1. Art and Diseño

W.T.Dickinson und H.Whiteìey
WATERSHED AREAS CONTRIBUTING TO RUNOFF
*W.T.DICKINSON and H.WHITELEY
SUMMARY
The response of streamflow to precipitation input on Blue Springs Creek, an I.H.D.Representative Basin, has been analyzed in light of the contributing area concept. The streamflow was
separated into two components of response, baseflow and storm runoff, the baseflow being
estimated from flows measured at springs on the basin. The minimum contributing area, defined
as that area which contributing 100 percent of the effective rainfall would yield the measured
storm runoff, was evaluated for each precipitation event. These area values were related to
measured soil moisture volumes.
The results revealed that the minimum contributingarea can vary widely,showing a range of
values from 1 to 50 percent for the events analyzed.The majority of values were below 10percent,
a median value for rainfall events being 5 percent. When compared with actual basin segments,
the minimum contributing areas appear to give a good indication ofthe approximate areal extent
of those portions of the Blue Springs Basin contributing to runoff.
RESUME
CONTRIBUTION DES SURFACES D’UN BASSIN A L’ÉCOULEMENT
L a réponse de l’écoulement du cours d’eau à la précipitation dans le Blue Springs Creek, un
bassin représentatifde la DHI,a été analysé à la lumière du concept des aires contribuant à la
formation de l’écoulement.L’écoulementde la rivière a été partagé en deux parties :l’écoulement
de base et celui de l’averse;l’écoulement de base étant déterminé par la mesure des débits des
sources du bassin. L’aire de contribution minimum, définie comme étant celle contribuant pour
100% de la pluie effective, donnant l’écoulement d’averse mesuré, a été évaluée pour chaque
pluie. Ces valeurs de surfaces furent mises en relation avec les volumes d’humiditédu sol mesurés.
Les résultats ont montré que l’airede contribution minimum peut largement varier,prenant
des valeurs de 1 à 50Yopour des pluies analysées. La majorité des valeurs étaient inférieures à
10%, la valeur moyenne s’élevant à 5 %O. En les comparant avec des parties de bassins existants,
les surfaces de contribution minima apparaissent comme donnant une bonne indication des
surfaces approximatives des portions du Bassin de Blue Springs qui contribuent à la formation
de l’écoulement.
INTRODUCTION
Estimation of the volume of streamflow which will result from a precipitation event
on a rural basin remains one of the most difficult tasks facing the hydrologist. N o
widely accepted conceptual model is available to describe adequately the processes
controlling the allocation of input water to storage and to output components on a
vegetated permeable basin.
Traditionally, infiltration approaches such as those suggested by Horton (1935),
Linsley and Ackermann (1941),Cook (1946),and Sharp et ul. (1949),have been found
to be useful on small plots but less applicable in watershed situations. Infiltration
approaches modified by soil moisture deficits have also been utilized by Linsley and
Crawford (1960)and Holtan (1961).
Lack of a firm physical concept has led to graphical correlations among seemingly
pertinent watershed and storm parameters. Examples of such methodology include
the correlations of Kohler and Linsley (1951), and Witherspoon (1962). These
empirical approaches acknowledge the importance of antecedent moisture conditions
* Assistant Professors,School of Engineering,University of Guelph,Guelph,Ontario,Canada.
12
1.12
Watershed areas contributing to rrrnoff
on the basin, as does the work of the U.S. Soil Conservation Service (1963) and
McCutchan (1960).
Recently, a runoff concept has been suggested by such workers as Moldenhauer
et al. (1960), Betson (19641,and Zavodchikov (1965), which hypothesizes that only
a relatively small portion of most catchment areas contributes direct runoff to the
stream flow. The properties of these “contributing areas” would vary from catchment to catchment according to topography, soil characteristics, and vegetation.
Further,there would be changes with time within a catchment due to variable soil
moisture conditions prior to and during storm events.
It has been the objective of this study to consider the storm related streamflow
volumes from the Blue Springs Basin, a research basin established in conjunction
with the International Hydrological Decade Program in Canada, in light of the
contributing area concept. The extent of minimum contributing areas are estimated
and their relationship to measured soil moisture volumes is considered for the basin.
CONTRIBUTING
AREACONCEPT
A detailed consideration of the contributing area concept, including a review of
literature,has been provided by Riddle (1969).A brief discussion of the main points
is given below.
