The influence of cloudiness on UV global irradiance (295–385 nm)

Two years of continuous measurements of UV (295–385 nm) irradiance recorded at Granada (37.18°N, 3.58°W, 660 m a.m.s.l.), Spain, were combined with concurrent synoptic cloud observations to establish the relative influence of clouds on UV irradiance. A marked influence of cloud cover on UV irradiance is evident, but negligible for fractional cloud coverage below 3 octas. The ratio of UV irradiance to the total solar irradiance increases with cloud cover, especially for cloud cover greater than 4 octas, highlighting the different effects of cloud on total solar irradiance and UV irradiance. In order to determine the effect of cloud on UV irradiance, we considered a cloud modification factor, defined as the ratio between the UV measurements and the corresponding clear sky UV irradiance that would be expected for the same time period and atmospheric conditions. It is shown that the effect of cloud for UV wavelengths is less than that for the whole solar spectrum and less than that for the visible part of the spectrum. On the other hand, the small influence of cloud cover for intermediate cloudiness was accompanied by low variability and variability increases with increasing cloud cover.     

Estimated and measured DNA, plant-chromosphere and erythemal-weighted irradiances at Barrow and South Pole (1979–2000)

In the last two decades as a consequence of ozone depletion there has been an increasing interest in the study of biological effects of ultraviolet radiation (UV). Spectral instruments, which provide detailed information on UV environmental conditions, have been in use systematically only for little more than a decade. These time series are still relatively short and information on spectral historical irradiance levels is not available. Many efforts have been carried out in inferring this information from other available data sets. One of them has been the use of statistical models. Spectral irradiances are available at South Pole (90°00′S 0) and Barrow (71°18′N, 156°47′W) from the NSF UV Radiation Monitoring Program since 1991. In the present paper, daily-integrated biologically weighted irradiances for these sites are inferred back to 1979 using a multi-regressive model, obtaining time series that extend near the beginning of the Antarctic ozone depletion. These datasets are unique since the daily-integrated irradiances were calculated from irradiance measured hourly at the earth’s surface. The biologically weighted irradiances are estimated from irradiance measured with broadband instruments, ozone, and solar zenith angles. From daily-integrated irradiance, monthly means were also calculated. The RMS errors between the estimated and measured daily-integrated irradiances range from 4.69 to 7.49% at South Pole and from 9.57 to 15.20% at Barrow, while the monthly mean errors vary from 2.07 to 3% and 2.95 to 3.91%, respectively. Completing the databases with spectral measurements, the resulting time series extend from 1979 to 2000. Analyzing monthly values an increase relative to 1979–1981 during all years is observed at South Pole. Largest increases are observed for DNA and plant-chromosphere weighted irradiances during October. Although at a lower rate, an increase is also observed at Barrow during the spring. Maximum monthly increase at South Pole during October is near 1200% relative to 1979–1981, while the increase at Barrow is near one tenth of that percentage. Daily-integrated irradiance shows that a slight increase was present during the spring at South Pole for the period 1979–1981 reflecting the beginning of the ozone depletion. Historical maximums of daily-integrated DNA weighted irradiance at South Pole (90°00′S, 0°00′) are about as large as summer maximums at San Diego (32°45′N, 117°11′W).     

Total reflected solar radiation calculated from multi-band sensor data

Reflected solar radiation is a significant term in the net radiation equation and is the one most strongly affected by surface conditions. The evaluation of net radiation over a heterogeneous area requires a detailed knowledge of the areal distribution of the reflected solar radiation. Remote sensing offers a means to obtain this areal distribution, provided that the total reflected solar spectrum can be estimated from discrete band multispectral radiometric data. A radiative transfer model was used to calculate the irradiance at the earth's surface for a number of atmospheric scattering and adsorption conditions. Response functions of two multiband radiometers were used to obtain the partial spectrum/total spectrum (P/T) ratio for each radiometer at each atmospheric condition. It was found that the P/T ratio was essentially independent of atmospheric scattering and only mildly dependent on water vapor absorption. Reflectance spectral distributions for 14 different surface conditions (bare soil to full green canopy) were used along with the irradiance data to determine the P/T ratio for reflected solar radiation. Multispectral data, with the appropriate P/T ratio, were used to calculate the total incoming radiation and the total reflected radiation from a wheat canopy. The calculated data differed from wide band pyranometer data by about 5%. It was concluded that both total incoming and reflected solar radiation can be evaluated from multispectral radiometric data. This development is a step towards regional net radiation maps, and possibly regional evapotranspiration maps.     

