Charles L. Walthall

Estimating Corn Leaf Chlorophyll Concentration from Leaf and Canopy Reflectance

Farmers must balance the competing goals of supplying adequate N for their crops while minimizing N losses to the environment. To characterize the spatial variability of N over large fields, traditional methods (soil testing, plant tissue analysis, and chlorophyll meters) require many point samples. Because of the close link between leaf chlorophyll and leaf N concentration, remote sensing techniques have the potential to evaluate the N variability over large fields quickly. Our objectives were to (1) select wavelengths sensitive to leaf chlorophyll concentration, (2) simulate canopy reflectance using a radiative transfer model, and (3) propose a strategy for detecting leaf chlorophyll status of plants using remotely sensed data. A wide range of leaf chlorophyll levels was established in field-grown corn (Zea mays L.) with the application of 8 N levels: 0%, 12.5%, 25%, 50%, 75%, 100%, 125%, and 150% of the recommended rate. Reflectance and transmittance spectra of fully expanded upper leaves were acquired over the 400-nm to 1,000-nm wavelength range shortly after anthesis with a spectroradiometer and integrating sphere. Broad-band differences in leaf spectra were observed near 550 nm, 715 nm, and >750 nm. Crop canopy reflectance was simulated using the SAIL (Scattering by Arbitrarily Inclined Leaves) canopy reflectance model for a wide range of background reflectances, leaf area indices (LAI), and leaf chlorophyll concentrations. Variations in background reflectance and LAI confounded the detection of the relatively subtle differences in canopy reflectance due to changes in leaf chlorophyll concentration. Spectral vegetation indices that combined near-infrared reflectance and red reflectance (e.g., OSAVI and NIR/Red) minimized contributions of background reflectance, while spectral vegetation indices that combined reflectances of near-infrared and other visible bands (MCARI and NIR/Green) were responsive to both leaf chlorophyll concentrations and background reflectance. Pairs of these spectral vegetation indices plotted together produced isolines of leaf chlorophyll concentrations. The slopes of these isolines were linearly related to leaf chlorophyll concentration. A limited test with measured canopy reflectance and leaf chlorophyll data confirmed these results. The characterization of leaf chlorophyll concentrations at the field scale without the confounding problem of background reflectance and LAI variability holds promise as a valuable aid for decision making in managing N applications.

Retrieval of land surface albedo from satellite observations : A simulation study

Land surface albedo is a critical parameter affecting the earths climate and is required by global and regional climatic modeling and surface energy balance monitoring. Surface albedo retrieved from satellite observations at one atmospheric condition may not be suitable for applicati to other atmospheric conditions. In this paper the authors separate the apparent surface albedo from the inherent surface albedo, which is independent of atmospheric conditions, based on extensive radiative transfer simulations under a variety of atmospheric conditions. The results show that spectral inherent albedos are different from spectral apparent albedos in many cases. Total shortwave apparent albedos under both clear and cloudy conditions are also significantly different from their inherent total shortwave albedos. The conversion coefficients of the surface inherent narrowband albedos derived from the MODIS (Moderate-Resolution Imaging Spectroradiometer) and the MISR (Multiangle Imaging Spectroradiometer) instruments to the surface broadband inherent albedo are reported. A new approach of predicting broadband surface inherent albedos from MODIS or MISR top of atmosphere (TOA) narrowband albedos using a neural network is proposed. The simulations show that surface total shortwave and near-infrared inherent albedos can be predicted accurately from TOA narrowband albedos without atmospheric information, whereas visible inherent albedo cannot.

Remote sensing of photosynthetic-light-use efficiency of boreal forest

Using a helicopter-mounted portable spectroradiometer and continuous eddy covariance data we were able to evaluate the photochemical reflectance index (PRI) as an indicator of canopy photosynthetic light-use efficiency (LUE) in four boreal forest species during the Boreal Ecosystem Atmosphere experiment (BOREAS). PRI was calculated from narrow waveband reflectance data and correlated with LUE calculated from eddy covariance data. Significant linear correlations were found between PRI and LUE when the four species were grouped together and when divided into functional type: coniferous and deciduous. Data from the helicopter-mounted spectroradiometer were then averaged to represent data generated by the Airborne Visible Infrared Imaging Spectrometer (AVIRIS). We calculated PRI from these data and relationships with canopy LUE were investigated. The relationship between PRI and LUE was weakened for deciduous species but strengthened for the coniferous species. The robust nature of this relationship suggests that relative photosynthetic rates may be derived from remotely-sensed reflectance measurements. ©2000 Elsevier Science B.V. All rights reserved.

