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N A T U R A LE N V I F O N M E NRTE S E A R CC
HO U N C I L
INSTITUTEof HYDROLOGY
REPORTNo 17 JULY 1972
A CONCEPTUAL
RUNOFFMODELFORTHECAMCATCHMENT
by
w. T. DtcKtNSON
and
J. R. DOUGLAS
I N S T I T U TO
EF H Y D R O L O G Y
HOWBERY
PARK
WALLINGFORD
BERKS
HI R E
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CONTENTS
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Page
t.
INTRODUCTION
2
1.1
Purpose at1d objectil€s
2
settiig
The lhlsio8laphic
2
1.3
Data networks
5
2,
HYDRO',oCI
CA.IJPNOCESSES
7
2.1
General vofuEtric
2.2
Mo}pholEtri
7
8
q^;
r€spcmse
c in ali ces
e+^Pqr.
B
Grounah,rater1eve1s
10
Gromalvater storage and outf[ov
13
I
F^i
c+r,!@
2.6
Cdrcl-usion frord process observatlods
3,
DIVXIoPMEtrT0I coNCrPTUAl IIODEL
A furEpednodel
optiDisation
of par€.neter€
2T
29
Interilependence of par&@ters
3.b
Prediction test
L.
CO,\C],L5IONSNND COMMENTS
5.
REIERENCES
39
43
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FIOURES
Pege
l.
The Can catchnoent above Der'nford Mill-
2.
Soifs snd su.rface drainage
catchlrent
3.
Estimated field capacity piofiles
observatio"r s.iLes
l+.
waler ta.o.Lecontours ror l). J.oy
5.
Ch€rrges in vater
cat chnent
6.
Groundva.ter alischarge region
7.
Con4)arati1/e lrater table ccntour3, titted
and quartic suxface, fox 25.3.69
8.
Ivbsn vater table level computed by i.ntegration
of
quartic surface, pLotted aeainst the arithrnetic
average vefl levef
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2
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T
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I
t
table
on the Can
c@ditions
for soil
at southen
of the
Estination
of stora€e coefficient
tater
level and outflov
data
by eye,
11.
Felationsbip
betreen
basellov
anal nean uater
12.
Relationship
bet{een
baseffov
and fagged rater
13.
Growah,rater storage
6nal baseflov
l\.
Three storage
1r.
Graphica.l representation
of nodel
to soil noistu.e store
16.
Sanple recoral of obserl€d snd predicted
17'22
Error
function
23.
Pr€dicted
hoi?c
fmction
table
20
le\.ef
table
2\
level
of recharge
the C&n catchmeni;
functions
runoff
values
betlreen
fo}^ 1966-1969
ilischalges
28
relating
s u.rfaces shoeing interdelenatence
nf
h
^dal
,
-_-_-!aratreters
s'lal observeal monthly
18
grormih-ater
Sanp.leseasonaf Datterns of soil wate! ccotent,
percolat j. on t o grounabrater, grorm dlwater s tor6,ge
a.nd baseflolr
vari6,'<
15
16
Ca& catchmerlt
fron
noite l for
1ll
enal of
10.
conceltu€"f
l1
raoistur€
12
gradient
as a linear
3
6
36-38
l+l
TABIES
1.
Mor-?honetri c paralleters
2.
le charge estinates
3.
Recharge fates required to yiefal estinateal
for storage coefficient
of 2.lr percent
f+.
Fr.mctions used in various
5.
Sets of optindsed larareter
vatues ! vith
indices of efficiency
anil vo-tune error
a.
Goodness of fit
of the Csln catchnent
from grounalsater
indices
ncdel
9
balance
volunes
22
for.lB
respective
33
-
1
INSTITUTE OF
HYDROLOGY
Report No. 17 J u l y 1972
A CONCEPTUAL RUNOFF mDEL FOR THE CAM CATmMWT
W,T,
Dickinson*
and
J.R. Douglas
ABSTRACT
The hy&ological response of t h e 200 km2 catchment o f
the River Cam above Dernford M i l l , Cambridgeshire,
is shown to be cansiderably more uniform than would
be expected for an area half covered by boulder
clw and h a l f exposed chalk. Percolation to the
grounhater system is found to take place over
p r a c t i c a l l y t h e whole area. As a consequence, a
'lumpedv conceptual model was thought adequate to
simulate t h e catchment response to p r e c i p i t a t i o n .
The resulting model simulates the groundwater sys tern
and i t s contribution to streamflaw adequately,
although t h e modelling of surface runoff events
proves more d i f f i c u l t , due to t h e a r t i f i c i a l controls
present in the catchment.
*School of Engineering, University of Guelph, Canada
1.
1.1
INTRODUCTION
Purposes and objectives
The simulatim of runoff processes of a catchment is an important a i d i n
f o r e c a s t i n g river flows and f o r managing t h e water resources of river basins.
Ideally, the parameters of such a s i m u l a t i a model should be determined
objectively and should be closely r e l a t e d to measurable or p r e d i c t a b l e physical
bas.in c h a r a c t e r i s t i c s .
Differences
ir,
parameter values m e y then be examined i n
l i g h t of p h y s i c a l changes on a basin, natural or man-made, or changes in b a s i n
characteristics from one catchment to another.
The C a m catchment, d r a i n e d by the River C a m above Dernford M i l l , h a s been
s e l e c t e d for t h e purpose of developing a runoff simulation m d e l
permeable basin.
for a
The o b j e c t i v e s of t h e s t u d y have been ( i ) to hypothesize a
conceptual r u n o f f model f o r the C a m catchment, and (ii) to t e s t t h e s u i t a b i l i t y
of t h e model f o r s e l e c t e d periods of measured i n p u t data.
2
The physiographic s e t t i n g
Located approximately t e n miles south of Cambridge, t h e Cam catchment
comprises 197 km2 of predominantly agricultural countryside.
The basin is
s i t u a t e d on the margin of the Fens ; in the headwaters and f u r t h e r to the
s o u t h e a s t l i e t h e t i l l covered Chalk uplands and to t h e northwest lies the
w e s t e r n plateau.
Tne c e n t r a l branch of t h e r i v e r rises in t h e Chalk uplands
s o d h of Saffron Walden, and flows s l i g h t l y west of north to the gauging
s t a t i o n near J k r n f o r d fill. A map of t h e catchment and its l o c a t i o n are shown
in Figure
1,
The Chalk uplands are characterized by a low Chalk plateau with a variable,
and often considerable, thickness of Boulder C l a y as a s u p e r f i c i a l d e p o s i t
(Sparks, 1957).
The p l a t e a u is bounded to the northwest by a gentle and
irregular escarpment.
The Boulder C l q , deposited to n t a x i m u m depths on t h e
emtern and w e s t e r n topographic d i v i d e s o f the catchment, stretches almost to
t h e present v a l l e y bottom
at a number of locations.
The general. topography
( F i g u r e 1) in c o n j u n c t i o n w i t h the d i s t r i b u t i o n of Boulder Clay (Figure 2 )
suggests t h a t the valleys are largely pre-Boulder Clay features , as is the
es carpmen t
.
.
