Estimation of Global and Diffuse Photosynthetic Photon Flux Density under Various Sky Conditions Using Ground-Based Whole-Sky Images
"> Figure 1
<p>Flow diagram of the derivation of the sky-condition factors of <span class="html-italic">CC</span>, <span class="html-italic">SA</span>, and <span class="html-italic">BI<b><sub>ws</sub></b></span> from whole-sky image processing.</p> "> Figure 2
<p>Observed monthly means of (<b>a</b>) the sky-condition factors of <span class="html-italic">CC</span>, <span class="html-italic">SA</span>, and <span class="html-italic">BI<sub>ws</sub></span>, (<b>b</b>) the <span class="html-italic">PF</span>, (<b>c</b>) <span class="html-italic">Q/E<sub>(g)</sub></span>, and (<b>d</b>) <span class="html-italic">Q/E<sub>(d)</sub></span>. The bars show the standard deviations.</p> "> Figure 3
<p>Scatter plots of the estimated and observed PPFD: (<b>left</b>) global PPFD and (<b>right</b>) diffuse PPFD (<span class="html-italic">n</span> = 17,438).</p> "> Figure 4
<p>Cumulative probability distributions of the observed and estimated PPFD with instantaneous values: (<b>left</b>) global PPFD and (<b>right</b>) diffuse PPFD.</p> "> Figure 5
<p>Scatter plots of the estimated and observed daily PPFD: (<b>left</b>) daily global PPFD and (<b>right</b>) daily diffuse PPFD (<span class="html-italic">n</span> = 59).</p> "> Figure 6
<p>Diurnal changes in the observed and estimated PPFD on 3 days with clear sky conditions (upper plots: 5 May, 25 June, and 5 November) and 3 days with fluctuating sky conditions (lower plots: 30 June, 10 July, and 30 August).</p> "> Figure 7
<p>Time series of the diurnal accumulation of the observed and estimated global and diffuse PPFD: (<b>a</b>) 25 June and (<b>b</b>) 30 June. The lower graph of each panel shows the diurnal changes of the <span class="html-italic">SA</span> (0,1), <span class="html-italic">CC</span>, and <span class="html-italic">BI<sub>ws</sub></span> in sky-condition factors.</p> ">
Abstract
:1. Introduction
2. Materials and Methods
2.1. Observations and Obtained Data
2.2. Method for Estimating Global and Diffuse PPFD
2.3. Sky-Condition Factors
- CC: CC is defined as the ratio of the area composed of clouds to the whole-sky area, which is given as a percentage. The whole-sky images were taken using a fisheye lens with an equidistant projection. In calculating the ratio of the areas on a photographic image, it is generally necessary to convert the equidistant projection to an equisolid angle projection. Therefore, the CC was calculated through a multiplication by a correction coefficient [28].
- SA: SA indicates the degree to which the sun can be seen and the status of the direct component. In cases of clear sky, the sun covers around 2300–4500 pixels with BI = 1. In cases where there are clouds near the sun, there can be more than 4500 pixels with BI = 1. The sun is thus determined as appearing when the number of pixels with BI = 1 is over 2300 (in the specific case of our camera and lens). This diameter of the sun when covering 2300 pixels corresponds approximately to 5.9° on the whole-sky image. This angle is almost equivalent to the field of view of a normal-incidence pyrheliometer; i.e., 5°. We used SA = 1 for the case where the sun is visible and SA = 0 for the case where the sun is hidden by clouds. Additionally, SA can be presented as the SA ratio during the integration time; i.e., an hourly timescale.
- BIws: The relative brightness index for the whole-sky image is related to the brightness of the sky; i.e., the diffuse component. The value of BI reflects the different levels of brightness of cloud and blue sky under various sky conditions; i.e., thick or thin clouds and dry or humid air, including the presence of aerosols. The value of BIws is the mean of the RGB digital numbers normalized by the maximum quantized digital number (8 bit: 255 values) in the whole-sky image, not including the area classified as sun.
- SEA: The sun elevation angle θ (SEA; degrees) is the main factor that determines the passage of the SR through the atmosphere, and it is strongly related to the direct and diffuse components under a clear sky. The value of the SEA can be calculated from the latitude and longitude of the study site and the time at which an image was acquired.