Moldenhauer et al. (1960) were among the first to suggest the possiblity that an
entire catchment might not contribute storm runoff to a stream. Hewlett (1961)
postulated that lower portions of a catchment normally exhibited higher soil moisture
levels than upslope portions, and could be expected to contribute runoff earlier in a
storm, Betson (1964) also indicated that storm runoff usually originates from a
small,relatively consistent,part of the catchment.
It has been further recognized that the contributing area of a particular catchment is dynamic in nature, varying primarily with soil moisture conditions, and also
stage of cover or growth. At the beginning of a storm event, the storm runoff
producing area is relatively small,and is a function of the stream surface area and
soil moisture near the stream.As the storm progresses, the area increases at a rate
which is a function of the rainfall intensity and the depletion of soil moisture storage.
Hewlett and Hibbert (1965) have proposed a dynamic area model which derives
its direct runoff from valley areas near the stream and a variable source area located
on the lower slopes of the hillsides. The Tennessee Valley Authority (1965, 1966)
have also derived an area model based on the antecedent soil moisture and total
soil moisture storage. Ragan (1968) observed the dynamic contributing area to be a
function of variations in catchment conditions prior to and during a storm, and to
variations in the rainfall characteristics.Zavodchikov (1 965) also observed a relationship between the contributing area and soil moisture.
Summarizing this discussion,it should be stated that the contributing area concept
is in essence a manner of interpretation of the runoff coefficient. The graphical
relationships of Kohler and Linsley (1951), and Witherspoon (1962) could be
considered in the same manner. The concept appears to be more physically realistic
than one suggesting that the entire basin contributes runoff in a uniform fashion.
Further,the concept is helpful for catchment model development.
GENERAL
DESCRIPTION
AND LOCAIION
OF BLUESPRINGS
CREEK
Blue Springs Creek is a perennial stream with attractions which include a hydrologic
regime largely unaffected by human activity; and this despite its location in one of
1.13
13
W.T.Dickinson und H.Whiteley
the longest established agricultural areas in Ontario.The creek lies about 80 km west
of Toronto and 10Okm north of Niagara Falls on the eastern extremity of the
Lake Erie drainage.
The closest streams to the north and west are also part of the Lake Erie drainage
but bordering streams to the south and east flow into Lake Ontario, crossing the
Niagara Escarpment which runs in a north south direction 16km to the east of
Blue Springs Creek.
The regional climate is subhumid continental with moderate winters and warm
summers. The mean temperature for February, the coldest month, is -6.3 O C , while
for July it is 20.2OC.The mean annual precipitation of 8 4 0 m m is quite uniformly
distributed through the year. The December to March precipitation is almost equally
divided between rain and snow.
GEOLOGY
AND TOPOGRAPHY
The rock which forms the top of the Niagara Escarpment,and which is responsible
for the presence of Niagara Falls,is a silurian limestone or dolomite. It is this rock,
appearing locally as Abermarle dolomite,which underlies Blue Springs Creek. It has
an important influence on the topography and hydrologic regime of the area.
The proximity of the relatively flat surface of this bedrock produces a subdued
surface relief over a large portion of the basin. But since the area is within the Paris
T
i
l
l Moraine (Chapman and Putnam, 1966), the presence of variable depths of
glacial deposits creates substantial local relief in other parts of the area.
The main stream channel occupies the downstream portion of a glacial spillway
channel. It is cut into the dolomite to a depth of 15m at the lower end of the basin
with a valley floor width of 300m or more.
DRAINAGE
AREA
The designation of a drainage area for this creek based on surface topography is
difficultand the usefulness of this description of the basin may be less than many
hydrologists customarily assume. Figure 1 shows a best guess approximation of the
topographically defined boundaries for the area “contributing” to the stream at the
downstream streamflow gauging station. There are three difficult sections in this
boundary.
The northern end of the drainage area has variations in relief of more than 50m
due to extremely hummocky kame deposits of mixed glacial material. This topography results in several drainage systems of a few hectares each which drain into
swampy potholes with no drainage outlets. The location of the boundary from A
to B is thus a matter of subjective judgement.
Between C and D a swamp occupies an old glacial spillway channel. There is an
outlet at each end of the swamp. One outlet leads to Blue Springs Creek, the other
to a stream draining to Lake Ontario. The line between C and D crosses the swamp
at a location chosen because there is no obvious surface drainage across it in either
direction.