FIGURE 1 THE CAM CATCHMENT ABOVE DERNFORD MILL
The Chalk
formation rests on the Cambridge Greensand, a concentrate
of coarse material derived from c l a y (Steers, 1 9 6 5 ) . The Greensand and the
u n d e r l y i n g G a u L t Clay e x h i b i t a low regional d i p t o t h e southeast, causing
outcrops i n the Chalk to form b e l t s l e n d i n g n o r t h e a s t to southwest.
Of
p a r t i c u l a r interest i s a h a r d , relatively impermeable band o f Greensand which
o u t c r o p s in t h e v i c i n i t y of Dernford M i l l ,
The presence of this band is
l i k e l y t o prevent t h e occurrence of much subsurface seepage northwards past
t h e r i v e r gauging s t a t i o n .
In t h e mass, the Chalk is very porous, h o l d i n g water in about one t h i r d
o f i t s total volume when saturated (Woodland, 1946).
However, the pores are
small and w a t e r i s held l a r g e l y by c a p i l l a r y a c t i o n .
4,s
a result, t h e
water c a r r y i n g capacity of the Chalk depends on the e x i s t e n c e and c h a r a c t e r
of fissures.
( ~ u n t eB
r lair,
The e f f e c t i v e p o r o s i t y , thought to be about 1.5 to 2.0 percent
1965 ; Steers, 1965), is l i k e l y t o be dependent therefore upon
t h e d i s t r i b u t i o n and s i z e of the fissures.
The Chalk is so porous and w e l l
f i s s u r e d t h a t ~ r t u a l l ya l l water f a l l i n g upon it p e r c o l a t e s , r e s u l t i n g i n
l i t t l e , if any, d i r e c t m o f f .
Only in t h e v a l l e y bottoms, where t h e forma-
t i o n nay be saturated to the surface, does surface r u n o f f occur.
On t h e other hand, the Boulder C l a y deposit is thought to be consider-
1946; Thomasson, 1969). T y p i c a l l y , it yields
l i t t l e water; however, it may become very pebbly in places, or contain patches
a b l y less permeable (Woodland,
of rubbly c h a l k or lqrers o f s a n d and gravel, l e a d i n g to t h e presence of small
if unreliable water supplies.
Whereas direct measurements o f p ~ r c o l a t i o nhave been conducted in t h e
C n a l k (Woadland, 1946), y i e l d i n g annual amounts of 150 t o 180
v a l u e s are available
for the Boulder Clay.
m, no measured
Woodland ( 1946) and Hunter B l a i r
(1965) assumed a p e r c o l a t i o n rate for t h e deposit of $0 mms p e r year. From
the f i s s u r e d nature of the Chalk and the variable composition of the Boulder
C l a y , it seems l i k e l y tha* p e r c o l a t i o n rates Tor both may vary considerably
over the catchment.
Such v a r i a b i l i t y is noted in t h e surface drainage
c h a r a c t e r i s tics given in Figure 2.
The presence of a b u r i e d channel beneath t h e c e n t r a l branch of the river
has been r e p o r t e d a n d documented (woodland, 1946; Steers, 1965).
'&li
S t a r t i n g at
ttles ford, t h e channel deepens southward, departing f r o m t h e present course
of the Cam near Quendon.
It is deep, narrow, steep-bided and filled t o depths
of 60 m with a chalky silty till.
Due to the nature of t h i s f i l l material,
and to t h e observation t h a t buried channels in the region are not graded t o
a base l e v e l b u t rather undulate unevenly in a series of rack b a s i n s , it
appears u n l i k e l y t h a t much undergromd seepage occurs aut of the catchment
by w a y of the buried channel.
The p r e c i p i t a t i o n pattern over the area is fairly uniform, with an
average r a i n f a l l of about 635 mms.
Woodland ( 1946) has noted the seasonal
d i s t r i b u t i o n for t h e period 1877 t o 1944, with a low i n March of 4 1.5 mas,
and the w e t t e s t months of July and October y i e l d i n g 68.8 mms and 65.6 ms
respectively.
The annud p o t e n t i a l transpiration f o r Cambridgeshire is approximately
530 mms ( b b i s t r y of Agriculture, Fisheries and Food, 1967).
The seasonal
pattern is very marked, with 460 mnas during the summer period f r o m April
to September, m d 70 mrns i n the winter months.
1,3
Data.networks
The network of hydrological instruments located in and around t h e C a m
catchment during t h e interval 1966 to 7970 is s h m in Figure I .
The data
collected from the various f i e l d instruments have been checked and u t i l i z e d
for t h e compukation of (i) river discharge, (ii) mean basin p r e c i p i t a t i o n ,
(iii) p o t e n t i a l evaporation arid t r a n s p i r a t i o n , and (iv) mean b a s i n s o i l
moisture status.
The river discharge has been determined for two hourly i n t e r v a l s from
t h e continuous r i v e r s t a g e record and a stage : dischare r e l a t i o n s h i p developed
by the Great Olrse River ~ u t h o r i t y - f i a i l y
mean basin precipitation h a s been
computed as the Thiessen average of the gauge network.
Hourly p r e c i p i t a t i o n
was obtained by d i s t r i b u t i n g t h e basin mean i n time aceoraing to t h e c o n t i n u ~ u s
_I P o t e n t i a l daily evaporation and
gauge record obtained at the climate station.
transpiration estimates have been determined f r o m t h e equations presented by
Penman ( 1948)
X~rorn
each s e t of s o i l moist-
condition has been determined.
data, a. mean b a s i n s o i l moisture
The above data are stored on magnetic tapes
*...
..
....
.
.
Cakanous gloy at;
Chdky Bouldmr Clay ;
Poor to free drainage.
8,-
.
..
.
.
....
*
.
a
m
uluP.wr d and
Brolrvn earth; Chalky drlft Ww
Chalk; Frw drainam.
Imperfrct to poar drainage.
Calcanous gamy mil;
Alluvium, locully overlyi nu peat;
Imperfect to poor dra~nage.
ml
0
1
Towns and viblag=.
1
FIGURE 2
Seal. in krn
2
3
4
rn
5
SOILS AND SURFACE DRAINAGE CONDITIONS ON
THE CAM CATCHMENT ( after Thomasson, 1969)
and are r e a d i l y available for computer analysis.
2.
HYDOLOGICAL PROCESSES
Examinaticol of t h e s e t t i n g of t h e Cam Catchment reveals two aspects
of t h e hydro1ogica.l f l o w regime which warrant some i n v e s t i g a t i o n prior t o
the development of a simulatim mdel. Firstly, the role which t h e Boulder
C l a y plws i n d e t e h n i n g d i r e c t runoff and recharge t o the groundwater
system requires some c l a r i f i c a t i o n .
SecbndJy, it would be helpful i f t h e
storage characteristics o f t h e Chalk and t h e i r areal variability within
the catchment were b e t t e r understood.
The data available for t h e
experimental catchment has allowed study of these aspects,
2 1
General volumetric response
Preliminary analysis of the runoff hydrographs for the period
January 1966 to December 1970 reveals t h a t an average of only 30 p e r c e n t
of t h e water f a l l i n g as p r e c i p i t a t i o n discharges i n t o the river during a
80 percent of this runoff occurs as baseflow, or as slow
response. Data collected between 1952 and 1962 ( ~ u n t e rB l a i r , 1965) also
year.
