Ratio of PAR to broadband solar radiation measured in Cyprus

Agricultural and Forest Meteorology 121 (2004) 135–140

Ratio of PAR to broadband solar radiation measured in Cyprus C.P. Jacovides a,∗ , F.S. Timvios a,b , G. Papaioannou a , D.N. Asimakopoulos a , C.M. Theofilou b a

Laboratory of Meteorology, Department of Physics, Division of Applied Physics, University of Athens, Panepistimioupolis, Building PHYS-V, Athens 157-84, Greece b Department of Agrometeorology, Meteorological Service of Cyprus, Nicosia, Cyprus Received 6 June 2002; received in revised form 6 October 2003; accepted 9 October 2003

Abstract The relationship between the two radiant fluxes is studied from almost a 3-year data archive of hourly photosynthetically active photon flux (QP ) and global solar irradiance (RS ) performed at Athalassa, Cyprus. These data are used to determine temporal variability of the ratio (QP /RS ) and its dependence on sky conditions. The seasonal variation of the ratio obtained from daily data ranges from 1.942 E MJ−1 (summer) to 1.892 E MJ−1 (winter) with an annual mean value of 1.919 E MJ−1 . The ratio increased from 1.865 to 2.01 E MJ−1 (daily values) or from 1.878 to 2.197 ␮E J−1 (hourly values), as sky conditions changed from clear to overcast. Effective atmospheric parameters such as sky clearness, brightness and path length were found to cause substantial changes to the PAR fraction. © 2003 Elsevier B.V. All rights reserved. Keywords: Photosynthetically PFD; Solar irradiance; Atmospheric parameters; Eastern Mediterranean

1. Introduction Photosynthetically active radiation (PAR) contributes significantly in comprehensive studies of radiation climate, remote sensing of vegetation, radiation regimes of plant canopy and photosynthesis. The PAR radiation covering both photon and energy terms lies between 400 and 700 nm or 380–700 nm in the solar spectrum (McCree, 1972). However, today’s more commonly accepted spectral interval 400–700 nm does not cause misunderstanding (Ross and Sulev, 2000).

∗ Corresponding author. Tel.: +30-210-727-6931; fax: +30-210-729-5281. E-mail address: [email protected] (C.P. Jacovides).

Most published experimental results (Table 1) use measured values of global PAR (RP ) and global solar radiation (RS ) to determining the PAR fraction of the broadband solar radiation. In the literature, there exist three distinct measuring techniques for determining PAR: (i) spectrally, by integrating spectral irradiance distribution measurements over the waveband 400–700 nm; (ii) indirectly, through combined filtered data; and (iii) directly, via spectral PAR measurements (400–700 nm) by means of quantum sensor (QP ). For further detail see the review article of Ross and Sulev (2000). Published values for the PAR fraction of global irradiance are around 0.45 or 2 ␮E J−1 for photon efficiency. Nevertheless, the range of the PAR fraction suggests the desirability for recalibration accounting for local climatic differences. Thus, the present analysis aims to quantify temporal vari-

0168-1923/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2003.10.001

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Table 1 PAR photon flux density to broadband solar irradiance correlations, at different locations References

Location

Wavelength limits (nm)

QP /RS

Britton and Dodd (1976)

Texas, USA

400–700

2.148–2.65 long photoperiod days 2.057–2.85 short photoperiod days

Howell et al. (1983)

California, USA

400–700

2.052, daily values 2.034–2.057, winter–summer values

Hansen (1984) Rao (1984)

Aas, Norway Oregon, USA

385–695 385–695

2.24–2.56, daily clear-cloudy 2.088, daily values 2.084–2.088, daily winter–summer 2.025–2.207, daily clear-cloudy

Alados et al. (1996) Papaioannou et al. (1996)

Almeria, Spain Athens, Greece

400–700 385–695

1.874–2.011, daily values 1.990, hourly values 1.915–2.038, hourly winter–summer 1.956–2.029, hourly clear-cloudy

Udo and Aro (1999)

Ilorin, Nigeria

400–700

2.079, daily clear 2.102–2.148, hourly clear-cloudy

This analysis

Athalassa, Cyprus

400–700

1.919, daily values 1.865–2.010, daily clear-cloudy 1.878–2.197, hourly clear-cloudy

ations of the PAR efficiency for various atmospheric conditions at Athalassa, Cyprus (35◦ 15 N latitude, 33◦ 40 E longitude, 165 m a.m.s.l.), and to identify reasons for such variations.

