Atmospheric optical depth effects on angular anisotropy of plant canopy reflectance

Abstract

A field experiment was conducted to determine whether changes in atmospheric aerosol optical depth would effect changes in bi-directional reflectance distributions of vegetation canopies. Measurements were made of the directionally reflected radiance distributions of two pasture grass canopies (same species, different growth forms) and one soya bean plant canopy under different sky irradiance distributions, which resulted from a variation in aerosol optical depth. The reflected radiance data were analysed in the solar principal plane in two narrow spectral bands, one visible (662 nm) and one infrared (826 nm). The observed changes in reflectance for both wavelengths from irradiance distribution variation is interpreted to be due largely to changes in the percentage of shadowed area viewed by the sensor for the incomplete canopies (pasture grass). For the complete coverage vegetation canopy (soya bean) studied, the effects of specular reflection and the increased diffuse irradiance penetration into the canopy are concluded to be primary physical mechanisms responsible for reflectance changes. Observed reflectivities were found to be lower on a hazy day (higher optical depth with a greater diffuse fraction) than on a clear day, with solar zenith angles at about 58^° on both days, for full-coverage soya bean canopies. The reduced reflectance most likely results from a diminished specular reflection and a greater diffuse radiation penetration into the canopy, which effects an increased energy absorption at large solar zenith angles. The opposite was true for fractional coverage grass canopies at solar zenith angles of about 56^° since the shadowing was less on the hazy day and, therefore, the soil/litter background was more fully illuminated. In the near-infrared waveband the changes in reflectance are much less than in the visible and, therefore, normalized difference vegetation index values differ substantially under clear and hazy sky conditions for the same vegetation canopy conditions. Thus, the influence of atmospheric optical depth must be considered for accurate remote sensing and in situ data interpretation.     

Irradiance measurement errors due to the assumption of a Lambertian reference panel

Total and diffuse global spectral irradiances, which are often required field measurements in remote sensing, are commonly obtained by measuring the radiance from a horizontal reference panel with assumed Lambertian properties. A technique is presented for determining the error in diurnal irradiance measurements that results from the non-Lambertian behavior of a reference panel under various irradiance conditions. Spectral biconical reflectance factors of a spray-painted barium sulfate panel, along with simulated sky radiance data for clear and hazy skies at six solar zenith angles, were used to calculate the estimated panel irradiances and true-irradiances for a nadir-looking sensor in two wavelength bands. The inherent errors in total spectral irradiance (0.68 μm) for a clear sky were 0.60, 6.0, 13.0, and 27.0% for solar zenith angles of 0°, 45°, 60°, and 75°. The technique can be used to characterize the error of a specific panel used in field measurements and thus eliminate any ambiguity of the effects of the type, preparation, and aging of the paint.     

Remote sensing reflectance and its relationship to optical properties of natural waters

Monte Carlo simulations of photon propagation through natural water have been utilized to determine the sub-surface remote sensing reflectance, R _RSW (the sub-surface value of the ratio of upwelling radiance from the nadir to the downwelling irradiance) as a function of water type (defined by the ratio of the backscattering coefficient to the absorption coefficient Bb/a), solar zenith angle, and incident radiation distribution (direct or diffuse). R _RSW, as opposed to volume reflectance, R _V (the sub-surface value of the ratio of upwelling to downwelling vector irradiance), is directly applicable to remotely sensed data collected over natural waters. It is shown that, for a nadir viewing direction, (a) R _RSW is essentially independent of solar zenith angle and incident radiation distribution and (b) the dominant factor in determining R _RSW is the optical nature of the water body itself (expressed as Bb/a). A relationship between the sub-surface remote sensing reflectance averaged over solar zenith angle between 15° and 89°, R _RSW and water type is found to predict R _RSW with an r.m.s. error of 9 per cent. Also addressed is the determination of the aquatic optical property, Bb/a, from the sub-surface remote sensing reflectance, R _RSW This capability along with the specific absorption and scattering coefficients of aquatic constituents can, through bio-optical models, be used to estimate the concentrations of these aquatic constituents in non-Case I waters. The empirical relationship obtained to estimate Bb/a (with a r.m.s. error of 9·3 per cent) from the nadir value of the sub-surface remote sensing reflectance is Bb/a = 0·0027 + 987R _RSW ? 34·5( R _RSW)² + 1534( R _RSW)³.     