Further,
show t h a t mly 25 to 30 percent of the r a i n f a l l runs off above Demford M i l l .
Such figures are i n d i c a t i v e of an extiremely permeable catchment.
Monthly runoff percentages pertaining to rapid or direct r u n o f f volumes
also denote h i g h permeability.
Values determined by d i v i d i n g the estimated
d i r e c t runoff by t h e p r e c i p i t a t i o n during each monthly interval range
between 0.4 and 19.0 p e r c e n t for t h e p e r i o d 1966 to 1970. me d i s t r i b u t i o n
of t h e s e values is p o s i t i v e l y skewed, t h e mean being 6.0 percent and the
median 4.0 percent.
Compared with similarly computed values in the
literature ( ~ i c k i n s o nand Whiteley, 1970), these index values for the Cam
catchment confirm that the basin does n o t generate much rapid runoff at any
time of' t h e year.
These i n i t i a l analyses s u g g e s t rather strongly t h a t mst of the area
of t h e catchment which is covered by Boulder C l a y deposits must be
relatively permeable material since if it were t i g h t and impermeable
considerable surface runoff w o u l d be generated, resulting in h i g h e r annual
and monthly runoff volumes.
I
- 8 -
2.2
B
8
I
Morphometric indices
A s m m q y of some morphometric parameters o f
presented in Table 1.
the Cam catchment is
Regions e x h i b i t i n g t h e most impermeable d r a i n a g e
I
c h a r a c t e r i s t i c s i n c l u d e Debden Water, Fulpen Slade , and t h e headwaters of
Wenden Brook (see Figure I )
.
The remaining major portion of the catchment
I
d i s p l a y s tributary p a t t e r n s exemplary o f extremely l i m i t e d surface runoff
conditions.
I
The proportian of e f f e c t i v e l y impermeable area r e f l e c t e d by the
This
geomorph~logicpattern i n d e x e d above appears t o be seven to ten p e r c e n t .
area is concentrated i n the headwaters above Saffron Wdden , w i t h a small
p o r t i o n on t h e western watershed. In general, such a p a t t e r n i s i n accord w i t h
t h e volumetric response observea in t h e preceding section.
2.3
Soil moisture storage
The data o b t a i n e d monthly from t h e various soil moisture observation s i t e s
have been e x m i n e d with regard to v a r i a b i l i t y in time and space.
As the s i t e s
were i n i t i a l l y selected to be representative of the mspped soil conditions (see
Figures 1 and 2 ) , and l o c a t e d under similar grassed v@geFtation,it w a s
a n t i c i p a t e d t h a t similarities and/or differences i n soil moisture storage
conditions might be observed.
Fox each of the seven sites for which soil moisture measurements were
c o l l e c t e d d u r i n g t h e i n t e r v a l 1967 t o 197'0,
estimated.
a f i e l d capacity p r o f i l e h a s been
These profiles ( ~ i g u r e3) were determined from f i e l d measurements
o b t a i n e d during the w i n t e r months of December to March before which no p r e c i p i -
t a t i o n had been o b s e r v e d f o r at least five days.
The v a x i a b i l i t y between such
s e t s o f measurements at each s i t e w a s found generally to be w i t h i n sampling
e r r o r , and the r e s u l t i n g p r o f i l e s are thought to be representative of f i e l d
capacity c o n d i t i o n s .
Figure 3 reveals t h a t , except for sites A and G, the estimated profiles
are remarkably similar.
S i t e A, l o c a t e d in the c h a n n e l alluvium, m i & t
expected t o behave uniquely.
be
On t h e other h a n d , the d m s t constant " s h i f t "
of s i x to eight p e r c e n t moisture volume f r a c t i o n e x h i b i t e d by site G i s
1
8
8
t
I
1
1
I
J
I
m
8
thought to be peculiar to the site and not necessarily representative of
As the neutron probe has n o t been c a l i b r a t e d for
each i n d i v i d u a l s i t e , it is possible for estimated moisture volumes to be
the soils in the area.
in error due to i n s t a l l a t i u n or site peculiarities.
Considering the v a r i a b i l i t y of soil conditions d i s c u s s e d in t h e
I n t r o d u c t i a n , t h e similarities in Figure 3 seem remarkable.
Although t h e
seven soil moisture observatim s i t e s represent a small s m p l e of the soils
in t h e catchment, nevertheless the above results f'wther suggest that some
o f t h e Chalk and Boulder C l a y areas s t o r e moisture in a similar way.
Correlations between observations at the various sites and an areal
w e i g h t e d mean f o r t h e catchment reflect the similarities observed above.
A l t h o w the Mole Hall location, s i t e E , w a s found t o be the best c o r r e l a t e d ,
t h e r e was very l i t t l e to choose between sites B, D, E, and F, located in both
Chalk and Boulder C l a y .
2.4
Groundtwater l e v e l s
Observations discussed in the p r e c e d i n g sections suggest t h a t , from a
h y d r o l o g i c a l respmse p o i n t of v i e w , much of t h e area of the Cam catchment
which is characterized by a s u p e r f i c i a l Boulder C l a y deposit may behave
similarly to regions of bare Chalk,
That is, t h e manner in which regions
generate d i r e c t runoff and store soil moisture appears similar.
Consideration
is now given to t h e groundwater system in order to study the recharge, storage,
md outflow patterns
from t h e saturated zme.
Water t a b l e contour maps ( ~ i g u r e4 ) have revealed t h e possible e x i s t e n c e
of a r e g i o n a l groundwater flow system and allowed the following observations to
(i) the regional water t a b l e d i v i d e generally coincides with t h e
surface topographic d i v i d e ; (ii) water table gradients over much of t h e area are
be made:
similar; and (iii)the growhater system discharges along the c e n t r a l river
course.
With the possible exception of t h e southern edge of the catchment, the
water table d i v i d e corresponds to t h e physiography.
F u r t h e r , t h e flow l i n e s
t e n d to converge on t h e o u t l e t s e c t i o n n e a r Demford M i l l .
A t the southern
end, t h e water t a b l e gradient becomes v e l y flat and t h e grounhater divide
FIGURE 3
ESTIMATED FIELD CAPACITY PROFILES FOR SOIL MOISTURE
OBSERVATION SITES
Water table contour
metres 0.0.
-6-
0
1
Scale in km
2
3
?
FIGURE 4
5
\
/
I
WATER TABLE CONTOURS FlTTf D BY EYE FOR DATA
OF MARCH 25th, 1989
8
Data from wells i n the vicinity suggest t h a t the d i v i d e mhy
in fact s h i f t its position seasonally (Figure 5). However, as the gradients
ill-defined.
are particularly l e v e l , seepage into or out of the catchment is l i k e l y t o be
of n e g l i g i b l e volume.
A similarity in water table gradients over much of the catchment area
implies t h a t recharge to the groundwater system and transmissibility throw
t h e saturated zme are somewhat uniform.
In small areas, where there are
local i r r e g u l a r i t i e s i n surface conditions, soils and the Chalk, this statement
is undoubtedly invalid.
