variations in the natural chemical concentration of river water during flood flows, and the lag...

8/2/2019 Variations in the Natural Chemical Concentration of River Water During Flood Flows, And the Lag Effect

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Journal of Hydrology, 26 (1975) 237--244 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

V A R IA T IO N S IN T H E N A T U R A L C H E MIC A L C O N C E N T R A T IO N O FR IV E R WA T E R D U R IN G F L O O D F L O WS, A ND T HE L A G E FFE C T :SOME FURTHER COMMENTS

D.E. WALLING and I.D.L. FOSTERDepartment of Geography, University of Exeter, Exeter (Great Britain)(Accepted for publication January 8, 1975)

ABSTRACTWalling, D.E. and Foster, I.D.L., 1975. Variations in the natural chemical concentrationof riverwater during flood flows, and the lag effect: some further comments. J. Hydrol.,26: 237--244.

Following upon the recent paper by Glover and Johnson (1974), the authors brieflypresent results from several Devon catchments which demonstrate the complexity ofvariations in the chemical concentration of river water during flood flows. Certain solutespecies increase rather than decrease in concentration during storm events and the"chemographs" of those species which exhibit dilution are often complicated by a"flushing effect" . Some solutes may exhibit variable response, evidencing increased anddecreased concentrations during different events. Even in small catchments, the "chemo-graph" response can lag behind the stream-flow response and examples of where thetrough precedes the stream-flow peak have also been documented. Values of "chemo-graph" lag between zero and 14.5 h have been found at a gauging station on a Devon riverand the variation can be tentatively explained in terms of catchment moisture status.Any attempt to account for the nature and timing of "chemograph" response mustconsider processes operat ing over the entire catchment in addition to those in the channel.

INTRODUCTIONIn a recent paper , Glover and Joh nso n (1974) have described measuremen ts

taken on the r iver South Tyne which demonst rate that varia t ions in the concen-t ra t ion of cer ta in dissolved const i tuents tended to lag behind associatedchanges in discharge and that this lag is increased progressively downstream.This la t ter phenomenon was ably explained on the basis of kinemat ic wavetheo ry s uppor ted by laborato ry experiments . The s imple dilut ion of base f lowby f lood water and the effects resul tant upon the di fferent ia l between f loodwave veloci ty and mean water veloci ty are , however, only two of the manycontrols act ing to influence the nature and t iming of variat ions in solute con-centrat ion in streams. Invest igat ions at present being carried out by the au-thors in several Devon catchments are increasingly demonst rat ing the complexi tyof this aspect of physical hydrolog y. Measurements taken on smal l s t reams canbe part icular ly useful in a t temp t ing to u nderst and the processes involved.


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I n m a n y r iv e rs i n d iv i d u a l s o l u t e s p e c i e s d o n o t a ll e x h i b i t a d r o p i n t h e" c h e m o g r a p h " a s s o c i a t e d w i t h a r is e i n d i s c h ar g e . N i t r a t e a n d p o t a s s i u mc o n c e n t r a t i o n s , i n p a r t i c u la r , o f t e n i n c r e a s e d u r i n g f l o o d e v e n t s a n d i n F i g . l Bt h e t r e n d o f p o t a s s i u m a n d m a g n e s i u m c o n c e n t r a t i o n s d u r i n g t w o s t o r me v e n t s i n a s m a ll D e v o n c a t c h m e n t is p o r t r a y e d . A u t o m a t i c p u m p s a m p l in ga p p a r a t u s s i m i l ar t o t h a t d e s c r i b e d b y W a l li ng a n d T e e d ( 1 9 7 1 ) w a s u s e d t oo b t a i n t h e s e r e s u l ts . I n t h is c as e , m a g n e s i u m c o n c e n t r a t i o n s e x h i b i t a c la s si cd i l u ti o n w h i ls t t h e " c h e m o g r a p h " f o r p o t a s s i u m s h o w s a m a r k e d i n cr e as e inc o n c e n t r a t i o n . I n o r d e r t o e x p l a in t h i s o c c u r e n c e , t h e s e v er al p a t h w a y sb e t w e e n t h e i n p u t o f p r e c i p i ta t i o n a n d t h e r e s u lt a n t c h a n n e l f l o w m u s t b ec o n s i d e r e d . T h e v a r i o u s r a t es o f s o l u t e p i c k u p , a n d t h e r e s u l t a n t c o n c e n t r a -t i o n s i n c o m p a r i s o n t o t h o s e i n t h e p r e c e d i n g b a s e f l o w w i ll g o v e r n t h e p r e c i s eb e h a v i o u r o f t h e " c h e m o g r a p h " .

