Wolf Creek Research Basin: Hydrology, Ecology, Environment
Partitioning of Energy During the Snow-free Season at
the Wolf Creek Research Basin
Raoul J. Granger
National Water Research Institute, National Hydrology Research Centre,
Environment Canada, Saskatoon, Saskatchewan, S7N 3H5
Abstract
The results of four years of observation of the partitioning of energy at three sites
in the Wolf Creek Research Basin are presented. The sites are representative of
the major land use classes in the basin, boreal forest, high bush tundra and alpine
tundra, and range in elevation from 750 to 1600 m. The instrumentation deployed
allows for the determination of the complete energy and water balances at these
sites. The data from these sites are also used in the development of
parameterizations for the modelling of evapotranspiration and the application of
remote sensing to the estimate of regional evapotranspiration.
The energy balances are derived and presented for the three Wolf Creek sites. Net
radiation and soil heat fluxes are measured directly, and the turbulent fluxes are
derived from measurements of temperature, humidity and wind speeds observed
at the sites. Results from the 1994-1997 snow-free periods are presented.
Seasonal trends, as well as the differences between the sites (land cover and
elevation), are demonstrated.
Introduction
Wolf Creek basin, near Whitehorse, Yukon Territory, has been the site of intense
investigations of the processes governing the energy and water balance
components in a high latitude environment. It has been designated as one of the
major study sites for the Canadian GEWEX programme; which has amongst its
objectives the achievement of an improved understanding of cold region, high
latitude hydrological and meteorological processes and the role that they play in
the global climate system. In the understanding of climatic change and the
assessment of its effects, the correct evaluation of the partitioning of energy at
the surface, and of the role evapotranspiration will play in a modified
hydrological cycle, becomes crucial in regions such as the Canadian Prairies and
the North. Most current operational evapotranspiration models ignore the effects
of changes in soil heat storage. In regions where the seasonal soil heat storage
effects are important, these models must be modified to take these effects into
account.
33
Granger - Evaporation
Wolf Creek Site Description and Instrumentation
The Wolf Creek basin, located 15 kilometres south of Whitehorse, Yukon
Territory, occupies a 195 km2 area which forms part of the headwater region for
the Yukon River. The basin elevation ranges from 750 to 2250 m. The area is
within the discontinuous/scattered permafrost zone with sporadic permafrost at
higher elevations and on north-facing slopes. The watershed has three broad
vegetation cover types of about equal proportions; these are alpine tundra, shrub
taiga and boreal forest consisting of mixed spruce, pine and poplar. The three
broad vegetation cover types present in the basin are representative of the major
vegetation cover types found from the boreal forest of Western Canada to the
high arctic tundra. The basin is thus a convenient, relatively compact outdoor
laboratory offering three of the major vegetation types found in the Mackenzie
Basin. As well, the presence of permafrost on some higher and north-facing
slopes makes this basin an appropriate site for undertaking comparative studies
of the hydrological effects of permafrost.
Micrometeorological instrument sites were established at representative
locations within each of the three vegetation zones in the basin. The alpine tundra
site, at an elevation of 1615 m, is situated on a wind-swept, high alpine tundra
plateau which defines part of the drainage divide along the northern edge of the
basin. A reasonably-level fetch extends for approximately 100 m in all
directions; to the north, south and west the terrain drops towards the respective
valleys, while to the east the terrain rises slightly to a rounded hilltop. Vegetation
is sparse and consists of mosses and lichens with occasional patches of scrub
willow and birch no more than 20 cm tall. The plateau is also strewn with
boulders.
The shrub taiga site, at an elevation of 1250 m, is located near the bottom of a
glacial valley in the upper reaches of the basin. There is a slightly sloping fetch
NE and SW for several km; to the SE the valley slopes up and to the NW the
valley drops somewhat before rising to the next ridge. The vegetation consists
mainly of tall willows and alders, ranging from 1 to 2 m in height, with very
scattered spruce.
