Measuring and modelling photosynthetically active radiation in Tibet Plateau during April October

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1 Agricultural and Forest Meteorology 102 (2000) Measuring and modelling photosynthetically active radiation in Tibet Plateau during April October Xianzhou Zhang a,, Yiguang Zhang a, Yunhua Zhoub b a Commission for Integrated Survey of Natural Resources, Chinese Academy of Sciences, P.O. Box 9717, Beijing , China b Institute of Geography, Chinese Academy of Sciences, Beijing , China Received 13 May 1999; received in revised form 12 December 1999; accepted 17 December 1999 Abstract Based on the measured data of spectral solar radiation in Lhasa, Tibet from 15 April to 15 October 1994, the ratio of photosynthetically active radiation (PAR) to solar global radiation was presented, i.e. η PAR =0.439±0.014, and 1 J energy of PAR is equivalent to 4.43 mol quantum. In the climatological estimation of Tibet Plateau, following equations can be used to estimate the daily total PAR energy flux density (MJ m 2 per day) and daily total PAR photo flux density U PAR (mol photon m 2 per day): = ( ln E )Q U PAR = ( ln E )Q Here, Q (MJ m 2 per day) is the daily global radiation. E*=E P 0 /P, E (hpa) is the water vapor pressure at site, P 0 (hpa) is the standard atmosphere pressure at sea level, P (hpa) is the atmosphere pressure at site Elsevier Science B.V. All rights reserved. Keywords: PAR; Tibet Plateau; Solar radiation 1. Introduction Photosynthetically active radiation (PAR) means the solar radiation in the waveband of nm which can be absorbed by photosynthetic system of plants. McCree (1972) showed that the photon flux in this waveband was an accurate estimation of PAR for both natural and artificial sources. Incident PAR is neces- Under the auspices of the National (G ) and CAS Key Project for Basic Research on Tibetan Plateau (KZ951-A1-204; KZ95T-05; KZ95T-06). Corresponding author. Fax: address: zxz@server.cisnar.ac.cn (X. Zhang) sary in order to estimate the intercepted light for the purpose of modeling photosynthesis of single plant leaves or complex plant communities. PAR is the general radiation term that covers both photon terms and energy terms. One is the photon flux density, U PAR, which is defined as the number of the incident photons in the waveband of nm per unit time on per unit surface. The other is the energy flux density,, which is defined as the number of the incident energy in the same waveband per unit time on per unit surface. Generally, PAR are not measured in routine observation but the PAR value is essential to analyze the plant growth and/or to evaluate the conversion from solar energy to chemical energy /00/$ see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S (00)

2 208 X. Zhang et al. / Agricultural and Forest Meteorology 102 (2000) Therefore, the calculation of PAR is very important. PAR is calculated based on the ratio of PAR to the solar global radiation which can be taken as a constant. However, the ratio is not a same figure according to different authors (Moon, 1940; Yocum, 1964; Mc- Cree, 1966; Yocum et al., 1969; Szeicz, 1974; Briton and Dodd, 1976; Stanhill and Fuchs, 1977; McCartney, 1978; Ross, 1981; Stigter and Musabilha, 1982; Rodskjer, 1983; Howell et al., 1983; Rao, 1984; Karalis, 1989; Papaioannou et al., 1993). In fact, the ratio is influenced by meteorological factors and varies with time and site. Therefore, in order to calculate the ratio, it is necessary to establish a estimation model. Different authors have studied in this respect (Zhou and Xiang, 1992; Alados et al., 1996; Zhou et al., 1996). The development of an appropriate model of the ratio could create a large data of solar global radiation without the substantial cost in observation network, especially in Tibet Plateau. Tibet Plateau is called the Third Pole of the earth owing to its significant features, such as high altitude, thin air, good air transparency, strong solar radiation. The solar global radiation is much higher there than that of the plain areas. The solar energy resource ranks the second in the world, only next to the Sahara Desert, so it is very significant and useful to study the solar radiation and its spectral characteristics in Tibet Plateau. In the Tibet Plateau, the spectral measurements before were not much and only focused on direct radiation (Tian et al., 1982). The purpose of this paper is to evaluate the relationship between PAR and solar global radiation in Tibet Plateau. Here we present the analytic results of spectral solar radiation on the basis of measurements recorded in Lhasa, Tibet, including its seasonal characteristics, the variations of the ratio of PAR to global solar radiation, the ratio weather-dependant function, estimation models of the PAR and the conversion coefficient from PAR energy to photon. 2. Materials and methods The measurements were made at Lhasa Agroecosystem Research Station, Chinese Academy of Sciences (Lhasa, Tibet, E, N, 3688 m above sea level) from 15 April 1994 to 15 October Spectral solar radiation measurements were carried out by six spectral radiometers: WG295 ( nm), GG400 ( nm), GG495 ( nm), OG530 ( nm), RG630 ( nm) and RG695 ( nm). GG400 is made in Jinzhou 322 factory in China. The others are Eppley Precision Pyranometers modeling PSP with outer domes of Schott filter glass made in America. The instruments are fixed at the meteorological observing site of Lhasa Agroecosystem Research Station, at 1.50 m above ground level and in a row from south to north without any shading each other. The output was recorded on a data logger made in Jinzhou in China and the sample was collected circulatively once every minute. The instrument exports hourly instantaneous and accumulative values from sunrise to sunset every day. Daily sums of these instantaneous measurements were used to calculate insolation. In order to determine the instrument sensitivity, the filter covers transparency and instrument s sensitivity were comparatively standardized in plain area (Beijing, China) and Tibet Plateau (Lhasa). The sensitivity of the spectral radiometer is standardized by standard direct radiometer with plane filter. The glass model of the filter should be as same as those of the semispherical covers. That is to say the determination method of instrument sensitivity is shading light method with a gobo. 3. Results and discussion 3.1. Distribution characteristics of spectral radiation energy during growing season (April October) For convenience, η λ is taken to express the relative flux density of spectral radiation. It is the ratio of Q λ (the solar radiation flux density with wavelength less than λ) to Q (the total global solar radiation flux density), i.e. η λ = Q λ /Q. Thus, the relative flux density of spectral radiation in waveband of λ 1 λ 2 (η λ )is η λ = Q λ Q = η λ 1 η λ2 (1) η λ can be used to assess the changing characteristics of spectral radiation energy. From the daily data of spectral radiation in Lhasa, the monthly averages of the η λ in six wavebands were calculated (Table 1). The averages of daily η λ