2. Experimental set-up and data analyses The analysis is based on hourly radiometric data collected at the semi-rural Athalassa site, Cyprus, for almost a 3-year period (September 1997–May 2000). It is worth noting that this time interval lies in a dry period observed in eastern Mediterranean basin during last decade, i.e., 1992–2001. Global solar irradiance was measured using a Kipp & Zonen model CM-11 (Delft, The Netherlands) while another Kipp & Zonen CM-11 with a polar axis shadowband was used to measure diffuse irradiance. The PAR photon flux was measured with a Licor quantum sensor LI-190SA (Lincoln, Nebraska, USA). The hourly data were checked for inconsistencies to eliminate problems associated with questionable measurements. Due to cosine response problems this analysis is limited to cases with solar elevation h > 5◦ . Solar radiant measurements have an estimated experimental error of 2–3%, while the quantum sensor has a relative error of less than 5%.

The well-known indices, sky clearness ε and brightness ∆ (Perez et al., 1990) are used to characterize the sky conditions. They are computed for each hour from irradiance values and defined as ε= ∆=

Rd + Rb Rd Rd R0n sin h

(1)

(2)

with Rd and Rb being the diffuse and direct beam irradiance on a horizontal surface derived from the measurements of diffuse and global irradiance and R0n being the extraterrestrial irradiance. Three sky categories have been considered: (i) overcast skies (∆ < 0.1, ε < 1.2); (ii) clear skies (∆ < 0.1, ε > 5.2); and (iii) intermediate skies (0.2 < ∆ < 0.3 and 1.2 < ε < 5.2). Linear regression analyses between QP and RS fluxes and several statistical indices are used to evaluate the correlation results. The correlation accuracy is evaluated using regression analysis of estimated versus measured values in terms of the root mean square error (RMSE), the mean bias error (MBE), the coefficient of determination R2 , and the standard deviation (S.D.). RMSE and MBE indicators are given as the percentage of the mean measured PAR value.

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Fig. 1. Measured daily values of photosynthetic photon flux density (QP ) and broadband solar radiation (RS ), in Athalassa, Cyprus.

3. Results and discussion The available hourly values of solar irradiance components are grouped into daily, monthly, and seasonal data sets. It is noted here that the ratios are calculated for each hour and then averaged for each day. Fig. 1 shows the daily variability of both fluxes, RS and QP , for the period of measurements. It is clear that QP is highly correlated with global solar radiation RS sug-

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gesting further that daily QP values can be obtained directly from RS . It has generally been the practice to express the measured hourly or daily spectral irradiance as fraction of the broadband solar radiation to detect systematic relationships between the two. Here, following several workers (e.g. Howell et al., 1983; Papaioannou et al., 1996; Udo and Aro, 1999) the linear relation Y = αX, is applied. Results of monthly and seasonal correlations between QP and RS are given in Table 2. It is clear that the monthly daily ratios vary from 1.945 (E MJ−1 ) in September to 1.860 (E MJ−1 ) in March and November, resulting in an annual variability of 4%, which is less than the measurement uncertainty. These low values are related to hazy conditions produced by dust loads brought by prevailing winds (SE and SW from the Saharan and Arabian desserts) and transition atmospheric conditions. Usually, the rainy season ends during March as the dry season sets in; whereas, during November the rainy season is just setting in due to active synoptic systems (depressions) approaching and traversing the island more frequently. In contrast, the high September value is due to the increased water vapor levels usually observed during this period. Water

Table 2 Ratio of PAR photon flux density to broadband solar radiation for individual months and seasons Daily values January February Marchc Aprilc Mayc Junec Julyc Augustc Septemberc Octoberc November December All data Clear days Intermediate Overcast Cold period (November–February) Warm period (May–September) a

