hydrology of subarctic canadian shield: soil-filled valleys

Hydrology of subarctic Canadian shield: soil-filled valleys

Christopher Spence*, Ming-ko Woo

School of Geography and Geology, McMaster University, Hamilton, Ont., Canada L8S 4K1

Received 15 April 2002; accepted 28 April 2003

Abstract

The Canadian Shield landscape includes bedrock uplands and valleys infilled with soil. A site near Yellowknife in subarctic

Canada was studied to elucidate the hydrological behaviour of soil-filled valleys. A suite of hydrological processes was found to

be important in the studied valley, including snowmelt and rainfall, ground frost development, evaporation, infiltration, lateral

inflow from adjacent uplands, and surface and subsurface flows. Valley storage requirements during snowmelt are met by local

meltwater infiltration, but lateral inflows in the summer are needed to satisfy the storage before runoff can be generated from the

valley. The valleys perform three functions of collecting vertical and lateral water inputs, retaining and losing the water held in

storage, and transferring water down the valley and generating local outflow. Unlike channel flows in humid areas, runoff from

the upper valley has to meet the storage demands of the lower valley segments, often causing seepage loss along flow paths to

render the flows intermittent. A fill-and-spill runoff system is proposed in which the valley physiography results in a series of

segments with varying storage conditions. As water is supplied to the valley, each segment has to be filled until its storage

threshold for runoff is exceeded. Then, subsurface or surface flows will be generated, but these flows may be arrested to furnish

water to satisfy the storage requirements of the segments downstream. Such a flow system applies also to other valleys in the

Shield environment.

q 2003 Elsevier B.V. All rights reserved.

Keywords: Runoff generation; Canadian shield; Northwest territories; Water budget; Storage; Subarctic

1. Introduction

Headwater basins in the Canadian Shield comprise

a mosaic of exposed Precambrian bedrock uplands

and soil-filled valley bottoms. Upon reaching the

valleys, runoff produced on the uplands is modified by

the storage and flow delivery mechanisms in the soil-

filled zone (Allan and Roulet, 1994; Buttle and Sami,

1992; Peters et al., 1995). There is an insufficient

understanding of the relationship between catchment

water balance, soil-zone storage and hydrological

linkages to account for the wide variations in the

runoff ratios reported for headwater areas (Branfireun

and Roulet, 1998; Landals and Gill, 1972). Further-

more, the subarctic portion of the Canadian Shield is

subject to intense winter coldness and ground frost

may add hydrological complications (Metcalfe and

Buttle, 2001; Thorne et al., 1994; Wright, 1979). It is

the purpose of this research to study the hydrological

processes in a subarctic Canadian Shield headwater

soil-filled valley and to elucidate the hydrological

0022-1694/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0022-1694(03)00175-6

Journal of Hydrology 279 (2003) 151–166

www.elsevier.com/locate/jhydrol

* Corresponding author. Address: Environment Canada, #301

5204 50th Avenue, Yellowknife, Northwest Territories, Canada

X1A 1E2. Fax: þ1-867-873-8185.

E-mail address: [email protected] (C. Spence).

http://www.elsevier.com/locate/jhydrol

connections in terms of water transfer from the

bedrock uplands and through the soil-filled valleys.

An improved understanding of the headwater hydrol-

ogy will permit realistic representation of the hydro-

logical processes in the modelling of shield

hydrology, and, given the considerable extent of

Precambrian bedrock landscape in Canada and in

northern Europe, have application for many northern

areas.

2. Study site

Pocket Lake basin, located 4 km north of the City

of Yellowknife in Canada’s Northwest Territories

(Fig. 1), has been studied intermittently since the

1970 s when it was identified as a field site as part of

Canada’s contribution to the International Hydrolo-

gical Decade. The physical setting of the area has

been described by Landals and Gill (1972), Reid

(1997) and Spence and Woo (2002). The present study

focuses on a soil-filled valley bottom of a headwater

basin downslope of the bedrock runoff plots investi-

gated by Spence and Woo (2002). The soil-filled

valley is 10,875 m2 in size. A surrounding exposed

bedrock upland 37,948 m2 in area drains into the

valley. Open woodlands of black spruce (Picea

mariana) grow at the edge of the valley along the

soil/bedrock border. Understory vegetation includes

dwarf willow (Salix spp.), Labrador tea (Ledum

Fig. 1. Pocket Lake soil filled valley with location map and inset picture. The valley is outlined in grey and the entire basin in black. Pocket Lake

is at the north end of the valley and is seen as the dark area at the bottom of the picture. Dotted arrows show direction of surface flow. Black lines

with letter labels denote well and piezometer transects. White squares are ablation lines. Stars show locations of bedrock runoff plots. Triangles

are trench locations. Weirs are represented by white boxes with a V notch. The rectangle is the soil/bedrock contact instrumentation. The white

circle is a thermistor string and the black circle is a climate tower.

