of prairie snowpack Spatial and temporal variations of the albedo

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Spatial and temporal variations of the albedo of prairie snowpack

A.D.J. O'Neill and Don M. Gray Saskatoon, Saskatchewan

CoZZege of Engineering, University of Saskatchewan,

ABSTRACT: In this paper, the results of studies conducted on the temporal and spatial variability of the albedo of shallow Prairie Snowpacks are presented. The albedo of these snowpacks is shown to remain relatively constant during the melt-free period; although affected by individual snow event activity. Evidence also shows that both "point" and "spatially-averaged" albedo values decay rapidly with time during the melt period in a manner quite dissimilar from the time-decay of albedo usually assumed for deep mountainous packs.

The agreement obtained in the temporal variation of albedo of the "spatially-averaged" values and "point" measurements suggest that the "point" values may be extrapolated to provide realistic estimates of the albedo over a watershed.

RESUME: Dans cette communication, on présente des résultats d'études faites sur la .variabilité temporelle et spatiale des neiges tassées, peu profondes, des Prairies. On démontre que llalbedo de ces neiges tassées reste relativement constant pendant la période hors fonte bien qu'il soit influencé par les impondérables liés aux neiges. On donne des preuves que les valeurs ponctuelles et les valeurs moyennes spatiales de l'albedo décroissent rapidement avec le temps pendant la période de fonte, dlune manière tout 2 fait différente de celles habituellement supposées dans le cas des neiges profondes tassées de montagne.

L'accord obtenu entre les valeurs ponctuelles et les valeurs moyennes spatiales des variations temporelles de 1' albedo suggère que

les valeurs ponctuelles peuvent être extrapolées pour donner des évaluations réalistes de l'albedo d'un bassin.

INTRODUCTION

Successful development of watershed snowmelt models that are based on the Energy Budget approach requires knowledge of both the spatial and temporal variability of the snowpack specifically during the snowmelt period. In many cases, reflected energy represents a major component in the Energy Budget. During snowmelt, albedo values may change in the range between 90 per cent and 20 per cent. Because

reflected energy is usually evaluated simply as the product of the albedo and the incident short-wave radiation to the surface, any error in the albedo term will result in a corresponding absolute error in reflected energy as a linear function of the incoming radiation. It is, therefore, important that the temporal and spatial variations in albedo be known. In this paper, an attempt is made to quantify these relationships for shallow snowpacks of the Canadian Prairies,

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INSTRUMENTATION

Study of spatial variability in albedo is most readily approached by utilizing airborne sensing techniques whereas temporal variability of the parameter may be amenable to measurement by either surface-based point measurements or airborne measurements. In the studies, Kipp and Zonen (hemispheric) Pyranometers were used for surface-based measurements. Airborne sensing was carried out with a specially constructed beam radiometer described by O'Neill and Gray

Dl

Over a homogeneous, isotropic, reflecting surface the relationship between fluxes measured by hemispheric and beam sensors i5,

FH

=

Fg/sin2ß

=

TI ₍₁₎

where FH

=

the f h x measured by the hemispheric sensor, FB

=

the flux measured by the beam sensor,

I

=

the radiant intensity of the reflected radiation, and f3

=

the half-angle of the beam sensor.

A narrow-beam system is incapable of measuring specular components of reflected solar radiation from level terrain except at llvery high"

sun angles. Thus, any departure from isotropic in the reflective properties of the ground surface means that the beam albedo and hemispheric albedo are no longer related by the 11sin2ß'1 factor.

Dutton [Z] in analysis of data on the reflection of artificial light from snow, obtained by Middleton and Munga11 [3], concluded that the specular component of reflection from new snow would result in a difference of less than 6.5 per cent in the values measured by compa- rable beam and hemispheric sensors.

The beam radiometer used in the study was calibrated by comparing its output with that from a Kipp and Zonen Pyranometer when both were inverted side by side over a uniform snow surface. The cali- bration was performed near solar noon and some compensation for specular effects was, therefore, implicitly obtained from the cali- bration procedure.

