Journal of Hydrology, 103 (1988) 27~292
Elsevier Science Publishers B.V., Amsterdam
275
Printed in The Netherlands
I31
S T U D I E S OF A SUBARCTIC C O A S T A L MARSH, I. HYDROLOGY
JONATHAN S. PRICE* and MING-KO WOO
Department of Geography, McMaster University Hamilton, Ont. L8S 4K1 (Canada)
(Received November 25, 1986; revised and accepted February 5, 1988)
ABSTRACT
Price, J.S. and Woo, M.K., 1988. Studies of a subarctic coastal marsh, I. Hydrology. J. Hydrol., 103:
275 292.
The hydrological processes of a subarctic coastal marsh undergoing rapid isostatic uplift were
studied to assess the links between water inputs and outputs. A water balance was performed for
the summer season. Precipitation was the major input, followed by surface inflow, but tidal inputs
were unimportant. Streamflow and evaporation were major outputs, while surface flow losses were
limited to the snowmelt period. Subsurface flows were insignificant. The water storage characteristics of the marsh regulated the flow processes and evaporation. Since they were strongly influenced
by the uneven peat development and pronounced seasonality, water storage, and therefore the
marsh hydrological system were highly variable in both time and space.
INTRODUCTION
Coastal salt marshes occur in different climatic regions ranging from the
tropics to the subarctic and provide a special habitat for terrestrial and aquatic
organisms. The hydrology of these marshes controls the spatial distribution of
nutrient and ecological attributes (Tyler, 1971; Casey et al., 1986; Price and
Woo, this volume). Apart from several detailed studies of limited scope (e.g.
Hemond and Fifield, 1982; Hemond et al., 1984), there have been few comprehensive studies of the hydrological regime of coastal marshes. This is especially
true of subarctic marshes, which are very extensive on a global scale
(Chapman, 1974). Specific to the subarctic environment are processes
associated with coldness, snow accumulation and melt, and a pronounced
decrease in the vegetative growth rate. In some cases there are the added
complications ofisostasy which affects the development time of the marsh soils
and vegetation. This has been observed in the subarctic marshes of North
America (Sims et al., 1982) and northwestern Europe including the Baltic and
Fennoscandia (Chapman, 1974), wh~ere isostatic uplift has led to a progressively
more advanced stage of peat development inland from the coast. There, the
presence of such coastal landforms as raised beach ridges affect the magnitude
* Present address: Department of Geography, Memorial University of Newfoundland, St. John's,
Nfld. A1B 3X9 (Canada).
0022-1694/88/$03.50
(~ 1988 Elsevier Science Publishers B.V.
276
and direction of water transfer, and vegetative succession modifies the snow
and frost distribution and evapotranspiration rates.
While the hydrological processes of the marshes can be identified easily, the
linkages which affect the variability of these processes need clarification. The
present paper will focus upon the seasonal and spatial aspects of the hydrological processes, and demonstrate their complex interaction in an emerging
subarctic coastal marsh. The area chosen for this study is located in the
Hudson Bay Lowland, an area that attains special significance as a migratory
bird breeding and staging ground because of its high plant productivity and
fertility (Prevett et al., 1979).
STUDY AREA
The study site (51°10'N, 79°47'W) located at the southern end of James Bay
(Fig. 1), has a continental subarctic climate, with mean J a n u a r y and July
temperatures at Moosonee, Ontario, of -20.0 and 15.5°C respectively. Mean
annual precipitation is 727mm, 30% of which falls as snow (Environment
Canada, 1982).
Paleozoic sedimentary bedrock (Hutton and Black, 1975) typically occurs at
a depth greater than 20-30 m (Ontario Hydro, unpub, drill log). Sparsely fossi-
i'll and
P,ezometer
tran,ect
stations
0
200
m
JAMES BAY
÷ Water table observation well
• Water level recorder
o Radiometer
" ~ -_ , -~
?
BACKSHORE ZONE
• Air ground temperature
AL-Overland flow collector
.~Y//'/
÷
÷
MHWMean High Water (Tide)
T Manual tide staff
+
+
+
--Contour
interval[]
/~£~
f-" +
~
2 .
----Water balance boundary
I
__ ~
FIRST BEACH RIDGE
Wooded beach ridges
>~J-,,,,
and levees
] //~----~,~"-,,.
-- ~ ~ + ~
~ - ~ L --.---,~ ~
+
5
+
~
+
÷
/
,,L. ~ 3 . o
~' ,_jSECOND BEACH RIDGE~,....._..