C. Spence, M.-k. Woo / Journal of Hydrology 279 (2003) 151–166152

groenlandicum) and rose (Rosa spp.) bushes. Ground

cover includes moss (Sphagnum spp.), lichen (Cla-

donia spp.) grass (Eriophorum spp.) and sedges

(Carex spp.). The upper part of the valley has a

gradient of 1%, while the lower section descends at

9%. The substrate consists of three distinct layers each

composed of organic soils, silty sands and a basal

layer of cobbles and boulders (Table 1) which pinches

out towards the valley sides. The bedrock hillslopes

rise abruptly from the soil-filled valley floor. Beneath

the soil, a bedrock sill stretches across the lower reach

of the valley. Down valley from the sill the bedrock

and ground surfaces have similar gradients.

The 1961–1990 climate at Yellowknife is charac-

terised by short cool summers with a July daily

average temperature of 16 8C and long cold winters

with a January daily average temperature of 229 8C.

Annual unadjusted precipitation for the same period

averages about 280 mm, with 55% of that falling as

snow (Wedel et al., 1990). Convective cells produce

much of the summer precipitation and so summer

rainfall is quite variable from year to year. As the jet

stream settles over the region in September, con-

ditions become cool and damp. If this shift begins

early, the annual precipitation will be higher than

normal due to the extended wet period.

3. Methods

Geophysical surveys were conducted to define the

surface and bedrock topography of the study site. The

bedrock surface beneath the soil-filled valley was

mapped using a combination of probing by hammer-

ing a steel rod into the sediment until encountering the

bedrock and a pulseEKKO IV ground penetrating

radar with a 400 V transmitter and an antenna centre

frequency of 100 MHz (Spence, 1996). Soil tempera-

ture was recorded at half hourly intervals using a

string of Campbell Scientific 107B thermistors

(accuracy ^ 0.4 8C) located at the foot of the hillslope

(Fig. 1). A steel rod was used to probe for the depth of

the frost table during the spring and early summer.

Daily valley water budget terms, including rainfall,

snowmelt, evapotranspiration, inflow from the bed-

rock upland, surface and subsurface runoff and change

in storage, were obtained between 10 May 2000 and

16 May 2001 with the instrumentation shown in Fig.

1. Here, magnitudes of all water budget components

are expressed as depth per unit valley-area, except

when noted. A meteorological tower on a bedrock

ridge above the valley was equipped with a Meteor-

ological Service of Canada Type B rain gauge to

measure rainfall (accuracy 5%). Rainfall intensity was

measured at the same tower using a tipping-bucket

rain gauge with signals recorded by a Campbell

Scientific CR10X datalogger. The snow water equiv-

alent of the spring snowpack was calculated from

snow density measured with an Eastern Snow

Conference snow sampler and snow depth measured

with an aluminium rod along snow survey transects,

following the method described by Pomeroy and Gray

(1995) (accuracy 15%). The snow survey included

five random survey lines each of which included at

least five snow density and twenty snow depth

samples. Daily snowmelt was calculated by measur-

ing the lowering of the snow surface and the snow

density of the surface snow layer, along four ablation

lines on the bedrock upland and two in the valley,

using the technique described by Heron and Woo

(1978) (accuracy 25%). Infiltration rates were deter-

mined under different ground frost conditions using

double-ring infiltrometers. Evapotranspiration was

estimated using the eddy correlation energy budget

techniques and instrumentation described by Spence

and Rouse (2002) who report an accuracy of 20% with

these methods. Lateral inflow from upslope exposed

bedrock was measured with volumetric measurements

and velocity-area flow calculations at the upslope weir

sites (Fig. 1), with velocity obtained using a Price type

pygmy current meter. These weirs captured runoff

from 88% of the bedrock upland. The error associated

with these runoff measurements is estimated con-

servatively at 20%. It was assumed that additional

inflow from bedrock side slopes outside areas

Table 1

Hydrological characteristics of the soil layers in the headwater

valley

Soil type Depths

(m)

Porosity Hydraulic

conductivity (m/d)

Organic 0–0.35 0.75 10

Silty sands 0.35–1.5 0.5 1

Cobbles and boulders 1.5–1.75 0.35 10

C. Spence, M.-k. Woo / Journal of Hydrology 279 (2003) 151–166 153

captured by the weirs could be estimated using runoff

data from the bedrock runoff plots described in

Spence and Woo (2002) because of their similar

drainage areas and physiography. The plots were

reported to measure runoff within 7%. Spence and

Woo (2002) classify their plots into bare and soil

covered. The relative areal extent of each across the

side slopes was delineated from air photos and side

slope inflow calculated as a portion of total lateral

inflow following:

Rbss ¼ðRbabÞ þ ðRscascÞ

abss

ð1Þ

where a is an area from which runoff, R; is generated.