Daily photosynthesis totals calculated from solar radiation histograms

Histograms of the number of hours in the day in which the photosynthetic photon flux density (PPFD) of solar radiation was within 5 preset bands, were constructed from readings made with a miniature battery-operated millivolt histogram recorder, connected to a photon sensor. The number of histograms showing a bimodal distribution was much fewer than was expected from a review of the literature, even in a site shaded by trees. This was attributed to smoothing of the data over a whole day, compared with published data which were mostly taken near noon. Bimodal distributions were found in a greenhouse under clear or partly cloudy conditions.The histograms were combined with published data for the instantaneous gross photosynthetic responses to PPFD of two bermudagrass swards, one of low leaf area index (LAI) and highly non-linear response, and the other of high LAI and more linear response. The daily photosynthesis totals of the swards were calculated by summing the products of the number of hours accumulated in each band by the photosynthetic rate in the center of the band.Because of the non-linearity of response, the photosynthesis total in the greenhouse was 75–82% of that calculated for an open site on a clear day, while the daily photon total was only 70% of the open site value. Also, for the same daily photon total, the daily photosynthesis total of each sward on the open site was greater on a day of light clouds (moderate PPFD) than on a day of sunshine alternating with heavy clouds (bimodal histogram of PPFDs). The difference between the two days was greater for the sward with the more non-linear response. These interactions between non-linearity of photosynthetic response and type of radiation histogram were in the expected direction, however, the differences were not very large (+ 10% for the sites and responses studied).The histogram recorder provides an inexpensive and convenient means of obtaining solar radiation data that can be used to calculate daily photosynthesis totals, either from experimental photosynthesis data or from canopy photosynthesis models. Because the response of photosynthesis is non-linear, linear integrals of solar irradiance do not provide sufficient information for such photosynthesis studies.     

Effect of row spacing,growth stage,and nitrogen rate on spectral irradiance in winter wheat

Soil reflectance affects spectral irradiance measurements taken in winter wheat at early stages of growth when percent cover is low. The objective of this study was to determine the critical percent vegetation coverage needed for forage nitrogen (N) uptake calibration with indirect spectral irradiance measurements. Two field experiments were conducted at Tipton and Perkins, OK in October 1996. The effect of row spacing (15.2, 19.0, 25.4, and 30.5 cm) and growth stage (Feekes 4 and 5) under various N fertilizer rates (0, 56, 112, and 168 kg N ha^‐1) on spectral irradiance measurements from wheat was evaluated. The normalized difference vegetative index (NDVI) was used to characterize wheat canopy irradiance. In general, NDVI decreased with increasing row spacing and increased with N fertilizer rate at Feekes growth stage 4. Row spacing and N rate were independent of each other since no significant interaction was found. High correlation (r=0.81–0.98) was observed between NDVI and vegetation coverage. Percent vegetation coverage was a good predictor of the other dependent variables including forage dry matter, and total N uptake, which could indirectly be determined using NDVI. The coefficients of variation (CV's) from NDVI values decreased with increasing vegetation coverage suggesting that less variable NDVI values (CV less than 10%) might be obtained from plots where vegetation coverage exceeds 50%.     

Two-dimensional solar radiation interception model for hedgerow fruit trees

A two-dimensional, hourly or daily time step model was developed, which takes canopy characteristics and row orientation into account to simulate solar radiation interception in hedgerow orchards. In order to determine the spatial and temporal distribution of soil irradiance across the tree row, the canopy path length through which the radiation must travel to reach a certain point on the soil surface is calculated. The model assumes leaves to be uniformly distributed within an ellipsoid, and radiation penetrating the canopy is attenuated according to Beer’s law. Beam or direct radiation and diffuse radiation for the PAR (photosynthetically active radiation) and NIR (near-infrared radiation) wavebands are calculated separately, as they interact differently with the canopy. The attenuation of beam radiation by the canopy is strongly dependent on canopy dimensions and architecture, zenith and azimuth angle, as well as row orientation. Radiation can penetrate neighbouring rows, so two rows on either side of the simulated row are considered. Validation of the model was carried out for a wide range of conditions (crops, row orientation, canopy density, tree size and shape). Field measurements included solar radiation, soil irradiance at different distances from the tree row with tube solarimeters, leaf area density, as well as canopy size and row orientation. Model predictions of soil irradiance were excellent in orchards with symmetrical and elliptical canopies having a uniform leaf distribution. In orchards where the canopy was non-symmetric and/or had non-uniform leaf distribution, errors in predictions of solar radiation transmittance occurred. As a result of these discrepancies, the overall MAE was 40% of the average measured value of radiant transmittance over the whole day.     