•
I
,
+ •
+
•
I-
+
i
i
Fig. 1. S t u d y a r e a a n d i n s t r u m e n t a t i o n . Note t h a t t h e p r e s e n t m e a n h i g h w a t e r m a r k ( M H W ) is
c u r r e n t l y 500 m c o a s t w a r d of t h a t s h o w n on this 1957 aerial p h o t o g r a p h . M a r s h C r e e k d r a i n s t h e
area within the water balance boundary.
277
liferous blue silty clay deposited during the post glacial Tyrrell Sea episode
(Lee, 1960), is found at depths of 1.7 2.2m and is overlain by more recently
deposited marine sediments of silt and fine sand. The topography is characterized by very low gradients of 1.0 m km 1. Subparallel raised beaches, formed
of reworked sand and silt (Martini et al., 1980), are superimposed on the coastal
plain (Fig. 1). At the average isostatic rebound rate of 0.7-1.25 m century 1
(Webber et al., 1970), the coast progrades one to two kilometres each century.
Raised beach ridges and interridge depressions form the two primary
landscape units. These terrain types have different physical properties and are
therefore hydrologically distinctive. The vegetation ranges from simple sedge
(Carex paleacea) communities near the coast, to the more complex stands of
woody vegetation inland (Salix sp., Alnus sp.), especially on the ridges (Ewing
and Kershaw, 1986).
METHODS
Hydrological data were obtained between 1984 and 1986, mostly during the
spring to late summer. Tidal data were also collected in July and August in
both 1985 and 1986. The deployment of the requisite instrumentation is shown
in Fig. 1.
A premelt snow survey was conducted in April 1984 using a method
described by Adams and Roulet (1982). Daily snowmelt was monitored at
several sites by measuring the daily lowering of the snow surface, and then
multiplying it by the surface snow density (Heron and Woo, 1978). Summer
rainfall was measured continuously with a tipping bucket rain gauge, supplemented by four manual rain gauges to determine the spatial variability of
rain storms.
Evaporation was measured directly using a 0.3m diameter clear plastic
floating pan lysimeter, and by weighing soil-can lysimeters (138 × 98mm
diameter) at a ponded and a nonponded site. Evaporation was also calculated
using the combination model of Priestley and Taylor (1972):
E -
~[a/(a + °/)] (Q* - Qg)/L;
(1)
where E is the evaporation rate, L is the latent heat of vaporization, p is the
density of water, a is the slope of the saturated vapour pressure-temperature
curve, 7 is the psychrometric constant, Q* is net radiation, Qg is ground heat
flux, and ~ is an empirical coefficient. To provide data for eqn. (1), net radiation,
air temperature, and ground heat flux were measured. In the Hudson Bay
Lowlands, Rouse et al. (1977) found ~ = 1.26 for saturated surfaces and ~ - 1.0
for drier surfaces. These values were acceptable for this study because the
evaporation calculated compared favourably with the lysimeter data.
Streamflow was determined at a small creek draining the depression behind
the first ridge (Fig. 1). Overland flow measurements were made in 1985 on the
second beach ridge using metal eaves troughs to direct overland flow into a 201
polyethylene bag attached to the apex of the gutters. The bag was allowed to
278
fill for five minutes and the volume of water collected was measured in a
graduated cylinder.
Water table elevation was measured periodically at 40 groundwater wells
constructed from 20mmi.d. PVC slotted pipes. Continuously recorded water
table measurements were also made (Fig. 1). Piezometric heads were obtained
using 6mmi.d. piezometer tubes with 0.1m slotted screened tips in 0.1m
diameter boreholes. A graded sand filter was placed around the piezometer tip
and the tips were isolated with bentonite plugs. Each piezometer station shown
in Fig. 1 consisted of a depth-integrated groundwater well and four or five
piezometers with their bottom openings set at depths of ranging from 0.5 to
3.0 m. Ten selected piezometers were filled with kerosine in the autumn of 1985
to prevent freezing. The kerosine was removed before piezometric measurements were made in April 1986.
Hydraulic conductivity was assessed with bail tests in the piezometers using
the method of Hvorslev (1951). Hydraulic conductivity of the organic layer was
computed by the same method, but using a 50mmi.d. tube slotted over the
thickness of the entire organic layer. Flow net analysis (Freeze and Cherry
1979) was used to determine the magnitude and direction of groundwater
fluxes.