The subscripts b, sc, and bss represent bare plots, soil

covered plots and bedrock side slopes, respectively.

Rb and Rsc are expressed in depth per unit runoff plot-

area and Rbss is expressed in depth per unit area of the

side slopes. Rbss was converted into millimetres over

the area of the valley, av; using:

Ibss ¼Rbssabss

av

ð2Þ

and added to Ibw; inflow measured at the weirs, in

runoff/unit area of the valley, to produce total lateral

inflow, I:

I ¼ Ibw þ Ibss ð3Þ

Peters et al. (1995) identified that runoff from

exposed bedrock enters soil along the bedrock

surface. At the bottom of a bedrock hillslope, daily

volumetric measurements were made at a runoff plot

that consisted of paired weirs, one at the soil-bedrock

contact and the other at the bedrock surface. The

partitioning of runoff into surface flow and flow along

the bedrock surface during both frozen and unfrozen

conditions at this location was assumed to occur

everywhere along the soil-filled valley bedrock

contact. At the outlet of the basin, stage recorded

continuously at a 908 V-notch weir was converted into

discharge by a rating curve obtained from periodic

discharge measurements using the velocity-area

method (accuracy similar to the inflow weirs at

20%). Subsurface flow was collected in two trenches

(accuracy 10%). They were operative only in the

summer as flooding prevented their usage during the

spring. Areas contributing to surface runoff were

mapped based on visible observations. A network of

piezometers along three transects enabled the deter-

mination of the direction of groundwater movement

within the soil-filled valley. Hydraulic conductivity

was calculated using pump tests as described in

Freeze and Cherry (1979).

The water table was measured continuously at two

wells, one at the edge and the other in the centre of the

valley along transect H (Fig. 1). Six additional wells

along transect G, three along transect H and two along

transect E were measured opportunistically. Field

calibrated Campbell Scientific CS615 time domain

reflectometry (TDR) soil moisture sensors were

installed at 0.05 and 0.3 m depths within the middle

of the soil-filled valley and to 0.3 m depth at the valley

edge. Daily change in storage, DS; at each point was

calculated as:

DS ¼ DSu þ DSs

¼ Du½z 2 zwðtÞ� þ Sy½zwðtÞ2 zwðt 2 1Þ� ð4Þ

where DSu and DSs are the changes in soil moisture

fraction in the unsaturated and saturated zones; Du is

change in soil moisture fraction in the unsaturated

zone; Sy is the specific yield of soil, measured here to

be 0.13; z is total soil thickness, zwðtÞ and zwðt 2 1Þ are

height of the water table (measured from the bedrock

upward) for the present ðtÞ and the previous ðt 2 1Þ

time periods. The valley edge was delineated using

the strip of trees visible in Fig. 1. The average valley

change in storage was a prorated calculation based on

the relative areas of valley edge and centre. The

change in storage calculation is expected to have an

accuracy of 25% based on the error of the calibration

of the TDR sensors and the estimate of Sy:

4. Hydrological processes in the soil-filled valley

4.1. Ground frost

The soil-filled valley has only seasonal frost.

Ground thaw proceeded evenly during the spring

and early summer of 2000, with rates that varied

within 20% of the average value, close to the accuracy

of the frost probe measurements. Thaw rates between

29 March and 1 June 2000 averaged 5 mm/d and


increased to 15 mm/d after 1 June. The frozen layer

disappeared between mid and late June. Surface soil

temperature reached a maximum in June and July, but

the entire column was warmest at the end of August.

The soil was frozen to approximately 1 m depth by the

spring of 2001, but periodic measurements indicated

that ground thaw was negligible during the snowmelt

period.

4.2. Snowmelt

A snow survey on 1 April 2001 measured

160 ^ 25 mm of snow water equivalent in the valley.

Ablation rates between 2 April and 19 April 2001

averaged 2.7 mm/d. Latent heat flux estimated from

tower measurements suggested that an average of

0.5 mm/d of the ablation during this period was

directed to the atmosphere. Meltwater produced in

these two weeks was not observed to runoff or enter

soil storage so it was either refrozen within the

snowpack or in surface depressions as a cold snap

ensued until 23 April 2001. After 24 April 2001

above-freezing air temperatures ripened the entire

snow cover and snowmelt increased to a peak of

16.9 mm on 5 May 2001. The rate of losses to

the atmosphere estimated from tower measurements

increased to an average of 1.9 mm/d after 23 April

2001. The entire snowpack in the valley was gone by

10 May 2001. Total valley snowmelt equalled

117 mm with the remainder, 49 mm, directed to the

atmosphere.