The forest site (elevation of 750 m) is in a mature white spruce stand on a flat
valley bottom; the vegetation height ranges from 15 to 18 m. The site is located
near the basin outlet.
Each of these sites contained the core instrumentation to monitor the vertical
water and energy balance; the following parameters were recorded: net all-wave
radiation, incoming and reflected short-wave radiation, air temperature*, relative
humidity*, wind speed* and direction, infrared canopy surface temperature, soil
heat flux, soil temperature*, rainfall, snowfall and depth of snow on the ground.
Barometric pressure was also observed at the Alpine station. (* Denotes those
parameters measured at two or more levels.)
34
Wolf Creek Research Basin: Hydrology, Ecology, Environment
Data were automatically-recorded every half hour; each recording represents the
average of the individual readings sampled at 15 second intervals. For the study
period, the data stream is continuous and uninterrupted. For the purpose of
obtaining the daily energy balance terms, the daily average values were
computed based on the 24-hr period from midnight to midnight Pacific Daylight
Time.
Energy Balance
The energy balance of a surface can be expressed by the equation,
(1)
where Rn is the net all-wave radiation to the surface, Qg is the heat flux into the
soil surface, LE and H are the turbulent latent and sensible heat fluxes above the
surface.
In Eq. 1, the net radiation term, Rn, was measured directly. Because daily time
periods were used, the changes in energy stored in the soil layer above the soil
heat flux sensors were ignored, and the soil heat flux sensor readings were used
directly to represent the soil heat term, Qg. The latent heat term, LE, was
estimated using an evapotranspiration model (described below), and the
sensible heat term, H, was determined as the residual in the balance equation
(1). Calculating the sensible heat as the residual in Eq. 1 means that it includes
(for the surfaces with significant vegetation) the change in heat storage within
the vegetation canopy; however, since daily totals are considered, this
component is small compared to the turbulent flux itself and is generally
ignored.
The components of the energy balance are expressed in equivalent millimetres of
evaporation by dividing each term by the latent heat of vaporization, L. The
energy balance equation presented in the form of Eq. 1 actually represents the
partitioning of the incoming radiation into the three major fluxes, such that a net
radiation flux directed toward the surface is given a positive sign, while the soil
heat, sensible and latent heat terms are considered positive if directed away from
the surface.
Evapotranspiration Model
The daily evapotranspiration, E, was estimated using an algorithm developed by
Granger and Gray (1989) and Granger (1991). The method is based on an
extension of the Penman (1948) equation to the non-saturated case. The method
is also consistent with the feedback approach described by Bouchet (1963). The
35
Granger - Evaporation
general equation for evaporation from non-saturated surfaces, as derived by
Granger and Gray (1989) is given as
(2)
where, ∆ is the slope of the saturation vapour pressure vs temperature curve; G
is a dimensionless relative evaporation parameter; γ is the psychrometric
constant and Ea is the drying power of the air, as defined by Penman (1948). The
fluxes, Rn, Qg, and Ea are in units of mm/d equivalent evaporation.
The drying power, Ea, can be obtained using a Dalton-type formulation:
(3)
where, ea is the actual vapour pressure of the air (expressed in kPa), e* is the
saturated vapour pressure of the air at the actual air temperature, and ƒ(u), the
vapour transfer coefficient, is a function of wind speed when applied to a daily
period:
(4)
where the average wind speed, u, is given in m/s. The constants a and b are
related to the surface roughness using the following expressions derived in the
southern boreal forest (Pomeroy et al., 1997):
(5a)
(5b)
in which the roughness length, Z0 , is given in cm.
