th e a t m o s p h e r e . A g a i nt h is i s a d i f f i c u lt a n d t i m e c o n s u m i n g t a sk .S u b s e q u e n t u s e o f r e m o t e l y s e n s e d d a t a f r o m s a t e ll i te s h a s t e n d e d t o a d o p t t h e th r e s h -o l d i n g m e t h o d b e c a u s e o f i t s s im p l i c i t y (R u n d q u i s t e t a l ., 1 9 8 7 ; M a n a v a l a n e t a l. , 1 9 9 3 ) .J o h n s t o n a n d B a r s o n ( 1 9 9 3 ) m a d e u s e o f t h e i n c r e a s e d s p a ti a l r es o l u ti o n o f L a n d s a t T Md a t a a n d s e l e c t e d a m i d d l e - i n f r a r e d b a n d f o r t h e i r a n a l y s i s . T h e y c o n c l u d e d t h a t t h em e t h o d g a v e e s ti m a t e s t h a t w e r e w i t h i n 2 0 % o f e s t im a t e s d e r i v e d f r o m m a p s .P re v i o u s s t u d i e s h a v e t e n d e d t o d e a l w i t h a r e a s o f s u b d u e d t o p o g ra p h y s o t h a t t h es i m p l e t h r e s h o l d i n g m e t h o d i s a b l e t o u n i q u e l y c l a s s i fy a r e a s a s wa t e r o r n o n -wa t e r . I na r e as o f m o r e p r o n o u n c e d t o p o g r a p h y , a r e a s o f s h a d o w c a n b e m i s c la s s if ie d a s w a t e r a n ds o s o m e m e t h o d i s r e q u i r e d t o s e p a ra t e t h e c o r r e c t l y c l a s s i f i e d a r e a s f ro m t h e r e m a i n d e rb e fo re t h e d a t a c a n b e u s e d t o e s t i m a t e wa t e r r e s o u rc e s . Ma n u a l i n t e rp re t a t i o n o f t h e

8/3/2019 Science 789

3/17

J.W. FincldJoumal of yddogy 195 (1997) 335-351 337processed data is implied in the studies quoted above but the development of GeographicInformation System (GIS) provides the oppommity for a more automatic methodappropriate to an operational system.

3. TheoryAnalysing the amount of reflected radiation in an infrared spectral band is a simple andreliable technique for detecting water because the amount of radiation reflected by water atthese wavelengths is usually less and more uniform than that from other types of landcover. The amount of infmred radiation reflected by a body of water is a combination ofreflection at the air-water interface, retlections from particles suspended in the watercolumn and reflections from the bottom. The thresholding technique assumes that the

amount of radiation reflected back to the sensor from these is negligible and the citcum-stances in which this assumption is justified are discussed in this section.High spatial resolution sensors mounted on satellites have narrow instantaneous fieldsof view, Table l., with the result that they are limited to near vertical observations. Thismeans that radiation reflected by water and detected by the sensor must emanate from anear xenith position (Cox and Munk, 1954) because water surfaces reflect radiation spec-ularly, i.e. incident radiation is reflected away at the same angle rather than being scat-tered. This is unlikely to occur except at a few times of year at latitudes between theTropics because the satellites cross the equator about 2 hours before solar noon. Anexception may occur when strong winds cause the development of waves on large bodiesof water resulting in some facets of the waves reflecting radiation to the sensor. Diffuseradiation emanating from a near-zenith position could be reflected from the water surfaceand detected by the sensor. However, diffuse radiation from the sky is much weaker thanthat directly from the sun, particularly at infmred wavelengths. In general, it is reasonableto assume that the amount of infrared radiation reflected from the water surface anddetected by the sensor is negligible. An exception can potentially occur with data fromthe SPOT satellite which has the ability to acquire data from a significant angle to eitherside of its track and so care may need to be exercised if this data is used.

The absorption of electromagnetic radiation by water is least at visible wavelengths,between 0.35 and 0.65 pm, and increases up to a maximum around 10 Frn after whichthere is a decrease (Szekielda, 1988). This means that it&red radiation reflected from theTable 1Clumxteristics of high spatial resolution satellite sensorsSatellite sensor Launched Spltial Instantaneous Repeat period Number of visibk

resolution (m) field of view () (days) and near-infmrcdbands

LandsatMss 23Krlfl2 79 11.5 18 4LandsatTM 16/07/82 30 14.9 18 6SpoTpan 22.02/86 10 4.1 8-26 1SPOTXS 22lw86 20 4.1 8-26 3IRS-1A 17KMial-t 72 9.3 22 4

8/3/2019 Science 789

4/17

338 J.W. Finch~Journalof Hydrology 195 (1997) 335-351b o t t o m o f wa t e r b o d i e s i s u n l i k e l y t o b e d e t e c t e d u n l e s s t h e d e p t h o f wa t e r i s l e s s t h a n0 .1 m . S i m i l a r ly s u sp e n d e d s e d i m e n t s a r e u n l i k e l y t o b e a s i g n i f ic a n t f a c t o r b e c a u s e t h ec o n c e n t r a t i o n s wo u l d h a v e t o b e h i g h t o r e f l e c t a n y s i g n i f i c a n t a m o u n t o f i n f r a r e d r a d i a t io nb a c k t o t h e s e n s o r (R i t c h ie e t a l ., 1 9 9 0 ) a n d t h i s i s u n l i k e l y e x c e p t s o o n a f t e r s i g n i f ic a n ti n f lo w t o t h e wa t e r b o d y fo l l o w i n g r a i n . I t is p o s s i b l e t h a t a p l a n k t o n b l o o m i n t h e w a t e rb o d y c o u l d c a u s e s o m e r e f l e c t i o n o f i n f r a r e d r a d i a t io n (D w i v e d i a n d N a ra i n , 1 9 8 7 ) , b u tt h is i s a l s o u n l i k e l y t o o c c u r t o a n y s i g n i f ic a n t e x t e n t .T h e a c c u r a c y w i t h w h i c h l a k e s u r f a c e a r e a s c a n b e e s t i m a t e d u s i n g t h e m e t h o d o ft l ir e s h o ld i n g in f r a r e d r e m o t e l y s e n s e d d a t a i s a fu n c t i o n o f t h e s i z e o f t h e l a k e , t h e g ro u n dr e s o lu t io n o f t h e d a ta a n d d i s c r im i n a t i o n b e t w e e n t h e w a t e r s u r f a c e a n d o t h e r l a n d c o v e rtypes , i. e . i t i s a func t ion o f the th resh o ld l eve l se t .3 .1 . T he e f f e c t o f t he t h re sho l d l e v e l