RESULTS
A comparison of the air temperature, precipitation, and net radiation for
1984 with the 30 year mean at Moosonee, 100 km east of the study site, indicates
the representativeness of the 1984 data (Fig. 3). The water inputs to the marsh
were snowmelt and rain, and surface and subsurface inflow from the inland
marshes, whereas the major outputs were evaporation and streamflow.
Tidal effects
Tidal inputs were limited to the backshore zone in 1984 and most of 1985
because the maximum tidal elevation did not exceed 1.9 m a.s.1. (Fig. 2). A storm
surge in October 1985 reached an elevation of 2.9m and was the only major
tidal inundation during the study period. Tidal surges occur most frequently in
autumn (Glooschenko and Clarke, 1982). Given the low frequency and
magnitude of tides with respect to the emerged wetland in this location, they
were not significant in terms of marsh hydrology beyond the mean high water
(MHW) zone. In the backshore zone above MHW, where occasional flooding
occurred, surface drainage was rapid, although infiltration of 60mmh 1
(measured with a ring infiltrometer) indicated that total saturation of the soil
would have occurred during tidal inundation. The low-permeability sediments
preclude the intrusion of tidal water much beyond MHW.
279
50
40-
I DAILY
'~
30-
~ESTIMATED DAILY MAX TIDES
I GREATER THAN 1.5 m
°-ud
2o-
~
lO-
Z
J
MAX\MIN
TIDE
-f
f
,
I
I
,
00-
II
~ ~ _10uJ
~Z
MAY
W
JUNE
T
1984
JULY
I
AUGUST
30
o
2o f ,
I
,
hIIIIIIIhmllhIIh ,,,,gllgl
ii
I
OOtl
4
II,llllllllllUllllJlllllllllllll 2
,~ <~ - I ° I
MAY
I
JUNE
~-~-
I
JULY
AUGUST
OCT
1985
Fig+ 2. Highest and lowest daily tides at the study site.
Snow accumulation and melt
A premelt snow s u r v e y (April 6, 1984) indicated a considerable r e d i s t r i b u t i o n
of snow by wind d u r i n g the winter. Raised beach ridges and stream banks with
willow and alder v e g e t a t i o n trapped 0.6-2.0m or more of snow [0.13~).45m
w a t e r e q u i v a l e n t (w.e.)] but the snow on the open i n t e r r i d g e depression was
scoured to 0.14m (0.05mw.e.). The areally weighted a v e r a g e was 0.5m
(0.15 m w.e.).
S n o w m e l t was c o n c e n t r a t e d within the spring period, with few m a j o r melt
events d u r i n g the winter. The snowmelt rates at open and willow/alder sites
were similar, r a n g i n g from 4 to 30 mm d 1 d u r i n g the main melt period. By late
April the melt rates for the residual s n o w b a n k s r e a c h e d as high as 40 mm d 1
Most of the snow in the depression melted by 14 April and m e l t w a t e r i n u n d a t e d
the low lying areas. Snow r e m a i n e d u n d e r the willow and the alder until 21
April,. t h o u g h late-lying deep s n o w b a n k s persisted until 6 May. Snow
m e l t w a t e r quickly satisfied the depression storage r e q u i r e m e n t s of the
wetland, and since the g r o u n d was frozen in the s a t u r a t e d state, little infiltration occurred.
Rainfall
After the melt period, rainfall became the m a j o r source of w a t e r input to the
marsh. In April, 20 mm of rain fell and this, with 150 mm of snowmelt, was the
largest m o n t h l y w a t e r i n p u t (Fig. 3). At this time the g r o u n d was still frozen
and e v a p o r a t i o n minimal, thus this w a t e r had little i n t e r a c t i o n with the env i r o n m e n t and little was stored. Rainfall in M a y was low (90mm) and the
i n t e n s i t y was u n d e r 10mm d 1. J u n e and J u l y had above a v e r a g e rainfall, with
280
MONTHLY
[]
PRECIPITATION
TOTALS
1984
Sum of Daily
Precip. > lOmm
[]
Evaporation
Mean Monthly
Precip
['7
II
Total
Precip.
E
z
O
t-<
p-
E.
IJJ
IT
t'~
e3
Z
©
F<
PC
O
G.
<
>
UJ
Niii @
APR
MAY
JUN
JUL
JUNE
I
AUG
2OO
3020-
0-
- Io- 20
APRIL
I
MAY
I
JULY
I
AUGUST
Fig. 3. Meteorological data for the 1984 field season at the study site, compared with the 30 year
climatic averages for Moosonee (shown as dashed lines). Precipitation and evaporation (top), net
radiation (middle), and air temperature (bottom).
