50
Northern Research Basins Water Balance
IAHS Publ. 290, 2 0 0 4
(Proceedings o f a workshop held at Victoria, Canada, March 2004).
Summer water balance in an Arctic tundra basin,
eastern Siberia
1
1
1
YOSHIYUKIISHII , YUJI KODAMA , NORIFUMI SATO &
HIRONORIYABUKI
2
1 Institute of Low Temperature
Science,
ishiiv(5).pop.lovvtem.hokudai.ac.ip
2 Frontier
Observational
Research
Hokkaido
System
for
University,
Global
Change,
Sapporo
Yokohama
060-0819,
236-0001,
Japan
Japan
Abstract To clarify the summer water balance of an Arctic tundra watershed
in eastern Siberia, hydrological and meteorological observations were made
during three summer seasons from 1997 to 1999. The weather conditions in
these summers were considerably different from year to year: wet in 1997, dry
in 1998, and first dry then wet in 1999. Both summer rainfall and snowmelt
amount were major input components, and they showed considerable interannual variability. Stream runoff was substantial as the output component, and
its high inter-annual variability, depended on that of input components. The
area of rocky terrain with lichen occupied 36% of the whole watershed, and
évapotranspiration from there was quite small. This led to the relatively small
amount of basin-averaged évapotranspiration compared with other compo­
nents. Change in storage within the thaw layer was small, because its
thickness was thin and soil moisture was kept in a nearly saturated condition
during the summer.
K e y w o r d s Arctic tundra; basin water balance; eastern Siberia; évapotranspiration; runoff
INTRODUCTION
The Arctic plays an important role in global hydrological and meteorological
processes. Tundra comprises the typical land surface in the Arctic, and it has a unique
water cycle due to the existence of frozen ground, drifting snow and tundra vegetation.
Various studies of basin water balance have been performed in the Arctic tundra
regions, especially in Alaska and the Canadian High Arctic (Church, 1974; Woo et al,
1983; Woo, 1986; Kane et al, 1991). However, in the case of the Siberian High
Arctic, our knowledge of the hydrological regime and processes is quite limited due to
the lack of scientific information in this area.
As a part of the GAME-Siberia project, hydrological and meteorological observa­
tions for a basin water/energy balance have been carried out in the east Siberian tundra
region near Tiksi since 1997. The objectives of these observational studies were: (a) to
characterize the water balance components in the small tundra watershed, (b) to clarify
seasonal and inter-annual variations in one-dimensional (1-D) energy and water vapour
fluxes over the tundra surface, and (c) to determine the areal distribution of ground
surface properties (Kodama, 2001).
Hydrological observations for the basin water balance were stalled at the
beginning of the summer in 1997, and we carried out intensive observations over the
next two summers (1998 and 1999), during which at least two or three scientists stayed
alternately at the site during the whole summer season (late May to early September).
Summer water balance in an Arctic tundra basin, eastern Siberia
51
The summer water balance in this basin has been reported in an earlier paper (Sato
et al, 2001a). However, at that time we had no information about the areal snow
distribution, and adopted a tuning parameter, a, in the water balance estimation, which
was the ratio of snow patch area to the whole watershed area. Change in storage was
calculated as a known value from the streamflow recession analysis. Thereafter, we
could obtain information on spatial surface conditions in this basin using helicopter
observation data in 1999 and 2000, and satellite data in 2001 (Sato et al, 2001b;
Yabuki, personal communication). Using these data, the summer basin water balance
was re-calculated in the normal manner, in which each water balance component was
estimated independently and the change in storage was obtained as a residual. In this
paper, results from the three summer seasons are reported.
STUDY AREA
2
The experimental watershed, 5.5 km in area, is located 7 km south of Tiksi (71°40'N,
128°50'E), Sakha Republic, Russia. It is near the mouth of the Lena River and 5 km
from the Laptev Sea coast (Fig. 1). The altitude of the watershed ranges from 40 to
300 m a.m.s.l. Permafrost completely underlies this region, and its thickness reaches
over 500 m (Fartyshev, 1993). The active layer thickness varies from 20 to 70 cm,
depending on the ground surface condition. The surface condition of snow-free areas
in the watershed is classified into three types: wet moss including sphagnum and
sedges, dry moss, and rocky terrain with lichens. Wet and dry moss regions are
distributed on flat plains and on the lower parts of slopes, where the gradients are
gentle. Rocky terrain is distributed on ridges and the upper parts of slopes. The
leeward side of ridges and hills, where snowdrifts are formed, is usually dry because
vegetation other than lichen cannot live there and no soils are developed.