From E to F the boundary is drawn to exclude an area containing pockets of
internal drainage to potholes because it is assumed that this area could never contribute flow to the creek.
14
14.1
Watershed areas contributing to runoff
,e-.,
BLUE SPRINGS CREEK
A
I.H.D.RESEARCH BASIN
Perennial Spring
A
0
Streamflow
gauging station
@ Spring with
measured flowrate
\L . 1
*-
FIGURE1. M a p of Blue Springs Creek I.H.D.Research Basin
TABLE
1. Surface Geology of Blue Springs Basin
Type
% of Area
Comments
Wentworth T
i
l
l
50
Stony,low clay content,medium to high permeability,soil profile > 1.5 m over dolomite,little
topographic relief.
Kames and Eskers
25
Highly variable material,some low permeability
pockets in potholes but generally highly permeable,
variable thickness,steep local slopes.
Abermarle
Dolomite
15
Outwash sand or gravel
I
Soil profile < 0.5 m over rock,rock in bulk highly
permeable due to solution cavities,low relief.
Highly permeable,medium to low relief.
Swamp and bog
3
Generally near perennial stream.
1.15
15
W.T.Dickinson and li. Whiteley
SURFACE
PROPERTIES
OF DRAINAGE
AREA
This boundary defines a drainage area of 44 km2.The distribution of surface
geology within this area is given in table 1 (Karrow, 1963). The land use
pattern, which is entirely rural, closely follows the surface geology. The 50% of
area in cultivated land is mainly on Wentworth Till. The remaining area is about
20% lightly grazed pasture and 30% forest.
DRAINAGE
PATTERN
Figure 1 shows the large number of perennial springs and resulting perennial streams
which characterize the Blue Springs Basin. It is a large spring near the centre of the
basin which gives the Creek its name. The term perennial is used here to mean that
flow has been continuous since field observations began on the basin in 1965.Nearly
all the springs occur either where there is a thinning of the cover of glacial material
over the bedrock dolomite or on a slope where a valley has been cut into the
dolomite.
Figure 1 also shows the occurrence of streams which are perennial at their source
at a spring but which become ephemeral before they reach the perennial stream system. Some of these streams such as G and H never reach the main stream but end in
depressions.Thispattern is typicalof the karstsystemswhich develop in dolomite areas.
The presence of high permeability soil and bedrock on the basin, the abundance
of springs and perennial streams,and the very slow recession curve of the creek in
dry summer periods, all suggest the importance of groundwater flow in the hydrologic regime of the creek. The importance of groundwater contributions to streamflow means, in turn, that the subsurface groundwater divide around the basin will
rank in importance with the topographic divide in determining streamflow characteristics. It is not yet possible to say whether the groundwater and surface topographic
divide coincide and this is one reason for questioning the use of surface drainage
area alone as a predictive areal unit.
W h e n attention is focused on the rapid response of flow in the stream to precipitation input,there is reason for looking critically at the basin to attempt to differentiate areas whose response may differ. Throughout the basin, and particularly in its
downstrpam portion, there are areas which have no surface drainage connection to
the stream. In the case of stream H,for example, there is a length of at least 1 km
between the downstream end of surfaceflow and the main stream.Thislengthof subsurface flow must greatly increase the lag time of response of the main stream to
precipitation occurring on the part of the total basin draining into this sink
In contrast, there are areas near the perennial stream (shown shaded on fig. 1)
which would be expected to provide fast streamflow response to precipitation. O n
Blue Springs Basin these areas have the following properties in common. They
border or surround a perennial stream; their surface elevation is rarely more than
2m above the elevation of the stream; the area is lower than surrounding ground,
and, in the case of the main valley, below surrounding bedrock level; bedrock is
close to the ground surface; and the area is covered by vegetation tolerant of long
periods of high soil moisture content. A dense growth of cedar trees is particularly
common.
All these characteristics are compatible with these areas being places of ground
water discharge. The areas under discussion are conspicuous on aerial photographs
as pockets of natural vegetation surrounded by cleared farm land. They have been
left untouched by the land owners because of their wet, poorly drained organic soils
and inaccessibility due to steep slopes leading down to the depressions or stream
valley floors they occupy.