F u r t h e r m o r e , d e t a il e d s t u d y o f t h e " c h e m o g r a p h s " o f t h o s e s o l u t e s p e ci ese x h i b i ti n g a d r o p d u r in g s t o r m e v e n ts o f t e n i n d ic a te s t h a t s o m e t h i n g m o r et h a n a s i m p l e d i lu t i o n is i n v o lv e d . I n m a n y i n s ta n c e s , a n d p a r t i c u l a r l y d u r i n ga u t u m n m o n t h s , t h e d e c r ea s e i n c o n c e n t r a t i o n m a y b e p r e c e d e d b y a r is e, o ra n i n i t i a l d r o p m a y b e f o l l o w e d b y a n i n c r e a s e b e f o r e g i v i n g w a y t o a s u b s e -q u e n t d e c r e a s e. T h is o c c u r e n c e h as b e e n t e r m e d t h e " f l u s h in g e f f e c t " b ys e v e ra l a u t h o r s ( e .g . , E d w a r d s , 1 9 7 3 ; W a ll in g , 1 9 7 4 ) a n d c a n b e a c c o u n t e d f o rb y t h e f l u s h i n g o f a c c u m m u l a t e d s o l u t e s i n t o th e s t r e a m b y t h e i n i ti a l p h a s e so f s t o r m r u n o f f . I n F ig . 1 A t h e o c c u r r e n c e o f th i s f l u s h in g e f f e c t i s d e m o n -s t r a t e d f o r a s m a l l D e v o n c a t c h m e n t , t h e d a t a a g a in h a v i n g b e e n c o l l e c t e dw i t h a n a u t o m a t i c p u m p i n g s a m p l e r. I n t h is c a se , t h e " c h e m o g r a p h s " f o rc a l c i u m , s o d i u m , a n d m a g n e s i u m a s s o c i a t e d w i t h t h e f i r s t s t r e a m - f l o w r is ee x h i b i t a n i n it ia l d e c r e a s e in c o n c e n t r a t i o n , f o l l o w e d b y a n i n c r ea s e a n d as u b s e q u e n t d r o p . D u r i n g t h e s e c o n d s t r e a m - f l o w r is e t h is f lu s h i n g e f f e c t isa b s e n t . I n F i g. 1 C , a c o n t i n u o u s r e c o r d o f sp e c if ic c o n d u c t a n c e h a s b e e n u s e dt o d e f i n e th e t r e n d o f t h e t o t a l d is s o lv e d s o li ds " c h e m o g r a p h " f o r t w o s t o r me v e n t s o n a la r g e r D e v o n r iv e r . In b o t h c a s e s a f l u s h i n g e f f e c t is e v i d e n t . T h ep r e c i s e e x t e n t o f th i s f l u s h in g e f f e c t d u r in g a n i n d iv i d u a l f l o o d e v e n t w il li n f lu e n c e t h e ti m i n g o f t h e d r o p i n t h e " c h e m o g r a p h " . A g a in th e p a t h w a y s t ot h e c h a n n e l t a k e n b y t h e s t o r m - p e r i o d r u n o f f m u s t b e c o n s i d e re d a n d t h ec h e m i c a l c h a r a c t e r o f t h e i n f l o w t o t h e c h a n n e l s h o u l d b e r e c o g n i s e d a s b e i n gt i m e - v a r i a n t a c c o r d i n g t o t h e d e ta i l e d t e m p o r a l a n d s p a t ia l p a t t e r n o f s o l u t ep i c k u p .