An a s s u m p t i o n o f t h e th r e s h o l Om g m e t h o d i s t h a t it i s p o s s i b l e t o s e t a l e v e l s u c h t h a t,f o r a p i x e l t h at i s a m i x t u re o f wa t e r a n d o t h e r l a n d c o v e r t y p e s , t h e p i x e l w i l l b e c l a s s if i e da s w a t e r i f th e p r o p o r ti o n o f l a n d c o v e r t h at i s w a t e r i n t h e c o r r e s p o n d i n g g r o u n d e l e m e n ti s g r e a t e r t h a n 5 0 % a n d n o n -w a t e r i f i t i s l e s s t h a n 5 0 % . T h i s i s t r u e p ro v i d e d t h a t o t h e rl a n d c o v e r ty p e s h a v e a u n i f o r m i n f r a r e d r e f le c t a n c e , b u t i n p r a c t i c e t h e r e a r e u s u a l l y m a n yt y p e s o f l a nd c o v e r , m o s t o f w h i c h a r e u n l i k e l y t o h a v e a c o n s t a n t i n f r a r e d r e fl e c ta n c e . T h ep ra c t i c a l s o l u ti o n is t o s e l e c t a t h r e s h o l d t h a t i s a p p ro p r i a t e t o t h e a v e ra g e r e f l e c t a n c e o f t h el a n d c o v e r t y p e s i m m e d i a t e l y a d j a c e n t t o t h e l a k es . T h e l a n d c o v e r s u r r o u n d in g t h e w a t e ri s o f te n e i t h e r s o m e f o r m o f v e g e t a t i o n d u e t o t h e p r e s e n c e o f h i g h e r s o il m o i s t u r e c o n t e n t so r b a re s o i l wh i c h i s e x p o s e d a s t h e l a k e l e v e l f a l l s. Th e re i s t h e p o t e n t i a l f o r s y s t e m a t i ce r ro r s i n e s ti m a t i n g t h e s i z e s o f t h e l a k e s i f t h e r e f l e c t a n c e o f t h e l a n d c o v e r a d j a c e n t t o t h el a k e d e v i a t e s f ro m t h is a s s u m p t i o n . I f Rw i s t h e r e f l e c t a n c e o f t h e w a t e r a n d P w i s t h ef r a c t i o n o f th e g ro u n d e l e m e n t fo r wh i c h w a t e r i s t h e l a n d c o v e r ty p e , R io i s t h e r e f l e c t a n c eo f t h e l a n d c o v e r a d ja c e n t t o t h e l a k e , a n d T is t h e t h r e s h o l d s e t t o d i s c r i m i n a t e b e t w e e nwa t e r a n d n o n -w a t e r , t h e n fo r a p i x e l t o b e c l a s s i f ie d a s wa t e r :

P w R w + ( 1 - P w ) R i c < T (1 )a n d fo r a p i x e l to b e c l a s s i fi e d a s n o n -wa t e r :

PwRw ( I - P w ) R l c > r ( 2)T h e threshold is selected so that the pixe l will be classified s wat er if the majority o ft h e l a n d c o v e r o f t h e g r o u n d e l e m e n t i s w a t e r , i.e. o r t h e c a s e w h e n P w is e q u a l t o 0 . 5, n dso:

T = 0 .5 (Rw +R ic) (3 )b u t i n t h e p re v i o u s d i s c u s s i o n i t is a s s u m e d t h a t n o i n f r a r e d r a d i a t i o n is r e f l e c t e d f ro m t h ewa t e r t o t h e s e n s o r, i .e . R w = 0 , a n d s o t h e t h r e s h o l d s h o u l d b e s e t t o h a l f t h e r e f l e c t a n c e o ft h e l a n d c o v e r ty p e a d j a c e n t t o t h e l a k e . I n t h i s c a s e ' a d j a c e n t ' c a n b e i n t e rp re t e d a s b e i n gt h e l a rg e r o f t h e l in e a r d i m e n s i o n s o f th e l a rg e s t g ro u n d e l e m e n t r e s o l v e d b y t h e s e n s or .T h i s i s a p p r o x i m a t e l y 3 0 m f o r L a n d s a t T M d a ta , 2 0 m f o r S P O T X S a n d 8 0 m f o r IR S o rL a n d s a t M S S .

8/3/2019 Science 789

5/17

J. W. Finch~Journalof Hydrology 195 (1997)335-351 339T h e a b o v e d i sc u ss io n i s f o r th e c a s e w h e n t h e r e i s a u n i f o r m l a n d c o v e r a d j a c e n t t o t h eLa ke s. De v i a t i o n s f ro m th is a s s u m p t i o n w i l l c a u s e u n d e r o r o v e r e s t i m a t e s o f t h e l a k e a r e a s

d u e t o e r r o r s o f o m i ss io n o r c o m m i s s i o n i n c l a s s if y i n g t h e p i x e l s c o v e r i n g th e l a k e e d g e s.I f t h e r e f le c t a nc e o f t h e a d ja c e n t l a n d c o v e r f a il s b e l o w t h e v a l u e o f t h e t h r e s h o l d t h e n t h ep i x e l s w i l l b e c l a ss if ie d a s wa t e r , i . e th e e r ro r s o f c o m m i s s i o n w i l l b e a m a x i m u m . W h e nt h e r e f le c t a n c e o f th e a d j a c e n t la n d c o v e r is g r e a t e r t h a n t h e a s s u m e d v a l u e t h e n e r ro r s o fo m i s s i o n i n c re a s e a s t h e r e f l e c t a n c e i n c re a s e s . I f t h e r e f l e c t a n c e o f t h e w a t e r i s z e ro a n d t h et h re s h o l d is s e t to h a l f t h e a s s u m e d r e f l e c t a n c e o f t h e l a n d c o v e r a d j a c e n t t o t h e l a k e , R t ,t h e n Eq . (1 ) b e c o m e s :

(1 -P w )R ~ < 0 .S R t (4 )Th e e q u a t i o n i s n o rm a l i s e d b y u s i n g t h e r a t i o , R, b e t w e e n t h e v a l u e a s s u m e d i n s e t t in g t h et h re s h o l d a n d t h e a c tu a l r e f l e c t a n c e o f t h e a d j a c e n t l a n d c o v e r t o g i v e :