3 X. Zhang et al. / Agricultural and Forest Meteorology 102 (2000) Table 1 Monthly average of daily η λ, relative flux of spectral radiation Color λ (nm) η λ (%) Average April May June July August September October UV Blue-violet Green Yellow-orange Red IR of spectral radiation during growth season are: UV ( nm) 5.3%, blue-violet light ( nm) 15.0%; green light ( nm) 5.7%; yellow-orange light ( nm) 17.0%; red light ( nm) 5.2%; IR ( nm) 51.8%. It is obvious that η λ of spectral radiation during growth season are various. Which is relevant to the monthly changes of the solar elevation, cloud amount, and water vapor content in the air and needs further research. E is in the range of hpa in Lhasa in P 0 is the standard atmosphere pressure at sea level (1013 hpa). P is the daily average value of atmosphere pressure at site, a and b are all statistical coefficients: a=0.3822, b= (see Fig. 2) Estimation model of energy flux density ( ) of PAR From the using the measured data of spectral radiation, the daily PAR can be estimated and the estimation models of PAR are established. Put (MJ m 2 per day), the solar radiation of the waveband of nm, and total solar radiation Q (MJ m 2 per day) in the following equation: = η PAR Q (2) Here η PAR is the PAR coefficient. It should be pointed out that the coefficient is not a constant but varies slightly with site, season and weather conditions. Fig. 1 illustrates the variation of daily η PAR in Lhasa. The average η PAR value with standard deviation is 0.429±0.013, and the variation coefficient is 3.0%. As mentioned earlier, η PAR fluctuates with the changes of meteorological elements. It can be estimated from the following empirical equation: η PAR = a + b ln E (3) Here, E*=E P 0 /P, is the daily average value of water vapor pressure at site. In the observing period, Fig. 1. The variation of daily η PAR in Lhasa, Tibet in Fig. 2. The relationship between the daily η PAR and E in Lhasa, Tibet in 1994.