C e r t a in i o n i c s p e c i e s c a n d e m o n s t r a t e v a r i a b le r e s p o n s e , e x h i b i t i n g i n c r e a s e dc o n c e n t r a t i o n s d u r i n g c e r t a i n s t o r m e v e n t s a n d d e c r e a s e d c o n c e n t r a t i o n sd u r i n g o t h e r s . N i t r a t e le v e ls i n p a r t i c u l a r o f t e n d e m o n s t r a t e v e r y c o m p l e xb e h a v i o u r o f th i s t y p e a n d a n o t h e r s p e c if ic e x a m p l e is p r o v i d e d b y f l u c t u a t io n si n p o t a s s i u m c o n c e n t r a t i o n i n a s m a l l s t r e a m ( F ig . 1 ). O n m o s t o c c a s i o n s ,p o t a s s i u m l e v e ls i n t h is s tr e a m i n c r e a s e m a r k e d l y d u r i n g s t o r m h y d r o g r a p h s( F ig . 1 B ) b u t , d u r i n g a s e ri es o f c lo s e l y s p a c e d s t o r m e v e n t s , c o n c e n t r a t i o n sa s s o c i a te d w i t h t h e l a te r h y d r o g r a p h s m a y d e m o n s t r a t e a d i l u ti o n e f f e c t( F ig . 1 A ) . T h e p r e c is e re s p o n s e o f t h e " c h e m o g r a p h " d u r i n g a s t o r m e v e n tw i ll v a r y f ro m s t o r m t o s t o r m a c c o r d in g t o m a n y c o n t r o l l i n g f a c t o r s .


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T h e a u t h o r s ' o b s e r v a t i o n s a l so s u g g e s t t h a t t h o s e s o l u t e s p e c i e s w h i c hc o n s i s t e n t l y e x h i b i t a d e c r e as e i n c o n c e n t r a t i o n d u r i n g a s t o r m e v e n t c a nr e s p o n d d i f fe r e n t l y a s r e ga r ds r e la t iv e t i m i n g a n d t h a t e v e n i n s m a l l c a t c h -m e n t s t h e " c h e m o g r a p h " r e s p o n s e c a n l a g b e h i n d t h e s t r e a m - f l o w r e s p o n s e .E x a m p l e s o f w h e r e t h e " c h e m o g r a p h " t r o u g h pr e c e d e s t h e s t r e a m - f l o w p e akh a v e al s o b e e n d o c u m e n t e d . F i g. 2 d e p i c t s t h e " c h e m o g r a p h s " o f c a l ci u m ,m a g n e s i u m a n d s o d i u m f o r t h r e e in d i v i d u al s to r m e v e n t s in a n o t h e r s m a l lc a t c h m e n t e q u i p p e d w i t h a u t o m a t i c p u m p s a m p l i n g a p p a r a t u s . I n t h i s c a t c h -m e n t , t h e " c h e m o g r a p h s " g e n er a ll y e x h i b i t a w e l l - d e f in e d m i n i m u m v a l u eo r t r o u g h a n d t h e a u t h o r s p r e f er t o d e f i n e l a g o n t h e b a s i s o f t h e r e l at iv et i m i n g o f t h e s t r e a m - f lo w p e a k a n d t h e s o l u t e t r o u g h , a s or ig in a ll y c o n c e i v e db y H e i d e l { 1 9 5 6 ) f o r s e d i m e n t . I n s p e c t i o n o f F ig . 2 p r e s e n t e d b y G l o v e r a n dJ o h n s o n { 1 9 7 4 ) s u g g e st s t h a t t h e r e m a y b e l i tt le d i ff e r e n c e i n t h e f in a l e s t im a t eo f l ag b e t w e e n t h e ir d e f i n i t i o n a n d t h i s o n e . C o m p a r i s o n o f F i gs . 2 A a n d 2 Ci n d i c at e s t h a t w h e r e as i n o n e e v e n t t h e " c h e m o g r a p h " t r o u g h s p r e c e d e d t h es t r e a m - f l o w p e a k , i n t h e o t h e r a l a g e f f e c t i s c le a r l y v i s ib l e . L a g e f f e c t s i n as m a l l c a t c h m e n t ar e a l so d e m o n s t r a t e d i n F ig . 1 A w h e r e t h e c a l c iu m , m a g n e -s i u m , a n d s o d i u m t r o u g h s la g b e h i n d t h e s t r e a m - f l o w p e a k s in b o t h e v e n t s .F u r t h e r m o r e , c l o s e i n s p e c t i o n o f F i g . 2 i n d ic a te s t h a t t h e i n d i v id u a l s o l u t ep a r a m e t e r s t e n d t o r e s p o n d d i f fe r e n t l y a s r e ga r d s t i m i n g . I n F i g. 2 B , t h es o d i u m t r o u g h o c c u r s b e fo r e t h e w a t e r p e a k, w h i l s t t h e c a l c iu m a n d m a g n e s i u mt r o u g h s l ag b e h in d t h e s t r e a m - f lo w m a x i m u m . C o n t r as t s i n t h e g en e r al f o r mo f t h e " c h e m o g r a p h s " f o r in d i v id u a l s o l u t e s p e c i es a re a ls o d e m o n s t r a t e d i nF i g . 2 A w h e r e c a l c i u m l e v e l s q u i c k l y r e t u r n t o t h o s e o c c u r r i n g b e f o r e t h es t o r m e v e n t , w h e r e a s m a g n e s i u m a n d s o d i u m c o n c e n t r a t i o n s a re r e s to r e d m o r es l o w l y .

