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The effect of viewing geometry and topography on viewable gap fractions through forest canopies
Author(s) -
Liu Jicheng,
Melloh Rae A.,
Woodcock Curtis E.,
Davis Robert E.,
Ochs Elke S.
Publication year - 2004
Publication title -
hydrological processes
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.222
H-Index - 161
eISSN - 1099-1085
pISSN - 0885-6087
DOI - 10.1002/hyp.5802
Subject(s) - zenith , nadir , remote sensing , radiative transfer , canopy , environmental science , geology , geometry , geography , physics , optics , mathematics , satellite , archaeology , astronomy
The fraction of the surface under forest canopies that is visible from above, or the viewable gap fraction (VGF), influences a number of significant physical processes, such as the longwave radiation budget of the surface and the magnitude of diffuse irradiance. In addition, it has significant implications for the remote sensing of the surface. The VGF is dependent on canopy structure, topography and viewing geometry. Although it is difficult to map VGF using current operational remote sensing systems, it is possible to estimate VGF using models based on the three‐dimensional structure of forest canopies. Results from hemispheric photographs taken in the field at Fraser Experimental Forest, Colorado, and a geometric optical (GO) model show a trend of rapid decrease in VGF as the view zenith angles diverges from nadir. Whereas there is general agreement between model estimates and the hemispheric photographs, the hemispheric photographs generally show higher VGF values for all view zenith angles. In particular, the higher values for VGF are apparent at high view zenith angles. Use of a more complicated GO radiative transfer model would add the effect of within‐crown gaps to those modelled by the GO model and will be used in future studies. VGF maps estimated using the GO model for the Fool Creek intensive study area show a significant decrease in VGF when view zenith angle is increased from 0° (nadir) to 30° viewing from the east. To produce VGF maps in mountain areas, the effect of topography must be taken into account, as changes in slope angle and azimuth are similar to changes in the view zenith angle. Hence, topography can serve either to accentuate or to minimize view zenith angle effects, depending on the slope orientation relative to the viewing position. Copyright © 2004 John Wiley & Sons, Ltd.