The appearance of s t e e p gradients in t h e northwest of t h e catchment
A similar concentration of low transmissibility
coincides w i t h t h e escarpment.
has been observed i n a l i k e location t o t h e east of the Cam catchment
(Woodland, 19461,
It is suggested that such zones mi&t have been the result
of t h e w a y i n which t h e Chalk s t r a t a were folded.
Examination of t h e water table contour maps i n conjunction w i t h t h e
topographic map reveals t h e major groundwater discharge region ( ~ i g u r e6 ) .
This regim appears to be continuous along the main stream channel, implying
that all t r i b u t a r i e s to this channel are likely to become dxy during the
summer months.
As
is
t o be expected, the discharge r e g i o n of Figure
6
corresponds closely to khe region of maximum water well y i e l d mapped by
Woodland ( 1946).
In addition to t h e i r variability in space, t h e w e l l levels w e r e c o n s i d e r e d
with regard to t h e i r f l u c t u a t i o n s in time. Although the range of seasonal
f l u c t u a t i o n can vary s d s t a n t i a l l y from one observation site t o another, a l l
w e l l s generally peak at t h e same t i m e in March or A p r i l , and reach a low
together in October or Noveniber.
No l a g between wells near t h e divide and
those in the valley is apparent, nor between those beneath Boulder Clay and
bare Chalk.
Recharge to t h e water table, therefore, seems to occur similarly
in time t h r o ~ o u t h e b a s i n .
The w e l l s exhibiting the greatest f l u c t u a t i o n s
in water l e v e l are l o c a t e d in t h e zone o f low t r a n s m i s s i b i l i t y .
2.5
Groundwater storage and out flaw,
In order that estimates could be made of the valme of groundwater
&
Y
-
1.5.
Gradient toward catehrmnt
5
o;
N
Gradient out of the catchment
A
4
$
1.0-
e
#
0
L*1
K
Z
m
2
D
rn
I
#"i
I
I
YI
A
0
t
2-0.5-
I
I
0
K
W
I
I
1
-1.0-
I
I
I
I
I
I
'1
FIGURE 5
I,
I
I
I
I
IYI
1
I t
I
t
,8' '
4'
I
I1
::
;
I
I
4
t
w
I
I
'
I
I
1
#\
'
''
I
I
a
'' , ,4'
r'
I
I
I
II
II
II
It
I
t
-1.5-
A
r l
1968
CHANGES IN WATER TABLE GRADIENT AT SOUTHERN
END OF CAM CATCHMENT
Scale in km
FIGURE 6
GROUNDWATER DISCHARGE REGION OF THE
CAM CATCHMENT
Eye fittinp, contours in m.
Quartic function fitting, oontours in m.
-
\
\
\
Weighting coefficient x 100.
a
-4.12
FIGURE 7
'.
f'
P
COMPARATIVE WATER TABLE CONTOURS FITTED BY EYE
AND A QUARTIC SURFACE FOR MARCH 25 th,I969
8
s t o r e d in the catchment, a quartic surface was f i t t e d to each s e t of water
level measurements.
A mean water t a b l e l e v e l was then deterwined a t t w o
weekly intervals, employing a s e t of w e i e t i n g coefficients f o r 38 well sites
to f a c i l i t a t e the volumetric integration (Chidley and Keys, 1970).
A
comparison of t h e q u a r t i c and eye f i t t i n g s t o a s a t of data is shown in
Figure 7, along with t h e weighting coefficients which ma^ be applied to t h e
appropriate well levels to. determine a basin mean water table level.
A comparison of the b a s i n m e a n water table l e v e l estimated f r o m t h e
quartic surface and from an a r i t h m e t i c awrage is presented in Figure 8.
Observation points appear to be s u f f i c i e n t J y numerous and w i d e l y d i s t r i b u t e d
in area t h a t differences in l e v e l from date to date are essentially i d e n t i c a l ,
Storage changes estimated by
i r r e s p e c t i v e of computation procedure.
arithmetic mean tend to be four percent l a r g e r than those y i e l d e d by t h e
i n t e g ~ a t i o nmethod.
Changes i n t h e mean water table level, itself
an index of t h e groundwater
storage condition over t h e catchment , w e r e considered together with estimated
groundwater runoff volurnt3s during t h e summer months to determine an e f f e c t i v e
porosity, or storage c o e f f i c i e n t , for t h e saturated zone.
selected (i.e.
For the data
during periods of little or no groun&ater recharge) the base
flow v o l w should approximate t h e change in grounChater storage. The s l o p e
of Figure 9, t h e r e f o r e , is an estimate of the s t o r a g e c o e f f i c i e n t . If it
could be assumed t h a t there w e r e no seepage or evaporation losses during the
time i n t e r v a l s considered, and t h a t t h e estimates of storage change and
r m o f f were w i t h o u t e r r o r , then the line used for determining the storage
c o e f f i c i e n t would be a lower envelope of t h e measured points.
However, in t h e
l i g h t of the data and computations used, t h e line was sketched as shown.
The storage coefficient having been estimated to be 2.4 percent, a
water balance of t h e groundwater system was performed, for which c o n t i n u i t y
was
expressee,
GPR
-
GRO
=
GS
where GPR is the recharge volume to groundwater in a
given ti=
interval,
2
E
k
53-
f
sz-
E
A
A
A
Y
3
2
51-
d
f
n
sox
Ez
49
-
I
I
A
49
47
l8
i0
ARlTHmTlC MEAN WELL LEVEL, METRES
FIGURE 8 MEAN WATER TABLE LEVEL COMPUTED BY INTEGRATION
OF QUARTIC SURFACE PLOTf ED AGAINST THE ARITHMETIC
AVERAGE WELL LEVEL
C H A N L IN GROUNDWATER STORAGE, MMS
FIGURE 9
ESTIMATION OF STORAGE COEFFICIENT FROM GROUNDWATER
WATER LEVEL AND OUTFLOW DATA
BASEFLOW,
MMS/DAY
GROUNDWATER*
STORAGE, MMS
PERCOLATION TO GROUNDWATER,
MMS / DAY
SOIL WATER
CONTENT, MMS
GAO is the groundwater outflow volume in the same i n t e r v a l ,
as estimated fmm the streamflow hydrograph,
GS is t h e change in groundwater storage, estimated from the
mean water table Levels and t h e storage c o e f f i c i e n t .
The patterns of recharge, storage and outflaw determined in the
balance are i l l u s t r a t e d i n Figure 10.
Soil nmietum observations have been
included for carsparison, along w i t h recharge, or percolation, estimated
from a storage c o e f f i c i e n t of four percent.
Figure 10 reveals two items of particular i n t e r e s t .
Firstly, an
increase in s t o r a g e coefficient to four percent changes t h e pattern o f
seasonal recharge, but does n o t alter t h e annual volume.
groundwater out f l a v hydrograph anticipates rather than 1-
storage condition.
E a c h i t e m is cansidered -her
Secondly, t h e
the groundwater
below.
The use of a storage c o e f f i c i e n t of 2.4 percent results in the
estimation of essentially no recharge during the summer months of 1967,
1969 and 1970, w i t h cmtinuous recharge throughout 1968.