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F i g. 3 . T w o e x a m p l e s o f t h e t r e n d o f s p ec i f i c c o n d u c t a n c e d u r i n g f l o od h y d r o g r a p h s o nt h e r i v e r C r e e d y ( A a n d B ) ; r e l a t i o n s h i p b e t w e e n s p e c i f i c c o n d u c t a n c e " c h e m o g r a p h " l a ga n d h y d r o g r a p h r i s e ( C ) , p r e c e d i n g f l o w l e ve l ( D ) a n d s o i l m o i s t u r e d e f i ci t ( E ) ; a n d t h es e a s o n a l d i s t r i b u t i o n o f l a g v a l u e s ( F ) .


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M o r e g e n e r a l i se d i n f o r m a t i o n o n t e m p o r a l v a r i a t i o n s i n s o l u t e l e v el s is a ls ob e i n g c o l l e c t e d f r o m l a rg e r ri ve rs i n D e v o n b y c o n t i n u o u s r e c o r d i n g o f s p e c if icc o n d u c t a n c e , a d i r e c t re f l e c t i o n o f t h e t o t a l d i s so l v ed s ol id s c o n c e n t r a t i o n i n as t re a m . S p e c i f ic c o n d u c t a n c e r e c o r d i n g a p p a r a t u s h a s b e e n o p e r a t e d a t t h eS o u t h - W e s t W a t e r A u t h o r i t y s t r e a m - f l o w g a u g in g s t a t io n o n t h e r i v e r C r e e d ya t C o w l e y n e a r E x e t e r s in c e A p r il 1 9 7 3. T h e c a t c h m e n t a r e a a b o v e th i s p o i n tis 2 6 2 k m 2. I n s p e c t i o n o f t h e r e c o r d s o f r iv e r st a g e a n d s p e c i f ic c o n d u c t a n c ef o r 3 2 c l e a r l y d e f i n e d s i n g le - p e ak e d s t o r m h y d r o g r a p h s i n d i c a te s t h a t t h et r o u g h s o f t h e c o n d u c t i v i t y t r a c e s t e n d t o l a g a f t e r t h e s t r e a m - f l o w p e a k s a n dt h a t t h e e x t e n t o f t h is l ag v a ri es f ro m a s m u c h a s 1 4 .5 d o w n t o z e r o h o u r sw h e r e t h e t r o u g h a n d p e a k c o i n c i d e . T w o e v e n t s w i t h la g t i m e s o f 1 0 . 0 a n d 0 . 5r e s p e c t i v e l y a re i l lu s t r a t e d i n F i g . 3 A , B . I t w o u l d s e e m d i f f i c u l t t o e x p l a i n ar a n g e i n l ag t i m e s o f t h is m a g n i t u d e c o u p l e d w i t h t h e o c c a s io n a l o c c u r e n c eo f z e r o l a g t i m e s s o l e l y i n t e r m s o f c h a n n e l r o u t i n g .A n a t t e m p t h a s b e e n m a d e t o d i s t i n g u i s h f a c t o r s w h i c h c o u l d a c c o u n t f o rt h e v a r i a t i o n i n t h e m a g n i t u d e o f t h e l ag a s s o c i a t e d w i t h i n d i v i d u a l f lo o de v e n ts . T h e a v a il a bl e d a t a e x h i b i t e d a g e n e r a l t e n d e n c y f o r s h o r t h e r l ag s t o b ea s s o c i a t e d w i t h t h e h i g h e s t s t r e a m - f l o w r is e s ( F i g . 