Sun-view angle effects on reflectance factors of corn canopies

The bidirectional reflectance characteristics of vegetation canopies vary with time of day and through the growing season. In this study the effects of sun and view angles on bidirectional reflectance factors from corn (Zea mays L.) canopies ranging in development from the six leaf stage to harvest maturity were examined. For nadir-acquired reflectance factors there was a strong solar angle dependence in all spectral bands for canopies with low leaf area index (LAI). A decrease in contrast between bare soil and vegetation due to shadows at larger solar zenith angles appeared to be the cause of this dependence. Sun angle dependence was least for well-developed canopies with higher LAI. However, for higher LAI canopies a moderate increase in reflectance factor was observed at larger solar zenith angles and was attributed to the presence of specular reflectance. Trends of off-nadir reflectance factors with respect to sun angle at different view azimuth angles indicated that the position of the sensor relative to the sun was an important factor for determining the angular reflectance characteristics of corn canopies. Reflectance factors were maximized for coincident sun and view angles and minimized when the sensor view direction was towards the sun. View direction relative to row orientation also contributed to the variation in reflectance factors.     

Conversion of sub-surface reflectances to above-surface MERIS reflectance

Factors for converting sub-surface reflectances to above-surface MERIS reflectances have been determined both as analytic functions and average numbers for solar zenith angles in the range 30°–75°, wind speeds up to 10 m s^?1, and the spectral domain 400–700 nm. The conversion factors have been obtained by numerical and statistical computations based on field observations of spectral radiance and irradiance, above and below the surface of the sea. The estimated maximum errors of the different algorithms range from ≤0.1% up to 10%, depending on the chosen method and the types of optical quantities that are available. The errors are smallest for solar zenith angles between 30° and 60° and increase when the solar zenith angle approaches 75°. The influence of the wind on the conversion factors is practically negligible. The algorithms, which have been derived for conditions representative of the Skagerrak and the adjacent seas, are assumed to be valid for both Case 1 and 2 waters.     

Characterization of the Reflectance Anisotropy of Three Boreal Forest Canopies in Spring–Summer

As a part of the Boreal Ecosystem-Atmosphere Study (BOREAS), measurements of the spectral reflectance anisotropy of three boreal forest canopies were studied for cloudless sky conditions at the phenological growth stages which were at or near maximum leaf area index at each site. The three sites were relatively homogeneous mature stands of black spruce, jack pine, and aspen located in the southern boreal zone of central Saskatchewan. Measurements of the spectal bidirectional reflectance factors with a 15° instrument field of view in three spectral bands centered at 662 nm, 826 nm, and 1658 nm were made with the PARABOLA instrument over a range of solar zenith angles typically varying from 35° (near solar noon) to 70°. The measured reflectance factors showed large anisotropy at all three sites and for all three wavelengths, with prominant backscatter peak reflectances, and strong retro solar view angle (hot spot) maximum reflectances in the visible (662 nm) and shortwave infrared (1658 nm) for the jack pine and black spruce sites, with a less pronounced hot spot at the aspen site. Pronounced effects of canopy and understory shadowing in the visible, as a function of solar zenith angle (SZA), were observed for the black spruce and jack pine sites, with resultant large linear increases in computed normalized difference and simple ratio vegetation indices as SZA increased for near-nadir view angles. Hemispheric spectral reflectances or spectral albedos were computed from angular integration of PARABOLA measured bidirectional reflectances. Visible (662 nm) hemispheric reflectances for the jack pine and black spruce canopies showed very little variation with solar zenith angle, while near-infrared hemispheric reflectances increased strongly with increasing SZA. Estimates were made of the total shortwave albedo for the aspen and jack pine sites from irradiance and reflectance weighting of the spectral hemispheric reflectances in the three measured wavelengths. Comparison of estimated to pyranometer measured total albedo showed all estimates to be biased high, but only by about 0.007–0.018, depending on which of two sets of pyranometer measured albedos were utilized for the comparison. The measured bidirectional reflectance factor (BRF) data sets reported in this study coupled with ancillary data of biophysical parameters collected at the same sites by BOREAS researchers provide a unique data set for the development and characterization of canopy bidirectional reflectance modeling and for the interpretation of remotely sensed data for boreal forest canopies.     