An increase in
storage c o e f f i c i e n t t o 4.0 percent reduces the sunner recharge i n 1968 t o
a n e g l i g i b l e amaunt, but results in a requirement for negative recharge
during t h e o t h e r three summers. Such negative values could he meaningful
either if seepage o u t of t h e c a t c h m n t into adjacent bagins w e r e to occur
at such times, or if s u b s t a n t i a l evaporation w e r e to take place from the
groundwater system.
It seems l i k e l y t h a t if any seepage occurs across the
catchment b o u n d q during the summer seasons, it is i n t o rather than out
of the Cam Catchment (see Figure 5).
Further, as t h e water table is a
cmsiderable distance below t h e ground surface in a l l but the immediate
discharge region of the main stream, large amounts of evaporation from the
groundwater system seem u n l i k e l y .
Therefore, m t h e basis of seasonal
=charge pattern, a storage coefficient of 2.4 percent appears r e a l i s t i c .
For values of storage coefficient in the range o f those alrea*
mentioned, annual recharge totals are very similar, as revealed in Table 2.
For such patterns of recharge, the annual v o l m is n o t a sensitive indicator
of an a p p r o p r i a t e storage c o e f f i c i e n t .
It is also u s e f u l t o consider at this point t h e relatim -itude
of
. RECHARGE ESTIMATES DETERMINED
TABLE 2.
FROM
GROUNDWATER BAUN CE
Recharge to groundwater
( m over catchment)
Storage c o e f f i c i e n t
of 2.4 percent
Time interval
Storage coefficient
of 4.0 percent
6th J m 1967 to 3rd Jan 1968
136
139
4th Jan 1968 to 3rd Jan 1969
1 86
20 1
3rd J m 1969 to 8th Jan 1970
150
133
124
143
9th Jan 1970 to 26th
TABLE 3.
me
1970
F3CHARI;E RATES REQUIRED TU YIELD ESTIMATED VOLUMES
M)R STORAGE: COEFFICIENT OF 2.4 P E R W T
Recharge rate t o groundwater
( n m over catchment axea noted)
Time interval
Area
of
91 h.2
Area of
155 kn?
Area of
197
3
6th Jan 1967 to 3rd Jan 1968
295
172
136
Jan 1969
40 4
236
186
3rd Jan I969 to 8th Jan 1970
326
190
150
9th Jm 1970 to 26th dune 1970
269
157
124
4 t h Jan 1968 to 2nd
the annual recharge volumes.
Table 3 shows the annual recharge rates
required if uniform percolation w e r e assumed to take place over 9 1 km2,
155 km2 and 197 km2, and
catchment.
no percolation existed over t h e remainder of t h e
The first a E a is approximately equivalent to that of t h e bare
Chalk; t h e second
mw
be obtained by considering the t o t a l area less 18 km2
of groundwater discharge fegion and less 23 km2 of e f f e c t i v e l y impermeable
soil; the f i n a l figure is t h e catchment area.
The recharge values presented,
when regarded w i t h previous estimates of 150 t o 180 mms percolating through
bare Chalk annually (Woodland, 1944) , t e n d to confirm t h e notion that at
l e w t 155 k$
of the c a t c h m e n t behave as permeable recharge area.
The manner in which groundwater outflow precedes, or a n t i c i p a t e s
s t o r a g e is curious!
,
This r e l a t i o n s h i p is a l s o demonstrated in Figure 1 1 by
t h e strong clockwise hysteresis e f f e c t .
This hysteresis is essentially
remved if the base flow is p l o t t e d against the mean w e l l levels lagged by
four weeks, as shown in Figure 12.
A second approach to r e m o ~ n gt h e
hysteresis i s t o c a s i d e r the storage to be a l i n e a r combinatkn of recharge
and baseflow, s h a m in F i g u r e 13.
The "backward lag" between groundwater storage and baseflow has a l s o
been observed by Hunter B l a i r ( 1965).
H i s explanation of t h i s phenomenon
(i) begin to rise
before the main catchment storage had begun to i n c r e a s e , and (ii begin Lo
w a s based on a bank storage hypothesis, causing f l o w to
decline before storage had achieved i t s peak.
As t h e water t a b l e w e l l
levels d i s c u s s e d e a r l i e r revealed n o l a g tendency in space, the above
hypothesis appears as acceptable as any, altho@
it can n e i t h e r be proven
nor dispraven w i t h the av&lable data.
Figure 12 a l s o reveaLs that, d t h o u g h a l i n e a r r e l a t i o n s h i p may
e x i s t between recharge, storage and baseflcru, the storage axis may s h i f t
If such s h i f t s do occur in tirw ,
p o s s i b l y r e f l e c t i n g the f i l l i n g up and depletion of secondaxy storage
during excessively wet or dry years.
systems , modelling of the grom&waterstorage system becomes extremely
tenuous
2.6
.
Conclusion f'rom process observations
Information on general volumetric response , geomrpholoery , soil
MEAN WATER TABLE LEVEL. M
FIGURE 11
RELATIONSHIP BETWEEN BASEFLOW AND MEAN WATER
TABLE LEVEL
Watw table levels are lagged 4 weeks behind mean daily basef lows
MEAN WATER TABLE LEVEL, M
FIGURE t 2
RELATIONSHIP BETWEEN BASEFLOW AND LAGGED WATER
TABLE LEVEL
I
I
t
I
&4
0
H
0
GROUNDWATEP STORAGE (RELATIVE TO MEAN LEVEL OF 51 M), MblS
0
a
d
0
N
0
W
0
I
L
0
misture and g r o u n h a t e r suggests that the Cam catchment is predominantly
a highly permeable area. There i s n o indication that the Boulder Clay d e p o s i t
as a whole behaves d i f f e r e n t l y from regions of bare Chalk.
Further, t h e
c h a r a c t e r i s t i c s of the Chalk appear uniform over much of t h e c a t c h m n t .
As
water storage and transmissim properties may be similar in many areas of
t h e b a s i n , it i s cmcluded t h a t t h e Cam catchment may be considered as a
s i n g l e homogeneous region for t h e purpose of modelling hydrologic response.
3.
3.1
DEVELOPMENT OF CONCEPTUAL M2DELS
A lumped model
On the basis of t h e preceding analysis, a lumped model ( i .e. lumped in
space) has been hypothesized for t h e Cam catchment.
The model is conceived
as three storage systems, as i l l u s t r a t e d in Figure 14.
The n a t u r e of each
system and the c h a r a c t e r i z a t i o n of fluxes i n t o , between and away from each
system are d i s c u s s e d below.
a)'
Surface s t o r e
The surface storage component accepts t h e incoming r a i n f a l l and p o t e n t i a l
evaporative demand and c a l c u l a t e s e f f e c t i w r a i n and a residual evaporative
demand to be passed rm to t h e soil moisture s t o r e , after s u b t r a c t i n g evapora-
tion from the surface store.
The p r e c i p i t a t i o n entering the store, R A I N (11)
is described by 6-hourly p r e c i p i t a t i o n t o t a l s obtained f'rom the raingauge
network ( s e e Section 1.3).