16
1.16
Watersliedareas contributing to runoff
RESEARCH
BASINDATA
The I.H.D. research study on the Blue Springs Basin is a joint effort of the University of Guelph and the Ontario Water Resources Commission. The latter group is
concentrating their attention on ground water and geological properties. Considerable assistance with data collection and instrumentation is provided by the Federal
government of Canada through the Inland Waters Branch and the Meteorological
Branch. In this study, use was made of precipitation,streamflow and soil moisture
data.
A coniinuous record of precipitation for the basin is given by two alter-shielded
weighing precipitation gauges. These gauges are supplemented in the months of
April to November with a tipping bucket recording raia gauge and 14 non-recording
Canadian Met. Standard rain gauges, read within a day after rain. These gauges are
fairly evenly distributed over the basin.
In the winter months of December to March, snow and rain are measured as
they fall in the two weighing gauges and there are also twice-daily readings of
depth of freshly fallen snow at two other locations. The accumulated depth of
water in the snow pack is measured by weighted core samples taken periodically
at 50 locations on the basin.
Hourly amounts of precipitation are taken from the recording gauge charts and
these hourly amounts are compared and adjusted to yield a basin average record
of hourly precipitation which totals to the arithmetic average of all gauge readings
for each storm.
Soil moisture readings are obtained with a neutron scatter soil moisture depth
probe with readings taken weekly at four locations on the basin as shown in
figure 1. Readings are taken to cover a depth from 19 c m to 200 c m in three
locations and to 150cm in the fourth where the access tube length is restricted by
the bedrock surface.
The moisture content in the top 19 c m of the soil is measured by a singIe
gravimetric sample taken at each site each week. The use of only one gravimetric
sample at each site introduces considerable error in the soil moisture estimate for
each site,but taking more samples on a regular basis would lead to a fast exhaustion
of surface sample sites near the access tube,and would take more of the technician’s
time than is available.
Streamflow values are available on a daily mean basis for the main gauging
station at the outlet of the research basin area and for two supplementary upstream
stations.The data for streamflow used in this study are for the “Boy Scout’s C a m p ”
gauging station (see fig. 1) which measures flow from the upper two-fifthsof the
basin (drainage area of 18 h2).
During periods of variable streamflow, average
values of flow for six hour long periods were calculated for this station for this
study.
SEPARATION OF RAPID
RESPONSE
STREAMFLOW
An inspection of flow hydrographs for any of the three gauging stations on Blue
Springs Basin suggests that two fairly distinct flow components are present. One
component rises and falls seasonally with long periods of slow recession. The other
component appears only after precipitation and has a fast rate of rise and rapid
recession. The geology of the basin makes a two component system physically
reasonable. The justification for making a two component separation will appear in
the results which are obtained.
1.17
17
W.T.Dickinson and H. Whiteley
The flow at the Boy Scout’s C a m p gauging station was separated into two
components by substracting from the measured streamflow rate an estimated
baseflow rate. The estimated baseflow rate was obtained through a comparison of
the flow at that station with a record of continuous flow at four springs whose
locations are shown in Fig. 1.
November 14, 1968 was selected as a day on which all the flow in the stream
was groundwater baseflow. This day follows a five day period with no precipitation
and 17 days with no daily amounts greater than 10mm.It was further assumed that
on this day there was little evaporation from baseflow in the stream channels and
groundwater discharge areas as the evaporation rate was less than 1 .Omm/day and
there was sufficient light rain in the first 12 days of the preceeding period to supply
evaporation from interception and surface storage for the five rain-freedays.
The flow measured at each spring on November 14, 1968 was taken as a standard
discharge for the spring.The ratio measured discharge to the November 14 discharge
was calculated for each spring for each day from August 1968 to December 1969.
The ratio of stream discharge (assumed to be baseflow) on three other dates,
October 30,1968,January 16, 1969 and February 21. 1969 to the stream discharge on
November 14, 1968 was calculated and a weighted mean value of the four spring discharge ratios for these dates was chosen to give as close a fit as possible to the
observed stream ratios on these dates. A weighting of 6,2, 1, 1, gave the best fit. The
results are shown in table 2.
TABLE
2. Estimate of Stream Baseflow Rates by Spring Discharge
Date
Oct. 30/68
Nov. 14/68
Jan. 16/69
Feb. 21/69
Observed Stream
Discharge
Ratio*
Observed Spring Discharge Ratio*
SH62
SH41
SHI1
SW72
Weighted M e a n
Basefiow
Estimate
1.10
1 .o0
1.13
1.38
0.93
1.00
1.06
1.28
1.06
1.00
1.39
2.08
1.00
1.00
1.31
1.81
1.99
1.00
1.02
1.06
0.97
1.00
1.15
1.47
*Ratio of discharge for given date to discharge a n Nov. 14, 1968.