Atmospheric effects on radiometry from zenith of a plane with dark vertical protrusions

Effects of an optically thin plane-parallel scattering atmosphere on radiometric imaging from the zenith of a specific surface type are analysed. The surface model was previously developed to describe arid steppe, where the sparse vegetation forms dark vertical protrusions from the bright soil plane. The analysis is in terms of the surface reflectivity to the zenith r_p for the direct beam, which is formulated as r_p = r_i exp(?s tan θ₀), where r_i is the Lambert-law reflectivity of the soil, the protrusion parameter 5 is the projection on a vertical plane of protrusions per unit area and θ⁰ is the zenith angle. The surface reflectivity r_P is approximately equal to that for the global irradiance (which is directly measured in the field) only for a narrow range of the solar zenith angles. The effects of the atmosphere when imaging large uniform areas of this type are comparable to those in imaging a Lambert surface with a reflectivity r_P . Thus, the effects can be approximated by those in the case of a dark Lambert surface (analysed previously), inasmuch as r_P is smalleSr than the soil reflectivity r_i for any off-zenith illumination. The surface becomes effectively darker with increasing solar zenith angle.

Adjacency effects of a reflection from one area and scattering in the instantaneous field of view (object pixel) are analysed as cross radiance and cross irradiance, The analysis is only for the case of a small object pixel embedded in a different terrain, extending to infinity as a uniform area. The effects of the cross radiance (which are dominant) are found to be smaller than those over a Lambert plane for the same surroundings-to-object-pixel contrast and atmospheric conditions. However, the adjacency effects are highly variable, because the effective contrast for our plane with dark protrusions is a function of not only the surface parameters but also of the solar zenith angle and the atmospheric conditions.     

Inferring hemispherical reflectance of the earth's surface for global energy budgets from remotely sensed nadir or directional radiance values

The importance of the hemispherical reflectance (albedo) of terrestrial surfaces to biospheric and atmospheric processes is briefly reviewed. It is proposed that satellite-borne instruments represent the only practical means of obtaining global estimates of surface albedo data at reasonable time resolution, the problem being how to relate the nadir or directional reflectance observations obtained from such sensors to the integrated hemispherical reflectance. This paper discusses results measured at ground level in which NOAA satellite 7/8 AVHRR data, Bands 1 (0.58–0.68 μm) and 2 (0.73–1.1 μm), were used to investigate 1) the relationships between directional reflectances (spanning the entire reflecting hemisphere) and hemispherical reflectance (albedo) and 2) the effect of solar zenith angle and cover type on these relationships. Eleven natural vegetation surfaces ranging from bare soils to dense vegetation canopies were considered in the study. The results show that errors in inferring hemispherical reflectance from nadir reflectance can be as high as 45% for all cover types and solar zenith angles. By choosing a time of observation such that the solar zenith angle is between 30 and 40° the same error is reduced to less than 20% in both bands. For both bands a view angle of 60° off-nadir and ±90° from the solar azimuth reduces this error to less than 11% for all sun angles and cover types. A technique using two specific view angles reduces this error to less than 6% for both bands and for all sun angles and cover types. These techniques may yield considerable dividends in terms of more reliable estimation of hemispherical reflectance of natural surfaces.     

Bidirectional effect on a spectral image sensor for in‐field crop reflectance assessment

Varying illumination geometry affects spectral measurements of a target reflectance and the intensity of solar radiation is the most important factor for in‐field spectral measurements. This paper reports the effect of bidirectional electromagnetic radiation on an image‐based reflectance sensor designed for plant nitrogen assessment. The results show the nonlinearity of reflectance as a function of the solar zenith angle. Ambient illumination was analysed and compensated for using fixed nadir‐view positions of a solar radiometer and a 3‐charge‐coupled device (CCD) multispectral imaging sensor (MSIS). A compensation algorithm was developed to correct for the nonlinearity of both sensors. The compensated reflectance remained linearly consistent with varying the solar zenith angle throughout the daytime within a maximum standard deviation of 0.62% at all three (green, red and near‐infrared) spectral channels, when testing with a 20% reflectance panel. The consistent reflectance was recovered under both sunny and cloudy conditions.     