Dutton [4] has clearly demonstrated the advantages of beam sys- tems in a study of the spatial variability of albedo. He concluded that the use of wide-beam or hemispheric instruments presents a vir- tually hopeless problem for measurement of isolated snow distribution features. Further, he stated that, "specially-designed instruments with fast, time-response are needed to measure the values of isolated features on the ground and to make studies of variations within small areas". The beam system described by D'Neill and Gray [l] has a high relative accuracy, linearity of response to intensity variations, a micro-second time constant, and is light, portable, and cheap to construct.

An indication of the response of the beam system to small-scale spatial variability in albedo is shown in Figure 1. In this figure, the large differences in albedo caused by vegetative types (deciduous clumps of trees and open fields) is clearly evident together with the smaller scale variations that occur over similar vegetative surfaces.

Data in this figure were obtained at a flight altitude of approximately 45m at which the field of view of the sensor is approximately

154m2.

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Exp e Y.imen ta

Z

Proce dure

Flights were made with a light, commercial, fixed-wing aircraft.

The beam sensor was mounted facing downward in the aircraft and used to measure reflected radiation. A simple photo-diode Solarimeter mounted on an adjustable bracket attached to the roof of the aircraft was used to obtain simultaneous records of the changes in intensity of incident global short-wave radiation during each flight. Some difficulty was experienced in maintaining the roof sensor level and hence these readings were only indirectly used in the final analysis of results. Ground-based readings of incoming radiation, in combina- tion with airborne measurements of reflected radiation, were used for albedo calculations. In addition, a 35" camera was mounted in the aircraft beside the beam radiometer and a series of overlapping photographs obtained in each flight. The photographs were used in interpretation of the albedo data.

Flights were made with the aircraft operating at a cruising speed of about 160km h-l, over typical Prairie terrain encountered in the semi-arid region of Canada. Approximately two-thirds of the area is under cultivation and one-third is summer fallow. Only isolated trees and farms appeared along the flight path and the maximum terrain elevation variation was approximately 75m.

imately 915m above ground level. Final selection of this altitude was based on the desire to first examine the large-scale structure of albedo and to provide a stable measurement platform by flying above the level of ground-induced turbulence. The area "seen" by the beam sensor with the aircraft operating at this altitude is approximately 6 hectares. Spatial averaging of albedo over an area of this extent has the advantage of reducing the influence of anomalous specular reflection from individual slopes within the area seen by the sensor, and provides a realistic average value of the albedo parameter, which may be applied in the Energy Budget approach for estimating runoff from a watershed. The method, however, does impose severe wavenumber restrictions on investigations of smaller scale spatial vari ab i li ty .

cloud edge effects, as experienced by Bauer and Dutton [5], and were confined to a period near solar noon.

Airborne measurements were carried out at an altitude of approx-

All flights were made on clear days to minimize the problem of

DATA EXTRACTION AND ANALYSIS

The extraction of data at regularly spaced intervals from an analogue recording should be conducted so as to provide adequate representation o f the signal output. Rational selection of the time- interval for data extraction may be based on; knowledge of the

frequency response characteristics of the measuring-recording system or on detailed preliminary examination of the data. The beam radio- meter system has a two-dimensional input (solar radiation reflected from a circular area on the ground) and a one-dimensional output (the trace on the recorder chart).

(see Dutton [4]). Thus, an empirical approach was taken in selecting the data-extraction interval.

1.75 seconds was extracted from the airborne albedo measurements taken on 22 January, 1971. This series was subjected to a power spectrum analysis using the Tukey method [6] and the results suggested that a 3.5-second archiving interval would be acceptable. A

The transfer process is complicated A special data series at intervals of

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3.5-second archiving interval implies folding about a frequency of approximately O. 25 cycles per second. The variance (power) contrib- uted to the signal by such folded frequencies was found to be less than 5 per cent of the total.

Figure 2.