P w > 1 - 0 . 5 R ( 5 )W h e n t h e a s s u m e d r e f l e c t a n c e o f t h e a d j a c e n t l a n d c o v e r i s e q u a l t o t h e a c t u a l r e f l e c t a n c eo f t h e l a n d c o v e r , i .e . = I , t h e n E q . ( 5 ) p r e d i c t s t h a t t h e ' a v e r a g e ' p r o p o r t i o n o f w a t e r t on o n - w a t e r i n t h e g r o u n d e l e m e n t s w i l l b e 0 . 5 , w h i c h i s t h e r e q u i r e d c o n d i t i o n . H e n c e , t h ep r o p o r t i o n o f p i x e l s a l o n g t h e w a t e r / n o n - w a t e r b o u n d a r y w r o n g l y c la ss i f ie d, P , c a n b ee s t i m a t e d f r o m :

= ~ - 0 . 5 ( 6 )T h i s e q u a t i o n p r e d i c t s t h a t t h e p r o p o r t i o n o f p i x e l s t h a t s h o u l d h a v e b e e n c l as s i f ie d a sw a t e r b u t a r e cl a ss i fi e d s n o n - w a t e r i n c r e a s e s a s R d e c r e a s e s u n t i l a l i m i t i s r e a c h e d w h e na ll t h e p i x e l s a r e c l as s if i ed s n o n - w a t e r . H o w e v e r , h a l f o f t h e s e a r e c o r r e c t l y c l as s i f i ed sn o n - w a t e r s o t h e l i m i t i s t h a t h a l f t h e p i x e l s w i l l b e w r o n g l y c la ss if ie d. h i s i s s h o w ni n F i g . I w h i c h i s a g r a p h o f t h e f u n c t i o n g i v e n i n E q . ( 6 ) . I t s h o w s t h a t t h e e r r o ri n e s t i m a t i n g t h e s u r f a c e a r e a o f a w a t e r b o d y i s g r e a t e r i f t h e a c t u a l l a n d c o v e rr e f l e c t a n c e i s h i g h e r t h a n t h e a s s u m e d v a l u e u s e d i n s e t t i n g t h e t h r e s h o l d t h a n i f i t i s

0 .5

i" O . S " " '0 .5

OMiSSiON

~ ooMmsfKm

I I I ,

2 3 5 10ItNUmslmgbetmtm/mt~ m flsets~o,RFig . 1. The proportion of misclaasified mixed water/non-waterpixels as a function of the threshold leve l set.

8/3/2019 Science 789

6/17

340 J. W. Finch~Journalof Hydrology 195 (1997) 335-351l o w e r b y t h e s a m e a m o u n t. T h i s s u g g e s t s t h a t, w h e n t h e r e i s u n c e r t a i n t y i n t h e v a l u e t h a ts h o u l d b e s e l e c t e d fo r t h e t h r e s h o l d , a l o w v a l u e s h o u l d b e s e l e c t e d a s t h i s is l i k e l y to r e s u l tin less misclass i f icat ion .3.2 . Th e s ize o f the lake

An i m p l i c i t a ss u m p t i o n i s th a t t h e l e n g t h o f t h e p e r i m e t e r o f th e l a k e i s s u f f i c i e n t ly l a rg et h a t t h e d is t r ib u t i o n o f t h e r a t io o f a r e a s o f w a t e r t o a r e a s o f n o n -w a t e r w i t h i n t h e g ro u n de l e m e n t s i s r a n d o m w i t h a m e a n o f 1 . F o r a n y i n d i v i d u a l l a k e t h i s m a y n o t b e t r u e ,p a r t i c u l a r l y w h e n t h e s i z e o f t h e l a k e a p p r o a c h e s t h a t o f t h e g r o u n d e l e m e n t o f t h er e m o t e l y s e ns e d d at a. H o w e v e r , i f t h e o b j e c t i v e o f a s t u d y i s t h e t o ta l v o l u m e o f w a t e rs t o re d i n a r e g i on a n d s o t h e t o ta l l e n g t h o f t h e w a t e r / n o n - w a t e r b o u n d a r y i s m u c h l a rg e rt h a n t h e g ro u n d e l e m e n t s i z e, i. e . th a t t h e re a r e m a n y l a k e s w i t h i n t h e r e g i o n , t h e a s s u m p -t ion wil l s t i l l be t rue.E r r o r s w il l o c c u r w h e n t h e a v e r a g e l e n g t h o f t h e m i n i m u m d i m e n s i o n o f a w a t e r b o d y isn o t a m u l ti p le o f th e s e n s o r 's g r o u n d r e s o l u t i o n e l e m e n t l i n e a r d i m e n s i o n . W h e n i t is l e ss ,m a n y o f th e p ix e l s c o v e t in g t he w a t e r / n o n - w a t e r b o u n d a r y w i ll h a v e t o o s m a l l a p r o p o r-t i o n o f w a t e r w i t h i n t h e g r o u n d e l e m e n t f o r th e p i x e l r e f l e c t a n c e t o b e b e l o w t h e t h r e sh o l da n d h e n c e fo r t h e p i x e l s to b e c l a s s i f ie d a s wa t e r . W h e n i t is g r e a t e r t h e n t h e c o n v e r s e w i llb e t ru e . Th i s e f f e c t i s d e p e n d e n t o n t h e s i z e o f th e l a k e a n d c a n b e i l l u s t r a te d b y t h e s i m p l ec a s e o f c i r c u l a r l a k e s . Th e s u r f a c e a r e a o f a c i r c u l a r l a k e i n c re a s e s a s t h e s q u a re o f t h era d i u s wh i l s t t h e l e n g t h o f i t s p e r i m e t e r i n c re a s e s l i n e a r l y w i t h t h e r a d i u s a n d s o f e we rp i x e ls w i ll c o v e r t h e w a t e r /n o n - w a t e r b o u n d a r y i n p r o p o r t i o n t o t h e t o t a l n u m b e r c o v e r i n ga l a k e a s t h e s u r f a c e a r e a o f t h e l a k e i n c re a s e s . He n c e , t h e re i s l e s s o p p o r t u n i t y fo r p ix e l s t obe misc lass i f i ed .3 .3 . E r r o r s d u e t o t h e i n c lu s io n o f o t h e r l a n d c o v e r t y p e s