The weighted mean spring discharge ratio was then calculated for each day of
the period and this ratio, multiplied by the observed streamflow on November 14,
1968,was a firstestimate of the baseflow rate at the gauging station on any day. As
stated earlier, this baseflow rate was to be substracted from the observed streamflow
to obtain the flow rate of the rapid responsecomponent.
This method of estimating baseflow gave very reasonable results for cold months,
as can be seen in figure 2.In the warm months from M a y to September,the baseflow
estimate based on springs shows the same slope as the measured streamflow recession
curve but consistently overestimates actual streamfiow by as much as 80%.
For periods when the spring estimated baseflow for the stream exceeded the
measured streamflow, the baseflow at the start of an isolated storm flow event was
taken as the actual streamflow and the slope of the spring estimated baseflow line
during the storm was used to draw the baseflow separation line during the storm
event,which was taken to end when the slope of the recession of measured streamflow matched the slope of the spring estimated baseflow.
A possible explanation for the failure of the springs to predict baseflow during
warm months is that evaporation from the groundwater discharge areas and the
stream channel takes water which in cold months, with low evaporation, appears as
measured streamflow. The four springs which were measured all discharged at the
18
1.18
Watershed ureas contributing to runoff
I
[Measured Stream Flow.
January '69
150
February
March
April
Base Flow estimated
from Flowrate
-
-
O
May '69
June
July
August
FJGURE2. Example of Base Flow Estimutedfrom Flowrate of Springs
r,
foot of abrupt slopes with no areas immediately up slope from the springs which
gave evidence of high water tables.
T o check this explanation, the area of possible groundwater discharge zones
upstream of the Boy Scout's C a m p gauging station was measured. The characteristics
of these zones have been listed earlier. Using this area (2.6km2) and an estimated
evaporation rate for late July 1969 of 4mm/day,the flow rate of evaporation would
be 120 liters/sec. compared with a difference between estimated and measured
streamflow of 58 liters/sec. at this time. It therefore seems quite possible that
evaporation from groundwater explains the difference and that either the average
rate of evaporation was lower than estimated or that a part of evaporation came
from soil moisture storage in layers of soil above the water table in portions of the
groundwater discharge areas.
STORM RUNOFF
After the afore-mentionedflow separation was performed,the remaining hydrograph
was defined as the storm runoff hydrograph. The runoff volumes for individual
storm events were determined and are presented in table 3. For each runoff event,
the effective storm rainfall was determined, and is also given in table 3. This
rainfall term is defined as the basin average rainfall for the storm event less an
amount for intercepted precipitation.This latter term was estimated to be approximately constant at 0.15cm for all rain storm events. It was determined by a consideration of occurrences of precipitation which resulted in no storm runoff.
1.19
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W.T.Dickinson and H. Whiteley
m
20
Watershed areas contributing to runoff
The minimum contributing area was evaluated as
where R
=
V =
P =
C
=
the minimum area which, contributing 100 percent of the effective
rainfall,would yield the measured direct runoff;
the volume of direct runoff
the depth of effectiveprecipitation;
a dimensionless coefficientdetermining the units of R.
The values obtained for the minimum contributing areas for the storm events
analyzed above are given in table 3 as a percentage of the total catchment area of
18.0km2.
An initial consideration of the minimum contributing area values reveals the
following points. The range of values for the storms analyzed is extreme, being from
approximately 1.0to 50 percent,with a mean of 10 percent and a median of 6.0percent. These measures of central tendency reflect the asymmetric distribution of
minimum contributing areas,with 80 percent of the values being less than 15 percent.
If streamflow from only rainfall events is considered, the range for the 1968 and
1969 seasons is 1.0 to 25 percent, with a mean of 6.5 percent and a median of
5.0percent. Eighty percent of the values for rain events lie below 10 percent. A more
complete representation of the statistical distribution of values for the Blue Springs
Basin requires the collection of additional field data.