On the Dependence of Discrete Ordinates Models for Layer Reflectance and Transmittance on Relative Optical Depth and Solar Zenith Angle

We investigate the effect of layer optical depth and solar zenith angle on recently developed discrete ordinates models for the diffuse reflectance and transmittance of an optically stationary atmospheric boundary layer. We start with a mathematical formulation of the atmospheric radiative transfer problems dealt with in this article, and we consider a discrete ordinates version of the governing equations. For the sake of continuity, we give a brief account of our recently developed models, and assuming steadiness with respect to the inherent optical properties, we work on the dependence of our models on two target parameters: the relative optical depth of the boundary layer and solar zenith angle. As a result, we get optimized relations for our models in terms of the target parameters. To illustrate the relevance of such relations, we present results of computer simulations using problem sets basic to solar-atmospheric science applications such as environmental remote sensing, radiation dosimetry, and solar power plants.     

The effect of anisotropic reflectance on imaging spectroscopy of snow properties

How does snow's anisotropic directional reflectance affect the mapping of snow properties from imaging spectrometer data? This sensitivity study applies two spectroscopy models to synthetic images of the spectral hemispherical-directional reflectance factor (HDRF) with prescribed snow-covered area and snow grain size. The MEMSCAG model determines both sub-pixel snow-covered area and the grain size of the fractional snow cover. The Nolin/Dozier model analyzes the ice absorption feature that spans wavelength λ≅1.03 μm to estimate snow grain radius when the pixel is fully snow-covered. Retrievals of subpixel snow-covered area with MEMSCAG are progressively more sensitive to the HDRF as grain size decreases, solar zenith angle increases, and fractional snow cover increases. The model overestimates snow cover in the forward reflectance angles by up to +20% and underestimates it in the backward reflectance angles by as much as −15%. Grain size retrievals from both MEMSCAG and Nolin/Dozier are more sensitive to anisotropy as grain size and solar zenith angle increase. MEMSCAG retrievals of grain size are insensitive to snow-covered area. The largest inferred grain sizes occur around a peak in the backward reflectance angles and the smallest generally occur at the largest view angles in the forward direction. Retrievals of albedo from MEMSCAG and Nolin/Dozier are similarly sensitive to anisotropy, with albedo errors up to 5% for a 30° solar zenith angle and up to 10% at 60°. The albedo differences between the two models are less than 0.015 for all grain sizes and solar zenith angles.     

Reflectance relations in row crop scenes

Abstract

Models that relate composite reflectance to its components are useful for inferring crop growth information from measured scene reflectance. Radiation measurements in Thematic Mapper bands (TM1, TM2, TM3, and TM4) were made from cotton, soybean, sunflower and grain sorghum at three stages of growth and used to evaluate three reflectance models. Two models, AIRM1 and AIRM2, assumed that scene components contribute in an additive independent manner to composite reflectance. The third model, TRIM, assumes that radiation transmitted through the canopy interacts with bare soil in two scene components. The AIRM2 and TRIM models divide the composite reflectance into canopy, bare soil, and shadow components, but AIRM1 considers only canopy and bare soil. Ranking of models in order of decreasing accuracy for predicting composite reflectance in bands TM3 and TM4 was AIRM2, TRIM, and AIRM1. The AIRM1 and AIRM2 models estimated average TM3 reflectance at full plant cover between 1 and 4 per cent for all crops. Their estimations in band TM4 were 60 per cent for cotton, soybean, and sunflower with grain sorghum being 50 percent.

Measured canopy and composite reflectances were graphically compared at the lowest and highest levels of canopy cover observed in each crop. Measured band TM3 canopy reflectance did not change with solar zenith angle, composite reflectance decreased with increasing zenith angle at the lowest canopy cover levels but was invariant at the highest canopy cover levels. Measured band TM4 canopy reflectance increased linearly with increasing solar zenith angle in all crops, but for composite reflectance this pattern was observed only at the highest canopy cover levels of cotton and soybean. The absence of a uniform pattern between band TM4 composite reflectance and solar zenith angle in grain sorghum is presumably due to large horizontal leaf angles and in sunflower to long vertical spacings between leaves. Predicted compared to measured band TM3 and TM4 composite reflectances of the AIRM1 and AIRM2 models were insensitive but the TRIM model was overly sensitive to zenith angle.     