A p ro b l e m wi t h a p p l y i n g s u c h a s im p l e m e t h o d a s th r e s h o l d i n g i s th a t o t h e r l a n d c o v e rt y p e s c a n h a v e r e f l e c t a n c e s l e s s t h a n t h e t h r e s h o l d l e v e l , e . g . a r e a s o f d e e p s h a d o wa s s o c i a t e d w i t h s t e e p t o p o g ra p h i c s l o p e s . I t i s i n t e r e s t i n g t o n o t e t h a t p r e v i o u s s t u d i e s ,s u c h a s th a t b y W o r k a n d G i l m e r ( 1 9 7 6 ), w e r e i n a r e a s w i t h a v e r y s u b d u e d to p o g r a p h yw h e r e s h a d o w s d i d n o t o c c u r. S o m e m e t h o d i s n e e d e d t o d i s c r im i n a t e b e t w e e n t h e a r e a s o fwa t e r a n d m i s c l a s s if i e d a re a s p r i o r t o t h e u s e o f t h e d a t a s e t t o e s ti m a t e t h e v o l u m e o f wa t e rs t o re d i n a r e g i o n . Th i s i s n o t a p ro b l e m a s t h e o b j e c t i v e o f t h e p re s e n t s t u d y is t o m o n i t o rk n o w n d a m s a n d s o o n l y a r e as t h a t c o i n c i d e w i t h t h e k n o w n p o s i t i o n o f a d a m l a k e n e e d b ec o n s id e r e d . A n e f f e c t i v e m e a n s o f a c h i e v i n g t h is i s t o m a k e u s e o f a G I S . T h e G I S c a n b eu s e d t o c o n v e r t t h e i m a g e o f t h e l a k e a r e a s f r o m r a s t e r f o r m a t o f t h e i m a g e t o v e c t o rfo rm a t , i . e . t h e l a k e a r e a s a r e r e p re s e n t e d b y a c o n t i n u o u s l i n e e n c l o s i n g t h e a r e a (B u r -ro u g h , 1 9 8 7) . Th i s d a t a s e t c a n t h e n b e c o m b i n e d w i t h a p r e v i o u s l y p re p a re d d a t a s e t t h a tc o n t a i n s p o i n t s w i t h l a b e l s o f t h e d a m i d e n t i f ic a t i o n c o d e . T h e p o i n ts a r e l o c a t e d w i t h i n t h el a k e a r e a s s o t h a t, w h e n p o l y g o n s a r e g e n e ra t e d f ro m t h e d a t a s e t, th e p o l y g o n s th a t f o rmt h e s u r f a c e a r e a s o f t h e l a k e s a r e fo rm e d w i t h t h e a p p ro p r i a t e d a m i d e n t i f i c a t i o n c o d ea s s o c i a te d . I t i s t h e n a s i m p l e p ro c e d u re t o d i s c a rd t h o s e p o l y g o n s t h a t d o n o t h a v e a d a mi d e n t i f ic a t i o n c o d e . , i . e. t h e y a r e a s s u m e d t o b e n o n -wa t e r .

8/3/2019 Science 789

7/17

J. W. Finch/Journal of Hydrology 195 (1997)335-3513.4 . C alcula t ing the capac i ty o f the dam s

M1

T h e a r ea o f e a c h w a t e r b o d y i s a u t o m a t i c a ll y c al c u l a te d b y t h e G I S a n d t h i s c a n b e i n p u ti n t o a su i t a b le a lgo r i thm to de t e r mi ne t he v o l um e o f t he l a ke . A s t udy o f t he sm a l l da m s i nt h r ee re g i ons i n Bo t swa n a ( M e i gh , 1995) de ve l ope d a l gor i t hm s f o r e s t ima t i ng the c a pa c i t yo f d a m s f r o m t h e s u r fa c e a re a o f t h e l a k e s . T h e s e r e g i o n s w e r e t h e c a t c h m e n t s o f t h r eema j o r pub l i c supp l y r e se r vo i r s , Ga bor one , Boka a a nd Sha she , a nd t he a l gor i t hms we r ed e v e l o p e d o n d a t a f o r 1 5 o u t o f 3 05 s m a l l d a m s i n th e r e g i o n s. T h e m a x i m u m c a p a c i ty o f ada m, Cf i n t housa nds o f m 3, i s e s t i ma t e d f r om t he su r f a c e a r e a o f t he l a ke , Af i n he c t a r e s ,using:Cf- -% 38A 25 (7 )

T h e v o l u m e o f w a t er s to r ed b y t h e d a m a t a n y o t h e r le v e l , C , i s c a lc u l a te d f r o m t h e s u r f a cearea o f the l ake a t tha t t ime , At , by:( A , ~ '6 s

C, ----Cf k~ffJ (S)

4 . The s tudy reg ionsTh e t w o r e g i ons use d i n t h is s t udy a r e l oc a t e d i n sou t h - e a s t Bo t sw a na , ( F i g . 2) . The a r e aha s a se m i - a ri d c l i ma t e w i t h a nnu a l r a i n f a l l ge ne r a l l y i n t he r a nge 45 0- 55 0 r a m, usua l l yf a l l ing f r om Oc t obe r t o A pr i l in t he f o r m o f shor t in t e nse s t o r m s t ha t a r e h i gh l y l oc a l ise d( M e i gh , 1995) . The r i ve r s a r e d r y muc h o f t he t i me , e ve n i n t he r a i ny se a son , a s t hesa nd y so i l s t ha t c ove r m os t o f the r e g i ons usua l l y ge ne r a t e s i gn i f ic a n t r un of f on l y a f t e rse ve r a l da y s o f s t o rms . R un of f oc c ur s t yp i c a l l y i n spa t e s l a s ti ng 2 o r 3 da y s , bu t c a n l a s t fo ru p t o a w e e k . O f th e a n n u a l r u n o f f , 7 5 % o r m o r e i s c o n c e n t r a te d in t h e p e r i o d f ro m t h eb e g i n n i n g o f D e c e m b e r u n t i l t h e e n d o f M a r c h , a n d t h e r e i s v e r y h i g h v a r ia b i li ty i n f l ow sf r o m y e a r t o y e a r. T h e a n n u a l o p e n w a t e r e v a p o r a ti o n , a b o u t 2 0 0 0 m m , g r e a t ly e x c e e d s th era infa l l .T h e m a j o r d a m s i n t h e r e g io n s , w h i c h a r e u s e d f o r m u n i c i p a l s u p p ly , h a v e c a p a c it ie s o fbe t w e e n 19 x 106 a nd 144 x 106 m 3 wh i l s t sma l l da m s a r e t yp i c a l l y be t we e n 1000 a nd100 000 m 3. The m a j o r i t y o f t he sma l l da m s a r e use d f o r s t oc k- w a t e r wh i l s t som e a r e use df o r sma l l - sc a l e i r r i ga t i on a nd o t he r pu r pose s . A l mos t a l l t he sma l l da ms a r e d r y a t t hebe g i nn i ng o f the r a i ny se a son . The se f il l up wi t h t he f i rs t r un of f e ve n t s , a nd subse que n tr u n o f f t e n d s t o p a s s t h r o u g h i n to t h e l a rg e d a m s i f s u c h r u n o f f f o l lo w s f a i r ly s o o n a f t e r th eda m s ha v e f i ll e d . H ow e ve r , i f t he r e i s a n e x t e nde d d r y pe r i od , du r i ng w hi c h t he l eve l s o ft he sm a l l e r da m s f a l l, subse que n t dow ns t r e a m r un of f e ve n t s wi l l a ga i n be c on s i de r a b l ya f f e c te d b y t h e s e d a m s .A t o ta l o f 3 0 5 d a m s w a s i d e n t if ie d i n t h e B o k a a a n d G a b o r o n e c a t c h m e n t s f r o mm a p s t ud ie s ( M e i gh , 1995) a nd t he su r f a c e a r e as o f t he se we r e e s t i ma t e d f r om t he1 : 50000 t opogr a ph i c ma ps . I n a dd i t i on , t he su r f a c e a r e a s o f 15 da ms we r e me a sur e d bys u r v e y i n g .