Slope, α (E MJ−1 )

R2

RMSE (%)a

MBE (%)a

S.D.a

Energy ratiob

80 76 89 86 89 60 60 58 85 85 84 82

1.908 1.894 1.860 1.907 1.927 1.938 1.928 1.911 1.945 1.938 1.861 1.898

0.990 0.994 0.992 0.992 0.996 0.998 0.992 0.990 0.992 0.991 0.994 0.995

3.36 2.91 3.77 3.25 2.26 1.07 1.89 2.41 2.71 2.92 3.48 2.57

0.008 0.0 0.030 0.00 0.00 0.003 0.003 0.008 0.003 0.001 0.032 0.005

0.018 0.025 0.039 0.026 0.011 0.026 0.027 0.025 0.018 0.021 0.037 0.023

0.418 0.414 0.407 0.417 0.422 0.424 0.422 0.418 0.426 0.424 0.407 0.415

934 189 492 79 267 392

1.919 1.865 1.926 2.010 1.892 1.942

0.997 0.994 0.996 0.996 0.994 0.994

3.11 2.62 2.98 4.40 3.43 2.57

0.011 0.007 0.021 0.002 0.0024 0.002

0.021 0.011 0.022 0.036 0.030 0.014

0.420 0.408 0.421 0.440 0.414 0.425

Number of days

For PAR estimates. Values have been obtained using McCree (1972) factor. c Dry season. b

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Fig. 2. Hourly correlations between photosynthetic photon flux density (QP ) and broadband solar irradiance (RS ), for three sky conditions are given. Regression lines shown correspond to clear (solid), intermediate (short-dashed), and overcast (long-dashed) skies, respectively.

vapor through absorption processes markedly affects the longer wavelengths, leaving the spectral PAR portion unaltered, thus, decreasing broadband solar radiation to a much greater extent than the spectral PAR; therefore, the ratio increases with increasing water vapor content (Alados et al., 1996). Table 2 indicates also an annual mean ratio value of 1.919 (E MJ−1 ), a warm period mean of 1.942 (E MJ−1 ) and a cold period mean of 1.892 (E MJ−1 ). Results of the linear regression between daily values of QP and RS are given in Table 2, while hourly correlations are displayed in Fig. 2. The ratio increases slightly with sky conditions changing from clear to overcast; daily ratio values varying from 1.865 to 2.01 (E MJ−1 ), suggest that the ratio on clear days is about 7% lower than that on overcast days. The respective hourly ratios (Fig. 2) vary from 1.878 to 2.197 (␮E J−1 ), indicating further that the ratio on clear days is 14.5% lower than that on overcast days. It is concluded, therefore, that clouds reduce the hourly global solar radiation proportionally more than the spectral PAR portion. The measured variation for both daily and hourly ratios found here is in line with the findings of Britton and Dodd (1976), Howell et al. (1983), Papaioannou et al. (1996) and Udo and Aro (1999). Another important consideration is the diurnal ratio pattern on specified clear days (Fig. 3). The most pronounced differences are expected to be between summer and winter solstices (21 June 1998 and 22 December 1998) and/or between spring and autumn equinox days (21 March 1999 and 21 September

1999). All days have similar sky parameters, i.e., ε ∼ 6.8, ∆ ∼ 0.10. Definite daily patterns during daylight hours are evident; higher ratio during sunrise/sunset and lower at noon hours. Having in mind Cyprus climatic characteristics, a more likely scenario is that surface heating causes an increase in surface moisture content in summer, thus, enhancing surface humidity-induced absorption processes at longer wavelengths, leading finally to higher ratios during this period. Similar results have been reported by Howell et al. (1983) and Udo and Aro (1999). Low levels of March are due to dense haze conditions, which enhance multiple scattering processes in the shorter wavelengths, thus, affecting the PAR

Fig. 3. Daily patterns of the ratio of photosynthetic photon flux dennsity to broadband solar irradiance (QP /RS ) for days throughout the year: winter (䉫) and summer () solstices, spring (×) and autumn ( ) equinoxes, and solar eclipse (䊉).