Spatial variability of photosynthetically active radiation in European beech and Norway spruce

The vertical and horizontal variability of solar radiation within a mature European beech (Fagus sylvatica L.)-Norway spruce (Picea abies [L.] Karst) mixed stand in Southern Germany is investigated. A large dataset with more than one million spectral measurements of photon fluence rates at six vertical levels within the stand is analyzed with respect to tree species, meteorological sky conditions, and the influence of solar elevation angle on canopy penetration. Irradiance probability density functions of the photosynthetically active waveband are used to describe the three-dimensional radiation field. For a quantification of umbra, penumbra, and sunfleck frequencies, in-canopy fractions of photon fluence rates within the photosynthetically active waveband are investigated. Different phenological stages of beech and their effects on the in-canopy light climate are compared. The results show that during overcast conditions (OVC) fractions of photosynthetically active radiation (PAR) are higher at all canopy levels than during clear sky (CS) conditions due to their exclusively diffuse character. The lowest median PAR level of less than 1% of above-canopy PAR can be observed in the shade crown of beech and at ground level. More PAR can penetrate the canopy at a higher solar elevation under CS conditions. This effect is more pronounced for spruce than for beech due to the conical crown shape of the conifers that allows photons from higher angles to enter the gaps inbetween trees in contrast to the more homogeneously closed beech canopy. Solar elevation is not an important factor at uniformly overcast conditions. Differences in the vertical distribution of umbra and penumbra can be detected when comparing species or different sky conditions. The frequency of sunflecks differs more by species and by the vertical position within the canopy than by sky condition.     

夏玉米冠层光合有效辐射垂直分布模型

吸收光合有效辐射分量（FPAR）是研究植被群体光合作用和光能利用的重要参数,弄清冠层内FPAR的垂直分布规律及其与冠层结构等参数之间的定量关系,可以为遥感定量反演冠层FPAR提供模型基础。该文基于平均冠层的辐射传输过程,结合冠层结构建立玉米冠层内FPAR垂直分布计算模型,并就模型的参数如植被组分光学特性、株型、太阳高度角以及天空散射光比例等对FPAR垂直分布结果的影响进行了分析。利用实测结构参数模拟了玉米冠层内FPAR,并与SUNSCAN测量值进行了比较,结果表明模型对封垄后的玉米冠层内FPAR垂直分布的模拟精度较高,RMSE均小于0.18,并能较好地计算封垄前冠层内FPAR的垂直分布趋势。     

Ultraviolet-B radiation in a row-crop canopy: an extended 1-D model

A decrease in stratospheric ozone may result in a serious threat to plants, since biologically active short-wavelength ultraviolet-B (UV-B 280–320 nm) radiation will increase even with a relatively small decrease in ozone. Numerous investigations have demonstrated that the effect of UV-B enhancements on plants includes reduction in grain yield, alteration in species competition, susceptibility to disease, and changes in plant structure and pigmentation. To determine the physiological effects on plants of any increases in UV-B radiation, the irradiances at the potential sensitive plant surface need to be known. A number of radiative transfer models exist but because of the importance of sky diffuse radiation to the global UV-B irradiance, models designed to estimate photosynthetically active radiation or total solar radiation may not accurately model the UV-B. This paper compares spatially and temporally averaged measurements of the UV-B canopy transmittance of a relatively dense maize canopy (sky view: 0.27°) to the estimations of two one-dimensional models differing mainly in the handling of sky radiance. The model that considered the distribution of sky radiance tended to underestimate the canopy transmittance, the model that assumed an isotropic sky radiance distribution tended to overestimate the canopy transmittance. However, the assumption concerning the sky radiance distribution accounted for only about 0.01 of the model error. Consequently, the sky radiance distribution is probably not important in modeling such dense crop canopies. The model that overestimated transmittance and had the generally larger errors, a modified Meyers model, used the assumption of uniform leaf angle distribution, whereas in the other model, designated the UVRT model, leaf angle distributions were estimated by sample measurements. Generally this model would be satisfactory in describing the statistically average UV-B irradiance conditions in the canopy. This model may also be applied to other dense plant canopies including forests.     