75% of r a i n f a l l i n g in e v e n t s e x c e e d i n g 10 m m d ~. A u g u s t r a i n f a l l w a s l o w , and
64% o c c u r r e d in e v e n t s g r e a t e r t h a n 10 m m d ~. R e c o r d s f r o m t h e m a n u a l r a i n
g a u g e s did n o t r e v e a l a n y n o t i c e a b l e s p a t i a l p a t t e r n .
Evaporation
Evaporation
increased
s t e a d i l y f r o m M a y to J u l y and t h e n d e c l i n e d in
281
August (Fig. 3). This overall trend reflects the net radiation regime. The
average daily evaporation from saturated surfaces between 4 May and 22
August was 2.8 mm, with a range of 0.7-5.0 mm, for a total of 303 mm. From the
first beach ridge to the coast the surface was normally unsaturated and the
evaporation ranged from 0.3 to 4.4 mm d-1, totalling 246 mm for the season (107
days). Local water storage characteristics affect the evaporation rates through
its control on the surface moisture supply. Thus evaporation is self-limiting on
the first beach ridge and backshore zone because it causes the water table to
drop below the surface. At more inland locations surface saturation is
maintained, allowing evaporation to proceed at the potential rate at all times.
Storage
Storage provides the link between the water inputs and losses of the marshes
and different storage mechanisms become more prominent during different
times of the year. Temporary water storage occurs as snow accumulation in
winter and as ponded water and soil water in the near surface zone during the
thawed season. Throughout the winter the surface layer of the marsh remains
frozen and surface and subsurface storage changes are small or negligible.
Snowmelt water was released from storage when the ground was frozen.
Surface depression storage was gradually replenished and considerable
outflow followed. Depression storage was ephemeral on the ridges but persisted
throughout the season in the interridge troughs. Between May and June, the
water storage level in the troughs increased (Fig. 4) despite low precipitation
and large evaporation rates. This rise was associated with the increased
hydraulic resistance to surface flow as the vegetation developed in the marsh,
thus drainage of the ponds and puddles was retarded. Evaporation had little
noticeable effect on the water table (i.e. storage) in May (Fig. 4). In June, slight
diurnal lowering of the water table on the first ridge was discernible; it became
prominent in July and extreme in August when high evaporation was coupled
with low rainfall. The exaggerated water table response on the first ridge
reflected the low specific yield of its soil (Sy = 0.04) compared to the ponded
water in the interridge depression (Sy ~ 1.0), because the water table change
is AS/Sy, where AS is the change in storage at a particular site.
Topography and peat development exert a strong control over the water
storage characteristics of the marsh. The water storage capacity of the first
ridge is small, as indicated by the large elevation range of the daily water table
positions (Fig. 5). Although the water table there rose into the organic soil
layer during rain events, it drained rapidly and evaporation reduced it further.
The water table of the first ridge'therefore resided primarily in the mineral soil.
Further inland, peat accumulation has resulted in a water table which is
sustained above the mineral soil. Because peat grows faster in the interridge
areas, the topographic advantage of the ridges in shedding water is eventually
eliminated. This enables surface and subsurface flow to be maintained across
the second ridge, and those ridges located further inland.
282
~
U
10
z
MAY
03-
E
I
JUNE
I
JULY
l
AUGUST
I
JUNE
'
JULY
'
AUGUST
STREAMFLOW
02-
1"
01
o
O0
MAY
280
z 270
(2_
u./
u~ 2 6 0 .
WATER
TABLE
MAY
\
I
JUNE
I
1984
JULY
I
AUGUST
F i g . 4. Precipitation, streamflow, and water table elevation from May to August 1984. T h e r e is no
record for the snowmelt period.
Surface and subsurface flow
Surface flow occurred over the entire marsh during the snowmelt period
when the water table was at its maximum observed elevation. The high water
table caused the first ridge to be completely inundated in some locations; hence
water flow followed the regional gradient towards the coast (Fig. 6a). After the
snowmelt period, rapid runoff and a diminished water supply resulted in a
general recession of the water table. The hydraulic gradients and hence flow
directions were then controlled by the local ridge and trough topography (Fig.
6b); this caused back-flow into the depression from the first beach ridge.
However, the groundwater "ridge" disappeared during dry periods, permitting
subsurface flow to resume toward the coast (Fig. 6c). At these times (e.g. Figs.
