Erratum to “Local-scale heterogeneity of photosynthetically active radiation (PAR), absorbed PAR and net radiation as a function of topography, sky conditions and leaf area index” [Remote Sensing of Environment 103 (2006) 324–337]
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Photosynthetically active radiation
Utilizing a quantum sensor and non-imaging hyperspectral spectrometer,2 cotton cultivars 4 level densities planting,in north Xinjiang,multi-temporal canopy hyperspectral data and photosynthetically active radiation data at cotton key growing stages were reoorded,and multivariate regression analysis method was used to analyze the correlated relationship between reflectance and cotton canopy FPAR,LAI,respectively.The results showed that the maximum correlation coefficients between hyperspectral data and FPAR,LAI occurred at 813 nm,758 nm wavelength,respectively;based on the linear regression equation between reflectance spectrum ρ813 and ρ758 and FPAR,LAI,respectively,their correlation coefficients were significant between measured FPAR and estimated FPAR,measured LAI and estimated LAI(rFPAR =0.7199**,rLAI =0.6430**,α=1%,n=70).The regression function accuracies were 96.5%,81.7%,respectively.The maximum value of correlation coefficient between the first derivative spectral data and LAI occurred at 734 nm wavelength,but the correlation coefficient was not significant between the first derivative spectral data and FPAR from 350 nm to 2500 nm wavelength.According to the first derivative spectral value ρ′734 estimate for LAI,measured LAI and estimated LAI was significantly relevant(rLAI=0.6947**,α=1%,n=70),the regression function accuracy was 82.4% and was nearly to the prediction precision of the LAI regression modeling at 758 nm wavelength reflectance.The study showed that canopy hyperspectral data can be used for real-time,nondestructive and quantitative estimate of FPAR,LAI.
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Because the leaf area index (LAI) is an essential parameter for understanding the structure and growth status of plant canopies, nondestructive and continuous estimation methods have been required. Recently, an LAI estimation method using the ratio of near-infrared radiation (NIR; 700–1000 nm) to photosynthetically active radiation (PAR; 400–700 nm) (NIR in /PAR in ) transmitted through a canopy has been proposed. However, because previous studies on this NIR in /PAR in -based LAI estimation method are limited to tall plants (e.g., forest and rice canopies), in this study, we applied this method to a short canopy (i.e., spinach) and investigated its validity. NIR in /PAR in and three other traditional indices for indirect LAI estimation—relative PPF density (rPPFD), normalized difference vegetation index (NDVI), and simple ratio (SR)—were measured in 25 canopies with different LAI. NIR in /PAR in showed better estimation sensitivity ( R 2 = 0.88) to the observed LAI than the other three indices, particularly when LAI was greater than 3 m 2 ·m −2 . In addition, the LAI estimated from NIR in /PAR in measured at 10-min intervals in the entire growth period could capture an increasing trend in the measured LAI throughout the entire growth stage (mean absolute error = 0.87 m 2 ·m −2 ). Errors in long-term LAI estimations may be caused by the sensor location and insufficient data due to unsuitable weather conditions for measuring NIR in /PAR in . The current study demonstrates the merits and limitations of the NIR in /PAR in -based LAI estimation method applied to low height canopies, thereby contributing to its practical use in horticultural crops.
Photosynthetically active radiation
Growing season
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Grassland is a major component of the Earth's available land. The vast area and remoteness of this ecosystem makes it difficult to assess its condition and monitor productivity by traditional niethods. Remote sensing potentially offers a rapid nondestructive approach for monitoring such ecosystems. A study was carried out in a tallgrass prairie site near Manhattan, Kansas, during the 1983 and 1984 seasons to investigate the feasibility of estimating light interception and green leaf area index (LAI) from measurements of canopy multispectral reflectance. Greenness (G, i) index was found to be strongly correlated with intercepted photosynthetically active radiation ( PAR). Two methods, a direct regression (RGR) and an indirect approach (IND), were used to estimate LAI from Goi index. The LAI values estimated by RGR method were consistently lower than the measured ones; however, good agreement was obtained between the LAI values estimated by IND method and the measured LAI. This suggests that Goi transformation of canopy spectral reflectance is more closely related to the fraction of intercepted PAR by green foliage than the quantity of green LAI.
