Browsing by Subject "Vegetation"
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Item Absorbed Photosynthetically Active Radiation of Steppe Vegetation and Sun-View Geometry Effects on Apar Estimates(IEEE, 1992-05-26) Walter-Shea, E. A.; Blad, B. L.; Mesarch, M. A.; Hays, C. J.; Deering, D. W.; Eck, ThomasInstantaneous fractions of absorbed photosynthetically active radiation (APAR) were measured at the Streletskaya Steppe Reserve of the Soviet Union in conjunction with canopy bidirectional-reflected radiation measured at solar zenith angles ranging between 37 and 74" during KUREX-91. APAR values were higher for KUREX-91 than those for FIFE-89 and the amount of APAR of a canopy was a function of solar zenith angle, decreasing as solar zenith angle increased at the Reserve. Differences in absorption are attributed to leaf area index (LAI) and leaf angle distribution and subsequently transmitted radiation interactions. LA1 was considerably higher at the Reserve than those at the FIFE site. In addition, leaf angle distributions of the Reserve approach a uniform distribution while distributions at the FIFE site more closely approximate erectophile distributions. Reflected PAR components at KUREX-91 and FIFE-89 were similar in magnitude and in their response to solar zenith angle (near constant value). However, transmitted PAR increased with increasing solar zenith angle at KUREX-91 and decreased with increasing solar zenith angle at FIFE-89. Transmitted PAR at FIFE-89 was considerably larger than those at KUREX-91. Spectral vegetative indices derived from canopy bidirectional reflected radiation are a function of solar and viewing directions indicating that solar and viewing directions must be considered in using remotely-sensed data to derive surface biophysical parameters.Item Bidirectional Reflectances of Three Soil Surfaces and Their Characterization through Model Inversion(IEEE, 1989-07) Deering, D. W.; Eck, Thomas; Otterman, J.Spectral bidirectional reflectances were measured over three natural soil sites using a specially designed radiometer called the PARABOLA. TWo of the sites were bare soils and the third had a sparse cover of desert scrub. The reflectances were strongly non Lambertian for all three surfaces but with markedly different patterns. The measured data were fitted with a quasi-physical reflectance model in which the surface back scattering and forwardscattering are separately formulated. A soil reflectance characterization was obtained by assessing the contributions of the forward, -backward and Lambertian components. This three-parameter characterization produced a satisfactory fit to the measured reflectances, and appears promising to provide a basis for soils categorization.Item Canopy Albedo And Transm1ittance In A Boreal Forest(IEEE, 1990-05-20) Eck, Thomas; Deering, D. W.The shortwave radiation balance of a forest stand of predominately spruce and hemlock in central Maine was investigated. Pyranomder measurements of insolation, reflected solar radiation from the top of the canopy, radiation incident on the forest floor, and radiation reftect ed from the forest floor were taken under both cloudless sky conditions and overcast cloud conditions. Measurements were made on consecutive days; insuring minimal change in forest canopy characteristics. The canopy albedo computed from half-day radiation totals, thus representative of a daily value, was 10.8 % on the clear day and 9. 8 % on the overcast day. The instantaneous albedo on the clear day ranged from 8.7 % at 43° solar zenith angle (solar noon) to 20.1% at 78° solar zenith angle, while on the overcast day the albedo was relatively constant due to the diffuse irradiance condition. Canopy transmittance for the daily period was 2.9% on the clear day and 4.2% on the overcast day. The canopy transmittance was nearly constant throughout the day on the overcast day at about 4.2% while sunfleck penetration on the clear day resulted in a higher transmittance of 4.5% from 43-47° solar zenith angles, that dropped to 1.8% at 68-72° solar zenith angles. Thus over the entire day, for the range of solar zenith angles measured, diffuse radiation on the overcast day penetrated further down into the forest canopy than the predominately direct radiation on the clear day. Keywords: albedo, transmittance, forest, pyranometer.Item Nadir and bidirectional surface measurements of Arctic tundra: site differentiation and vegetation phenology early in the growing season(IEEE, 2002-08-06) Vierling, L.A.; Deering, D.W.; Eck, ThomasGround-based nadir and bidirectional radiometric measurements were collected on the North Slope of Alaska in early June to determine their utility in differentiating Arctic tundra vegetation community types and examining differences in vegetation community structure and phenology. Three common Arctic vegetation types were sampled: lowland wet sedge, tussock tundra essentially devoid of woody vegetation, and tussock tundra with dwarf woody shrub vegetation. Nadir measurements alone allowed spectral differentiation between the wet sedge site and the two tussock tundra sites, but did not allow dear discrimination between the two tussock tundra sites. Bidirectional reflectance measurements, however, permitted good discrimination between the two tussock tundra sites. This outcome is largely due to the fact that normalized difference vegetation index (NDVI) values derived from measurements taken at large view zenith angles were sensitive to the woody plant foliage that extends above the hummocky tussock tundra surface early in the growing season.Item Shinnery oak bidirectional reflectance properties and canopy model inversion(IEEE, 1992-03) Deering, D. W.; Eck, Thomas; Grier, T.The authors present field measurements and the results of a three-dimensional canopy model inversion for sand shinnery oak. Spectral bidirectional radiance measurements in three spectral channels, 0.65-0.67 mu m, 0.81-0.84 mu m, and 1.62-1.69 mu m, encompassing both the complete land surface and sky hemispheres, were acquired for a sand shinnery oak plant community in west Texas. The changes in canopy reflectance that occur with variations in solar zenith angle and view direction and for two seasons of the year were evaluated. A three-dimensional radiation interaction model (TRIM) was then inverted to estimate oak leaf area index (LAI) and canopy density, expressed as percentage of cover, from the bidirectional reflectance data.Item Spectral Bidirectional and Hemispherical Reflectance Characteristics of Selected Sites in the Streletskaya Steppe(IEEE, 1992-05-26) Eck, Thomas; Deering, D. W.Measurements of plant canopy bidirectional reflectance made by the PARABOLA (Portable Apparatus for Rapid Acquistion of Bidirectional Observations of the Land and Atmosphere) instrument in three spectral bands (662,826,1658 nm) are analyzed for steppe grassland sites of differing productivity levels. The variation of spectral reflectance and the normalized difference vegetation index in the solar principal plane is presented. Comparisons are made with PARABOLA measurements from selected FIFE grassland sites in the Konza Prairie, Kansas. The Streletskaya Steppe sites showed no strong hot spot reflectance, while this effect was present in some FIFE sites but absent in others. The hot spot effect seems to be dependent on canopy geometry and background reflectance characteristics of these sites. Spectral hemispherical reflectance was computed from the angular integration of the directional measurements for the steppe sites. Total shortwave albedo was estimated from these hemispherical reflectance measurements and compared to albedo measured by pyranometers. The albedo estimates from PARABOLA were found to be from approximately 12-17% higher than the pyranometer measurements.