The relative evaporation, G, is related to a dimensionless relative drying power,
D. D is defined as:
(6)
and
(7)
Eq. 7 was derived using data obtained over a variety of surfaces including bare
soil, grass, growing wheat, and aspen forest, and it appears to be applicable to all
these surfaces; the differences in surface characteristics being accounted for in
36
Wolf Creek Research Basin: Hydrology, Ecology, Environment
the terms Ea (surface roughness) and Rn (albedo). Eq. 7 is limited to those
situations for which the value of the relative drying power, D, falls between zero
and unity, that is, only for those conditions in which the period available energy
(Rn-Qg) is positive. This, however, is a limitation which applies to all
evapotranspiration models which utilize a form of the Penman equation. When
working with daily time periods, especially during the growing season, the
available energy is almost always positive. For those periods when the available
energy is negative, an alternative approach must be used; a regression approach,
such as the Priestley-Taylor (1972) approach can be used if suitable coefficients
have been derived for the surface conditions encountered. Based on comparisons
with eddy correlation measurements obtained over a short grass surface on the
prairies, a P-T coefficient of 0.65 was found to be appropriate for estimating the
night-time vapour transfer.
Monthly Energy Balance
In order to investigate the seasonal trends, daily values of the terms in Eq. 1 were
calculated for the three sites, and these were summed for each month of the study
period. Table 1 presents the monthly totals of the energy balance terms,
expressed in mm equivalent for the three sites, for the observation periods May
through September, for the seasons 1994 through 1997. Also included are the
observed precipitation values.
Table 1. Monthly totals of the energy balance terms (mm equivalent) for the
observation periods May through September, for 1994, 1995, 1996 and 1997.
37
Granger - Evaporation
With the exception of the precipitation amounts, the respective values presented
in Table 1 did not vary greatly from year to year. The average values of the
monthly energy balance terms for the four study years were calculated, and these
are presented graphically in Fig.1. The figure allows for a comparison of the
typical energy balance conditions for the three land covers. Fig. 1 shows that the
net radiation input at all sites peaks in June or July and drops off quickly in
August and September. The amounts of radiation received at the three sites
differs significantly, with the alpine site receiving the least and the forest site
receiving the most net radiation. Although variations in cloud cover can account
for differences in net radiation, the most significant factor affecting the net
radiation input is the surface albedo; the observed average albedo values at the
alpine, taiga and forest sites were 0.21, 0.18 and 0.09, respectively.
Figure 1. The monthly energy balance, expressed in equivalent mm of evaporation,
for the alpine, shrub taiga and forest sites for the period May through September. The
values are the means of the study years 1994 - 1997.
Partitioning of Energy: Latent and Sensible Heat
The behavior of the latent and sensible heat terms appears to be somewhat more
complex. Although the magnitude of the net radiation term drops off rapidly in
August and September, the reduction in the latent and/or sensible heat term may
be less abrupt. For the alpine site both the evaporation, E, and the sensible heat,
H, follow the net radiation trend, indicating that the sparse vegetation at this site
has very little effect on the partitioning of the incoming energy. The shrub taiga
and forest sites show some divergence between the evapotranspiration and the
sensible heat. At these sites, from June to September, the latent heat is greater
than the sensible heat, with the difference being greatest for the forest site.
38
Wolf Creek Research Basin: Hydrology, Ecology, Environment
To better demonstrate the partitioning of energy Figure 2 shows the fraction of
the incoming net radiation used for the other energy balance components for the
alpine, taiga and forest sites. The figure shows the average results for the four
years of observation. The difference between the respective sites is quite visible.
At the alpine site the latent and sensible heat terms are approximately equal
throughout the snow-free season (Bowen ratio ≈ 1); the very sparse vegetation at
this site cannot exert any significant control on the partitioning of energy and,
with similar turbulent diffusivities the latent and sensible heat fluxes are similar.
At the shrub taiga and forest sites, with significant vegetation, more energy is
being diverted to evapotranspiration than to heating of the air; the difference
between the two turbulent energy terms increases throughout the season at these
sites, and the divergence is greatest for the forest where, for the month of
September, approximately 70% of the incoming energy is being used for
evapotranspiration. The average September Bowen ratios for the shrub taiga and
forest sites are 0.7 and 0.5, respectively.