8/3/2019 Science 789

8/17

342 J.W. Finch/Journal of Hydrology 195 (1997) 335-351

3OOOO0 35OOOO "4OOOOO 4.~0000

~ - - O u t l i n e o f L a n d s a t T M s c e n e- - - - C a r .h m e m t b o u n d m ~

R i v e r a n d s u ~ a m

F i g . 2 . L o c a t i o n o f t h e t w o s t u d y r e g i o n s r e l a t iv e t o t h e L a n d s a t T M s c e n e ( g r i d l i n es a r e a t 5 0 k m i n t er v a ls ) .

8/3/2019 Science 789

9/17

I .W. F inch/Joum alof H ydr ology 195 (1997) 335-351 3435. Data acquisition

The higher spatial resolution of data from SPOT XS would allow water bodies withsurface areas down to 0.1 ha to be resolved, compared to 0.3 ha using Landsat TM data.However the lack of SPOT XS data in the aarchive of the South African receiving stationfor the period of the study meant that Landsat TM data was used.The two catchments are covered by a single Landsat TM scene, path/row 172/077, withthe exception of the very southern end of the Gaborone catchment, where seven smalldams are located. Scenes for three different dates were obtained. The scene for 5 April1991 was selected at the end of a season of above average rainfall, 587 mm at Gaborone,with most of the rainfall in the latter part of the rainy season. Rainfall in January wasparticularly heavy with significant amounts also occurring in February and March, and soit could be assumed that the dams were at their maximum capacity and hence the lakes attheir greatest extent. In addition, there was no cloud cover over the area on that date. Theother two images, acquired on 4 May 1990 and 25 September 1990, allowed the areas ofthe lakes to be estimated after a wet season with about average rainfall (547 mm atGaborone) and towards the end of the subsequent dry season.

6. Data processingImage processing was carried out using ERDAS Imagine version 8.1 and the GIS was

ARC/Info. The remotely sensed data covers an area of approximately 210 by 195 km, (Fig.2). The relevant section covering the Gaborone and Bokaa catchments was extracted forprocessing. The image acquired on 5 April 1991 was geo-registered to the 1:50000topographic maps, which use the Universal Transverse Mercator (UTM) projection.This was achieved by identifying control points of locations that could be identifiedunambiguously on both the maps and the remotely sensed data. A second order polynomialwas fitted to the control points and this was used to estimate the position of each pixel inthe output, geo-registered image. The size of the picture elements (pixels) was maintainedat 30 m by 30 m and so a nearest neighbour sampling routine was employed. The other twoimages were then registered to the first using the same procedure.The data for all three images were converted from digital numbers (DN) to radianceusing the tables published by Markham and Barker (1986). It is necessary to apply acorrection for atmospheric scattering to compare the images acquired on different dates.The objective of the study was to test a system that could be applied simply and rapidly toremotely sensed data, so the use of radiative transfer models (Tame et al., 1990; Kneizys etal., 1983) was not appropriate. Instead, the dark-object subtraction method given byChavez (1988) was used. The method assumes that, within an image, there are a fewpixels that should represent zero reflectance, e.g. areas of deep shadow or deep clear water,and so no radiance should be detected. The sensors actually record non-zero radiance atthese pixels and so the value recorded is assumed to be due to atmospheric scattering and issubtracted from the data. The value for each image was identified from the histogram ofradiance values and subtracted from the data.The histogram of the image radiances, after being corrected for atmospheric effects, is

8/3/2019 Science 789

10/17

3442.5

J.W. Finch/Journal of Hydrology 195 (1997) 335-351

! 1.5o

0.5

o 0 20 40 60 80 1 O0 1 " uRa dia nce (W m'SStef~/Jm~)F i g . 3 . Hi s tog ra m o f ne a r - i n f ra re d r a d i a nc e va l ue s fo r t he s c e ne a c qu i re d on 5 Apr i l 1991 .

s h o w n i n F i g . 3 . T h e b u l k o f t h e v a l u e s l ie w i t h i n t h e r a n g e 1 8 - 1 2 7 W m -2 sr -~ ~ m -n w i t ha m e a n o f 6 8 W m -2 s r - ] ~ m -1 . H o w e v e r , t h e r e i s a s m a l l g r o u p o f v a l u e s t h a t r a n g eb e t w e e n 0 . 0 a n d 2 . 5 W m -2 s r - ~ / ~ m -1 w h i c h a r e d u e t o t h e p i c t u r e e l e m e n t s , p i x e l s , r e p r e -s e n t in g s o l e l y w a t e r . I f i t c a n b e a s s u m e d t h a t t h e l a n d c o v e r s u r r o u n d i n g t h e l a k e s i sr e p r e s e n t a t i v e o f t h e la n d c o v e r w i t h i n t h e e n t i r e i m a g e t h e n , th e a n a l y s i s g i v e n a b o v es u g g e s t s th a t h a l f t h e m e a n r a d i a n c e v a l u e w o u l d b e t h e a p p r o p r i a t e t h r e s h o l d l e v e l , i .e .34 W m -2 s r -~ /~m -~ . H o w e v e r , i t is a l s o p o s s i b l e t o c a l i b r a t e t h e t h r e s h o l d l e v e l u s i n g d a t af r o m t h e d a m s w h o s e a r e a s h a d b e e n d e t e r m i n e d b y s u r v ey i n g .