4.3. Rainfall

Rainfall, assumed uniform over the entire basin, in

the summer of 2000 totalled 154 mm (Fig. 2), almost

half of which fell between 14 August and 29 August.

Much of the remaining amount came from three

thunderstorms on 20 and 23 June and 14 July. While

May, July and September were drier than normal, wet

conditions dominated June and August. With an

average May to September rainfall of 141 mm,

Yellowknife is drier than the other Canadian Shield

locales where hydrological studies were undertaken

(e.g. 321 mm at Thompson, Manitoba, 360 mm at the

Experimental Lakes Area in northern Ontario,

358 mm at Schefferville, Quebec and 444 mm at

Muskoka, Ontario) (Metcalfe and Buttle, 2001;

Wright, 1979; Thorne et al., 1994; McDonnell and

Taylor, 1987).

Fig. 2. 10 May–25 September 2000 cumulative rainfall time series at Pocket Lake.


4.4. Infiltration

Double-ring infiltrometer measurements yielded

infiltration rates of about 1 m/d regardless of whether

the soil was frozen or otherwise. This is attributed to

the frozen but unsaturated conditions of the porous

soils prior to spring melt. Such non-limiting infiltra-

tion capacities for frozen soil (Gray et al., 2001)

allowed snowmelt and upslope runoff to percolate

through the frozen layer. Shallow soil moisture

measurements show that despite the ability of the

soil to accept infiltration, a large fraction of rainfall

(,0.6) could be intercepted by the ground vegetation

mat of lichen and moss because of its dryness (Bello

and Arama, 1980) during much of the summer. A lack

of soil moisture response after small (,12 mm)

rainfall events at 0.3 m depth implies that this

intercepted water was directed back to the

atmosphere.

4.5. Inflow from exposed bedrock upland

Summer inflow from the upland was intermittent

(Fig. 3). All inflow entered the valley along the soil-

bedrock interface and this is in agreement with

Peters et al.’s (1995) observation near Muskoka that

most lateral inflow from upslope exposed bedrock

enters Canadian Shield soil zones along the bedrock

surface. In the spring, inflow also travelled along the

bedrock surface as the frozen soil did not have much

ice to seal its pores. Spring inflow at the contact plot

began on 28 April 2001. A lag time between peak

bedrock snowmelt on 18 April and peak lateral

inflow to the valley on 3 May was mostly due to two

periods of sub-freezing temperatures during which

the meltwater was refrozen in the snow or in shallow

depressions on the bedrock surface.

4.6. Evapotranspiration

Between May and July 2000, evapotranspiration

from the soil-filled valley averaged 1.8 mm/d. Evapo-

transpiration exceeded rainfall in May, July and

September and almost equalled the June precipitation.

August was the only month when rainfall exceeded

evapotranspiration, as cool and wet conditions

reduced the evapotranspiration rate to 0.8 mm/d.

The similarity in calculated evapotranspiration and

change in storage during a dry period between 6 and

17 July 2001 shows that in the absence of rainfall

Fig. 3. 10 May–25 September 2000 daily lateral inflow from exposed bedrock to the soil filled valley.


input, evapotranspiration was sustained by moisture

storage in the soil (Fig. 4). This is in agreement with

observations at other subarctic Canadian Shield sites

(Spence and Rouse, 2002).

4.7. Storage

Soil moisture in the unsaturated zone at the valley

edge responded more readily to rainfall events than at

Fig. 4. Cumulative daily evapotranspiration and change in storage during a dry period in the middle of the 2000 growing season.

Fig. 5. Daily soil moisture measurements at different locations and depths in the soil filled valley for July through September 2000.


the centre (see smaller peaks in Fig. 5). Valley edge

soil moisture always increased more than observed

rainfall because it was often augmented by lateral

inflow. In contrast, increases to shallow valley centre

soil moisture averaged only 60% of rainfall inputs. At

0.3 m depth in the centre of the valley, the soil

moisture content reacted only to the largest rainfall

events (.12 mm).

As the summer of 2000 progressed, evapotran-

spiration loss exceeded rainfall and lateral inflow,

leading to a depletion of soil moisture storage. During

dry conditions the water table cross sections across the

valley were flatter than during wet conditions (Fig. 6).