A typical plot of the data obtained from each flight is shown in

AIRBORNE DATA

-

BEAM ALBEDO

Me an Coeff. Coeff.

Date Albedo Variance of of

(%I

Skewness Kurtosis

22/1/71 72.6 42.2 -0.3 6.6 1/3/71 62.3 28.1 -1.2 5.7 6/4/71 49.7 292.6 -0.6 2.2 8/4/71 43.3 208. O O 2.0 12/4/71 16.6 20.8 0.7 3.1

VARIATION OF ALBEDO WITH HEIGHT OF SENSOR

SURFACE DATA Climatological Station Albedo (Solar Noon)

(%I

74.3 63.3 48.7 22.5”

21.9 Bauer and Dutton [5] noted a height dependency in albedo which they attributed largely to the increase in global shortwave radiation with altitude and partly to a corresponding decrease in the measured intensity of reflected shortwave radiation. In this study, use of ground-based measurements of incident global shortwave radiation eliminated the first effect. Tests carried out by successive flights over a frozen, snow-covered lake yielded the data presented in Figure 3. These data suggest that the use of the beam measurements give reflected radiation within 2 per cent of ground-level values.

SEASONAL VARIATION IN SNOWPACK ALBEDO

Figure 4 shows the changes with time in the point and spatial albedo measurements over a typical Prairie snowpack during the winter and spring of 1971. Table 1 summarizes the major statistical features of the time-variance of the albedo measurements.

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free, the variance decreased again, skewness became positive, and the kurtosis approached normal,

averaged and point Ifsolar noon" albedo measurements.

the point measurements, the albedo values exhibit an erratic pattern during the melt-free period because of the occurrence of fresh snow- falls. During this period the albedo rarely fell below 70 per cent and frequently exceeded SO per cent.

albedo decreased at an accelerated rate until the ground was snow- free. Further, in considering only the point measurements, it can be concluded that the rfconcave-downwardlf shape of the curve representing the temporal variation of albedo of shallow Prairie snowpacks during the period is indicative of the increasing control exerted by the ground on the albedo as the snow depth decreases.

In comparing the general shape of the spatially-averaged values with point measurements it is interesting to note that the values are

in fairly close agreement and their variation with time follows the same characteristic shape. The spatial albedo values represent inte- grated values which are influenced by many factors such as: depth and extent of snow cover, slope, aspect, etc. Figure 2 illustrates the spatial condition on April 6, 1971 (mid-way through the melt process) in which the albedo of a completely snow-covered area was about 60 per cent while that of nearby areas of shallower and patchy snow cover was considerably lower. Where deeper accumulations persist, the albedo apparently remains relatively high during much of the melt period. It is suggested that the clean conditions and small grain-size of Prairie Snowpacks may explain the fact that in melt season albedo of the deeper residual snow patches is higher than that commonly-experienced in mountainous areas.

the percent of snow cover per se but also the depth-of-cover is an important factor which controls the albedo of Prairie Snowpacks whereas the normal metamorphic processes may play a secondary role.

At this stage of the study, the absolute magnitude of each factor as it affects spatial values cannot be evaluated.

On the basis of the results obtained it may be postulated that albedo measurements taken from one or more representative points within a Prairie Watershed may provide reasonable estimates of the time-variability of albedo. That is, the results give credence to the possibility of extrapolating point measurements in space. This result, in itself, provides the hydrologist the potential of

successful application of Energy Budget models for snowmelt predic- tion and forecasting in the Prairie Region.

Figure 4 illustrates the temporal variation in both spatially- As indicated by

During the melt period the

It appears that not only

COMPARISON OF ALBEDO VALUES FROM DEEP AND SHALLOW SNOWPACKS The preceding results suggest that the use of specific "albedo- time models", for example, those developed for deep, mountainous packs may lead to significant errors in albedo calculations if di- rectly applied to the Prairie situation. To demonstrate the differences which may exist, the change in albedo with time for the Prairie condition was plotted in the more "conventional" manner with the age of snow (during the melt period) together with the exponential decay curve for deep snowpacks constructed from information taken from the United States Army, Corps of Engineers [7] (see Fig. 5). In comparing the curves for the two cases, it is evident that the shapes are distinctly different; the shallow pack exhibiting a much more rapid decay than the deep pack.