2 .5

1 .5

1

0 .5

t I , I = i 0 I 0 I I28 30 32 34 38 38 40 42t h m d l o k l m d lm m ~ O V m " l m m ' ~ m " )F i g . 4 . R _M S d i f f e r e n c e b e t w e e n t h e a r e a s o f 1 5 d a m s d e t e r m i n e d b y s u r v e yi n g a n d e s t im a t e d f r o m r e m o t e l ys e n s e d n e a r - i n f t m e d d a ta a s a f u n c t i o n o f t h e t h r e s h o l d l ev e l .

8/3/2019 Science 789

11/17

8/3/2019 Science 789

12/17

8/3/2019 Science 789

13/17

Z W. Finch/Journal of Hydrology 195 (1997) 335-351 347

72240OO

7 2 2 2 0 0 0

7218000 78 7

l 4 M a y 1 9 9 O2 5 S e p t e m b e r 1 9 9 0

I s ~ , 1991Ri v e r and ~ Ueom l i nes

Fig. 7. Areas of surface wa ter selected using a GIS for three dates.

8/3/2019 Science 789

14/17

348 J.W. Finch/Journal of Hydrology 195 (1997) 335-351t h a t th e r e a re o n l y s m a ll d i f f e r e n c e s b e t w e e n t h e r a d i a n c e s o f t h e l a n d c o v e r s d u r i n g t h ey e a r .

7 . R e s u l t s a n d d i s c u s s io nT a b l e 2 g i v e s th e s u r f ac e a re a s e s t i m a t e d f r o m t h e r e m o t e l y s e n s e d d a t a a c q u i r e d o n 5

A p r i l 1 9 91 a n d f r o m t o p o g r a p h i c m a p s a n d t h e a r e a s m e a s u r e d b y s u r v e y i n g . T h e r e a r es ig n if ic a n t d i f f e r e n c e s b e t w e e n t h e m a p p e d a n d r e m o t e l y s e n s e d e s t im a t e s a n d t h e s u r -v e y e d a r e as . I n th e c a s e o f t h e r e m o t e l y s e n s e d d a t a , t h e w a t e r l e v e l m a y h a v e b e e n l o w e rt h a n t h e m a x i m u m a t t h e t i m e t h e i m a g e w a s a c q u i r e d . I n th e c a s e o f t he s u r f a c e a re a se s t i m a t e d f r o m m a p s t h i s m a y a l s o b e t h e c a s e . T h e m a p s w e r e c o m p i l e d f r o m a e r i a lp h o t o g r a p h y a n d s o t h e e x t e n t o f w a t e r m a p p e d w o u l d h a v e d e p e n d e d o n t he s e a s o n w h e nt h e a e r ia l p h o t o g r a p h y w a s f l o w n . I f t h e a e r ia l p h o t o g r a p h y w a s o b t a i n e d s o o n a f t e rr a in f a ll t h e n th e a c tu a l w a t e r b o d i e s w e r e p r o b a b l y m a p p e d w h e r e a s , i f t h e s u r v e y w a sf l o w n la t e in t h e d r y s e as o n, t h e a r e a s w e r e m o r e l i k e l y t o h a v e b e e n e s t i m a t e d f r o m t h ec h a n g e s i n t h e s o il c o lo u r s o r t h e h e i g h t o f t h e g r o u n d i n f e n ~ l f r o m t h e s t e re o p h o t o -g r a p h y . T h e r e m o t e l y se n s e d d a t a s h o w l e ss v a r i a n c e a s t h e y w e r e o b t a i n e d o n a s i n g led a t e . Th e s u r f a c e a r e a s d e r i v e d f ro m m a p s a r e a l s o l i k e l y t o s h o w v a r i a t io n s a s th e a r e a ss h o wn o n t h e m a p m a y b e s y m b o l i c , r a t h e r t h a n a c t u a l a r e a s , f o r t h e s m a l l e s t d a m s . Am e a s u r e o f a c c u r a c y is t h e R M S d i f f e r e n c e b e t w e e n t h e e s t i m a t e d a n d s u r v e y e d a re a s , a s ap r o p o r t io n o f th e s u r v e y e d ar e a . F o r t h e a r e a s e s t i m a t e d f r o m m a p s , t h e R M S d i f f e r e n c e i s0 . 5 0 wh i l s t f o r t h e e s t i m a t e s d e r i v e d f ro m re m o t e l y s e n s e d d a t a i t i s 0 . 4 3 . Th i s s u g g e s t st h a t t h e e s t i m a t e d s u r f a c e a r e a s d e r i v e d f r o m r e m o t e l y s e n s e d d a t a a r e c l o s e r t o t h o s eo b t a i n e d b y s u r v e y t h a n th o s e e s t i m a t e d f r o m t h e t o p o g r a p h i c m a p s . A l t h o u g h t h i s is n o t ac o m p a r i s o n o f t o t al l y in d e p e n d e n t m e a s u r e s , b e c a u s e t h e s e v a l u e s w e r e u s e d t o c a l i b ra t et h e a n a l y si s o f t h e r e m o t e l y se n s e d d a t a, i t d o e s g i v e a n e s t i m a t e o f t h e a m o u n t o f v a r ia n c et h a t c a n b e e x p e c t e d d u e t o t h e s i z e o f t h e l a k e s r e l a t i v e t o t h e s p a t i al r e s o l u t i o n o f t h er e m o t e l y s e n s e d d a t a .Table 2Comparison of surface areas measured by survey, map and remote sensingDam code Survey area (ha) Map area (ha) Remotely sensed area (ha)1009 11.40 23.00 11.11060 0.97 1.00 1.01078 0.53 0.80 0.91126 3.01 0.50 2.72001 57.80 48.00 50.72002 28.00 34.00 28.32009 10.10 7.50 3.72014 9.12 4.50 7.12016 5.44 3.90 3.42018 2.52 3.50 0.62026 2.14 2.25 2.02053 1.02 0.63 0.62067 0.29 0.50 0.5

8/3/2019 Science 789

15/17

J. W. Finch/Jownal o f Hydrology 195 (1997) 335-3 51 34910 ,000

1 , 0 0 0 "

i 1 0 0 ,10 o I' f.1 ~

0 .0 1 a a a a0 .0 1 0 .1 1 1 0 1 0 0 1 , 0 0 0 1 0 ,0 0 0m ( H e )Fig. 8. Scatter-plot o f surface water areas estimated from t~m otely sensed data acquired on 5 April 1991 againstareas determined from 1:5000 0 topograph ic maps.

