Soil Optical Properties Research Articles

Operational relationships between NOAA‐advanced very high resolution radiometer vegetation indices and daily fraction of absorbed photosynthetically active radiation, established for Sahelian vegetation canopies

To improve the estimation of primary production at a regional scale, an assessment of the utility of fraction of absorbed photosynthetically active radiation (fAPAR) estimated from spectral vegetation indices (VI) for the case of the Sahelian vegetation was studied. Simulations using a three‐dimensional radiative transfer model were conducted for two types of structurally distinct vegetation canopies: millet crop (regularly distributed clumps of vegetation over bare soil) and savanna (mixture of a grass layer and a sparse woody layer). A realistic range of values is extracted for each vegetation input variable (leaf area index, ground cover, height and spatial distribution) from published literature. Bidirectional reflectance factors were calculated in the NOAA‐advanced very high resolution radiometer (AVHRR) spectral bands for a geometric configuration representative of the NOAA satellite series. Two vegetation indices were tested: normalized difference vegetation index (NDVI) and modified soil adjusted vegetation index (MSAVI). The simulations indicate that the fAPAR‐VI relationship is sensitive to the geometry of measurement and soil optical properties, especially in the case of a millet crop. Simple linear models that include this variability and are directly applicable to atmospherically corrected AVHRR data are proposed, and the error of estimation of fAPAR is evaluated. MSAVI gives better results, especially when the brightness of soil is known. For natural vegetation the relationship between fAPAR and MSAVI is then reduced to a coefficient that is a constant (∼2.2). For millet canopies this coefficient is a function of Sun and view angles. Finally, the utility of our approach is illustrated with two examples of fAPAR‐VI relationships applied to a NOAA time series.

Remote Determination of Exposure Degree and Iron Concentration of Lunar Soils Using VIS-NIR Spectroscopic Methods

On the Moon, space weathering processes such as micrometeorite bombardment alter the optical properties of lunar soils. As a consequence, lunar soil optical properties are a function not only of composition, but of degree of exposure on the lunar surface as well. In order to accurately assess the compositional properties of the lunar surface using remotely acquired visible and near-infrared spectroscopic data, it is thus necessary either (1) to compare optical properties only of soils characterized by similar degrees of exposure or (2) to otherwise normalize or remove the optical effects due to exposure. Laboratory spectroscopic data for lunar soils are used to develop and test remote spectroscopic methods for determining degree of exposure and for distinguishing between the optical effects due to exposure and those due to composition. A method employing a ratio between reflectances within and outside of the 1 μm Fe 2+ crystal field absorption band was developed for remotely identifying highland soils that have reached a steady-state maturity. The relative optical properties of these soils are a function solely of composition and as such can be directly compared. Spectroscopic techniques for accurate quantitative determination of iron content for lunar highland soils are investigated as well. It is shown that approximations of the 1 μm Fe 2+ absorption band depth using few to several channel multispectral data or spectroscopic data of inadequate spectral range cannot be used with confidence for compositional analysis. However, band depth measurements derived from continuum-removed high spectral resolution data can be used to calculate the weight percent FeO and relative proportion of iron-bearing silicates in mature lunar highland and mare/highland mixture soils. A preliminary effort to calibrate telescopic band depth to laboratory soil measurements is described.

Modeling the bidirectional reflectance distribution function of mixed finite plant canopies and soil