METHOD
Precipitation
Two tipping bucket raingauges without windshields were used in this study. One was set
up over rocky terrain near the meteorological observation site MT (see Fig. 1), where it
was easy to keep the raingauge horizontal. The other one was set up at the top of the
mountain, PA, to check the spatial variability of summer rainfall in this watershed.
Slight rainfall events are often observed in the Arctic region. As the minimum
volume of the tipping bucket raingauge was 0.5 mm in height, it was deemed
inadequate for such slight rainfall events. Thus, a storage-type raingauge with a
windshield was installed near the tipping bucket gauge at site MT, and both rainfall
records were compared.
Runoff
A water level recorder using a pressure transducer was placed at the outlet of the
watershed (site HY in Fig. 1), where the stream channel was stable and the uniform
Yoshiyuki Ishii
52
m
0
1000
2000
3000
Fig. 1 Location map of study site and detailed topographic map of the experimental
watershed. MT, HY, and PA denote a meteorological observation site, streamflow
gauging and water-level monitoring site, and raingauge site, respectively. The broken
line shows the boundary of the watershed (5.5 km"), and thin lines show the snow
courses for the snow stake measurement.
flow assumption was easily applicable. Each year, streamflow gauging was made one
or two times a day in the snowmelt season and two or three times a month in the
summer season. Water level records were converted to continuous streamflow data
using empirically determined stage-discharge rating curves.
Snowmelt amount
The snowmelt amount was estimated by the degree-day method. Three snow courses
were set up in the watershed, as shown in Fig. 1. Snowmelt amounts were measured
using snow stakes at 7-10 sites along the courses during the snowmelt season in 1997
Summer water balance in an Arctic tundra basin, eastern Siberia
53
and 1998, and the relationship between the accumulated air temperature and the daily
snowmelt amount was established as follows:
m = 5.9T
(1)
where m is the total daily snowmelt amount (mm) and T is the daily mean air
temperature (°C). Snow stake measurements were not made in 1999, but equation (1)
was adequate for the different snow conditions in 1997 and 1998. Then we used
equation (1) to estimate the amount of snowmelt in 1999.
To clarify the spatial distribution of snow-covered areas and their decline as the
melt season progressed, helicopter observations were carried out over the watershed
for the summer of 1999-2000 and satellite image analysis was used for the summer of
2001. These results were roughly used to estimate the snow-covered area and its
seasonal change in 1997, 1998 and 1999. The basin-wide snowmelt amount could be
estimated by the multiplication of the point snowmelt amount and the snow-covered
area for each day.
Evapotranspiration
Satisfactory observation of the meteorological elements and the energy fluxes at site
MT were started in September 1997. Then évapotranspiration over the wet moss
regions was calculated by the Penman method, using the Polyarka Hydrometeorological Station data after the needed correction in 1997. Meanwhile, the Bowen ratio
method was used in 1998 and 1999 to estimate the évapotranspiration over the wet
moss regions. In this case, the meteorological data set obtained by the tower
observation was used for calculation. Sato et al. (2001b) clarified the relationship
between the potential and actual évapotranspiration over the wet moss and also the
relationship between the actual évapotranspiration over the wet moss and the rocky
terrain as follows:
£ w = 0.71£p
(2)
£'d = 0.18£w
(3)
where Ew is actual évapotranspiration over the wet moss tundra, £p is potential évapotranspiration, and Eà is actual évapotranspiration over the rocky terrain. According to
Sato et al. (2001a), the ratios of wet moss and rocky terrain to the total watershed area
were 0.64 and 0.36, respectively. We assumed that no net sublimation or evaporation
occurs from the snow surface, and snow patches exist only on the rocky terrain. Then
the watershed mean évapotranspiration, E, could be calculated using the ratio of snowcovered area, A, as follows:
£ = 0.64 J5W +0.36 £d
0.71 [ 0 . 6 4 - ( A - 0 . 3 6 ) ] Ep,
A>036
(4)
0.71 [0.64 + 0.18 (0.36 -A)]
Ep,
A< 0.36 0
Yoshiyuki Ishii
54
Water balance
The water balance of the watershed is expressed by the following equation:
P + M=Q + E + dS
(5)
where P is precipitation, M is snowmelt amount, E is areal averaged évapotranspira­
tion, and àS is change in storage. Since P, M, Q and E were calculated independently,
dS" was obtained as a residual.