A n e x a m i n a t i o n o f al l t h e a r e a s e s t i m a t e d f r o m t h e r e m o t e l y s e n s e d d a t a a n d t h o s ee s t im a t e d f r o m m a p s s h o w e d t h a t t h e r e a r e c o n s i d e r a b l e v a r ia t io n s . S o m e o f th e p o s s i b l er e a s o n s fo r th i s h a v e b e e n d i s c u s s e d a b o v e . In a d d i ti o n , s y s t e m a t i c d i f f e r e n c e s m a y h a v eo c c u r r e d b e t w e e n t h e s u r f a c e a r e a s e s t i m a t e d f r o m t h e r e m o t e l y s e n s e d d a t a a n d t o p o -g ra p h i c m a p s . Th i s w o u l d o c c u r i f t h e r a i n fa l l d i s t r i b u t io n d u r i n g t h e r a i n y s e a s o n h a db e e n n o n - u n i f o r m s p at ia ll y. T h e i m a g e r y w a s s e l e c te d a t t h e e n d o f a p a r t ic u l a rl y w e t r a in ys e a s o n s o th a t, o v e r t h e w h o l e o f t h e a r ea , t h e w a t e r l e v e l s b e h i n d t h e d a m s w o u l d b e a tt h e ir m a x i m u m p o s s ib l e a n d s o t h e s u r f a c e a r e a w o u l d a l s o b e m a x i m u m . I f t h e r ai n f al l ins o m e a re a s h a d b e e n s i g n i f ic a n t ly le s s , t h e n t h e s u r f a c e a r e a s e s t i m a t e d f ro m t h e r e m o t e l ys e n s e d d a t a w o u l d b e c o n s i s t e n t ly l es s t h a n t h o s e e s t i m a t e d f r o m t h e m a p s . T h i s h y p o t h -e s is w a s t e s t e d b y p l o tt in g a m a p s h o w i n g t h e l o c a t i o n o f t h e d a m s a n d i n d i c a ti n g w h e t h e rt h e s u r f a c e a r e a e s t i m a t e d f r o m t h e r e m o t e l y s e n s e d d a t a w a s g r e a t e r o r l e s s t h a n t h a te s t i m a t e d f ro m t h e m a p s . Ex a wi n i n g t h i s m a p s h o ws t h a t t h e d i s t r i b u t i o n i s e s s e n t i a l l yr a n d o m a n d s o i t m a y b e c o n c l u d e d t h a t t h e r a i n fa l l w a s s u f fi c ie n t ly h i g h o v e r t h e w h o l ea re a t o e n s u re t h e d a m s w e re f i l le d t o c a p a c i t y .

F i g . 8 s h o ws a s c a t t e r p l o t o f t h e s u r f a c e a r e a e s t i m a t e d f ro m t h e t o p o g ra p h i c m a p sa g a i n s t t h a t f ro m t h e r e m o t e l y s e n s e d d a t a . Th e c o r r e l a t i o n c o e f f i c i e n t i s 0 . 9 5 2 a n d t h es c a t t e r o f p o i n t s a ro u n d t h e 1 :1 l i n e i n d i c a t e s t h a t t h e re a r e n o s y s t e m a t i c d i f f e r e n c e sb e t w e e n t h e e s t i m a t e s o f s u r f a c e d e r i v e d f r o m t h e r e m o t e l y s e n s e d d a t a c o m p a r e d t ot h o s e d e r i v e d f r o m t h e m a p s . H o w e v e r , th e r e a r e q u i t e la r g e d i s c re p a n c i es b e t w e e n t h ee s t im a t e s o f t h e a r e a s o f s o m e d a m s . T h e s e o c c u r m o r e f r e q u e n t l y f o r t h e s m a l le s t d a m sa n d r e p re s e n t t h e f i m i t i m p o s e d b y t h e s p a t i a l r e s o l u t i o n o f t h e r e m o t e l y s e n s e d d a t a .A l t h o u g h r e m o t e l y s e n s e d d a t a m a y n o t p r o v i d e a n a c c u r a t e e s ti m a t e o f t h e a r e a o f a s i n g led a m , t h e a g g re g a t e d v a l u e s a r e a c c e p t a b l e .

S o m e o f t h e d a m s l is t e d f r o m t h e m a p s , n o t d e t e c t e d a s h a v i n g a n y w a t e r o n 5 A p r i l1 9 91 , w e r e c h e c k e d m a n u a l l y o n a f a ls e c o l o u r c o m p o s i t e i m a g e a n d n o w a t e r c o u l d b e

8/3/2019 Science 789

16/17

350 J. W. F i n c h ~ J o u r n a l f H y d r o l o g y 1 9 5 (1997) 3 3 5 - 3 5 1d e t e c t e d . T h e r e w e r e m o r e e m p t y d a m s i n t h e B o k a a c a t c h m e n t t h a n i n t h e G a b o r o n e .P r e s u m a b l y t h e s e e m p t y d a m s a r e n o l o n g e r m a i n t a i n e d b e c a u s e o f, f o r e x a m p l e , f a il u r e ft he da m wa l l o r a swi t ch t o a g r o und w a t e r supp l y . T h i s a ga i n i l lu s t r a te s t he po t e n ti a l f o rr e m o t e l y s e n se d d a t a to m o n i t o r c o n t i n u o u s l y t h e u s e o f s m a l l d a m s .The s t o re d wa t e r vo l ume o f t he da m s e s t i ma t e d f o r the t h r e e da t e s de c r e a se d i n t he o r de ro f 5 Apr i l 1991, 4 M a y 1990 , 25 Se p t e m be r 1990 , a s w ou l d be e xpe c t e d . I n Se p t e mbe r1990 , on l y t he l ar ge s t da ms s t i ll c on t a i ne d a ny s i gn i f i c a n t a m oun t s o f wa t e r . The ma x i -m um s t o r a ge o f wa t e r i n t he tw o c a t c hm e nt s w a s 104 x 106 m3 , a s de t e r mi ne d a t the e nd o ft he 1990/91 r a i ny se ason f o l l ow i ng a bove a v e r a ge r a i n f a ll . On 4 M a y 1990 i t wa s 89 xl 0 s m 3, f o l l owi ng t he 1989 /90 r a i ny se a son w hi c h wa s a bou t a ve r a ge a n d , by 25 Se p t e mbe r , it ha d de c l i ne d t o 65 x 106 m 3 o f w h i c h m os t wa s he l d i n t he l a r ge da ms .