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Fig. 4. Ratio of photosynthetic photon flux density to broadband solar irradiance (QP /RS ) vs. (a) sky clearness, ε; (b) sky brightness, ∆; and (c) atmospheric depth for the solar beam (sin h). Seasonal sin h variation is indicated by lines: 1, equinoxes; 2, summer solstice; and 3, winter solstice. Mean values and standard deviation intervals are given.

band more than the broadband spectrum. The daily ratio pattern of 11/8/99 corresponding to the last solar eclipse, given in Fig. 3, indicates a substantial ratio jump during this event. To further detect the influence of a particular parameter on the PAR fraction, hourly ratios versus sky clearness (ε), brightness (∆) and path length (sin h), are given in Fig. 4. As shown by several investigators (Alados and Alados-Arboledas, 1999; Gonzalez and Calbo, 2002), atmospheric parameters such as air mass, sky clearness and brightness, and water vapor

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content are identified as the most influential variables on the PAR fraction followed by aerosol load. From Fig. 4a and b, dependency of QP /RS on both sky parameters, (ε) and (∆), respectively, is evident (Alados et al., 1996; Papaioannou et al., 1996). Further, as solar elevation h (sin h) increases the ratio decreases up to h = 32◦ (Fig. 4c); thereon increases with increasing sin h (Mottus et al., 2001). It is noted here that at sin h < 0.5, the leading factor for changing QP /RS is sin h, i.e. reduced moisture levels occur in winter with low h values. At higher h values, sin h > 0.5, other factors dominate, i.e. high humidity levels occur in summer with high h values. Such a behavior is in accord with Alados et al. (1996) findings. Next, most of the experimental results included in Table 1 show quite important site-to-site variation. It is worth noting that though a variety of definitions of PAR wavelength limits appeared in the literature, comparisons are made against to those used either 385–695 nm or 400–700 nm spectral PAR limits. Ratios calculated from both daily and hourly data are lower than any value reported in the literature (Table 1). This can be explained as being caused by peculiar atmospheric conditions observed during the measurement period. However, mean daily ratios calculated for clear to overcast data are in line with Alados et al. (1996) findings for Almeria–Spain, a seashore Mediterranean site. It is notable also that Udo and Aro (1999) hourly cloudy data for Nigeria are in line with the present findings. Nevertheless, individual results for a specific site include their characteristics in expressing the correlation between PAR and broadband solar radiation. As underlined by several researchers detailed determination of the ratio needs local calibration to account for local climatic differences.

4. Concluding remarks The analyses of hourly or daily measurements of PAR photon flux density and broadband solar radiation covering the period September 1997 to May 2000 in Athalassa, Cyprus, are briefly summarized as follows: An annual mean value of 1.919 (E MJ−1 ) has been found for the daily PAR efficiency. The ratio (QP /RS ) exhibits seasonal dependence, high values in summer (1.942 E MJ−1 ) and low and more variable values

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(1.892 E MJ−1 ) in winter. The highest PAR fraction occurred in September (1.945 E MJ−1 ) while the lowest observed in March and November (1.860 E MJ−1 ). The QP proportion of RS increased from 1.865 to 2.010 (E MJ−1 ) for daily values, or from 1.878 to 2.197 (␮E J−1 ) for hourly values, as sky conditions changed from clear to overcast, respectively. The analysis of hourly values also reveals significant diurnal variation of the ratio during daylight hours. The sky clearness and brightness indices and path length caused substantial changes in the PAR fraction.

Acknowledgements The regional editor of the journal, Dr. J.B. Stewart, is highly acknowledged for his comments and suggestions in improving the revised manuscript. References Alados, I., Foyo-Moreno, I., Alados-Arboledas, L., 1996. Photosynthetically active radiation: measurements and modeling. Agric. For. Meteorol. 78, 121–131. Alados, I., Alados-Arboledas, L., 1999. Validation of an empirical model for photosynthetically active radiation. Int. J. Climatol. 19, 1145–1152.

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