6b and c), surface flow could not cross the beach ridge but drained perpendicularly to the regional gradient toward the creek. The groundwater "ridge"
was re-established following rain events of sufficient magnitude (Fig. 4),
causing water flow back into the marsh. This process is analogous to the
283
FREQUENCY (d)
40 30 20
I
[]
~
I
I
10
I
0
I
10 20 30 40 50
I
I
I
I
I
Days with water
table at elevation
shown
w
>
Peat
<
w
co 3.1
w
i
~
Si,t
-
RIDGE
1
~
4 DEPRESSION
26m
4:
z
O
w
.J
w
1 BEACH
RIDGE
3.0-
2.9
2.8 m ~
• Well Location
o 200 400 m
JAMES BAY
2.DEPRESSION
2•
40 30 20
i
10 0 10 20 30
FREQUENCY (d)
Fig. 5. Daily frequency of mean daily water table elevation at four locations along a transect
perpendicular to the coast. The contrast between ridge and depression can be seen, as can the effect
of increasing peat thickness.
v a r i a b l e s o u r c e a r e a c o n c e p t ( D u n n e a n d B l a c k , 1970). O n t h e s e c o n d a n d
s u b s e q u e n t b e a c h r i d g e s , c o n t i n u o u s s u r f a c e a n d s u b s u r f a c e flow o c c u r r e d
from the adjacent landward marsh where thicker peat sustained a higher water
t a b l e ( F i g . 5). I n 1985 s u r f a c e flow w a s m e a s u r e d h e r e a n d a r a t i n g c u r v e w a s
d e v e l o p e d . T h i s w a s a p p l i e d t o t h e 1984 w a t e r l e v e l d a t a t o e s t i m a t e 1984
s u r f a c e flow, w h i c h a v e r a g e d 0.25 m3d-1 p e r m o f r i d g e . A t t i m e s o f h i g h w a t e r
l e v e l i t r e a c h e d 0.75 m 3 d - l m 1. T h e a v e r a g e s u b s u r f a c e flow t h r o u g h t h e 0.3 m
o r g a n i c l a y e r w a s c a l c u l a t e d u s i n g D a r c y ' s l a w t o be 0 . 0 0 1 m 3 d - l m ~. T h e
a v e r a g e h y d r a u l i c c o n d u c t i v i t y o f t h e p e a t w a s 10 5m s 1.
284
+
Observalion
:: : Wooded
JAMES
Point
BAY
Levees
and Beach Rfdges
+
_ _
Water
fable
Contours
I • lowFDirect}on
0
+
+
+
200m
+
+
+
+
0 @ ~ ..
2.9
+
::::::
i.
i
..... :
÷
\\
!:
+
÷
÷
+
"
. . . . . . . .
al
+
:
÷
¢
. . . . . . .
¢
;
2...
t
:
......
Fig. 6. Water table elevation and water flow directions for the marsh during (a) snowmelt; (b) early
summer; and (c) summer dry period.
Deep groundwater
Deep groundwater refers to the water in the marine sediments underlying
the peat. The average hydraulic conductivity of the silt and sand of the upper
several metres is 10 7m s 1, whereas the underlying clay of the Tyrrell Sea unit
has a value of 10 1°ms 1. The piezometer transect across the second beach
ridge indicated a general flow toward the coast (Fig. 7, top). However, there was
a slight downward flow componenet on the seaward side of the ridge and an
upward component where it joins the trough. Wet or dry periods in summer did
not significantly alter the flow direction or the piezometric gradients. The
water table mound of the first beach ridge suggests that downward flow also
occurs there except in the dry periods. Deep groundwater flow toward the coast
in summer was estimated to be 3.5 x 10 5m3d-lm 1.
A different hydraulic head distribution was encountered in winter (Fig. 7,
bottom). The windswept interridge depression was relatively free of snow and
was frozen more deeply (1.7 m) than beneath the ridges (0.35 m). A desaturated
285
4-
Observation
:
Wooded
Points
JAMES BAY
Levees
: : and Beach Ridges
m
Water
Table
Contours
. Flow
+
. 5 1 j
J
Direction
0
200 m
+
,4"
j
+
+
.÷
zone developed beneath the frozen layer of the depression because soil freezing
induced an upward migration of moisture from the lower layers to the freezing
front (Smith, 1984; Kingsbury and Moore, 1987). In Fig. 7b it was assumed t hat
the piezometric head of the unsaturated zone beneath the frozen layer was
equal to the elevation head and that the pressure head was zero. Under the
winter condition, coastward flow of groundwater was limited to a 0.4m
unfrozen u n s atu r a t e d layer. The rate of flow under these conditions is at least
several orders of magnitude smaller than during summer. From the observations made in summer and winter, it is further concluded that the transport
of deep groundwater t hr oughout the marsh is insignificant compared with
surface and near surface flows.