An analytical model of the bidirectional reflectance for optically semi‐infinite plant canopies has been extended to describe the reflectance of finite depth canopies with contributions from the underlying soil. The model depends on 10 independent parameters describing vegetation and soil optical and structural properties. The model is inverted with a nonlinear minimization routine using directional reflectance data for lawn (leaf area index (LAI) is equal to 9.9), soybeans (LAI, 2.9) and simulated reflectance data (LAI, 1.0) from a numerical bidirectional reflectance distribution function (BRDF) model (Myneni et al., 1988). While the ten‐parameter model results in relatively low rms differences for the BRDF, most of the retrieved parameters exhibit poor stability. The most stable parameter was the single‐scattering albedo of the vegetation. Canopy albedo could be derived with an accuracy of less than 5% relative error in the visible and less than 1% in the near‐infrared. Sensitivity tests were performed to determine which of the 10 parameters were most important and to assess the effects of Gaussian noise on the parameter retrievals. Out of the 10 parameters, three were identified which described most of the BRDF variability. At low LAI values the most influential parameters were the single‐scattering albedos (both soil and vegetation) and LAI, while at higher LAI values (> 2.5) these shifted to the two scattering phase function parameters for vegetation and the single‐scattering albedo of the vegetation. The three‐parameter model, formed by fixing the seven least significant parameters, gave higher rms values but was less sensitive to noise in the BRDF than the full ten‐parameter model. A full hemispherical reflectance data set for lawn was then interpolated to yield BRDF values corresponding to advanced very high resolution radiometer (AVHRR) scan geometries collected over a period of nine days. The resulting retrieved parameters and BRDFs are similar to those for the full sampling geometry, suggesting that the limited geometry of AVHRR measurements might be used to reliably retrieve BRDF and canopy albedo with this model.

Investigation of soil influences in AVHRR red and near-infrared vegetation index imagery

The effects of soil optical properties on vegetation index imagery are analyzed with ground-based spectral measurements and both simulated and actual AVHRR data from the NOAA satellites. Soil effects on vegetation indices were divided into primary variations associated with the brightness of bare soils, secondary variations attributed to 'color' differences among bare soils, and soil-vegetation spectral mixing. Primary variations were attributed to shifts in the soil line owing to atmosphere or soil composition. Secondary soil variance was responsible for the Saharan desert 'artefact' areas of increased vegetation index response in AVHRR imagery.

Crop biomass evaluation using radiometric measurements

Crop biomass can be evaluated from radiometric measurements either by relating biomass to instantaneous measurements or by relating an integral of biomass to a radiometric value integrated over the corresponding portion of the growth period. In this study, the success obtained by using these two methods is discussed. A simple radiative transfer model was used in conjunction with experimental results to demonstrate the universality of the relationship between the normalized difference vegetation index ( ND) and leaf area index ( LAI) or the photosynthetically active radiation ( PAR) absorbed by the crop. It shows that the relationship between ND and absorbed PAR is less dependent on leaf orientation than the relationship between ND and LAI. A remaining problem is the sensitivity of those two relationships to soil optical properties. Nevertheless, temporal integration of radiometric data using the absorbed PAR concept appears to be a more promising approach than one-time measurements.

A physical model for predicting bidirectional reflectances over bare soil

Radiative processes at the Earth's surface are affected by the soil optical properties. A good understanding of radiative transfer over a bare soil surface is therefore a prerequisite to addressing more complex cases of discontinuous vegetation canopies. While most attempts at retrieving surface characteristics are made by fitting empirical functions to the data, we have applied an optimization technique to a physically based surface reflectance model initially developed to study planetary surfaces. This inversion procedure allows the direct estimation of the following five parameters: the single scattering coefficient, two parameters describing the hot spot phenomenon, and two parameters describing the scattering phase function. The approach is validated by testing the inversion procedure against both synthetic data and actual observations. The model is able to predict the observed bidirectional reflectances, as well as the directional-hemispherical reflectances, and to build the complete field of radiances over the upward hemisphere. The anisotropic behavior of bare soils is investigated and the prospect for inferring other basic soil properties, such as the soil porosity, is discussed.

Hemispherical Reflectance Variations of Vegetation Canopies and Implications for Global and Regional Energy Budget Studies

The hemispherical reflectance (HR) variations of vegetation canopies are studied as a function of solar zenith angle, wavelength, and canopy characteristics (leaf area index, leaf orientation distribution, and leaf and soil optical properties). The radiative transfer model of Kimes (1984) is used to explain the radiative transfers that give rise to variations in the HR for various vegetation canopies as a function of sun angle. The results of this model are compared with those calculated from Sellers' (1985) analytical two-stream approximation. It is noted that the findings have significant implications for a wide range of global and regional energy budget studies.