Narrowband to broadband conversions of land surface albedo: II. Validation

Abstract In the first paper of this series, we developed narrowband to broadband albedo conversion formulae for a series of sensors. These formulae were determined based on extensive radiative transfer simulations under different surface and atmospheric conditions. However, it is important to validate the simulation results using independent measurement data. In this paper, the validation results for three broadband albedos (total-shortwave, -visible and -near-IR albedos) using ground measurement of several cover types on five different days at Beltsville, MD are presented. Results show that the conversion formulae in the previous paper are very accurate and the average residual standard errors of the resulting broadband albedos for most sensors are around 0.02, which meets the required accuracy for land surface modeling.

Atmospheric correction of Landsat ETM+ land surface imagery. II. Validation and applications

For pt.I see ibid., vol.39, no.11, p.2490-8 (2001). This is the second paper of the series on atmospheric correction of Enhanced Thematic Mapper-Plus (ETM+) land surface imagery. In the first paper, a new algorithm that corrects heterogeneous aerosol scattering and surface adjacency effects was presented. In this study, our objectives are to (1) evaluate the accuracy of this new atmospheric correction algorithm using ground radiometric measurements, (2) apply this algorithm to correct Moderate-Resolution Imaging Spectroradiometer (MODIS) and SeaWiFS imagery, and (3) demonstrate how much atmospheric correction of ETM+ imagery can improve land cover classification, change detection, and broadband albedo calculations. Validation results indicate that this new algorithm can retrieve surface reflectance from ETM+ imagery accurately. All experimental cases demonstrate that this algorithm can be used for correcting both MODIS and SeaWiFS imagery. Although more tests and validation exercises are needed, it has been proven promising to correct different multispectral imagery operationally. We have also demonstrated that atmospheric correction does matter.

Profiles of photosynthetically active radiation, nitrogen and photosynthetic capacity in the boreal forest: Implications for scaling from leaf to canopy

Profiles of photosynthetically active radiation (PAR), leaf nitrogen per unit leaf area (Narea), and photosynthetic capacity (Amax) were measured in an aspen, two jack pine, and two black spruce stands in the BOREAS northern study area. Narea decreased with decreasing %PAR in each stand, in all conifer stands combined (r=0.52) and in all stands combined (r=0.46). Understory alder had higher Narea for similar %PAR than did aspen early in the growing season. Amax decreased with decreasing Narea, except for the negative correlation between Narea and Amax during shoot flush for jack pine. For the middle and late growing season data, Narea and Amax had r values of 0.51 for all stands combined and 0.60 for all conifer stands combined. For similar Narea the aspen stand had higher Amax than did the conifer stands. Photosynthetic capacity expressed as a percentage of Amax at the top of the canopy (%Amax0) decreased with %PAR similarly in all stands, but %Amax0 decreased at a much slower rate than did %PAR. To demonstrate the implications of the vertical distribution of Amax, three different assumptions were used to scale leaf Amax to the canopy (Acan-max): (1) constant Amax with canopy depth, (2)Amax scaled proportionally to %PAR, and (3) a linear relationship between Amax and cumulative leaf area index derived from our data. The first and third methods resulted in similar Acan-max; the second was much lower. All methods resulted in linear correlations between normalized difference vegetation indices measured from a helicopter and Acan-max (r=0.97, 0.93, and 0.97, respectively), but the slope was strongly influenced by the scaling method.

Prairie grassland bidirectional reflectances measured by different instruments at the FIFE site

Land surface reflectance measurements were acquired during the First ISLSCP Field Experiment (FIFE) field campaigns using a variety of ground-based and airborne spectral radiometers. To examine the validity of the assumption that the values acquired by the several different instruments and teams were interchangeable, the surface radiation measurement teams converged on a common site for 1 day during the fifth intensive field campaign (IFC 5) in 1989. The instruments compared for near-surface measurements included two ground-based Spectron Engineering SE59Os, one helicopter-mounted SE590, one ground-based and one helicopter-mounted Barnes modular multiband radiometer (MMR), and the portable apparatus for rapid acquisitions of bidirectional observations of land and atmosphere (PARABOLA) field radiometer. Comparisons were made for nadir measurements over a range of solar zenith angles and a range of off-nadir viewing angles. The bidirectional reflectances from the different instruments were generally found to be quite comparable. For example, for a 52° solar zenith angle, the nadir red and near-infrared spectral reflectance factors ranged from 3.5 to 4.5% and 28.2 to 31.9%, respectively. At the smaller solar zenith angles, however, the differences were somewhat greater (red, 4.5–6.1%; near-infrared (NIR), 25.0–28.9%), and the coefficients of variation for the samples taken by all of the instruments increased. Off-nadir viewing caused major departures from nadir bidirectional reflectances (30% reflectance at nadir compared with 55% at 60° off nadir in the NIR, for example), but all of the instruments captured the effects reasonably well. Spectral vegetation indices were found to have a considerable dependence on both solar zenith angle and sensor viewing angle. In spite of the general agreement between most instruments and teams, the lack of a more consistent band-to-band agreement resulted in appreciable differences in the spectral vegetation index values, which could potentially affect the accuracy and precision of remote sensing assessments of biophysical parameters.