3 C a n d T a b l e I ) . T h i s i s i na g r e e m e n t w i t h t h e f in d i n g s o f G l o v er a n d J o h n s o n ( 1 9 7 4 ). H o w e v e r , i na d d i t i o n t o t h e c h a n n e l r o u t i n g c o n t r o l s a s s o c ia t e d w i t h f l o o d - p e a k m a g n i t u d e ,c o n s i d e r a t i o n c a n b e g i v en to h y d r o l o g i c a l c o n d i t i o n s w i t h i n t h e e n t i r e w a t e r -s h e d . In t h is c o n t e x t i t c o u l d b e s u g g e s t ed t h a t " c h e m o g r a p h " l ag s ar e in v e r se l yr e la t e d to a n t e c e d e n t c a t c h m e n t w e t n e s s a n d s t o rm m a g n i t u d e . S i g ni fi c an ts ta t is ti c al r e l a t io n s h i p s ( T a b le I ) h a v e b e e n e s t a b l is h e d b e t w e e n " c h e m o g r a p h "l ag a n d t w o i n d ic e s o f a n t e c e d e n t m o i s t u r e c o n d i t i o n , n a m e l y , f l o w le v elp r e c e d i n g t h e s t o r m h y d r o g r a p h ( F ig . 3 D ) a n d s o i l - m o i s t u r e d e f i c i t ( F i g. 3 E ) .B o t h t h e s e r e la t io n s hi p s d e m o n s t r a t e t h a t m a x i m u m " c h e m o g r a p h " la gs a rea s s o c ia t e d w i t h d r y a n t e c e d e n t c o n d i t i o n s a n d i n s p e c t i o n o f t h e d a t a i n d i c a te sa c o r r e s p o n d i n g s ea s o na l t r e n d i n l ag m a g n i t u d e w i t h m a x i m u m v a lu e s o c c u r -r in g i n s u m m e r a n d m i n i m u m v a lu e s i n w i n t e r . A s im p l e h a r m o n i c c u r v e h a sb e e n f i t t e d t o t h e a n n u a l d i s t r i b u t i o n o f l a g v a l ue s (F i g. 3 F ) a n d a s i g n i f ic a n tr e l a ti o n s h i p e x i st s b e t w e e n l a g m a g n i t u d e a n d t h e a s s o c ia t e d s e a so n a l o rc o s i n e i n d e x f o r t h e d a y ( d ) o n w h i c h t h e e v e n t o c c u r r e d ( T a b le I ). A s t e p w i seT A B L E IC o r r e la t i o n b e t w e e n " c h e m o g r a p h " l ag ~ a n d o t h e r h y d r o l o g i c a l v a r i a b l e sV a r i a b l e C o r r e l a t i o n c o e f f i c i e n t 2h y d r o g r a p h r is e I - - 0 . 8 0p r e c e d i n g f l o w l e v e l ~ - - 0 . 7 9s o i l - m o i s t u r e d e f i c i t 0 . 7 1s e a s o n i n d e x ( c o s 2 n I ~ ] ) - - 0 . 7 2i V a r ia b l e s t r a n s f o r m e d t o lo g1 0 .: A l l c o r r e l a t i o n c o e f f i c i e n t s s i g n i f i c a n t > 9 9 % l e v e l.


7/8

2 4 3

T A B L E I IS t e p w i s e m u l t i p l e r e g r e s s i o n e q u a t i o n r e l a t i n g " c h e m o g r a p h " l a g t o t h e h y d r o l o g i c a lv a r i a b l e s" c h e m o g r a p h " l a g = 1 0 . 3 6 4 - - 3 . 6 3 6 X 1 - - 1 . 0 8 3 X 2 - - 2 . 9 6 5 X 3

R = 0 . 9 0w h e r e : X I = l o g h y d r o g r a p h r i se ( m 3 s e c -1 )

c o s 2 ~ r d - - 3 12 = s e a s o n i n d e x ( I ~ l )X ~ = l o g p r e c e d i n g f l o w l e v e l ( m 3 s e c -~ )