Assessing impacts of off-nadir observation on remote sensing of vegetation: use of the Suits model

Off-nadir remote sensing of vegetation can cause undesirable variability in measured spectral reflectance resulting from non-Lambertian characteristics of the canopy. The Suits model of radiative transfer in a vegetation canopy was evaluated as a means to simulate this variability. Comparison was made between model calculations and reflectance of a salt marsh cord grass canopy measured under a variety of solar- and viewing-angle conditions using an in situ radiometer. The model was effective in simulating both the sense and magnitude of reflectance changes due to variable angles of observation. However, the model did not reproduce the observed dependence of nadir canopy reflectance on solar zenith angle. A simple subtractive normalization procedure resulted in high correlation of modelled red and infrared reflectance with values measured at observation angles varying from 10 to 60° off-nadir and with solar zenith angles ranging from 18 to 55°. The modelling procedure was extended to simulate view-angle effects on aircraft scanner imagery of a coastal marsh with good results despite significant variability in biomass and leaf-area index of cord grass within the imaged area. Modelling appears to have potential in predicting view-angle effects and in reducing angular variability in remotely sensed data derived from aerial and orbital sensors.     

Soil and Sun angle interactions on partial canopy spectra

Abstract

The spectral behaviour of an incomplete cotton canopy was analysed in relation to solar zenith angle and soil background variations. Soil and vegetation spectral contributions towards canopy response were separated using a first-order interactive model and consequently used to compare the relative sensitivity of canopy spectra to soil background and solar angle differences. Canopy reflectance behaviour with solar angle increased, decreased or remained invariant depending on the reflectance properties of the underlying soil. Sunlit and shaded soil contributions were found to alter vegetation index behaviour significantly over different Sun angles.     

Anisotropic Reflection by Melting Glacier Ice: Measurements and Parametrizations in Landsat TM Bands 2 and 4

This article deals with the anisotropic reflection of radiation by melting glacier ice. Ground-based measurements of the directional distribution of the reflected radiation over the hemisphere (so-called BRDFs=bidirectional reflectance distribution functions) were made on the Morteratschgletscher (Switzerland) in Landsat TM bands 2 (520–600 nm) and 4 (760–900 nm). These BRDFs cover a wide range of solar zenith angles (26–75°) and surface characteristics (quantified by a variation in the spectrally integrated albedo between 0.14 and 0.50). All BRDFs exhibit a similar pattern with a minimum in the nadir direction and a maximum in the forward limb, but the amount of anisotropy increases with increasing wavelength, with increasing solar zenith angle and with decreasing albedo. The data were used to derive parametrizations (one for each TM band) which relate the bidirectional reflectance (the reflectance in a specific direction) to the albedo for a given solar-view geometry. Specific parametrizations (one for each TM band) for “near-nadir reflection” are also presented. All these parametrizations can be used to convert satellite-derived bidirectional reflectances into surface albedos and thus to correct for anisotropic reflectance. The residual uncertainty in the albedo due to inaccuracy of the correction is estimated to be 0.02 in both TM bands.     

Panicle contribution to bidirectional reflectance factors of a wheat canopy

Bidirectional reflectance factors (BRFs) of crop stands are strongly influenced by canopy architecture. In wheat, as well as in many other crops, canopy architecture changes dramatically with the phenological development of the plant community.

A ground-based experiment was performed to examine the effect of panicles of winter wheat (Triticum aestivum L.) at the flowering stage on canopy BRFs. Reflectance factors were measured in the field with a portable radiometer in the red (0-63-0-69 μm) and near-infrared (0-76-0-90 μm) wavelength intervals. Observations were made at three viewing angles and 14 solar zenith angles during two consecutive days on a control target and on a target where panicles had been removed.

Panicles did not contribute significantly to the red nor to the near-infrared (NIR) reflectance factors computed from nadir observations. Off-nadir NIR reflectance was also not altered by the presence of panicles, but was moderately sensitive to illumination angle. Off-nadir red reflectance in the backscattcring direction was higher in the canopy with panicles than in the canopy without panicles: at a solar zenith angle of about 50° the difference in the reflectance of the two targets reached a maximum of about 39 per cent.

These findings imply a potential to identify crops and their phenological development by more fully exploiting reflectance at several different viewing and solar angles.     