Streamflow
Streams fed primarily by surface and near-surface water were observed (12
November 1985) to dry up at the onset of freezing of the marsh. Stream channels
received a larger snow accumulation than the surrounding marsh during the
winter because windblown snow was trapped by riparian vegetation and in-
286
÷ Observation
:Wooded
Point
Levees
and Beach Ridges
--
Water
JAMES BAY
1
L F
Table
Contours
"--
I I ~ F low Direction
0
200 m
÷
÷
~+
III-
÷
÷
'I"111
4-
+
÷
÷
÷
÷
•
÷
÷
\
F i g . 6. ( c o n t i n u e d ) .
filled the channels. These snow drifts had a high density (over 300 kg m 3) and
became barriers to streamflow in spring. Such snow jams were found in the
Washkugaw River and in the smaller creeks draining the marsh (Woo and
Heron, 1987), causing meltwater to flood the entire marsh. When the jams were
eroded the ponded water was released rapidly to produce the highest annual
discharge.
After the spring freshet, low flows of 5-81 s 1 were frequently encountered
in the small stream during extended dry periods (Fig. 4). Low flows were
maintained partially by surface drainage from the depression (Fig. 6) and
partly from groundwater discharge towards the stream channel. The latter was
facilitated by local increases in the piezometric gradient adjacent to the
stream, which were an order of magnitude greater than elsewhere in the marsh.
During the wet periods both overland flow and flow within the organic layer fed
the stream directly (Fig. 8, left). In the dry periods the water level receded
below the organic layer, halting the surface and near surface flows to the
stream (Fig. 8, right). However, the piezometric gradient beneath the stream
increased, yielding higher groundwater discharge rates, thus maintaining
streamflow throughout summer. Groundwater flow to the streams in this area
is relatively small, representing only 1% of the seasonal flow (DiCenzo, 1987).
287
SUMMER
VERTICAL EXAGGERATION 50:1
• PIEZOMETER/WATERTASLE
3.5--
[]
SILT
[]
PEAT
o 3.0co
cv
.
.
o
25-
(2_
z
20-
'~
15--
~
1.0-
.
.
.
.
.
o
0.5-
~):::
i
3 -! f i , .
Y
//~1 V//A
0
//~
-~
I
I
/"
]
I
I WINTER
3.5--~
=- 3 o ~ E
I
~--LOWER BOUNDARY OF FROZEN LAYER
/
" - - , ~
/i
i
~
r±~r ::v
\ ,to
r
= ........
0
0
I
100
]
200
I
300
I
400
I
500
600
DISTANCE (m)
Fig. 7. P i e z o m e t r i c i s o p o t e n t i a l a c r o s s t h e s e c o n d b e a c h r i d g e for t h e s u m m e r (top), a n d w i n t e r
(bottom).
WATER BALANCE
Water balance relationships indicate the relative importance of different
hydrological processes through time. The water balance equation for the marsh
can be written as:
P
+
T + Qsi + Qss~ -
Q~o -
Qsso -
Q-
E
=
AS
+ ~
(2)
where P is precipitation, T is tidal inflow, Q~i and Vssi a r e surface and
subsurface inflow from interior marshes respectively, Q~o and Q~o are surface
and subsurface outflow respectively, Q is streamflow, E is evaporation, AS is
change in storage, and ~ is the residual error term.
The water budget was calculated for an area of the marsh (0.91 km 2) bounded
by the crests of the first and second beach ridges, by the Washkugaw River to
the west and by a subdued drainage divide to the east (Fig. 1). Water balance
computations were made for the period 4 May-22 August 1984 which excludes
the snowmelt season. The width of the second beach ridge that forms the water
balance boundary was assumed to be the input face (1340m) along which
surface and subsurface inputs occurred. Results presented in the previous
section show that Qssi, Q~o, Qsso, and T were negligible. For the calculation of
evaporation, 15% of the area was unsaturated at the surface most of the time,
288
3.0
g
E
>,
c~
2.0
:5
2
.~_
,7,
1.0
0
20
[]
PEAT
15
10
5
£)istance from stream (m)
LAYER
[]
CLAY
[]
0
SILT
20
15
Distance
10
5
from stream (m)
--ISOPOTENTIAL
HEAD
-----WATER
(m above arbitrary datum)
0
TABLE
Fig. 8. Piezometric isopotential across a small creek draining the depression during a wet period
(left) and a dry period (right).