A l l v a r i a b l e s a n d r e g r e s s i o n c o e f f i c i e n t s s i g n i f i c a n t > 9 5 . 0 % l e ve l

m u l t i p l e r e g re s s io n r o u t i n e h a s b e e n e m p l o y e d t o d e v e l o p a m u l t i v a ri a ter e la t io n s h i p b e t w e e n " c h e m o g r a p h " l ag an d t h e f o u r v ar ia b le s d e s c r i b e d inT a b l e I a n d t h e r e s u l t a n t e q u a t i o n i n w h i c h t h r e e v a r i a b le s a r e s i g n i fi c a n t isp r e s e n t e d i n T a b l e I I. T h e v a l u e o f t h i s r e l a ti o n s h i p f o r p r e c i s e i n f e r e n ti a l p u r -p o s e s is r e s t ri c t ed b e c a u s e o f t h e o b v i o u s i n t e r d e p e n d e n c e w i t h i n t h e i n d e p e n -d e n t v a ri a b le s . N e v e r t h e l e s s , it i n d i c a t e s t h a t " c h e m o g r a p h " l ag s c a n b e p re -d i c t e d t o a r e a s o n a b l e d e g r ee o f a c c u r a c y f r o m m e a s u r e s o f a n t e c e d e n t m o i s t u r ec o n d i t i o n , m a g n i t u d e o f t h e h y d r o g r a p h r is e a n d s ea s o n . F u r t h e r m o r e , l agt i m e r e m a i n s i n v e r s e l y r e l a t e d t o a l l t h r e e v a r i a b l e s .

D a t a f r o m t h e r iv e r C r e e d y i n d ic a te s t h a t " c h e m o g r a p h " l ag v a ri es m a r k e d l ya c c o r d in g t o a n t e c e d e n t m o i s t u r e s t at u s . T h is t e n d e n c y is a ls o s h o w n w h e n t h es t r e a m - f l o w a n d s p e c if ic c o n d u c t a n c e r e c o r d s f o r a c l o s el y s p a ce d s e q u e n c eo f s t o r m e v e n t s a re e x a m i n e d , i n t h a t l a g t i m e s p r o g r e s s iv e l y d e c r e a s e t h r o u g ht h e s e ri es o f h y d r o g r a p h s . A t e n t a t i v e c o n c e p t u a l e x p l a n a t i o n o f t h is o c c u r -r e n c e c a n b e s o u g h t i n t h e p r o c e s s e s i n t e r p o s e d b e t w e e n r a in f al l i n p u t a n dc h a n n e l f l o w a n d t h e ir c o n t r o l b y m o i s t u r e c o n d i t i o n s . D u r i n g c o n d i t i o n so f l o w s o i l- m o i s tu r e s t at u s , t h e d r y g r o u n d s u r f ac e a n d u p p e r h o r i z o n s o f th es o il w i ll c o n t a i n a c c u m u l a t i o n s o f r e a d i l y s o l u b l e m a t e r i a l. S t o r m r u n o f f w i llf l u s h t h i s m a t e r i a l i n t o t h e s t r e a m s a n d a n y s o i l w a t e r s h u n t e d t o t h e s t r e a m b yt h r o u g h f l o w m e c h a n i s m s ( e. g. , H e w l e t t a n d H i b b e r t , 1 9 6 7 ) w i ll , b y v i rt u e o fl o n g r e s i d e n c e t im e s , b e h i g h l y c h a r g e d w i t h s o l u t e s. F u r t h e r m o r e , r a p i de x p a n s i o n o f a re a s c o n t r i b u t i n g t o s t o r m r u n o f f ( e .g ., C a r so n a n d S u t t o n ,1 9 7 1 ) w i ll i n t r o d u c e f r e s h s o l u t e s o u r c e s a s t h e s t o r m e v e n t p r o c e e d s . T h es o l u te c o n t e n t o f t h e s t o r m r u n o f f w il l r e ac h a m i n i m u m t o w a r d s t h e e n d o ft h e e v e n t w h e n t h e s u p p l y o f re a d i l y a v ai la b le s o l u te s h a s b e e n e x h a u s t e d o rr e d u c e d a n d b e f o r e t h e i n c r e a s e in s o l u t e l ev e ls a s s o c ia t e d w i t h t h e l o n g e rr e s id e n c e t i m e o f r u n o f f f e e d i n g t h e r ec e s s io n l im b o f th e h y d r o g r a p h o c c u r s.T h e t r o u g h o f t h e " c h e m o g r a p h " m a y t h e r e f o r e la g a p p r e c i a b l y a f te r t h es t r e a m - f l o w p e a k . U n d e r c o n d i t i o n s o f h i gh s o i l - m o i s t u r e s t a t u s , t h e s u p p l yo f re a d i ly s o l u b l e m a t e r i a l i s l i k e ly t o h a v e b e e n d e p l e t e d b y p r e v i o u s s t o r me v e n t s an d t h e c o n t r i b u t i n g a r ea w il l b e a p p r o a c h i n g i ts m a x i m u m e x t e n t .S o l u t e c o n c e n t r a t i o n s i n s t r e a m f l o w w il l f al l r a p i d l y a t t h e o n s e t o f t h e s t o r me v e n t , in r e s p o n s e to t h e d i l u t e n a t u r e o f t h e s t o r m r u n o f f , a n d t h e t r o u g h o ft h e " c h e m o g r a p h " w il l c o i n c i d e w i th , o r o c c u r v e ry s h o r t l y a f t e r, t h e s tr e a m -