The p o t e n t i a l daily pan evaporation, EVAP (11), is
e s t i m a t e d from t h e climate s t a t i o n data and apportioned i n t o 6-hourly i n t e r v d s
with the w e i g h t i n g factors 0.697, 0.138, 0.0, and 0.165 for t h e p e r i o d s
09- 15,
15-2 1 , 2 1-03 and 03-09 hours respectively.
Evaporation from t h e surface
s t o r e , ES, is assunted to be a f'unction of t h e potential evaporation, EVAP (111,
and t h e e f f e c t i v e r a i n discharged t o the s o i l moisture store, ERAIN, a
Function of the incoming p r e c i p i t a t i o n , and t h e surface storage moisture s t a t u s ,
SS-DS.
b)
S o i l moisture store
Three hydralogical processes are conceived t o be associated with t h e s o i l
-
11
-
-
I
I
I
I.
SURFACE
STORE
r r r r - - - - r r - r r e I L
SOIL MOISTURE
STORE
DCL
1
GROUND WATER
STORE
FIGURE 14 THREE STORE CONCEPTUAL MODEL
FOR CAM CATCHMENT
GS
moisture storage comgonent
.
These are a c t u a l t r a n s p i r a t i o n , direct runoff
md percolation to t h e groundwater store.
A c t u a l t r a n s p i r a t i o n is assumed
to be a f'unction of the evaporative demand on t h e s o i l moisture s t o r e ,
EEVAP, and the moisture status of the store, DC.
t h a t not ~ a t i s f i e dby t h e surface s t o r e .
The evaporative demand is
Direct runoff, ROFF, and p e r c o l a t i o n
to groundwater, GRP, are a l s o characterized as simple Arnctions of the soil
moisture storage.
c)
Groundwaterstore
Storage in t h i s component of the system is assumed to be a function of
input to t h e s t o r e , GPR, and output, GRO.
A s i m p l e r form r e l a t i n g only
storage and outflow has also been considered.
The moisture s t a t u s of the
s t o r e is monitored as ES.
d)
Routing of runoff values :
The direct runoff volume, ROW, is routed through a single l i n e a r
reservoir of storage, S , and lagged by a time i n t e r v a l , DEL, t o produce the
The r o u t e d d i r e c t runoff, R , is then added to
d i r e c t r u n o f f contribution.
t h e groun8water outflow component, GRO , which
be lagged by the interval
GDEL, to yield the estimated six-hourly outflow Q.
The various functional forms utilized in the model are o u t l i n e d in Table 4.
Some of t h e relationships are presented graphically in Figure 15.
3.2
Optimisatian of parameters
The error f u n c t i o n , or residual variance, has been defined as the cumulated
sum of squares of difference between computed and observed six-hourly runoff
increments.
That is, @
n
(Q'
- Q)',
eh?.
efficiency w a s considered as the
proportion of the "no model" variance accounted f o r by the model, R~ =
(F$ - + ) / F O ~ , where Fo2 was t h e "no-model" residual variance ash and
S u t c l i f f e , 1970).
The four years of r e c o r d , 1966 t o 1969, were divided into two equal. periods
of two years' duration.
Various forms o f t h e model w e r e optimised on t h e
TABLE 4
FUNCTIONS USED IN VARIOUS mDEL F
O
M
Model 1
1 Effective r a i n f a l l
ERAIN=RAIB(II) +
m-ss
2 Evaporation from surface
ES=FS. EVAP(II)
3 Residual evap. demand
EEVAP=EVAP(TI) - ES
Parameters: FS, SS
C o n s t r a i n t s : FS= I . 0
4 A c t u a l transpiration
ET=ECP. EEVAP
where
E CP=FC ( DCT-DC)
( DCT-DCS
-
30
Model. 2
Model 3
bdel 4
+
+
+
+
+
*
j
+
+
-
+
+
5 Direct runoff
generation
ERAIN
ROF-ROP
where
.
ROP=RC/ ( EIS+DC)
+
6 Percolation to groundwater
GPFPGLR ( 1.0
- zM
nc )
+
constrained
+
Constraints :
BOP s ROE4
+
+
Parameters : C S M
Parameters :
GSM, GF
8 S=DRK.R
an d
R delwed by Dm
GRO delwe d by GDEL
Parameters : DRK ,
DEL, GDEL
.
+
No delay of
GRO
GDEL deleted
.
+
+
GS=GSM [GF.GPR
*( 1-GF) GR3 ]
*
T1OP=RC. e-FS DC
_C,
GS=GSM. GRO
+
- -
Parameters: FC, DCT,
DCS, RC, RS, ROM,
GLR, GDM
7 Flaw from groundwater
i.
FS n o t
+
-
ROM deleted
fro c o n t r a i n t s
_CS
+
+
+
j
+
*
j
_e,
a) Actual transpiration
b ) Ditoct runoff
ROM
ROP
e ) Percolation to groundwater
GLR
GPR
0
GDM
DC
FIGURE 1 5
GRAPHlCAL REPRESENTATION OF MODEL FUNCTIONS
RELATING TO SOIL MOISTURE STORE
first two y e a r s ' record, and t h e r e s i d u a l variance, e f f i c i e n c y , and
o p t i m i s e d parameter values were noted.
A f i n a l form of the model was then
a p p l i e d w i t h a s e t of' optimm values to the next two years of data to o b t a i n
a second and independent assessmnt of residual varimce and efficiency.
The o p t i m i s a t i o n process used w a s t h a t of 0' Carznell et a2 ( 1970).
Instead of optimising all parameters simultaneously, groupings of up
t o e i g h t parameters were o p t i m i s e d while the remainder w e r e f i x e d a t
seemingly appropriate or previously optimised values.
In t h e l i g h t of
e x i s t i n g interdependence among p a r a t e r s , this approach, u t i l i z e d i n
conjunction with t h e mapping of t h e error f u n c t i o n for various combinations, proved e f f i c i e n t and economical.
The various f u n c t i o n a l forms of the model presented in Table 4,
optimised for the 1966-1967 period of record, yield the results shown in
Table 5 .
The s e t of parameter values l i s t e d for %&el
5 w a s subjectively
s e l e c t e d , aFter i n s p e c t i o n of the e a r l i e r optimised values and mappings
of the e r r o r function.
Regarding t h e general "goodness of fitf'of predicted flows to t h o s e
observed, t h e higher n u d e r e d mdels appear to describe grotmawater f l o w
conditions adequately b u t to encounter difficuley w i t h the direct runoff.
Further, the problem w i t h direct runoff appears to be associated m o r e
with t h e time d i s t r i b u t i o n than w i t h the occurrence and volume o f flow.
G e n e r a l l y , the observed hydrograph is more peaked than t h e p r e d i c t e d f l o w
record.
These observations may be noted in t h e sample period of observed
and p r e d i c t e d f l o w s sketched i n Figure 16.
3+3
Interdependence of parameters
A s noted by Mandeville
e t aZ, (1970), t h e o p t i m i s a t i o n and utility
of p a r t i c u l a x model parameters can most e a s i l y be studied if the para3lleters
are o t h o g o n d ( i. e
.a
change in one does n o t involve a change in another)
.