Figure 2. The partitioning of the incoming energy, expressed as a fraction of the net
radiation, for the alpine, shrub taiga and forest sites for the period May through
September. The monthly values are the means of the study years 1994 - 1997.
That the vegetated sites partition a greater amount of energy to evapotranspiration is not unusual; what is surprising, however, is the seasonal trend.
Pomeroy et al.( 1996, 1997) showed that for the southern boreal forest the mature
stands produced more evapotranspiration than cleared surfaces or regenerating
stands; they showed that for the water-limited southern forest, the partitioning of
energy between latent and sensible heat terms responded to the available
moisture, that is, months with greater precipitation produced greater
evapotranspiration. This appears (Fig. 2) not to be the case for the northern
boreal forest vegetation, where the evaporative fraction increases throughout the
39
Granger - Evaporation
growing season. Fig. 2 represents the average of the four study years; however,
the same trend is evident for each year, with little response to variations in
monthly precipitation.
Soil Heat Flux
The soil heat flux, as is the case for most situations, is the smallest term in the
energy balance. Contrary to the radiation inputs, however, the soil heat fluxes are
largest at the alpine site and least at the forest site. This is as expected, since the
presence of surface cover and vegetation tends to reduce the contribution to soil
heat storage. The mid-summer soil heat term is typically 10%, 6% and 3% of the
net radiation term for the alpine, taiga and forest sites, respectively. These values
(percentage of the net radiation contributing to soil heat) remain relatively
constant throughout the growing season, dropping off only in September when
the heat flux changes direction as soil refreezing begins. This seasonal trend is
quite different from that observed at southern boreal forest sites (Pomeroy et al.,
1996), where the soil heat fluxes are largest immediately after the snow melt, and
decrease continuously throughout the growing season. Although there is some
permafrost within the basin, none of the study sites has a permafrost subsoil. The
soil heat flux regime under permafrost could be significantly different from those
observed here.
Water Use
Pomeroy et al. (1996, 1997) suggested that the southern boreal forest has adapted
its water use strategy to the dry conditions, increasing its evapotranspiration as
water is made available; the observed partitioning of energy demonstrates the
forests adaptation to local conditions. The northern forest is energy-limited; the
peak energy supply (net radiation) occurs shortly after the beginning of the
growing season and drops off quickly (Fig. 1), and the soil temperature regime
limits the ability of the vegetation to transpire effectively early in the season. In
order to maximize its productivity, the northern forest must maintain a relatively
high water use; with a declining energy input, this results in a seasonal trend for
the partitioning of energy as shown in Fig. 2.
This may be further demonstrated by showing the relationship between
evapotranspiration and precipitation inputs. Fig. 3 is a comparison of the average
monthly values of the ratio of evapotranspiration to precipitation for the three
sites; the 1994 data have been excluded because of missing values. Fig. 3 shows
that for May all sites use more water than the amount supplied by precipitation
during the period; all sites are making use of the snowmelt water. For the alpine
site the ratio drops quickly to near unity. The shrub taiga utilizes 1.5 to 2 times
the precipitation amount during the summer; the ratio declines steadily
40
Wolf Creek Research Basin: Hydrology, Ecology, Environment
throughout the season. The forest site continues to utilize from 3.3 to 4.2 times
the available precipitation throughout the growing season. In September, when
defoliation occurs, the ratio drops significantly at all sites.
Figure 3. The average monthly values of the ratio of evapotranspiration to
precipitation for the Forest, Shrub Taiga and Alpine sites (1995 - 1997).