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f l o w p e a k w h i c h i t s e lf r e p r e s e n ts m a x i m u m s u r f a c e r u n o f f r at e s a n d i n m a n yc a s e s t h e c e s s a t i o n o f s t o r m r a in f a ll .

V a r i at io n s in t h e n a t u ra l c h e m i c a l c o n c e n t r a t i o n o f c h a n n el f l o w d u r i n gf l o o d e v e n t s i n b o t h s m a ll a n d la rg e c a t c h m e n t s c a n e x h i b i t m a r k e d c o m p l e x i t y .T h e v a r i e t y o f s o l u t e r e s p o n s e d u r i n g i n d iv i d u a l s t o r m e v e n t s e v i d e n c e d b yt h e s e a nd o t h e r c a t c h m e n t s u n d e r i n v e st ig a t io n b y t h e a u t h o r s s u g g e st t h a ta f u ll u n d e r s t a n d i n g o f th e " c h e m o g r a p h " r e c o r d e d in a r iv e r c h a n n e l w i llo n l y b e a c h i e v e d if t h e p r o c e s s e s i n t e r p o s e d b e t w e e n s t o r m r a in f a ll a n d c h a n n e li n p u t a n d o c c u r i n g o v e r th e e n t i r e d r a in a g e b a s i n a re c o n s i d e r e d . T h e e f f e c t so f ch a n n e l r o u t i n g a r e o n l y o n e o f m a n y c o n t r o l s o n t h e d e t a i l e d f o r m o f t h e" c h e m o g r a p h " .ACKNOWLEDGEMENTS

T h e a u t h o r s a r e g r a te f u l t o t h e N a t u r a l E n v i r o n m e n t R e s e a r c h C o u n c i l f o rp r o v i d in g a R e s e a r c h G r a n t ( D . E . W . ) in s u p p o r t o f w o r k a t E x e t e r, a n d aR e s e a r c h S t u d e n t s h i p ( I .D . L . F . ) a n d to t h e S o u t h - W e s t W a t e r A u t h o r i t y f o rp r o v i si o n o f s t r e a m - f l o w re c o r d s an d c o o p e r a t i o n w i th t h e p r o g r a m m e o fw a t e r q u a l i t y m o n i t o r i n g .

REFERENCES

Carson, M.A. and Sut ton, E.A., 1971. The hydrologic response of the Eaton River Basin,Quebec. Can. J. Earth Sci., 8: 102--115.Edwards, A.M.C., 1973. The variation of dissolved constituents with discharge in someNorfolk rivers. J. Hydrol., 18: 219--242.Glover, B.J. and Johns on, P., 1974. Variations in the natural chemical concent rat ion ofriver water during flood flows, and the lag effect. J. Hydrol., 22: 303--316.Heidel, S.G., 1956. The progressive lag of sediment conce nt ra tion with flood waves.Trans. Am. Geophyso Union, 37: 56--66.Hewlett, J.D. and Hibbert, A.R., 1967. Factors affecting the response of small watershedsto precipi tatio n in humid areas. In: W.E. Sopper and H.W. Lull (Editors), Inte rnat iona lSymposium on Forest Hydrology. Pergamon, Oxford, pp. 725--736.Walling, D.E., 1974. Suspended se diment and solute yields from a small catchm ent prior tourbani zat ion . In: K.J. Gregory and D.E. Walling (Editors), Fluvial Processes in Instru-mented Watersheds. Inst. Brit. Geogr. Spec. Publ., 6: 169--192.Walling, D.E. and Teed, A., 1971. A simple pum pin g sample r for research in to suspendedsediment transport in small catchments. J. Hydrol., 1 3: 32 5- -3 37

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