Moreover, in order to know the water storage capacity within the thaw layer and
its seasonal change, thaw depths were measured at 11 sites of wet/dry moss and rocky
terrain around site MT. Soil moisture measurements at three different depths—namely
0, 15 and 30 cm—were also made continuously using the TDR sensors only at the wet
moss region near site MT in 1998 and 1999.
RESULTS AND DISCUSSION
Input components
Since two raingauges were not sufficient to examine the spatial variability of summer
rainfall, we tried to use two more of these gauges at the middle part of the slope and at
the valley bottom, near the centre of the watershed, in 1998. However, we could not
consistently obtain data due to sensor trouble and theft. In addition, more than 10
plastic containers (15 cm x 20 cm) were placed at the various sites in the watershed to
measure the total rainfall amount during one event and to compare its spatial variation
from early July to early August in 1999. However, we did not observe any significant
rainfall event in this period. Figure 2 shows a comparison of the total precipitation in
11 events in 1998, which were measured at site MT and PA. As no remarkable
difference was recognized between the sites, we assumed that the summer rainfall
within the watershed was uniform and its spatial variability was small. The cumulative
precipitation for the storage-type gauge was slightly less than the tipping bucket
results, but those differences were considered to be too small to be significant (Fig. 3).
Therefore, we calculated the basin-averaged precipitation using the tipping bucket
raingauge data at site MT.
Figure 4 shows the seasonal changes of snow-covered areas in 1999, 2000 and
2001. Because the helicopter observation and the satellite image analysis could not be
performed so often, there are only sparse data in this figure. However, two types of
seasonal change could be roughly recognized: heavy snow years like 2001, and light
snow years like 1999 and 2000. Two polynomial approximated curves were fitted to
the heavy and light snow years, respectively. Although there was no clear definition
for the light and heavy snow years, we judged the snow condition according to
whether the snowpatch size in the valley floor was large or not in the period just before
the snowmelt season. Then we assumed, from the results of a field survey, that 1997
was a heavy snow year and 1998 was a light one. Basin-averaged snowmelt amount on
a daily basis was estimated by the multiplication of snow-covered areas determined by
those curves in Fig. 4 and daily snowmelt amounts calculated by equation (1).
Summer water balance in an Arctic tundra basin, eastern Siberia
55
20
• 1999
1998
o
0
v.
0
1
,
•
1
5
10
15
20
Lower site, M T (mm)
Fig. 2 Comparison of total rainfall at the lower site (MT) and upper site (PA) for the
17 events in 1998 and 1999.
Tipping Bucket Type
(mm)
Fig. 3 Comparison of total rainfall measured by the tipping bucket type and by the
storage type.
o
» 1999
x 2000
• 2001
\
x\
\*
|
0
f
1
i
20
40
60
80
Days from June 1
nr
100
Fig. 4 Seasonal changes of snow-covered area in 1999, 2000 and 2001. Thick and thin
solid lines indicate the polynomial approximated curves for the heavy and the light
snow years, respectively.
Yoshiyuki Ishii
56
Output components
Figure 5 shows the stage-discharge rating curves at site HY for each year. These
curves covered all measured streamflow data from the early snowmelt season to the
late summer. In the beginning of the snowmelt season, these curves could not be used
because of the existence of ice bodies in the streambed, or ice jams. Then we assumed
the streamflow values measured once or twice a day to be the daily mean value.