太阳总辐射的分光测量及其能量分配

1987年至1988年,在北京利用MS-800型精密分波长日射计和带有不同波长范围滤光片的七种传感器测定了太阳分光辐射。根据观测资料,分析了不同光谱区的能量分配及其季节变化,讨论了太阳高度角、云量等因子对它的影响,建立了分光辐射与总辐射间的关系式。     

太阳辐射减弱对冬小麦叶片光合作用膜脂过氧化及同化物累积的影响

为了探究气溶胶的太阳辐射减弱效应对农作物叶片生理特性及同化物累积的影响,以扬麦13为供试材料,在大田试验条件下系统研究了不同辐射减弱处理（100%、60%、40%、20%、15%）对冬小麦叶片光合作用、膜脂过氧化水平和同化物积累的影响。结果表明：太阳辐射减弱至自然光的60%-15%时,冬小麦叶片净光合速率减少15.8%-70.1%,光合色素的含量（叶绿素a、b和类胡萝卜素）显著提高,其中叶绿素b含量的增加最明显。太阳辐射减弱会抑制冬小麦叶片膜脂过氧化程度,减少细胞膜透性,且辐射减弱的程度越高,影响越大。当太阳辐射为自然光的60%-15%时,冬小麦叶片的可溶性糖含量比对照降低了32.22%-76.45%,可溶性蛋白含量降低了19.02%-48.12%,但总游离氨基酸含量比对照增加了47.46%-177.87%。上述结果表明,太阳辐射减弱后会减弱冬小麦光合作用,增加光合色素含量,抑制膜脂过氧化程度,从而导致作物体内同化物的累积受阻,糖类及蛋白质的含量显著降低,必然会影响作物的产量及品质形成。     

基于气象资料的日辐射模型在中国西北地区适用性评价

地表总辐射(Rs)是作物生长模型率定、蒸散量估算、灌溉制度制定和太阳能资源利用的重要基础数据。为有效提高辐射资源利用率,该文基于中国西北地区10个气象站点1993-2016年气象数据对9种不同日辐射模型进行适用性评价。采用非线性回归分析法对Bristow-Campbell(B-C)模型进行参数属地化修正,得到B-C校正模型。模型适用性评价结果表明:9种模型在西北地区的辐射模拟值和实测值均呈极显著相关(P0.01);基于日照时数的日辐射模型(?ngstr?m-Prescott、Ogelman、Bahel、Louche、Almorox-Hontoria、Glower-Mc Culloch,其R2介于0.875~0.954)计算精度高于基于温度的模型(Hargreaves-Samani、Annandale、Bristow-Campbell,其R2介于0.652~0.813);其中基于日照时数的模型中Bahel模型精度最高,其次是Ogelman和Glower-Mc Culloch模型,其RMSE分别为2.282、2.309和2.313 MJ/(m~2·d),n RMSE分别为14.0%、14.2%和14.2%,MAE分别为1.666、1.701和1.697 MJ/(m~2·d),Nash-Sutcliffe系数(NS)分别为0.905、0.903和0.902;基于温度的日辐射模型中B-C校正模型精度最高,其RMSE为3.819 MJ/(m~2·d),n RMSE为23.3%,MAE为2.680 MJ/(m~2·d),NS为0.741。因此,西北地区日辐射计算当仅有日照时数资料时推荐使用Bahel模型,当仅有温度资料时推荐使用Bristow-Campbell校正模型。     