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2005年8月对贝加尔湖地区的太阳分光辐射(总辐射Q、紫外辐射UV、可见光辐射VIS、近红外辐射NIR)、光合有效辐射PAR、气象参数等做了短期测量，初步得到了Q、UV、VIS、NIR、PAR等的变化特征。观测表明，小时累计值之比UV/Q、VIS/Q、NIR/Q、PAR/Q、PAR/VIS等相对稳定，并表现出明显的日变化和逐日变化，它们的平均值分别为0.054、0.435、0.513、2.118、4.874。基于对测量数据的分析，利用辐射和气象台站常规的测量数据，建立了计算实际天气PAR、VIS小时值的经验公式以及PAR与VIS转换关系式，计算值与观测值符合得较好。PAR、VIS计算值与观测值的相对偏差小于4.5%。考虑到辐射资料的可用性，建立了使用不同参数情形下的经验公式，并讨论了有关参数的使用。 Integrated observations were made of solar radiation (including global radiation (Q), UV, visible radiation (VIS), near infrared radiation (NIR)), photosynthetically active radiation (PAR), and me-teorological parameters at Baikal Lake in August 2005. The characteristics of Q, UV, VIS, NIR, PAR were obtained. The results show that the ratios of hourly sum UV/Q, VIS/Q, NIR/Q, PAR/Q, PAR/ VIS were relatively stable and displayed evident diurnal and daily variations. Their averages were 0.054, 0.435, 0.513, 2.118 and 4.874, respectively. Based on analyzing observational data and uti-lizing routine observational data at solar radiation and meteorological stations, the empirical formulas for calculating hourly sums of PAR and VIS, and relationship between PAR and VIS were developed under all sky conditions. The calculated PAR and VIS were in good agreement with measured, and their relative biases were less than 4.5%. Considering the availability of different solar radiation data, the empirical formulas using different parameters were developed, and the option of selecting different parameters was also discussed.
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This paper compares by land cover type seasonal and spatial variations of MODIS leaf area index (LAI) and fraction of photosynthetically active radiation (0.4–0.7 μm) absorbed by vegetation (FPAR) from 2.5 years with those from the Common Land Model (CLM) and investigates possible reasons for notable differences. The FPAR value is mainly determined by LAI in MODIS and both LAI and stem area index (SAI) in CLM. On average, the model underestimates FPAR in the Southern Hemisphere and overestimates FPAR over most areas in the Northern Hemisphere compared to MODIS observations during all seasons except northern middle latitude summer. Such overestimation is most significant in winter over northern high latitudes. The MODIS LAI is generally consistent with the model during the snow‐free periods but may be underestimated in the presence of snow, especially for evergreen trees. The positive FPAR bias is mainly attributed to CLM SAI of deciduous canopy and higher LAI than MODIS for evergreen canopy as well. The negative FPAR bias results from several factors, including differences in LAI and soil albedo between CLM and MODIS or limitations of the geometric optics scheme used in the model. Therefore the MODIS algorithm needs to better represent the winter LAI retrievals, while the model needs to better quantify LAI and SAI. Since stems will not have the same single‐scattering albedo as green leaves, it may be inappropriate for the model to treat LAI and SAI the same in the FPAR and albedo parameterizations. If so, the role of SAI in these parameterizations needs reformulation.
Photosynthetically active radiation
Albedo (alchemy)
Moderate-resolution imaging spectroradiometer
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This study aimed to estimate the tomato leaf area index (LAI) by comparing two methods (destructive and interception of photosynthetically active radiation) and the consequent relationship to fruit yield and quality. The experiment was carried out in a greenhouse with tezontle (red volcanic scoria) as the substrate and a drip irrigation system. The experiment consisted of three treatments: T1, T2 and T3, with one, two and three stems per plant, respectively. The LAI was measured with a ceptometer that estimates the intercepted radiation above and below the canopy. Maximum LAI was found at 1413 cumulative growing degree days (CGDD). Those indexes were 3.69, 5.27 and 6.16 for T1, T2 and T3, respectively. Individual correlation models were fitted linearly between the two methods. The R2 values were 0.98, 0.99 and 0.99 with yields of 20, 18 and 17 kg m-2 for T1, T2 and T3, respectively. In addition, T1 produced better fruit size quality with approximately 69, 23 and 8% classified as first, second and third class, respectively. Only 1% was classified as a small fruit. Increasing the number of stems per plant increased the LAI and fruit number but decreased fruit size. Highlights: The leaf area index (LAI) is a very important variable for growth and development of crops. The ceptometer proved to be a fast, useful and statistically reliable method to estimate LAI. The increase of photosynthetically active radiation favors the photosynthetic efficiency per unit area. The number of stems per plant increase the LAI, dry matter accumulation, plant height and the number of fruits. However, the size of fruit decreases.
Photosynthetically active radiation
Interception
Shading
Specific leaf area
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Growing season
Biometeorology
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