Estimation of shortwave hemispherical reflectance (albedo) from bidirectionally reflected radiance data

Abstract Albedo is the ratio of reflected solar radiation from a surface to that incident upon it. Due to the spatial and temporal resolution of satellite remote sensing instruments, many formulations have been developed to convert remotely sensed data into estmates of albedo. Most of these equations depend upon the assumption of isotropic reflection and, therefore, use only nadir measurements; only in recent years have investigators attempted to model the anisotropic nature of terrestrial surfaces. A Barnes Modular Multiband Radiometer (MMR) was used to collect remotely sensed data from prairie vegetation at seven view zenith angles in the solar principal plane. An equation to estimate albedo from bidirectional reflectance data is proposed and evaluated in this paper. The estimates of albedo were greater than values obtained with simultaneous pyranometer measurements: a more conventional approach. The overestimation was systematic. Potential sources of error are discussed and include: 1) extrapolation of the bidirectional reflectance data out to a view zenith angle of 90°; 2) use of inappropriate weighting coefficients in the numerator of the albedo equation; 3) surface shadowing caused by the A-frame intrumentation used to measure the incoming and outgoing radiation fluxes; 4) errors in estimates of the denominator of the proposed albedo equation (i.e., incoming shortwave radiation); and 5) a “hot spot” contribution in bidirectional data measured by the MMR.

Spectral Discrimination of Cannabis sativa L. Leaves and Canopies

The growing of marijuana (Cannabis sativa L.) on public lands poses problems to the environment and the public. Remote sensing offers a potential way of monitoring public lands for the production of marijuana. However, very little information on the spectral properties of marijuana is available in the scientific literature. Our objectives were to characterize the spectral properties of the leaves of marijuana and various other plants that occur where marijuana is grown in the eastern United States, simulate canopy reflectance, and identify wavebands for discriminating marijuana from other plants. In a series of replicated field experiments, the basic factors affecting marijuana growth and reflectance, including planting date, plant density, and N-fertilization were varied. Leaf optical properties were measured periodically during the growing season with a spectroradiometer and integrating sphere. As N-fertilization rate decreased, the marijuana plants produced leaves with lower chlorophyll concentrations and higher reflectance values in the visible wavelength region, particularly at 550 nm. The reflectance spectra of the herbaceous dicot species examined were very similar to the spectrum of marijuana. The reflectance spectra of the monocots and the trees differed significantly from the spectrum of marijuana, particularly in the green and near-infrared wavelength regions. Canopy reflectance spectra of marijuana and several representative species were simulated for a wide range of LAI and background reflectances. Major differences in canopy reflectance of marijuana and other plants were observed near 550 nm, 720 nm, and 800 nm. Dense canopies of maryjuana were more spectrally discriminable from other vegetation than sparse canopies. Thus, based on measured leaf spectra and simulated canopy reflectance spectra, we would choose several relatively narrow (i.e., 30 nm or less) spectral bands in the green (550 nm), red (670 nm), red edge (720 nm), and the near-infrared (800 nm) to discriminate marijuana leaves and canopies from other species. Much of the leaf spectral information is also available in the canopy reflectance data.

Evidence of hot spot directional signature from airborne POLDER measurements

The POLDER instrument was flown during the BOREAS experiment over various sites and at various altitudes in the Canadian boreal forest and other nearby targets. The instrument design permits the acquisition of the directional signature of any surface cover. In particular, the high directional resolution of POLDER allows it to measure, with an unprecedented accuracy, the hot spot signature of natural targets. The authors present some typical examples of such highly anisotropic reflectance directional signatures. The ratio of the maximum reflectance (hot spot direction) to the minimum reflectance (broad area in the forward scattering hemisphere) varies with wavelength and canopy. It can be as large as six in the visible and three in the near IR.