A large rainfall event beginning 23 August 2000

(Fig. 2) generated the largest daily lateral inflow from

the bedrock upland during the summer of 2000 (Fig. 3)

which caused distinct spatial differences in valley

water table response. The water table rose more

rapidly at the sides of the valley and peaked 2 h before

the centre because lateral inflow was quicker than

rainfall percolation, much of the latter also being

intercepted by the ground mat of lichen and moss

(Fig. 7). This same event also caused uneven rises in

the water table along the valley. By 27 August 2000

the water table at transect H rose an average of 0.32 m

but the water table at transect G rose by only 0.1 m

because of higher lateral inflow inputs close to

transect H.

After 29 August, the water table declined, first at

the sides, then in the middle. Throughout the winter of

2000–01, groundwater possibly drained through the

fractures of the bedrock underneath the soil-filled

valley. Immediately before snowmelt (end of March),

the water table profile across the valley was flat and

appeared similar to the dry summer condition. The

first rise of the water table (0.6 m) occurred on 18

April in the middle of the valley, indicating that the

water source was from snowmelt in the valley and

the meltwater was able to infiltrate the frozen soil. The

water table at the sides of the valley did not rise until

28 April when lateral flow began in earnest. Estimates

of daily snowmelt, rainfall and lateral inflow show

that before 28 April, 70% of these inputs entered

valley storage and the remainder was lost to the

atmosphere. The water table reached the topographic

surface on April 29 and remained there until the end

of the study period.

4.8. Subsurface runoff

Pumping tests and direct measurement of flow at

the trenches indicate a hydraulic conductivity of

approximately 1 m/d for the soil, regardless of its

thermal state. Piezometric measurements across

transects G and H showed that water from the bedrock

upland consistently drained towards the centre of

the valley and then down the valley. The presence of

Fig. 6. The water table across and along the soil filled valley during

wet (27 August 2000) and dry (16 August 2000) conditions. The

locations of other transects when crossed are referenced on each

cross section. Information on transect H only covers the western 16

m of its length. The white circles denote locations of piezometers or

wells. The locations of the transect H wells with data illustrated in

Fig. 7 are noted. Refer to Fig. 1 for locations of transects.


a bedrock sill in the lower valley (Fig. 6) restricted

subsurface runoff for much of the summer, a

situation similarly reported by Allan and Roulet

(1994) at another Canadian Shield watershed in

Ontario. Subsurface runoff was first observed at the

lower trench when the water table rose above the

sill on 26 August 2000 in response to 40 mm of

rainfall, and continued until 25 September when the

water table dropped below the elevation of the sill

(Fig. 8).

4.9. Surface runoff

A large storm event generated 58 mm of rainfall

and 66 mm of lateral inflow between 14 August and

27 August. This triggered saturation overland flow at

0800 on 27 August as the water table rose above

ground at the western valley edge at transect

H. Surface runoff followed the valley thalweg, along

a water track similar to that described by McNamara

et al. (1998). A rapid rise in near surface soil moisture

0.5 m downslope of transect G on transect E indicated

that this flow reached that position between 0900 and

0930 (Figs. 5 and 8). The piezometric heads along and

adjacent to the flow path showed that part of the

surface runoff infiltrated the soil as it travelled

downstream, suggesting the influent nature of this

intermittent stream. The cessation of rainfall and a

reduction of inflow from the bedrock upland caused

the water table to recede on 29 August, but the loss to

stream influence continued. The stream-flow segment

retreated up-valley until surface flow ceased

altogether. The overall effect of the surface flow

process is therefore the formation of an intermittent

stream that expanded from and contracted to the

source of saturation overland flow near transect H.

Runoff in the spring of 2001 followed two phases.

Despite 93 mm of precipitation, snowmelt and inflow,

only 8 mm of surface runoff was generated by 1 May.

Most of the spring runoff (396 mm) from the 1.1 ha

valley was produced after the cold spells in early May

and was fed by 377 mm of inflow from the 3.8 ha

bedrock upland (Fig. 9). The large lateral inflow

volume is a result of the relative size difference

between the larger upland and smaller valley (Eq. (2)).

The concordance of daily flow rates between the

surface inflows and surface outflows may imply that

the soil-filled valley served mainly as a conduit for

runoff without significantly altering the flows through

soil storage. However, as the frozen ground had little

effect on infiltration and soil hydraulic conductivity,

inflow from the upland could enter the saturated soil-

filled valley after 29 April, to mix with the water

already residing in the valley soils. An analysis of

Fig. 7. Half hourly measurements of water table at the valley edge and valley centre.


water collected during previous spring melt events at

Pocket Lake [from a study by and using analytical

methods described in Gibson et al. (1998)] revealed

that surface runoff was a chemical mixture of

snowmelt event water and pre-event groundwater.