It is of interest to compare the above results with similar computations for other
basins in Ontario and with catchment results considered in the literature. Minimum
contributing area values were determined for South Parkhill Creek, a 4 2 h 2 basin
situated on lacustrine clay deposits on the eastern shore of Lake Huron. For rain
storm events occurring between October 1965 and November 1969, the minimum
contributing areas ranged from O to 59 percent, with a mean value of 20 percent,
and a median value of 10percent:
Riddle (1969)has summarized values of contributing area found in the literature.
These are summarized in table 4,along with the result of Riddle’s investigation on
TABLE
4. Summary of ContributingArea Values Noted in the Literature
Author
Betson (1964)
Catchment Area
@m2)
Catchment
Characteristics
Contributing Area
Characteristics
0.015
Pasture cover
Mean value
4.6%
0.020
swamp
Area denuded
of vegetation
Mean value
85.8%
+2%
Betson (1964)
Tennessee Valley
Authority (1965)
Zovodchikov (i 965)
Ragan (1968)
Riddle (1969)
0.019
1000 to 1500
0.460
24
Riddle (1969)
28
Heavily grazed pasture
Springmelt conditions
Forested
Agricultural;
intermittent stream
Agricultural;
perennial stream
Range
5 to 20%
Range
20 to 60%
Range
1.2 to 3.0%
Median value 2.2%
Range
0.2 to 40%
Median value 2.7%
Range
0.5to 8%
~~
1.21
2.1
W.T.Dickinson and H.Whiteley
two Ontario basins. The median contributing area values for the results on these
latter basins were approximately2.5percent.
A consideration of these results suggests that, although the basins investigated
have been of variable characteristics, the distributions of contributing area values are
very similar in certain regards. The majority of rain-producing streamflow events
exhibit contributing areas less than 10 percent. Particular basin conditions, such as
total forest cover, total lack of vegetation, and springmelt periods, lead to understandable variations. It is interesting to note that the springmelt values of Zavodchikov (1965) are very similar to those obtained for similar seasonal conditions on the
Blue Springs Basin.
BASINMOISTURE
CONDITIONS
A consideration of the literature, and field observation of the behaviour of small
catchments,reveals that for a particular basin, the most significant factors affecting
the volume of storm runoff from an input event are the basin moisture conditions
at the outset and during the progress of the event. The latter factor is highly dependent on the rainfall and/or snowmelt rates and their distribution in time. Therefore,
the question arises: how can basin moisture conditions be adequately quantified?
As weekly soil moisture observations have been made throughout the year on the
Blue Springs Basin, it was decided to utilize these to obtain an initial index of
moisture status on the basin. A graph of the temporal distribution of soil moisture
stored in the upper 200 c m of soil at four sites on the basin is presented in figure3.
60
P- 55
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5
O
50
8 45
L
al
P
Q
4c
al
Y
3
Y-
0
35
E
o
30
FIGURE3. Time Series of Moisture Content Measured in Upper 200 em of Soil Profile
As the soil and vegetative properties at each observation site are somewhat varied,
the absolute and relative changes in soil moisture also vary. However, the basic
trends in time are very similar. It was decided to select the observed values at the
Donne1 site, for a soil profile depth of 114cm, as indices of the soil moisture status
22
I .22
Watershed areas corttributing to runof
of the basin. It should be emphasized that these moisture values have not been
interpreted in this study as absolute measures of moisture in the soil. Nor have they
been used to determine volumetric soil moisture deficiencies. Rather, they are considered as indices.
For each storm runoff event analyzed, a basin moisture index was computed as,
P
M=M,S2
= the basin moisture index for the storm event,in cm;
M,= the soil moisture stored in the upper 114c m of soil at the Donne1
where M
site on the day prior to the storm,in cm;
P = the effective storm precipitation,in cm.
The moisture indices so obtained are plotted in figure 4 in relation to the corresponding minimum contributing area values. The line through the scatter of data
points bas been sketched by eye.
A number of comments should be made with regard to the sketched relationship
of figure4.
FIGURE4. Experiinental Relationship between Miniinurn Contributing Area and a Basin Moisture
Index
(i) For the Blue Springs Basin, the relationship between minimum contributing
area and moisture index is nonlinear in nature,with a basin moisture index value of
approximately 30 c m behaving as a threshold value.Soil moisture and storm situations
combining to exhibit a moisture index less than 30 cm account for 70 percent of the
events analyzed and the corresponding minimum contributing areas are small. For
indices greater than 30-cm,the minimum contributing areas show a marked and
rapid increase in value.
