Solar Energy, VoL 31, No, 2, pp. 233-234, 1983 0038-092X/83[080233-02503.0010 Printed in Great Britain. Pergamon Press Ltd

TECHNICAL NOTE

E s t i m a t i n g pho tosyn the t ica l ly ac t ive radiation from measured solar irradlance

WILLIAM J. BLACKBURN Crop Production Division, Regional Development and International Affairs Branch, Agriculture Canada, Ottawa,

Canada KIA 0C5

and

JOHN T. A. PROCTOR Department of Horticultural Science, University of Guelph, Guelph, Canada N 1G 2WI

(Received 10 July 1982; in revised/otto 21 July 1982)

l. INTRODUCTION Studies in agricultural science often require irradiance data to quantify the energy regime of a particular crop canopy, tree or plant. Although total solar irradiance (K ~ ) can often be obtained from standard meteorological stations or alternatively measured on-site, values of the photosynthetically active radiation (PAR) in the waveband 0.3-0.7 ttm are usually not available. For several agricultural applications, especially crop photosynthesis, a knowledge of the latter is often crucial. Hence, a reliable method of estimating PAR from available irradiance data is of interest.

Monteith[l] suggests for practical purposes that the energy content of photosynthetically useful radiation can be taken as half of the total solar irradiance. Szeicz[2, 3] showed that this was a good approximation regardless of the atmospheric aerosol and water vapour concentrations. Subsequent work by Suckling et al. [4] applied this relationship to derive empirical relationships between K ~ and PAR to estimate PAR above a tree canopy.

In this study, hourly measurements of K ~, were made above a dwarf apple canopy on 186 days between 1 November 1980 and 14 June 1981 in a research orchard at Guelph, Ontario (43°3YN, 80°IYW). In addition, solar energy in the waveband excluding PAR was measured hourly and values of PAR were obtained by residual. These data were used to examine the diurnal and seasonal variation of PAR as a component of total solar irradi- ance.

2, METHODS Measurements of K ~ were made with an Eppley Precision

Spectral Pyranometer. A second Eppley pyranometer fitted with an RG695 hemispheric filter (formerly RG8) was used to measure the solar energy beyond 0.7 ~m; hence, the solar spectrum is subdivided into near-IR and visible components. Values of PAR were obtained as the difference between the two measured irradiances. Both instruments were mounted and levelled on a tower above the tree canopy, each with an unobstructed view of the atmosphere. All sensor signals were integrated on an hourly basis and recorded on cassette tape using a Campbell CR5 automatic data logger. The radiation sensors were calibrated at the National Atmospheric Radiation Centre prior to the measurement period.

3. DISCUSSION Daily totals of K ~ and PAR were computed for all 186 days

representing a variety of sky conditions including overcast, partly cloudy and virtually cloudless days in both summer and winter. Figure i shows the relationship between daily totals of PAR and K ~ for all days on which measurements were made. Linear regression and correlation analyses were performed to derive empirical relationships between the two irradiances. Results presented in Table 1 show that PAR and K J, are strongly linearly correlated (r = 0.99) on a daily basis, with PAR

20

15

N

E io

5 I0 15 20

K~, (MJ m z day ~ )

0 25 30 B5

Fig. 1. Correlation between daily totals of photosynthetically active radiation, PAR, and total solar radiation, K ~, for 186 days between 1 November 1980 and 14 June 1981.

233

234 Technical Note

5 0 0

4 0 0

N 3 0 0

'E

~ 200

] o o

i I

I i

: I

i

T I

- I I

t I ! ~ W I N T E R - C L O U D Y ( 2 4 F e b r u o r y ) • - • W I N T E R - C L E A R ( 17 M o r c h )

o S U M M E R - C L O U D Y ( l O M o y ) i • S U M M E R - C L E A R ( 17 M o y )

Ioo zoo soo 400 500 600 700 800 900 1000

K¢ ( W m -z )

Fig. 2. Correlation between hourly values of photosynthetically active radiation, PAR, and total solar radiation, K ~, for selected cloudy and clear days in summer and winter during 1980 and 1981.

Table 1. Linear regression and correlation analyses results for photosynthetically active radiation and solar

irradiance

findings agree very well with those reported by McCree[5] who showed that the active fraction of energy between 0.3 and 0.7 tzm as a per cent of K ~ varied with sky conditions, ranging from 47 per cent on a clear day with high sun to about 58 per cent in very heavy overcast. n b s .e . r

Daily totalst 186 0.47 0.23 0.99 Hourly data:~ 1. Winter-cloudy 8 0.50 4 0.99 2. Winter-clear 12 0.44 5 0.99 3. Summer-cloudy 13 0.58 1 0.99 4. Summer-clear 15 0.45 8 0.99

n = sample size, b =average slope for best fit line through the origin, s.e. = standard error, r = correlation coel~cient.

tUnits for s.e. are MJ m-2 day ~. ~:Units for s.e. are W m 2.

on average, comprising approximately 47 percent of K ~[. The standard error is negligible (0.23 MJ m 2 day t).

The analysis was repeated using hourly data for four selected days (24 February, 17 March, 10 May and 17 May 1981) which were assumed typical of winter cloudy, winter clear, summer cloudy and summer clear days, respectively. Figure 2 shows that the correlation between the two fluxes remains high on an hourly basis and the scatter in the data is small. The statistical results are given in Table 1. On average, hourly values of PAR ranged between approx. 44 and 58 per cent on individual days depending on cloud amount. The results suggest that the PAR proportion of K J, increased on average from 45 to 54 per cent for days when overcast conditions are approached. This increase is higher than that reported by Suckling et aL[4] in describing the difference between clear and cloudy days where PAR was about 1 per cent higher for cloudy skies than for clear skies. The latter relation- ship was derived from a limited data base which included 8 bright summer days and 1 overcast summer day. However, the current

4. CONCLUSIONS

]'his study shows that: (1) On a daily basis, PAR is approximately 47 per cent of K [ . (2) On an hourly basis, PAR/K ~ increases with cloud amount

from 45 per cent for relatively clear (cloudless) skies, to around 54 per cent for cloudy skies.

Acknowledgements--We are indebted to Mr. Dean Louttit of the University of Guelph for field assistance during the measurement program, Mr. Wayne Pierce of the Simcoe Research Station, OMAF, for loaning necessary instrumentation and to Dr. J. A. Davies of McMaster University for reviewing the manuscript. The study was supported by the Ontario Ministry of Agriculture and Food through a Provincial Lottery Research award.

REFERENCES

1. J. L. Monteith, Principles o[ Environmental Physics. Arnold, London (1973).

2. G. Szeicz, Solar radiation for plant growth. J. AppL Ecol. 11, 617-636 (1974).

3. G. Szeicz, Field measurements of energy in the 0.4-0.7 micron range. In Light as an Ecological Factor (Edited by R. Bain- bridge, G. C. Evans and O. Rackman), pp. 41-52. Blackwell, Oxford (1%6).

4. P. W. Suckling, J. A. Davies and J. T. A. Proctor, The transmission of global and photosynthetically active radiation within a dwarf apple orchard. Can. J. Bot. 53, 1428-1441 (1975).

5. K. J. McCree, A solarimeter for measuring photosynthetically active radiation. Agric. MeteoroL 3, 353 (1%6).