Research Article

The effect of viewing geometry and topography on viewable gap fractions through forest canopies

  1. Jicheng Liu1,*,
  2. Rae A. Melloh2,
  3. Curtis E. Woodcock1,
  4. Robert E. Davis2,
  5. Elke S. Ochs2

Article first published online: 13 DEC 2004

DOI: 10.1002/hyp.5802

Issue

Hydrological Processes

Hydrological Processes

Special Issue: Eastern Snow Conference/Western Snow Conference

Volume 18, Issue 18, pages 3595–3607, 30 December 2004

Additional Information

How to Cite

Liu, J., Melloh, R. A., Woodcock, C. E., Davis, R. E. and Ochs, E. S. (2004), The effect of viewing geometry and topography on viewable gap fractions through forest canopies. Hydrological Processes, 18: 3595–3607. doi: 10.1002/hyp.5802

Author Information

  1. 1

    Department of Geography, Boston University, Boston, MA 02215, USA

  2. 2

    ERDC—Cold Regions Research and Engineering Laboratory, Hanover, NH 03755, USA

*Department of Geography, Boston University, Boston, MA 02215, USA.

Publication History

  1. Issue published online: 13 DEC 2004
  2. Article first published online: 13 DEC 2004
  3. Manuscript Accepted: 11 SEP 2004
  4. Manuscript Received: 15 JUN 2004

Funded by

  • US Army

SEARCH

SEARCH BY CITATION

ARTICLE TOOLS

Get PDF (674K)

Keywords:

  • viewable gap fractions;
  • view zenith angle;
  • GO model;
  • remote sensing of snow

Abstract

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.

Get PDF (674K)