= 1.0 for eqn. (1), and the r e m a i n d e r was always s a t u r a t e d (~ = 1.3). S t o r a g e
c h a n g e (AS) was c a l c u l a t e d using the specific yield (Sy) and c h a n g e in w a t e r
level e l e v a t i o n (Ah), such t h a t AS = Ah Sy. In the ponded areas Sy ~ 1.0, and
was 0.04 in the m i n e r a l soil of the first b e a c h ridge.
Over the w a t e r b a l a n c e period the largest single c o m p o n e n t was precipit a t i o n (429 mm) (Fig. 9). Rainfall was low in May, with no e v e n t g r e a t e r t h a n
10 mm d 1. This c r e a t e d a large storage deficit even t h o u g h e v a p o r a t i o n had not
yet r e a c h e d its peak. H e a v y p r e c i p i t a t i o n d u r i n g J u n e and J u l y replenished the
storage, while A u g u s t e x p e r i e n c e d a large w a t e r deficit because of high evapo r a t i o n and low prec.ipitation (Fig. 3). Surface inflow was only 15°/5 of precipitation, but this was e q u i v a l e n t to 22% of e v a p o r a t i o n , and was t h e r e f o r e a
significant w a t e r source which partially offset storage losses.
Streamflow was the largest o u t p u t (71% of precipitation) followed closely by
e v a p o r a t i o n (68% of precipitation). The response of streamflow to rainfall was
g o v e r n e d by storage. F o r example, w h e n storage was low, a 15.9mm rainfall on
3 J u n e 1984 yielded in a m a x i m u m streamflow of 1.8 mm d 1, whereas 15.9 mm
of r a i n on 27 J u n e p r o d u c e d a peak flow of 10.7mmd -1 because of wet
a n t e c e d e n t conditions. A l t h o u g h the flat m a r s h y area c o n t a i n s a considerable
v o l u m e of water, it has little d y n a m i c storage capacity; thus it c a n n o t r e t a i n
m u c h w a t e r to sustain prolonged recession flow.
289
600
P * Q.,
(~lss
j
aS
DE
w -200OQ9
o~:
Q
--if)
Q+E
?~ -4ooQ)
-
600 I
MAY
I
JUNE
I
JULY
I
"
AUGUST
Fig. 9. Components of the marsh water balance for the period 4 May 22 August 1984. Numbers in
brackets represent seasonal totals (ram).
E v a p o r a t i o n loss d u r i n g the w a t e r b a l a n c e period was c o m p a r a b l e to
s t r e a m f l o w in m a g n i t u d e but was less v a r i a b l e t h r o u g h time. E v e n at low flow
periods, e v a p o r a t i o n c o n t i n u e d to w i t h d r a w w a t e r from w e t l a n d s t o r a g e and
this deficit h a d to be e l i m i n a t e d before r a i n f a l l could r a i s e the flow again. A
s i m i l a r p h e n o m e n o n was o b s e r v e d in an a r c t i c w e t l a n d by R o u l e t and Woo
(1986a).
N e t s t o r a g e c h a n g e o v e r the w a t e r b a l a n c e period w a s small, r e p r e s e n t i n g
only 12% of p r e c i p i t a t i o n . T h e e s t i m a t e d s t o r a g e c h a n g e ( - 5 0 m m ) differed
from s t o r a g e c h a n g e c a l c u l a t e d as a r e s i d u a l ( - 1 0 6 m m ) . The difference
(56 mm) is the e r r o r t e r m (~) in eqn. (2) a n d is 13% of p r e c i p i t a t i o n . S o m e of this
e r r o r is a t t r i b u t a b l e to the m e t h o d of d e t e r m i n i n g s u r f a c e a n d s u b s u r f a c e
inflow w h i c h m a y n o t h a v e b e e n r e p r e s e n t a t i v e of the e n t i r e ridge. T h e r e w e r e
p r e f e r r e d flow zones c r o s s i n g the ridge w h i c h w e r e n o t b e e n a c c o u n t e d for, so
t h a t the v a l u e of Qs: is p r o b a b l y u n d e r e s t i m a t e d . H o w e v e r , as this c o m p o n e n t
is small in m a g n i t u d e , the e r r o r i n t r o d u c e d is small r e l a t i v e to the o t h e r
c o m p o n e n t s . M e a s u r e m e n t s of p r e c i p i t a t i o n , e v a p o r a t i o n , a n d s t r e a m f l o w are
290
approximately + 15% under ideal conditions (Gray, 1970; Stewart and Rouse,
1976; Ackers et al., 1978). Thus the error in the water balance lies within the
acceptable limits based on the measurement techniques used.