基于6S模型的GF-1卫星影像大气校正及效果

高分一号(GF-1)卫星是中国高分系列卫星的首发星,自2013年4月成功发射以来,在中国农业遥感业务工作中得到了广泛应用,已成为中国大宗农作物种植面积遥感监测的主要数据源。该文基于6S(second simulation of a satellite signal in the solar spectrum)辐射传输模型原理,设计并实现了适合于GF-1卫星数据大气校正算法与程序。算法以GF-1卫星1级数据、元数据及传感器公开参数为输入数据,不需要其他外源辅助数据,经过辐射定标,计算各波段平均太阳辐射值、表观反射率,通过选择大气模式,驱动6S模型获取表观反射率转换为地表反射率的参数,逐像元计算影像地表反射率。在算法研制的基础上,应用Fortran和IDL语言编写了大气校正批处理程序,实现了大气校正过程的批处理。该文采用2014年4月3日、6月28日、11月2日,以及2015年1月19日4个时相北京地区GF1卫星WFV(wide field view)数据,分别代表春夏秋冬4个季节,通过与ENVI软件的FLAASH(fast line-of-sight atmospheric analysis of spectral hypercubes)大气校正结果对比进行评估。2种方法 4个时相各波段全年相对偏差为3.26%,蓝光波段偏差最大为11.21%,其次是红、近红和绿光波段,分别为1.19%、0.73%和0.24%。作物覆盖区平均相对误差为12.99%,冬季最高为17.40%,秋季和春季分别为15.02%和14.15%,夏季相对差异最小为8.31%。各波段地表反射率的整体校正情况并未有太大差异,但6S校正后各波段反射率普遍比FLAASH校正结果略微偏高。2种校正结果计算的NDVI也基本一致,相对偏差0.64%;除水体外,绝对值差值的平均值均在0.0548以内。从计算效率来分析,6S模块实现了商用软件FLAASH模块中未提供的批量计算,在相同硬件环境下计算效率提高了75.0%以上。研究结果表明了该文开发的大气校正程序能够稳定批量处理GF-1卫星数据,可以作为农业遥感监测业务流程的组成部分。     

光合有效辐射及其传感器研究进展

植物利用约400～700 nm波段的光驱动光合作用,但不同波长的光驱动效率不相同,而且随着植物类型及生长阶段的不同而变化。因此,准确获取被植物捕获并用于驱动光合作用的光辐射成为困扰科学家的难题。当前,光量子传感器被普遍接受并用于评价光合作用潜力,可测量400～700 nm波段的光量子通量密度或光量子通量,其光谱响应函数为直线。该文回顾了经典光合有效辐射（photosynthetically active radiation,PAR）定义的形成过程,介绍了PAR传感器的演化路径,讨论了PAR及其传感器的应用现状。由于测量对象及应用环境的多样化,PAR的定义仍然没有完全统一,且早期研究对光谱响应函数的度量不充分。随着当前人工光照明与植物生长发育相关研究的深入,发现植物光合作用吸收的光波长范围比400～700 nm要宽,不同的光谱能量分布（波长配比,能量配比）、光周期等对光合作用影响显著,并且很难将光辐射对光合作用的影响和光形态效应区分开,因此PAR的定义及其传感器的研发仍处于不断发展中。理想的PAR应该从植物光合作用的角度来定义,未来PAR传感器的光谱响应函数应与植物光合作用的能力曲线相一致,并能依据测量对象及应用需求而调整。与此相适应,未来PAR传感器应向用户可对光谱响应函数编程的方向发展。     

MODIS数据辅助的GF-1影像晴空光合有效辐射反演

面向农作物产量监测对中高分辨率遥感数据光合有效辐射(photosynthetically available radiation,PAR)反演的实际需求,该文选择山东省禹城市2014年1月至2014年12月共13景GF-1/WFV卫星影像作为数据源,基于中分辨率成像光谱仪(moderate-resolution imaging spectroradiometer,MODIS)地表反射率产品作为辅助数据源,开发了适于业务运行的WFV数据气溶胶光学厚度(aerosol optical depth,AOD)及PAR的反演算法。算法核心是采用6S(second simulation of satellite signal in the solar spectrum)大气辐射传输模型,建立包括AOD在内的大气参数与查找表(look-up table,LUT),结合大气顶层太阳入射辐照度及卫星入瞳处辐射亮度值反演地表反射率数据,通过与WFV蓝光波段地表反射率数据对比获取大气参数。通过反演的大气参数计算400~700 nm连续光谱区间的PAR值,并建立WFV数据离散红、绿、蓝光波段与连续光谱区间PAR的转换系数,实现WFV数据PAR的反演。其中,WFV蓝光波段反射率数据与MODIS地表反射率数据关系、离散到连续谱段PAR的关系可以从美国地质勘探局(United States Geological Survey,USGS)提供的典型地物波谱库数据理论计算获取。利用中国生态系统研究网络(chinese ecosystem research network,CERN)禹城站地面观测值进行验证结果表明,该文提出的算法总体精度达到92.63%,平均绝对误差为14.56 W/m~2,平均相对误差7.37%,具有业务应用的潜力。     