Albedo of vegetated surfaces: its variability with differing irradiances

The albedo of four vegetated surfaces was investigated to derive its variability with differing distributions of the irradiance. The results are based on measured values of the spectral biconical reflectance factor, which are combined with calculated spectral irradiances for low and high atmospheric turbidity. The solar zenith angle is varied from 0° to 80°. The derived spectral albedos are then integrated with respect to wavelength in order to achieve the albedo. It is found that the variability of the albedo with respect to the atmospheric turbidity is less than 0.01 in nearly all cases. The variability of the albedo with respect to the solar elevation angle, however, is larger than 0.02 in many cases. For solar elevation angles from 20° to 60°, the variability of the albedo of the four surfaces can be represented by a mean curve which fits the individual variabilities with an accuracy of 0.015.     

Influence of soil surface roughness on soil bidirectional reflectance

The non-Lambertian behaviour of soil surfaces depends on its roughness at micro-scale and larger scales, as well as on the incident angle of the direct solar beam on the surface. A geometrical model, taking into account the diffuse as well as the specular component of energy leaving soil surfaces in the visible and near-infrared, is used in the paper to describe the influence of soil surface roughness, caused by soil aggregates or soil clods, on the soil bidirectional reflectance distribution. A rough soil surface in the model is simulated by equalsized opaque spheroids lying on a horizontal surface. The model was tested in outdoor conditions on artificially formed soil surfaces made of two spectrally different soil materials: a mineral loam, and a loam with high organic matter content. The spectral data were measured by a field radiometer in the three SPOT (HRV) bands. The model predicts that at specific illumination conditions, soils surfaces with the highest roughness, expressed by the minimum distances between soil aggregates, can show lower variation of reflectance in the view zenith angle function than soil surfaces of a lower roughness.     

基于模型模拟的植被NDVI与观测天顶角和LAI的关系

利用PROSPECT和SAIL模型模拟了不同叶绿素含量、不同LAI和不同观测天顶角下的植被冠层反射率,分析了NDVI随LAI、观测天顶角和叶绿素含量的变化规律。结果表明:叶绿素影响冠层反射率主要在可见光波段,冠层反射率随叶绿素含量的增加而下降;冠层反射率随观测天顶角的增加而增加,而LAI较高时,其受观测天顶角的影响较小。观测天顶角相同时,随叶绿素含量的增加NDVI呈上升趋势;叶绿素含量一定时,NDVI随LAI的增加而增加。LAI为1时,在不同叶绿素含量下,随观测天顶角的增加,NDVI呈先下降后上升的趋势,拐点在观测天顶角65°或70°处,而LAI为3、5和7时,NDVI呈现下降趋势。叶绿素含量较高时,NDVI受观测天顶角的影响较小。当LAI较大和叶绿素含量较低时,NDVI随观测天顶角的增加(>70°)下降较快。     

Canopy bidirectional reflectance dependence on leaf orientation

Abstract

The bidirectional reflectance patterns of a complete (dense) canopy are examined as functions of canopy architecture, as specified by azimuth angle δ_e and zenith angle ψ for a leaf normal. The leaves are assumed to be opaque Lambertian reflectors, all with identical orientation and reflectance properties throughout the canopy, and randomly distributed with respect to the irradiation field and the viewing direction. Multiple reflections are not considered and irradiation is by direct beam only. Simple analytical expressions for the bidirectional reflectance factor are presented and analysed. The nadir reflectance (expressed as a fraction of the leaf reflectance) for canopies whose leaves face the sun, δ_e = 0, is bounded by cos ψ and 1/2; cos ψ. The nadir reflectance initially increases with increasing ψ, but then decreases when ψ reaches moderate to large values. For a δ_e = π canopy, on the other hand, the much lower nadir reflectance is bounded by ½ cos ψ and 0, and decreases with increasing ψ throughout the entire range of ψ (0 to ½π). The maximum bidirectional reflectance occurs at large viewing zenith angles (i.e. close to the horizon). The maximum reflectance is always higher for a δ_e = 0 canopy than for a δ_e = π canopy, but the differences become small when ψ approaches ½π. The bidirectional reflectance thus depends on the leaf azimuth as well as the zenith angle. Leaf-area azimuthal distributions should be considered when conducting model inversions to infer canopy characteristics and architecture.