The water budget indicates that streamflow and evaporation are of equal
importance as water sinks. Similar results were reported by Brown et al. (1968)
in an Alaskan wetland. Runoff, however, is more important in spring and
during flood events, while evaporation plays a more prominent role later in the
summer. Surface inflow from inland marshes partially offsets evaporation
losses during dry periods and thereby reduces the storage deficit that must
otherwise be eliminated by rainfall. Streamflow can therefore respond more
quickly. Surface inflow is less important compared with some wetlands in
arctic Canada where surface inflow cascades from lakes, snow meltwater and
stream overflow to maintain a high water table throughout summer (Roulet
and Woo, 1986b).
DISCUSSION AND CONCLUSIONS
Subarctic coastal marshes have several characteristics distinguishable from
other types of wetlands. Water may be introduced to the marsh tidally,
although tidal inputs to the study area were largely restricted to autumn storm
surges because isostatic rebound has raised the marshes well above the normal
tidal range. These surges contribute little to the water storage because the peat
is already saturated during this period of low evaporation and frequent rain
events. Additional tidal water will merely run off. Coastal emergence due to
isostatic rebound imparts other special properties to these wetlands. Raised
beach ridges control the direction and velocity of the surface and groundwater
flow, while the storage level in the depressions determines the flow magnitude.
As peat thickness increases inland, the water storage capacity is enhanced,
affecting both evaporation (saturated or unsaturated conditions) and runoff
processes (surface, subsurface, or deep groundwater flows).
The temporal variability of the marsh hydrology is regulated by seasonal
changes in water inputs and storage. In the spring, meltwater in the marsh and
inflows from interior wetlands release water which cannot be absorbed by the
low storage capacity of the frozen ground. Total inundation is accompanied by
rapid surface flow ir~ the regional direction toward the coast. The presence of
snow jams along the coastal section of the stream leads to overflowing of the
marsh creeks and rivers, resulting in an uninhibited exchange of the stream
and marsh waters. Following snowmelt, the high water table in the ridge acts
as a drainage barrier and surface and subsurface flows converge in the
depression and then discharge as streamflow. This barrier is removed during
periods of high evaporation and low rainfall, so that regional groundwater flow
can resume. At this time storage declines and streamflow recedes to a baseflow
level but evaporation remains important. In winter there is no surface or
near-surface flow, and groundwater flow in the regional direction is minimal.
The integrated effects of spatial and temporal changes on marsh hydrology
291
a r e r e f l e c t e d i n t h e p a t t e r n of s t r e a m f l o w . T h i s is t h e m a j o r o u t p u t f r o m t h e
h y d r o l o g i c a l system c o n t r o l l e d by p r e c i p i t a t i o n , inflow, storage a n d evapor a t i o n loss. S t r e a m s of t h e s u b a r c t i c c o a s t a l m a r s c h do n o t e x h i b i t t h e m u s k e g
r e g i m e p r o p o s e d b y C h u r c h (1974) i n w h i c h h i g h flows a r e a t t e n u a t e d b y t h e
w e t l a n d s . I n s p i t e of t h e l a r g e t o t a l w a t e r s t o r a g e i n t h e s e w e t l a n d s , t h e s m a l l
dynamic storage capacity means that snowmelt and rainfall are transmitted
q u i c k l y t h r o u g h t h e s y s t e m u n l e s s p r e c e d e d by a l o n g d r y p e r i o d w h e n evapor a t i o n h a s c r e a t e d a s t o r a g e deficit.
ACKNOWLEDGEMENTS
T h e f i n a n c i a l a s s i s t a n c e of t h e N a t u r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c h
C o u n c i l c o l l a b o r a t i v e g r a n t , a n d of t h e D e p a r t m e n t of I n d i a n a n d N o r t h e r n
A f f a i r s ' N o r t h e r n T r a i n i n g G r a n t is g r a t e f u l l y a c k n o w l e d g e d . I n v a l u a b l e h e l p
i n t h e field w a s r e c e i v e d f r o m Drs. R. H e r o n a n d K. E w i n g ; a n d P. S t e e r , P.
d i C e n z o , P. L a f l e u r , a n d S. H a r d i l l . G r o u n d h e a t flux d a t a w a s g e n e r o u s l y
p r o v i d e d b y Dr. W.R. R o u s e , a n d a d v i c e o n t h e p i e z o m e t e r n e t w o r k w a s k i n d l y
g i v e n by Dr. J.A. C h e r r y .
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