作物株丛间的太阳辐射测试仪

本文介绍一种节电型的太阳辐射量测试装置,内容包括传感器的制作和用实时时钟控制的单片机数据采集系统。这个装置主要用于在作物冠层中测量接收到的太阳辐射量的百分比。     

设施番茄外观形态及物质累积分配模型构建与验证

为构建适用于日光温室与塑料大棚的设施番茄生长模型,该研究利用保温塑料大棚春茬试验数据,建立以辐热积为尺度的番茄外观形态及物质累积分配模型,并利用保温塑料大棚秋茬和日光温室越冬茬的试验数据验证模型的准确性。结果表明：1）番茄株高模拟值的决定系数R2和均方根误差（Root Mean Square Error,RMSE）分别为0.907 4和13.66 cm;2）番茄整株及各器官的干物质质量模拟值的决定系数R2范围为0.854 1～0.975 1,RMSE为2.87～6.98 g/株;3）番茄整株、地上部以及果实鲜质量累积的模拟值的决定系数R2范围为0.887 2～0.905 0,RMSE为109.83～171.16 g/株。综上可知,该研究建立的模型可较准确地预测番茄株高与干鲜质量物质累积值,模型的实用性较强,可为设施番茄生产提供理论依据和决策支持。     

Estimating the three-dimensional structure of canopy foliage based on the light measurements in a Betula ermanii stand

In order to more accurately evaluate the functional activity of forest stands by canopy production and evapotranspiration, we improved the methods for field measurements and statistical modeling to estimate foliage configuration (spatial distribution of leaves) while simultaneously reconstructing the three-dimensional photosynthetically active photon flux density (PPFD) distribution (PPFD pattern) in a forest canopy. By using a sensor (photodiode) array, a PPFD pattern was observed in summer 2002 under the canopy in an even-aged, pure stand of Japanese mountain birch Betula ermanii Cham. (17-years old) in Hokkaido, northern Japan. A Markov chain Monte Carlo (MCMC) sampling technique is developed such that a set of foliage configurations generated by the model referred to as the Gibbs foliage canopy (GFC) approximates the field-measured PPFD pattern. The posterior distribution of the foliage configurations is generated by the parallel tempering MCMC of eight independent series of foliage configurations. The GFC model generated the posterior distribution of the LAI estimates (mean 4.56) that appeared to be appropriate in comparison to other LAI estimates of the B. ermanii stand based on the indirect and nondestructive methods by LAI-2000 (LAI = 3.43) and litterfall traps (LAI = 5.56) because they could be under- and overestimated, respectively. Our evaluations of the canopy production and evapotranspiration rates suggest that the relationship between LAI and canopy functions was not very simple because it depended on the nonlinear functional forms of the leaf responses of photosynthesis and transpiration to PPFD. The current study demonstrates an application of MCMC techniques that can generate a set of possible structures of unobserved/unobservable objects based on the high-resolution dataset obtained by some indirect (or remote-sensing) methods.     

Canopy architecture,irradiance distribution on leaf surfaces and consequent photosynthetic efficiencies in heterogeneous plant canopies. Part II. Results and discussion

The influence of canopy architecture on the irradiance distribution on leaf surfaces in horizontally heterogeneous canopies was investigated using a procedural radiative transfer model. Four model canopies were abstracted as plant stands of 10 rows and 20 plants per row, and the procedural model was executed in conjunction with these digitized canopies. From the resulting distribution of the penetration function, the energy incident on leaf surfaces along the normal was calculated. The irradiance distribution was characterized by formally defining three kinds of irradiance — normal, diminished and null. Gross and net photosynthetic efficiencies due to irradiance in each energy class are presented. Canopy architecture was found to significantly influence the efficiency of energy use by affecting the interception pattern of solar radiation.