Streamflow regimes for the three summer seasons are shown in Fig. 6 with hourly
precipitation. Ishii et al. (2001) compared the characteristics of rainfall-runoff response
in this watershed to those of Imnavait Creek in Alaska reported by McNamara et al.
(1998). As pointed out by Church (1974), rainfall-runoff response usually shows a
quick rise and a quick recession of hydrograph in the Arctic tundra region. However,
in the case of Imnavait Creek, McNamara et al. (1998) reported a quick rise by lateral
flow through the water tracks on the slope and slower recession by subsurface flow
through the highly porous moss layer. In this study, the rainfall-runoff response
showed nearly the same characteristics as those of Imnavait Creek. Although
streamflow changed in correspondence with the rainfall events during the summer, the
baseflow of late August was remarkably high in 1997 and 1999. There was no serious
eiTor within the use of the stage-discharge relationship in these periods, and we
confirmed the high baseflow by in situ observation of stream water levels. The weather
condition of late August in 1997 was wet; however, it was comparatively dry in 1998
and 1999. We also made streamflow observations at the adjacent river during the
summer in 1998 and 1999; this was the main river of our experimental stream, and its
watershed area was about 40 km . This main river showed a reduced baseflow
tendency not only in 1998, but also in 1999. Although why the high baseflow of late
August occurred in some years was not clear, we considered that it might be a sitespecific phenomenon in our experimental watershed.
2
Summer water balance in an Arctic tundra basin, eastern Siberia
57
10
0.01
-I
5-Jun
1
,
25-Jun
1
1
15-Jul
1
1
4"Aug
1
1
1
1
24-Aug
Fig. 6 Seasonal variations in summer rainfall and streamflow during the three summer
seasons from 1997 to 1999.
Yoshiyuki Ishii
58
The daily mean energy fluxes over the wet moss tundra in the summer of 19971999 are shown in Fig. 7. Each flux decreased gradually from the beginning of
summer to its end. The latent heat flux was usually larger than the sensible heat one,
and the Bowen ratio was less than 1.0. Figure 8 shows the seasonal variation of daily
évapotranspiration. The weather conditions of these summers showed high year-toyear variation; that is, a wet summer in 1997, dry in 1998, and first dry, then wet in
1999. However, the interannual variation in évapotranspiration was relatively small.
1997
20-Jun
1-Jun
5-Jul
16-Jun
20-Jul
1-Jul
4-Aug
16-Jul
19-Aug
31-Jul
3"Sep
15"Aug
30"Aug
Fig. 7 Seasonal variations in daily mean energy fluxes over wet moss in the summer
of 1997-1999. Rn, H, IE, and G denote net radiation, sensible heat flux, latent heat
flux and soil heat flux, respectively. Sensible heat fluxes in 1997 were not calculated.
Summer water balance in an Arctic tundra basin, eastern Siberia
1-Jun
16-Jun
1-Jul
16-Jul
31-Jul
15-Aug
59
30-Aug
Fig. 8 Seasonal variations in daily évapotranspiration in the summers of 1997-1999.
Water balance
Figures 9 and 10 illustrate the 10-day average and the cumulative daily value for each
water balance component over the summer, respectively. Change in storage, dS, was
calculated as a residual of each component in these figures. Table 1 presents a
summary of the water balance components during the whole summer in 1997, 1998
and 1999. Summer rainfall and snowmelt amounts show a large year-to-year variation;
as inputs to the water balance, the components have the same importance in terms of
magnitude. Stream runoff is a major output component in every year. Its year-to-year
variation depends on both the summer rainfall and the snowmelt. Meanwhile, basinaveraged évapotranspiration was quite small due to the existence of rocky terrain,
which occupies 36% ofthe watershed.
Yoshiyuki Ishii
60
mm d
be
<!
Fig. 9 Seasonal variations in the ten-day average of each water balance component
during summers of 1997, 1998 and 1999. P, M, Q, and E denote summer precipitation,
snowmelt amount, stream runoff and évapotranspiration, respectively.
Seasonal variations of thaw depths for the three typical ground surfaces in 1998
and 1999, and of soil moisture measured by TDR sensors at three depths on the wet
moss, are shown in Figs 11 and 12, respectively. In general, the thaw layer was thick at
the rocky terrain, and thin at wet moss sites. However, the water storage capacity of
the thaw layer was small because of the high saturation rate at all places and depths.