A sys t e m ba se d on t he se te c hn i qu e s r e qu i r e s r e mo t e l y se nse d da t a f o r c l oud- f r e e da ys ,qu i c k l y a nd c ons i s t en t l y a va i l a b le . I n p r a c t ic e , m a n y se m i - a r id r e g i ons ha v e v i r tua l l yc l oud- f r e e sk i e s du r ing t he d r y se a so n , w he n t he da t a a r e ne e de d . A no t he r poss i b l e l im i t a -t i on is t he de l a y i n t he p r ov i s ion o f r e mo t e l y se nse d da t a a f t e r i t ha s be e n a c qu i r e d by t hesa te lf it e . W i t h spe c i a l a rr a nge me nt s , t he da t a c a n be ob t a i ne d i n l e s s t ha n a we e k , r a t he rt ha n t he long e r pe r i ods c us t om a r y f o r mor e r ou t i ne da t a pu r c ha se s .

8 . C o n c l u s i o n sThi s s t udy shows t ha t r e mot e l y se nse d da t a , ob t a i ne d f r om sa t e l l i t e s , c a n p r ov i dee s t ima t e s o f wa t e r su r fa c e a r e a s t o a n a c c e p t a b l e a c c ur a c y . Ho we v e r , some f o r m o f g r oundt r u th i s e s se n t ia l t o c he c k a n d c a l i b r a t e t he a na l ys i s . I t is i m por t a n t t ha t t he i ma ge r y use d t od e t e rm i n e t h e m a x i m u m c a p a c i ty o f t h e d a m s i s a c q u i r e d d i r e c tl y a f te r a v e r y w e t r a in yse a son t o e nsur e tha t the wa t e r e x t e n t i s a t a ma x i mu m .The c om bi na t i on o f ima g e p r oc e ss i ng a n d GI S t e c hn i que s a pp l i e d to h i gh spa t ia l r e so -l u t ion r e mot e l y se nse d da t a a l low s t he c a pa c i t y o f the da m s t o be m oni t o r e d a nd t h i s c a n bec a r d e d o u t d u r i n g t h e d r y s e a s o n , p r o v i d e d c l o u d c o v e r i s m i n i m a l . T h e t h r e s h o l d i n g

me t hod i s a s i mpl e a nd e a s i l y a pp l i e d me t hod f o r de t e r mi n i ng t he e x t e n t o f t he l a ke sa nd , i f no o t he r i n f o r m a t i on i s a va i l a b le , t he t h r e sho l d sho u l d be h a l f the m e a n i n f ra r e dr a d i a nc e o f the i ma ge . Us i ng a GI S a l l ows non- w a t e r bod i e s to be d i sc r i mi na t e d , a nd t hel a ke a re a s , a nd he n c e t he c a pa c i ti e s o f t he da m s t o be e s t i ma t e d qu i c k l y .W i th r e m o t e l y s e fi se d d a t a a n a r e a c a n b e m o n i to r e d c o n s t a n t ly t o i d e n t if y n e w d a m sa nd e x i s t i ng da ms t ha t a r e no l onge r i n use . Th i s i s no t poss i b l e us i ng ma ps whi c h a r eupda t e d a t i n f r e que n t i n te r va ls . Ae r i a l su r v e y w ou l d be p r oh i b i t ive l y e xpe ns i ve bu t da t aob t a i ne d f r o m sa te l li te m oun t e d se nsor s p r ov i de a c os t - e f f e c ti ve so lu t ion .

R e f e r e n c e s

Bun'ough , P.A., 1987. Principles of geographical inform ation systems for lan d resources assessment. ClarendonPress, Oxford.Chavoz, P.$., 1988. A n improved dark-object subtraction tech niqu e for atmospheric scattering correction ofmultispectral data. Remote Sensing Environ., 24: 459--479.

8/3/2019 Science 789

17/17

J. W. Finch/Journal of Hydrology 195 (1997) 335-351 35 1Cox, C. and M unk, W ., 1954. Measurement of the roughness of the sea su rface f rom photographs of the sun 'sOt ter . Journal o f the Optical Society of Am erica, 44: 838 -850.Dw i vedi, R.M . and Neraln, A., 1987. Re m ote sensing of phytoplankton - an attem pt from the Landsat thematicmapper. Int. J . Rem ote Sensing, 8: 1 563 -156 7.Johnston, R.M . and Barson, M.M., 1993. Remote-sensing o f Australian w etland s - an evaluation o f Landsat TMdata for inventory and cimdfication. Anst. J . Mar. Freshwat. R es., 44: 2 35 -25 2.Kneizys, F.X., Shettle, E.P., Gallery, W .O., Chetw3md, J . H , Abreu, L.W., Selb y, J .E.A., Clough, S.A . and Penn,R.W., 1983. Atmospher ic t ransmit tance/radiance: com puter code LO W TR AN 6(V). HD -AI3 7 786, AirforceGeophysics Lab., l- lanscom, M A.Manavalan, P., Sathyanath, P. and Rajegowda, G .L.; 1993. Digital imag e analysis techniques to estimate water-spread for capacity evaluations of reservoirs. Photogramm . Eng . and R em ote Seam., 59: 1389-1395.Markham, B.L. and Barker, J .L., 1986. Landsat M SS and T M post-calibration dyn am ic ranges, exoatmosphericreflectances and a t satellite temperatures. In l .amdsat Tec hnica l No tes, EO SA T, pp. 3 -8 .Meigh, J . , 1995. The intpact of small farm reservoirs on urban water supplies in Botswane~ Natural R esourcesForum, 19: 71-83.Ritchie, J .C., Cooper, C .M. and Schiebe, F.R., 1990. Th e relationship of MS S an d TM digital data with suspendedsediments, chlorophyll, and temperature in Moo n Lake, Mississippi. R em ote Sensing Environ., 33: 137-148.Randquist, D.C., Lawson, M.P., Qu een, L.P. and Ce rven ey, R.S., 1987. The relationship between sum mer-seasonrainfall events and lake-surface area. Wa ter Resources Bull., 23: 493 -5 08 .Settle, J .T. and Drake, N .A., 1993. Lin ear m ixing and the estimation of groun d co ve r proportions. Int. ] . Rem oteSensing, 14: 1159--1177.Szekielda, ILH., 1988. Satel li te mo nitod n 8 o f the earth. Joh n W iley, New York.Tanre, D ., Dem o, C., Duhaut, P., Herman, M ., Morcrette, J .J . , Pe~oos, J . and De$champ$, P.Y., 1990. Descriptionof a com pute r code to simulate the satellite signal in the solar spectrum: the 5S code. Int. | . R em ote Sensing,

1 l : 6 5 9 - 6 6 8 .Work, E.A. and Giimer, D.S., 1976. Utilization of satellite data for inven toryin g prairie ponds and lakes. Photo-granun. E ng. and Rem ote Sens. , 42: 685-694 .