Unfortunately, such o r t h o g o n a l i t y is not g e n e r a l l y experienced in
h y d r o l o g i c a l conceptual models and interdependence of parameters ~ 0 m 3 n l y
i n c r e a s e s with t h e number of parameters.
TABLE 5
SETS OF OPTIMISED PARAMETER VALVES, WITH FQSPECTIVE
I N D I C F S OF EFFICIENCY AND VOLUME EAROR
Parmeter
Model 1
Model 3
Model 2
Model 4
Model
SS
14.3
14.3
9.29
9.5
9.5
FS
1.0
1.0
2.42
1.4
1
FC
0.65
0.65
0.65
0.65
0.60
Dm
16.86
16.86
16.86
16.86
DCT
128.60
128.60
104.5
-------
104.5
- - - -- - RC
I1S
4.40
4.40
4-5
18.92
18.92
I 5. 0
ROM
0.30
0.30
0.27
0.27
-5
17.0
100.0
0.24
0,24
0.029
0.20
0.029
-
0.27
0.27
0.28
57.54
57.54
-
------GLR
GDM
50.0
57-54
65. 0
------GS M
100.0
-
GF
GEL
600. 0
539.5
-0.15
-0.13
-
-0.13
-
-
-
4.0
539.5
560. 0
-0.075
------DEL
2-32
2 - 32
2.32
2.32
2.32
DRK
4.85
4.85
4.85
4.63
4.55
--Efficiency as
a. percentage
E r r o r in runoff
volme as a
per c e n t age
61
+
4.2
72
+ 1.8
76
-
5
0.19
77
-
0.66
78
+ 1.5
FIGURE 16
-
-
-P
z
SAMPLE RECORD OF OBSERVED AND PREDICTED
RUNOFF VALUES
-- --- --- -- *---- - 4
-
+
---- ----
Ad-
--*
--*
-
- ---+
----*
1-
4
i
5
&
+.a
.
OBSERVE0 nrOROGRAPH
PREMCTZD H-APH
---
B
;
$
a
1
a
h
E
rx 0.5
+
z
I
S
s
z
w
A
\
("J
-
Y
Y
-\
--%
---
1.0
1
'
2
'
3
'
4
'
I ' 6 ' 7 ' 6 ' 9
' 1 0 ' 1 1 ' 1 t ' 1 3 ' 1 4 r 1 l ' 1 6 ' 1 7 ' I 8 ' W ' ~ ' 2 ' 2 ~ ' 2 3 ' U ' 2 5 ' ~ ' n ' t l ~ 1 ' ? ' 3 ' 4 ' 5 ' 6 ' 7 ' 8 ' 9 ' * 0 '
FEBRUARY In68
MARCH 1866
A considerable range of interdependence conditions w a s found to
e x i s t in various portions of t h e model forms cansidered above.
S-le
mappings of t h e error f u n c t i o n , p r e s e n t e d i n Figures 1 1 to 22, reveal t h e
situation. Such mappings w e r e found to be very u s e m as guides to the
selection of an optimum s e t of parameter values.
The h i g h e s t interdependence was observed among the parameters
concerning t h e surface s t o r e and evaporation from the s o i l moisture stom.
The parameters are not only dependent within each of these storage phases,
but a l s o between phases.
For a catchment in which 70 percent of the
incoming annual precipitation is evaporated, it is perhaps to be expected
that a l l parameters i n v o l v i n g evaporation should be highly interdependent.
The parameters describing the surface s t o r e and a c t u a l transpiration
are conceived t o g e t h e r as modelling the t o t a l evapotranspiration process.
For t h e p r e s e n t , they seem d i f f i c u l t t o i n t e r p r e t physically.
What a
surface s t o r e of approximately t e n mms capacity physically characterizes,
arid w h e t h e r evaporation from agricultural surfaces occurs at one and a
half times the p o t e n t i a l r a t e , remain unanswered questions.
FS and SS parameters
In fact, the
merely d e s c r i b e a black box which suitably
disposes of moisture for t h e rest of t h e model.
The parameter DCT t e n d s to assume a value such that evaporation *om
the soil moisture store is possible at all times.
That is, DCT c o i n c i d e s
w i t h the 1argest soil moisture d e f i c i t experienced in t h e p e r i o d of record.
Whether such a condition indicates continuous evaporation *om
the soil, or
whether it r e f l e c t s the need f o r a modelling mechanism t o evaporate
considerable volumes of water, is open to speculation.
The direct runoff, percolation and groundwater storwe parameters
are reasonably independent and more amenable to physical i n t e r p r e t a t i o n .
Althoueh, the e r r o r f u n c t i o n surface for the d i r e c t runoff parameters, RS
and RC, is r e l a t i v e l y flat, t h e r e is a c l e a r l y defined optimum.
These
parameters appear tk be f a i r l y stable when o t h e r parameter values are
modified.
The r a g e of r u n o f f percentages y i e l d e d by the
FE and RC values
obtained are comparable w i t h those i d e n t i f i e d in S e c t i o n 2.1.
Figures 2 1 and 22, showing error function contours f o r percolation
ERROR FUNCTION SURFACES FOR THE EVAPORATION
PARAMETERS OF THE M O M 1
FIGURE 17
FIGURE 18
THE FS/SS SURFACE
THE FS/DGf SURFACE
I
I
1.2
1.3
I
rn
FS
1.4
1.5
FIGURE 19 ERROR FUNCTION SURFACE SHOW l NG I NTERDEPENOENCE
BETWEEN THE TWO EVAWRATJON REOUCT!ON FACTORS
FIGURE 20 INTERDEPEND€NCE BETWEEN THE SURFACE RUNQFF
GENERATION PARAMETERS (ROP= R C / ~ D C x R S )
ERROR FUNCTION SURFACES FOR VARIOUS
GROUNDWATER PARAMETE~S
0.21-
0.27-
GLR
0.25-
0.2%-
I
1
49.54
1
53.54
1
1
1
57.54
61.54
65.54
GDM
FIGURE 21 THE GLR/GDM SURFACE SHOWING
INTERDEPENDENCE BETWEEN PARAMETERS IN
THE PERCOLATION MODEL
FIGURE 22 THE GF/GSM SURFACE SHOWS THE LOW
SENSITIVITY OF THE GROUNDWATER RESERVOIR
COEFFICIENT, GSM
and groundwater storage parameters, are both relatively flat.
In the
former an optimum is r e l a t i v e l y clear; in the latter, the optimum of GF
is clear, while the e r r o r function is less sensitive to GSM. A suitable
value of GSM may be obtained from cansideration of well l e v e l and
baseflow data such as that of Figure 13. The parameter GLR may
correspond t o a physically based maximum rate of percolation through the
upper hori zans
.
Prediction test
Having obtained an optimum s e t of parameter values from t h e 1966-
1967 period of record (i.e. h d e l 5 ) , the model w a s used to pre&ct f l o w s
given rainfall and evaporation, for 1968 and 1969. Indices of the
goodness of fit are given in Table 4 and a comparative p l o t of discharges
is presented in Figure 23.