I .23
23
W.T.Dickinson und IT. Whiteley
(ii) For storm events over Blue Springs Basin exhibiting moisture indices less than
30 cm, the minimum contributing area appears rather insensitive to the moisture
index. Although it is physically realistic to expect the sketched relationship to have
a positive slope in this range of index values, the points developed from storm data
do not exhibit a significanttrend.
(iU) A consideration of other storm and watershed parameters such as rainfall
intensities,their distribution in time, and the state of vegetative cover, did not yield
much further information.There is a slight tendency for storms exhibiting maximum
hourly rainfall intensities greater than 1.25cm to plot above the line. However, a
regression analysis utilizing these additional parameters failed to reveal any significant levels of correlation.
(iv) The moisture indices for situations involving snow or ice cover and snowmelt
were difficult to establish due to a paucity of accurate soil moisture data during the
winter and spring periods. It is unlikely that such a simple moisture index as has
been utilized could adequately describe the snow and ice situations.Nevertheless, it
is of interest to view the relative positions of the spring runoff events to the
remaining population.
BASINBEHAVIOUH
The small yields of storm runoff which frequently occur suggest that only a small,
rather consistent,portion of the basin responds rapidly to all inputs.For most events,
the majority of the basin plays little or no role in contributing to storm runoff
amounts. The streamflow response is controlled by the distribution of storage
capacity over the basin relative to the stream network, and the moisture status of
the storage elements.
/--
y-/
/
/
/
I
I
I
I
Basin moisture index, increasing
-
L
I
FIGURE5. Hypothetical Relationdiil,between M i n i m u m Contributing Area and ci Basin Moisture
Index
24
1.24
Watershed urem contributing to 1.11noff
The results of streamflow analysis from Blue Springs Basin suggest that some
10% of the basin has a low storage capacity for precipitatioti input, while a great
portion of the catchment has a large capacity for storage. Further, this large
capacity is well indexed by soil moisture storage. Once the larger storage body
approaches capacity, the portion of the basin contributing to storm rutloff rapidly
increases.
It is further suggested that, if more data were available at the higher levels of
moisture, the contributing area vs. moisture iiidex relaíionship might have the
appearance of figure 5. That is, there may well be a second threshold value for
some basins signifying the approach to an upper limit of area which physically can
contribute to direct runoff. Such an upper limit would rise with iccrease in input
but could be considered approximately constant for all but extremely improbable
input rates. Further, this upper limit may be significantly less than 100% of the
catchment area, as defined by surface topography. In fact, for the Blue Springs
Basin,it may be in the order of 60%.
MINIMUM
CONTRIBUTING
AREA
The question arises: how meaningful is the term miilimum contributing area as an
absolute area term? D o the numerical values represent basin segmeiits which are the
prime contributors to storm runoff?Does the basin actually behave in this maimer?
Although these questions cannot yet be answered directly, the research program
on the Blue Springs Basin has offered qualitative confirmation of the contributing
area concept.The possible groundwater recharge areas shown on figure 1 most likely
make the initial contribution of storm runoff to the stream.These aieas surrounding
the perennial stream channels represent 14.5 percent of the catchment area,while if
similar areas surrounding the intermittent streams are included, the total area
represents 20.0 percent. As the majority of values computed for minimum contributing area are below 10 percent, and all but two rainfall events result in values less
than 20 percent,it seems highly probable that minimum contributing area values do
in fact indicate the approximate areal extent of the portion of the catchmect contributing to storm runoff. This conclusion has been qualitatively verified with the field
observation that it is hard to find evidence of storm runoff occurring outside these
areas.
As the moisture conditions increase above the threshold index value of 30 cm, it
seems likely that an area greater than the minumum contributing area aciivcly
contributes storm runoff. The extent of this extra area is determined by surface
drainage and storage conditions, and could be sensitive to input rates, i.e. infiltration
properties could be important. The large values of contributing area for frozen
conditions are more likely due to low infiltration raies on noimally permeablc soil
than to high moisture storage values.
ACKNOWLEDGEMENTS
The work upon which this paper is based was supported in part by funds provided
by the Ontario Department of Agriculture and Food, and the National Research
Council. The assistance of Professor H . D . Ayers, Director of the School of Engineering,University of Guelph,is gratefully acknowledged.
1.25
25
W.T.Dickinson and H. Whiteley
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