Hence, change in storage was small when compared with rainfall, snowmelt amount,
and runoff, as shown in Fig. 9 and Table 1. In the late summer of 1997 and 1999,
change in storage became slightly more significant than in 1998, but additional surface
drainage from the lakes was not found in either year.
Summer water balance in an Arctic tundra basin, eastern Siberia
P
mm
1-Jun
16-Jun
o Q
1-Jul
x E
16-Jul
•
M
61
—dS
31-Jul 15-Aug 30-Aug 14-S
Fig. 10 Cumulative daily precipitation (P), streamflow (Q), évapotranspiration (E), snow­
melt amount (M), and change in storage (dS) during summers of 1997,1998 and 1999.
Table 1 Summary of water balance components in mm.
Year
Period
P
M
1997
1998
1999
23 June to 31 August
5 June to 26 August
5 June to 31 August
217
90
98
130
129
83
Q
338
186
144
E
dS
75
33
54
-66
0
-17
P, summer rainfall; M, snowmelt amount; Q, stream runoff; E, évapotranspiration; and dS, change in
storage.
Yoshiyuki Ishii
0
1998
-20
A
40
-M
ft
ca
Q
60
—•— dry moss
...o--.
t moss
—x—rocky terrain
w e
24-May
13-Jun
3-Jul
23-Jul
12-Aug
1-Sep
Fig. 11 Seasonal variations in thaw depths for wet moss, dry moss and rocky terrain in
1998 and 1999.
60
/ \A
CD
B 40
^
20
15
V :
30
cm
^
~ H \
'
\ \M—JjjmxJ
" /
••-<>-••
-—. ^ ^ ^ ^
/
\
1
i
60
•0
% of volume
I
i
1998
I
I
I
% of volume
1999
3
tan"
ffihTTrmi
I NU 11 UJUJ-OST^
3
•71
3
bo
o
CO
Fig. 12 Seasonal variations in saturation rate of soil water for three different depths in
1998 and 1999.
63
Summer water balance in an Arctic tundra basin, eastern Siberia
Estimation errors
Estimation errors are likely to lie mainly in the assumptions of snow-covered area and
of basin-wide precipitation. We considered only two curves of seasonal variation in
snow-covered areas, for heavy and light snow years. If the spatial distribution of snowcovered areas shows a large interannual variability depending on the winter and
summer weather conditions, these curves may not be satisfactory as the generalized
relationships. Evapotranspiration might also have large errors in estimation. However,
it was small as compared with precipitation, snowmelt amount and runoff, making its
estimation error small in the water balance. Since this experimental watershed is
located in a mountainous region, the catchment boundary is easily determined. The
estimation error of the stream runoff is smallest among the components.
CONCLUSION
To clarify the summer water balance of the Arctic tundra watershed in eastern Siberia,
hydrological and meteorological observations were made during three summer seasons
from 1997 to 1999. The weather conditions in these summers were considerably
different from year to year: wet in 1997, dry in 1998, and first dry then wet in 1999.
The results of water balance estimation is summarized as follows:
(a) Both summer rainfall and snowmelt amount were major input components, and
they showed considerable interannual variability.
(b) Stream runoff was substantial as the output component, and its high inter-annual
variability depended on that of input components.
(c) The area of rocky terrain with lichen occupied 36% of the whole watershed, and
évapotranspiration from there was quite small. This led to the relatively small
amount of basin-averaged évapotranspiration as compared with other components.
(d) Change in storage within the thaw layer was small, because its thickness was thin
and soil moisture was kept at a nearly saturated condition during the summer.
Acknowledgements This study was supported by the GAME-Siberia project, which is
funded by the Ministry of Education, Culture, Sports, Science and Technology of
Japan and the Frontier Observational Research System for Global Change. We would
like to thank Prof. Tetsuo Ohata of Hokkaido University for giving us the opportunity
to carry out this research. We are also grateful for valuable discussions and substantial
logistical support provided by all the members ofthe GAME-Siberia project.
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