4 210 X. Zhang et al. / Agricultural and Forest Meteorology 102 (2000) It should be pointed out that due to the limitation of the instrument, the observed PAR waveband is nm in Lhasa, while the usual waveband adopted internationally is nm. This small difference in wavebands bring systematical errors of η PAR. In order to compare with those in other areas, our data from Tibet Plateau were corrected to standard waveband. The PAR irradiances in the waveband of nm and nm were set as Q 1PAR and Q 2PAR respectively. Then κ can be determined as follows: κ = Q 2PAR (4) Q 1PAR Obviously, κ is related to the spectral structure of solar radiation, i.e. relevant to observation time, site, season and weather conditions. In order to obtain an average value, κ, daylight spectral data (Judd et al., 1964) adopted by CIE were used to calculate the κ at different correlation color temperature T cc. From the distribution graph of T cc frequency (F) on the basis of observed data all over the world (Zhou et al., 1996), κ can be obtained from the following formula: κ = (κf) = (5) Fig. 3 shows the relationship of F, T cc and κ. To make the temperature abscissa convergent fast, the temperature reciprocal (common in chromatology) is used as abscissa. The temperature reciprocal is defined as the reciprocal of T cc, whose unit is Mireds (1 Mireds=10 6 /T cc, the unit of T cc is K). Strictly speaking, κ calculated in Eq. (5) is not necessarily suitable to Tibet Plateau. Fortunately, κ is very little value and only varies slightly with T cc. Therefore, κ as a climatological correction coefficient doesn t bring about a big error in the results. So from Eqs. (4) and (5), Eq. (6) can be obtained: Q 2PAR = Q 1PAR (6) By Eq. (6), the spectral radiation in nm can be corrected to that in nm, the standard waveband. After correction, the daily average η PAR value with standard deviation change into 0.439±0.014, the two empirical coefficients a and b change to and in Eq. (3) in Lhasa. It is reported that the observed η PAR value from April to October in Beijing is 0.48 (Zhou et al., 1996), the average η PAR at Yucheng in Shandong Province, China, is 0.45 (Zhou and Xiang, 1992). They all prove the fact that η PAR in Tibet Plateau is not higher than that in plain areas, although the solar radiation is stronger and PAR energy flux density is higher in Tibet Plateau. By the combination of Eqs. (2), (3) and (6), the climatological estimation model of the daily (MJ m 2 per day) can be obtained: = ( ln E )Q (7) Here, Q is the daily solar global radiation (MJ m 2 per day) Estimation model of photon flux density (U PAR ) of PAR There are somewhat inherent relationship between U PAR and which could be described by the following equation: U PAR = u (8) Fig. 3. Relationship between correction coefficient κ, distribution frequency F and correlation color temperature T cc. Here, u is the quanta number in per unit PAR energy and defined as the quantum conversion coefficient with the unit of mol photon J 1. Supposing a monochromatic light with wavelength λ, u λ, the quanta number per unit energy, is λ/nhc. N is mol 1, the Avogadro constant, h

5 X. Zhang et al. / Agricultural and Forest Meteorology 102 (2000) is J s, the Planck constant, and c is ms 1, the light velocity. For the incident solar global radiation down to the ground, u can be expressed by the following equation: u = U PAR = 1 Nhc λ Q λ dλ (9) Here, Q λ is the spectral irradiance. In calculation, Eq. (9) can be written in the other form: u = U PAR = 1 Nhc ki=l ( λi Q λi ) (10) Here, Q λi is the irradiance in waveband of λ i, λ i is the mean wavelength in the waveband, κ is the divided waveband number of PAR. In our spectral measurement of solar global radiation in Lhasa, the waveband nm was divided into four parts: nm, nm, nm and nm. That is to say κ=4, λ 1 =444 nm, λ 2 =505 nm, λ 3 =575 nm, λ 4 =660 nm, Q λ1 =Q 400 Q 488, Q λ2 =Q 488 Q 522, Q λ3 =Q 522 Q 627, and Q λ4 =Q 627 Q 693. Here, Q 400, Q 488, Q 522, Q 627 and Q 693 are the spectral irradiance radiation at the wavelength from 400, , 627, and 693 to 3000 nm, respectively. u can be obtained in the following equation: u = 1 Nhc λ 1 (Q 400 Q 488 ) + λ 2 (Q 488 Q 522 ) +λ 3 (Q 522 Q 627 ) + λ 4 (Q 627 Q 693 ) Q 400 Q 693 (11) Using the measured data of spectral radiation in Lhasa, Tibet, u can be easily calculated with Eq. (11). In calculating, the daily values were used. The results show that the daily value u are very close to each other (Fig. 4). The average value u and standard deviation is 4.43±0.016 mol photon J 1 with variation coefficient 0.4%. This means that in April October in Lhasa, a constant PAR quantum conversion coefficient i.e. u=4.43 mol photon J 1 can be used to convert PAR energy into quantum. In plain areas, McCree (1972) reported that the ratio of the photon flux to the energy flux was 4.57 mol photon J 1, approximately 3% higher than that u in Lhasa, Tibet Plateau. With the combination of Eqs. (7), (8) and (11), the climatological estimation model of the daily total PAR Fig. 4. The variation of daily average u values in Lhasa, Tibet in quantum U PAR (mol photon m 2 per day) can be determined: U PAR = ( ln E )Q (12) Here, Q (MJ m 2 per day) is the daily solar global radiation. 4. Conclusions From the measurement of spectral solar radiation in Lhasa, Tibet in the period of April to October in 1994, the relative flux densities of different wavebands of solar global radiation can be calculated. The PAR coefficient η PAR, the ratio of PAR energy to the solar global radiation, is a very important parameter to determine the energy flux density. According to the observed data, its average value is 0.439, which is not higher than that in plain areas although the solar radiation in Tibet Plateau is much stronger. In Lhasa, the energy of 1 J of PAR is equivalent to about 4.43 mol quantum. With climatological calculation and analysis, this value would be very useful in conversion between energy and quantum in Tibet Plateau. The statistical models of energy flux density ( ) and photo flux density (U PAR ) were set up respectively. References Alados, I., Foyo-Moreno, I., Alados-Arboledas, L., Photosynthetically active radiation: measurements and modelling. Agric. For. Meteorol. 78,

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