The results of such a prediction t e s t the
u t i l i t y of t h e model,
As the flow during 1968 and 1969 was cmsiderably more variable than
in 1966 and 1967, as shown i n the f'no-m~del" variance terms, a f i n a l
optimisatim of t h e d i r e c t runoff parameters RC, IE and DRK was performed
an t h e t o t a l four y e a r s of record. The resulting values were 0.267, 0.026
and 0.152 respectively.
As noted above, the RC and RS parameters remained
stable ; however, the DFK parsmeter changed s i g p i f i c a n t l y
changes in e f f i c i e n c y are shown in Table
.
Tbe corresponding
6.
The model has predicted baseflaw in 1968 and I969 as w e l l as it did
in 1966 and 1967.
This is reassuring, as som earlier results (Section 2.5)
suggested the possibility of a somewhat unstable groundwater system.
As a
consequence of an adequate basefluw fit, errors in volume remain fairly
small.
On
the o t h e r hand, p r e d i c t i o n o f
s,
suitable distribution of d i r e c t
runoff is d i f f i c u l t t o achieve, I t is worth n o t i n g t h a t the l o w e f f i c i e n c y
obtained in 1968, and t h e change i n DRK resulting i n an improved efficiency
r e l a t e t o a sin@.e major f l o ~ devent in September of t h a t year. The
remainder of t h e record is i n s i m i f i c a n t l y affected.
*
Period of
record
Annual
rTno-mdel,,
v a r i an c e
T e s t r u n I*
Ann u d
efficiency
T e s t run 2%'
Pe rcent age
error in
runoff volume
Annul
efficiency
Percent age
error in
run off vol m e
-
4.6
75.6
5.47
77.9
78. t
+
8.1
78.1
- 2-95
+ 9-10
15.83
67.2
-
2.0
75 4
-
13.05
73.9
+
3.7
74.0
+ 3.83
1966
8-79
1967
1968
1969
.
1.58
"Model optimised an 1966-1967 record and u t i l i s e d for
p r e d i c t i o n during 1968- 1969.
**Par=ters
reCOT d.
RC, RS, and DFK optimised on the f o u r
years
of
FIGURE 23
PREDICTED AND OBSERVED MONTHLY
DISCHARGES FOR 1966 -1969
points lie within
thew bounds)
PREDICTED MONTHLY DISCHARGE, MMS
4.
CONCLIEIOHS AND COMMENTS
The Cam catchment has been observed to behave hydrologically as an
e x t r e m e l y permeable area.
Regions o f s u p e r f i c i a l Boulder Clay deposit and
bare Chalk appear t o behaw in s u r p r i s i n g l y similar f a s h i o n s w i t h regard to
t h e i r generation of d i r e c t runoff, storage of moisturn and a b i l i t y to
recharge t h e groundwater system.
The b e h a v i o u r of t h e s a t u r a t e d f1.m regime
i n d i c a t e s t h a t much of the Chalk s t o r e s and transmits moisture r a t h e r
uniformly, exhibiting a storage c o e f f i c i e n t of approlcimately 2.5
percent.
The lumped conceptual model hypothesized for the catchment predicts
the b e s e f l o w response both in volume and time d i s t r i b u t i m q u i t e adequately
d u r i n g wet and dry periods.
This r e s u l t is most encouraging for the u t i l i t y
of t h e model for general water resources management.
F u r t h e r , i f more
d e t a i l e d s t u d i e s of t h e groundwater regime w e r e to be considered, the model
might be. used t o generate a uniformly d i s t r i b u t e d recharge i n p u t .
For t h e purpose of p r e d i c t i n g the time d i s t r i b u t i o n of f l o o d e v e n t s ,
the model is less s a t i s f a c t o r y .
Whether problems in this area are associated
w i t h the behaviour of t h e surface s t o r e component and/or the routing portion
is n o t c l e a r l y understood.
These two aspects of t h e model, also the m o s t
d i f f i c u l t to i n t e r p r e t physically, warrant further thought and modification.
I
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-
t
I
I
t
t
I
I
I
I
t
I
I
I
I
t
I
I
t
I
slld IGYS, K.M. 19?0.
J. Hfdrol.,
iainfaLl.
12,
-
X E F E I E N CE S
6.
dlIDLnY, T.i.E.
qJ
A .epid nethod of coeputing areal
15-2\.
DIctINsoN, W.T. and WHITS,EY, ir. 1970. water€heal ar:eas contributing
to
SUnp, on the tesults of reeea?ch on ?epresentatiue @rd etperinental
b($'i|ts, tlel l,ington! N.2., vaL I, IASH/UNESCA,IASH PubL 96! 12-26,
llrLoff,
HUN1ER
BLAIR, A. 1965. The hydiologr
Queens Uni1€rsity
of the River Cajo. M.Sc. Ihesis,
of Belfast.
M,A1',IDEYILLE
r A.N., orCoNNIltL,F.f.,, 'UTCLITFI, J,V. srrd NASH,J.E. 19?0.
niver flal. forecasting through coneeptuaf noalels. Part III - Ihe Rqy catchnent
at Grendon Lhlderarood. J. Hldnol.
11, 1A9-128.
IISHEffES lXD I00D. 1967. Pa tei tiaI, t?@rspi ta rt on.
M .TISTRYOF AGRICUI.,TUnE,
Tech.su1l. 16, HI4SO.
NASH, J.E.
moilels,
snd SUTCLM'E, J.V-
Parl I - e discussion
19?0.
Ri!.er flotf forecastin8
of principles,
J. IfAdvoT''
through conceptusl
10' 282-290.
P.8., NASH,J.E. and IAXR!!L, J.P. 1970. Ri!.er flov folecasting
OTCONNXLL,
through colceptus,l rdodefs. Part II - the Srosna catchhent at I'erbsne.
J. HUdt'aL., 10, 317-329,
PENIM, H.L. 19118, Natural evaporation flon oI)en vater,
P|oc. RoV. Soc, Lqnd. (D 193, Pa-1\r.
SPnnKS,B.W., 195?. {he evofution of the relief
Ceog". J.,
bare 6oi1 anal grass '
of the Caft Valley.
123, 188-207.
STEEFS,J.A.
1965. fhe Cafuddge Region.
Britis}]
Association,
caDbridge
( UniYercity Pr€ss),
lltoMAssoN,A.J, 1969. soiL6 of the So,ff?da alden Dlsttict,
Iiarpenalen.
speciaf surv. 2,
I
t
WOoDLAND,
A.ht. 19t5.
Ipslrich Ust?.ict.
I'late" steplA fton undetgz,owtd ooLnces of, C@rby.Ld,ge
Ceol., Sufv. Wa"tine palphlet 20.
Further Reaalinc
HoDGX,C.A.H. and SE.AIE, R.6. 1966. The eoils
Men. SoiI Surv. Gt Br., I{arpentlen.
of the diet?ict
a"owd Cafu?iage.
MrI{ISTRY0I't{OUSIlrc .Al,tDLoCAL GOVEXT{MENT.
1960. Rive" eteat juae
HUdtolog'i&L Sun eU. H}'fC\
VJOFSSA.I4,
B.C. snd lAYLOR, J.H.
Inst.
Geol. Sci.,l4ee.
1969.
geol. Surv. Gt
5eolory
of the colmtrV arolutd C@th?idge.
Br., H!6o.
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