3. Results and Discussion
3.1. Instability of PF and Q/E
3.2. Effects of Sky-Condition Factors on PF, Q/E, DR, and CI on an Hourly Timescale
3.3. Estimation of Global and Diffuse PPFD with Instantaneous Values
3.4. Validity of PPFD Estimation with Instantaneous Values
4. Conclusions
Author Contributions
Funding
Conflicts of Interest
References
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Data | Instrument | Interval | Period | Number of Original Data (Every 2 min) |
---|---|---|---|---|
SR | LI-200SB | 30 s | 18 April 2005–1 February 2006 | 103,162 |
CM6B | 30 s | 10 October 2005– 1 February 2006 | 38,977 | |
Global PAR/PPFD | MS-700 | 2 min | 24 February 2005–7 February 2006 | 117,030 |
Diffuse PAR/PPFD | MS-700, PRB-100 | 2 min | 24 February 2005–7 February 2006 | 117,030 |
Whole-sky image | E4500, FC-8 | 2 min | 22 February 2005–7 February 2006 | 104,845 |
Variables | CC | SA | BIws | SEA | Intercept | R2 | |
---|---|---|---|---|---|---|---|
PF | p.r.c. | 0.018 | −0.021 | −0.233 | 0.002 | 0.436 | 0.375 |
s.p.r.c. | 0.089 | −0.164 | −0.634 | 0.584 | |||
Q/E(g) | p.r.c. | 0.016 | 0.041 | 0.049 | −0.0004 | 4.529 | 0.343 |
s.p.r.c. | 0.146 | 0.583 | 0.239 | −0.296 | |||
Q/E(d) | p.r.c. | 0.206 | −0.008 | 0.075 | 4.328 | 0.834 | |
s.p.r.c. | 0.883 | −0.058 | 0.177 | ||||
DR | p.r.c. | 0.448 | −0.330 | −0.0002 | 0.557 | 0.919 | |
s.p.r.c. | 0.474 | −0.556 | −0.018 | ||||
CI | p.r.c. | −0.216 | 0.270 | 0.639 | −0.003 | 0.376 | 0.851 |
s.p.r.c. | −0.262 | 0.520 | 0.426 | −0.265 | |||
CIpar | p.r.c. | −0.224 | 0.259 | 0.709 | −0.003 | 0.386 | 0.873 |
s.p.r.c. | −0.269 | 0.494 | 0.469 | −0.248 |
Standard Partial Regression Coefficient | CC | BIws | SEA | DR | R2 |
---|---|---|---|---|---|
CIpar | −0.438 | 0.261 | 0.156 | ||
DR | 0.680 | 0.119 | −0.127 | 0.520 | |
Q/E(g) | 0.713 | −0.055 | −0.278 | −0.943 | 0.509 |
Q/E(d) | 0.783 | 0.104 | 0.083 | 0.741 |
Standard Partial Regression Coefficient | CC | BIws | SEA | DR | R2 |
---|---|---|---|---|---|
CIpar | −0.136 | 0.939 | −0.653 | 0.599 | |
DR | 0.452 | −0.353 | 0.127 | 0.273 | |
Q/E(g) | 0.678 | 0.091 | −0.218 | −0.727 | 0.564 |
Q/E(d) | 0.665 | 0.132 | −0.067 | 0.455 |
Statistics | PARg (W m−2) | PARd (W m−2) | PPFDg (μmol m−2 s−1) | PPFDd (μmol m−2 s−1) |
---|---|---|---|---|
Mean | 142.24 | 84.64 | 649.74 | 381.33 |
Mean error | 0.51 | 3.16 | 1.77 | 14.00 |
RMSE | 37.73 | 16.40 | 174.55 | 75.07 |
RMSE (%) | 26.6 | 20.1 | 26.9 | 20.4 |
Statistics | Daily Global PPFD | Daily Diffuse PPFD |
---|---|---|
Mean (mol m−2) | 24.36 | 14.29 |
Mean error | 0.22 | 0.61 |
RMSE | 1.98 | 1.57 |
RME (%) | 8.2 | 11.5 |
Date | 5-May | 25-June | 30-June | 10-July | 30-August | 5-November |
---|---|---|---|---|---|---|
CV of CIpar | 0.254 | 0.219 | 0.736 | 0.706 | 0.907 | 0.180 |
Obs. PPFDg | 42.83 | 48.20 | 30.90 | 26.63 | 16.58 | 24.83 |
Est. PPFDg | 44.12 | 47.63 | 30.77 | 29.25 | 17.74 | 26.10 |
Error % | +2.9 | −1.2 | −0.4 | +9.0 | +6.5 | +4.9 |
Obs. PPFDd | 18.67 | 22.62 | 18.86 | 21.20 | 13.04 | 8.68 |
Est. PPFDd | 20.20 | 21.18 | 19.64 | 21.47 | 13.49 | 10.03 |
Error % | +7.6 | −6.8 | +4.0 | +1.3 | +3.3 | +13.5 |
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Yamashita, M.; Yoshimura, M. Estimation of Global and Diffuse Photosynthetic Photon Flux Density under Various Sky Conditions Using Ground-Based Whole-Sky Images. Remote Sens. 2019, 11, 932. https://doi.org/10.3390/rs11080932
Yamashita M, Yoshimura M. Estimation of Global and Diffuse Photosynthetic Photon Flux Density under Various Sky Conditions Using Ground-Based Whole-Sky Images. Remote Sensing. 2019; 11(8):932. https://doi.org/10.3390/rs11080932
Chicago/Turabian StyleYamashita, Megumi, and Mitsunori Yoshimura. 2019. "Estimation of Global and Diffuse Photosynthetic Photon Flux Density under Various Sky Conditions Using Ground-Based Whole-Sky Images" Remote Sensing 11, no. 8: 932. https://doi.org/10.3390/rs11080932