Table 2 shows that ground ice and groundwater from

the soil-filled valley were relatively enriched with

oxygen-18 (18O) and deuterium (2H) and had

Fig. 8. Inflow from exposed bedrock, the response of soil moisture at the centre of the soil filled valley at transect G and runoff during the 23

August rainfall event.

Fig. 9. 2001 Spring melt cumulative water budget.


significantly higher electrical conductivity ðCÞ when

compared to snow. Surface runoff had isotopic and

chemical values between those of the snow and

ground ice and groundwater, suggesting that at least a

part of the valley contributes its pre-event water to the

spring runoff.

The sources that dictated the timing and volume of

runoff differed even though similar runoff generation

processes occurred in the spring and summer events.

In the spring, over half the valley snowpack was

directed to soil moisture storage and evaporation, so

the timing and magnitude of inflow from the bedrock

upland controlled the outflow hydrograph of the soil-

filled valley. In contrast, there was insignificant

vertical input to replenish the summer storage deficit

which then consumed much of the lateral inflow. This

resulted in a significant delay between lateral inflow

and valley outflow, a phenomenon not exhibited

during the spring melt.

5. Hydrological linkages between upland

and valley

The hydrological importance of the bedrock

uplands to the soil-filled valley is demonstrated by

the water balance considerations for the snowmelt and

the summer periods. In the spring (2 April–16 May)

of 2001, the valley received 117 mm from local

snowmelt, 53 mm of direct rainfall, but an inflow of

390 mm from the upland. This permitted 50 mm of

evaporation and 404 mm of outflow to be generated,

and a calculated increase of soil storage of 106 mm

(cf. measured storage increase was 108 mm) (Table 3).

In the summer (May–September) of 2000, direct

rainfall on the valley was 155 mm and inflow from

the uplands was 106 mm. These amounts sustained an

evapotranspiration loss of 164 mm and an outflow of

34 mm, as well as recharged the soil storage by

64 mm (cf. measured storage change was 73 mm)

(Table 3 and Fig. 10). These values show that inflow

from the uplands constitute a significant portion of

total inputs to the valley.

In the snowmelt season, as exemplified by the

spring of 2001, the bedrock upland snow cover melted

and provided the first contributions to surface runoff in

the basin (Fig. 11). At the same time, melting of the

valley snow cover percolated the dry frozen soil and

replenished storage. A rise of the water table at the

sides of the valley first occurred when many parts of

the upland contributed meltwater in support of the peak

inflow. Valley margins were the only sections of the

valley that produced surface runoff. Initial saturation

overland flow from these sites then followed water

tracks along the valley but it was subject to infiltration

losses, so that there was insufficient flow to reach the

valley outlet. Both the eastern and southern portions of

the valley eventually contributed to outflow, but as

inflows from the upland decreased, these intermittent

streams receded back to the up-valley areas.

The summer of 2000 was mostly dry. Rainfall and

inflow from the uplands were needed to replenish the

soil storage before runoff could begin in the valley.

Little rainfall reached the soil because of interception

loss to the ground vegetation. As inflow from the

upland was along the bedrock-soil interface, it could

avoid interception losses to the valley vegetation and

Table 2

Average values of selected chemical characteristics of water from

the Pocket basin site from 1995 to 2000. Stable isotope values are

presented in standard d notation as deviations per mille from

Vienna—SMOW (standard mean ocean water) such that dsample ¼

1000{ðRsample=RSMOWÞ2 1� where R is 2H/1H and 18O/16O

18O 2H C (mS/cm)

Snow 228 2215 7

Snowmelt runoff 224 2191 106

Ground ice/groundwater 220 2162 330

Table 3

2000 growing season and 2001 spring melt water budgets. All units

are in mm. (Qs and Qg are surface and subsurface outflow, M is

snowmelt, P is precipitation, E is evapotranspiration, I is lateral

inflow and DS is change in storage). The p included with Qg for the

2001 spring melt denotes unavailable because of flooding in the

trenches

Month Qs Qg M P E I DS DS

(calc.)

May 2000 0 0 3 29 1 211 226

June 2000 0 0 52 50 13 19 17

July 2000 0 0 25 40 7 252 28

August 2000 12 6 66 26 74 163 96

September 2000 2 14 9 19 11 245 215

2000 summer 14 20 155 164 106 74 64

2001 spring 404 p 117 53 50 390 108 106


could replenish soil storage directly. Thus, it was

lateral inflow rather than direct rainfall that controlled

the rise of the water table and the occurrence of

saturation overland flow in the soil-filled valley.

This was evidenced by the storm event of 23 August

2000. At the beginning of the rainstorm, the upland

bedrock areas generated runoff first, as their low

storage capacities were quickly satisfied (Fig. 12).