1

so

Further the errors which would

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evolve if either curve were "universally" applied to all snowmelt conditions are obvious. Not unlike most other empirical hydrologic relationships, the results indicate that judicious care and caution must be exercised in extrapolating "albedo-time" snowmelt relationships obtained from a particular physiographic region to another physiographically dissimilar region.

SUMMARY The results of the study suggest:

1. The albedo of shallow Prairie Snowpacks remains relatively constant during the melt-free period, although affected by individual snow occurrences.

2. Temporal variations or "time-decay" of the albedo of shallow snowpacks show an accelerated rate of change with time quite dissimilar to the shape of the relationship usually assumed for deep packs during snowmelt.

3.

values of shallow snowpacks are in close agreement and therefore suggest that under Prairie conditions "point" measurements may give realistic estimates of spatially averaged values.

Temporal variations of point and spatially averaged albedo

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial assistance provided by the Canada Department of the Environment (Atmospheric Environment Service, Toronto, and Inland Waters Directorate, Ottawa) in support of this study.

RE FE REN CES

O'Neill, A.D.J., and GRAY, D.M. (1972). Solar radiation pene- tration through snow. Proc. of the International Symposia on the Role of Snow and Ice in Hydrology, Banff, Canada.

DUTTON, J.A. (1962). ATI addition to the paper "Albedo varia- tions measured from an airplane over several types of surface".

J. Geophys. Res., 67, 13, pp. 5365-5366.

MIDDLETON, W.E.K., and MUNGALL, A.G. (1952). The luminous directional reflectance of snow. J. Opt. Soc. Am., 42, 8, DU'ITON, J.A. (1962). Space and time response of airborne radiation sensors for the measurement of ground variables. J.

Geophys. Res., 67, 1, pp. 195-205.

BAUER, K.G., and DUTTON, J.A. (1962). Albedo variations measured from an airplane over several types of surface. J , Geophys. Res., 67, 6, pp. 2367-2376.

BLACKMAN, R.B., and TUKEY, J.W. (1959). The measurement of power spectra. Dover Publications, Inc., New York, N.Y.

CORPS OF ENGINEERS, U.S. ARMY (1956). Snow hydrology. U.S.

Dept. of Commerce, Office of Technical Services.

pp. 572-579.

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DISCUSSION

D.M. Hockwood (U.S.A.)

-

I was particularly interested in the final graphic representation of the comparative data on the change of snowpack albedo with time. As you pointed out, the USCE (United States Corps of Engineers) graph represents the change in albedo of a deep mountain snowpack, whereas your graph represents the change for shallow prairie snowpacks. I congratulate Mr. O'Neill for doc- umenting these very interesting and worthwhile data.

A.M. Mustapha (Canada)

-

During the melt period did you have any corresponding point measurements of snow depth?

A.D. J. O'Neill (Canada)

-

No, we did not record the depth on a day-to-day basis. We do have some curves o f albedo versus depth which will be discussed in our paper in the next Unesco Session.

K.S. Davar (Canada)

-

Fig. 5 in your paper demonstrates that although the USCE curve of albedo variation with time permits an exponential estimation to be made, your measured curve €or the Prairies shows that sudden changes do occur (as on day 8). Thus, the only way of obtaining reliable values for albedo would be to use measured values. Would you agree?

A.D.J. O'NeiZZ (Canada)

-

Depth variations on the prairies can range from zero on wind-blown parts of summer fallow fields to several feet in gullies, so the ground effect on point albedo will be quite variable. However, spatial average albedo measurements from our aircraft agreed very well with the point measurements taken over a snow- field that was 1-1.5 ft deep.

186