Precipitation vs. Elevation
The precipitation data provided here represents only three seasons, and as such
is likely insufficient to derive specific relationships with elevation. However, the
data demonstrate that the usually-accepted notion of precipitation increasing
with elevation does not always hold. Although the three-year averages of the
values from Table 1 show precipitation increasing with elevation, the individual
monthly values generally do not necessarily show this trend. For example, in
June 1996, the reversed trend is true, with precipitation decreasing with
elevation. On a single event basis, the scatter is even greater. The data do suggest
then that caution must be exercised when attempting to distribute precipitation
over a mountainous basin based on valley bottom observations. Further analysis
is required in order to develop useful relationships for distributing precipitation;
such relationships will likely require some knowledge of storm type, storm track
and topography.
41
Granger - Evaporation
Summary
Energy balance data collected at three representative sites (alpine tundra, shrub
taiga and pine forest) in the Wolf Creek Research Basin during four snow-free
seasons are presented and analyzed.
The amounts of net radiation received at the three sites differs significantly, with
the alpine site receiving the least and the forest site receiving the most net
radiation. These variations are due to the large differences in the surface albedo.
As expected, the soil heat fluxes are largest at the alpine site and least at the
forest site.
The seasonal pattern of partitioning of energy to latent and sensible heat fluxes
does not appear to be governed by the seasonal pattern of precipitation, as is the
case in the southern boreal forest where the relative contribution to the latent heat
term is significantly reduced during the dry months. At the higher latitude sites
the forest and shrub vegetation employ a water use strategy for maximum
productivity that is adapted to the energy limited conditions encountered in the
north.
There appears to be no clear relationship between the precipitation at the three
observation sites (the boreal forest in the valley bottom, the shrub taiga above the
tree line and the high elevation alpine tundra), indicating that the application of
an elevation correction factor to precipitation observations made at the climate
station (valley bottom) could lead to significant errors in the estimation of the
precipitation distribution within the basin. Relationships taking into account such
factors as storm type, track and topography must be developed.
Acknowledgements
Funding assistance received through the Canadian GEWEX programme is
gratefully acknowledged. The cooperation of Richard Janowicz and of his
technical staff at the Water Resources Branch in Whitehorse is greatly
appreciated. The collaboration provided by Dr. John Pomeroy and the proficient
technical support provided by Newell Hedstrom and Dell Bayne, are also
recognized and appreciated.
42
Wolf Creek Research Basin: Hydrology, Ecology, Environment
References
Bouchet, R.J., 1963. Évapotranspiration réelle et potentielle, signification climatique.
IAHS Proceedings. Berkeley, CA, Symposium., Publ. No. 62: 134-142.
Granger, R.J. and D.M. Gray, 1989. Evaporation from natural non-saturated surfaces.
J. Hydrology, 111: 21-29.
Granger, R.J., 1991. Evapotranspiration from nonsaturated surfaces. Ph.D. Thesis.
Dept. of Agricultural Engineering, University of Saskatchewan, 141p.
Penman, H. L., 1948. Natural evaporation from open water, bare soil and grass. Proc.
R. Soc. London, Ser.A, 193: 120-145.
Pomeroy, J.W., R.J. Granger, N. Hedstrom, B. Toth, J. Parviainen and A. Pietroniro,
1996. Quantification of Hydrological Pathways in the Prince Albert Model
Forest, 1995-1996 Annual Report. Environment Canada (NHRI) Report to the
Prince Albert Model Forest Association. NHRI Contribution Series CS-96007.
60pp.
Pomeroy, J.W., R.J. Granger, A. Pietroniro, J.E. Elliott, B. Toth and N. Hedstrom,
1997. Hydrological Pathways in the Prince Albert Model Forest, Final Report to
the Prince Albert Model Forest Association. Environment Canada, NHRI
Contribution Series No. CS-97004. 154p + appendices.
Priestley, C.H.B. and R.J. Taylor, 1972. On the assessment of surface heat flux and
evaporation using large-scale parameters. Monthly Weather Rev., 100: 81-92.
43
Granger - Evaporation
44