The valley section contributed runoff only after lateral

inputs enabled saturation at the upper valley and along

the water track. The eastern arm of the valley did not

yield surface runoff during this storm because lateral

inputs from exposed bedrock were not as high relative

to available storage. This attests to the importance of

upland contribution in runoff generation from the soil-

filled valley and confirms that the lower reaches of the

valley acted primarily as a conduit for surface runoff.

6. Hydrological behaviour of the soil-filled valley

Soil-filled valleys are an integral part of the Shield

landscape, performing the triple functions of (1)

Fig. 10. Cumulative water budget of the soil filled valley over the summer of 2000.

Fig. 11. Map of areas contributing surface runoff during the 2001

spring melt. Each shade represents the only areas contributing to

surface runoff on that date.


collecting vertical and lateral water inputs, (2)

retaining and losing the water held in storage, and

(3) transferring water down the valley and generating

local outflow. The interactions among these functions

show that inputs to the valley must fill up the storage

requirement before runoff is spilled to areas down-

stream. Conceptually, valley hydrology then can be

examined in terms of water sources, storage and

spillage processes.

6.1. Water sources, storage and runoff

In view of the smaller surface area of the valley

relative to the adjacent upland catchment, vertical

inputs (rainfall and snowmelt) are small in compari-

son to the lateral inflows. The latter are generated

from the upland which represent a dynamic contribut-

ing zone, the hydrological behaviour of which is the

focus of a future paper.

Available storage, being a function of antecedent

moisture and evapotranspiration loss, determines the

minimum input required to exceed the thresholds for

subsurface and surface flow generation. Variable soil

depths caused by bedrock topography generally offer

the valley edges a shallow soil that has a lower

saturation threshold than the middle of the valley.

The proximity of the edge zones to the bedrock slopes

increases the opportunity for large lateral inflow and

greater likelihood to reach storage capacity than the

other parts of the valley.

Soil thickness and its spatial uniformity is

important for runoff delivery because it determines

whether shield hillslopes remain hydrologically

coupled through the dry periods (Buttle et al., 2000;

Branfireun and Roulet, 1998; Devito et al., 1996).

Both Dorset (Buttle and Peters, 1997; Peters et al.,

1995) and the Experimental Lakes Area (Branfireun

and Roulet, 1998) in the humid temperate zone of

Ontario have similar soil depths and catchment areas

as those at Pocket Lake, yet experience seasonal

runoff ratios twice those at our study basin. The

difference is attributed to the precipitation amount,

with the Ontario sites receiving two and a half to three

times the summer rainfall at Yellowknife. Buttle et al.

(2000) suggested that deeper soils in a shield

environment permit more storage, better hydrologic

connections and, in turn, higher runoff. This does not

apply to the dry landscape around Yellowknife, where

high available storage in deep soils requires a larger

amount of water input to saturate. The flow linkage is

affected by water availability and soil storage status

and the presence of thick soil does not necessarily

imply that hydrological connections are intact.

Flow generation is a function of where, when and

how the soil storage capacities are satisfied. Subsur-

face flow often follows the bedrock surface (Peters

et al., 1995), moves within the soil matrix (Devito et al.,

1996) or drains along macropores, but the saturated

zone is unevenly distributed in the valley and the water

table has to rise above any bedrock sills to generate

subsurface outflow. Saturation overland flow is the

dominant surface runoff mechanism in shield valleys

(Buttle et al., 2000) and flow production depends on

local saturation of the valley soil. The 2001 spring melt

runoff ratio was 0.4 higher than the August 2000 rain

event. This demonstrates the variability of flow

generation due to changing spatial storage demands

in the valley. There is also a feedback between

overland flow and valley soil recharge. Runoff

from an upper valley segment may encounter

Fig. 12. Map of areas contributing surface runoff during the August

2000 rain event. Unlike Fig. 11 the shading indicates the cumulative

expansion of the contributing area.


a non-saturated lower segment and the water will

infiltrate along the flow path until the lower valley

storage is satisfied or until all surface flow is lost to

seepage. In the latter case, the stream becomes

intermittent.

6.2. The fill and spill flow concept of runoff generation

Central to the fill-and-spill concept is the spatially

variable valley storage that needs to be satisfied before

water spills to generate either surface or subsurface

flow. Storage capacity in the valley is spatially variable

owing to topographical diversity, soil heterogeneity

and uneven thickness, and seasonal presence of ground

frost. Soil storage status in the valley, enriched by

rainfall, snowmelt and lateral inflow, but lost to

evaporation and downstream drainage, is temporally

dynamic. Thus, along segments of a soil-filled valley,

filling will continue until (1) the local storage spills over

the threshold created by any bedrock sills to permit

subsurface flow, and (2) the water table rises above the

topographic surface to generate overland flow.

The fill-and-spill flow system differs from the

normal modes of flow in a humid climate where

the channel flow in the valleys is permitted to leave

the catchments with little significant interruption. The

importance of lateral inflows and variable storage

present a system that departs greatly from typical

saturation overland flow processes in humid areas

such that contributing areas do not necessarily grow

upslope from the stream channel. The shield valley

represents a series of storage reservoirs with the flow

Fig 13. An illustration of conceptualised fill-and-spill runoff generation. A is a longitudinal profile of the valley and B is a cross section. P is

precipitation; t is time at step 1, 2 or 3. SSSF is subsurface stormflow and SOF is saturation overland flow. A is the contributing area at t2 or t3:


cascading down the valley, filling individual segments

to satisfy their deficits until the thresholds are reached;

then spillage resumes to continue the flow down-

stream. Fig. 13 illustrates this idea. At t1; rain begins

and the valley water table is below the topographic

surface. By t2; lateral inflow has entered from bedrock

uplands, prompting a water table rise to the topo-

graphic surface and saturation overland flow at the

valley sides. Storage demands downslope interrupt the

saturation overland flow before it can reach the valley

outlet. As event inputs continue, the storage reservoirs

in the valley between the sides and outlet saturate so

that by t3 an ephemeral stream of saturation overland

flow reaches the outlet and discharges runoff.

6.3. Upscaling of the flow system

The fill-and-spill flow system can also be applied

to the large catchments of the Canadian Shield. For

ephemerally and perennially draining wetlands, Buttle

and Sami (1992) and Branfireun and Roulet (1998)

noted a progression of water table rise down valley

during runoff events, indicating that each segment is

first filled by lateral inflows before saturation overland

flow can continue downstream. During the summer in

a southern Ontario swamp, Devito et al. (1996)

observed that lateral surface inflows exceeded surface

outflow, suggesting that some of the surface flow

seeped underground to address storage demands. Of

particular importance is the frequent presence of lakes

in the Shield that through their well known role in

flow regulation functions, exaggerates the storage and

release functions of the hydrologic system as has been

reported by Fitzgibbon and Dunne (1981) and Spence

(2000). Such mechanisms of fill-and-spill therefore

can be upscaled and we recommend that hydrological

modelling of shield catchments should take account of

the cascading behaviour of runoff and its interaction

with in-valley storage.

7. Conclusions

In this research we examined the hydrological

processes in a subarctic Canadian Shield headwater

soil-filled valley and clarified the hydrological con-

nections in terms of water transfer from the bedrock

uplands to and through the soil-filled valleys.

Evapotranspiration was a key controller of storage

amount in the valley. The importance of evapotran-

spiration has not been clearly identified in the

previous studies on the Canadian Shield that have

focused on the wetter environments in Manitoba,

Ontario and Quebec. Lateral inflows at this site were

also a crucial component of the valley water budget.

Our results demonstrate that water budgets at down-

slope positions may be controlled by upslope

locations where the latter’s contributing areas are

large enough. It was previously known that the state of

the hydrologic connection between uplands and

valleys influences basin runoff magnitude, but the

controls on this connection are now quantified.

Results of this study showed that the status of

available storage at the valley edge relative to the

magnitude of lateral inflow controls this connection

and it has significant implications on how water is

transferred down the valley. Valley edges tend to have

lower storage capacities because of shallower soils

and the highest inputs from their adjacent lateral

inflows, causing saturation overland flow to begin at

the upper or edge locations in valleys. Some of this

saturation overland flow would be lost to infiltration at

downstream locations until storage capacities are

satisfied. Such hydrological linkage functions of the

headwater valleys can be conceptualised as a fill-and-

spill runoff mechanism in which segments of the

valley represent reservoirs that have to be filled above

their capacity before spillage allows flow to continue

downstream. Such mechanisms can be upscaled to

large catchments and its representation should

improve the performance of hydrological models of

the shield environment.

Acknowledgements

We thank Shawne Kokelj, Andrea Czarnecki,

Kerry Walsh and Mark Dahl of Environment Canada,

and Claire Oswald of McMaster University and Iain

Stewart and Derek Steadman for their assistance in the

field. Environment Canada and the Mackenzie

GEWEX Study funded this work. Miramar Giant

Mine kindly provided access to the study site. We

acknowledge Shauna Sigurdson and Jesse Jasper of

Environment Canada for supporting this research.


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hydrology of subarctic canadian shield: soil-filled valleys

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