SolTran simulates the transmission of
solar radiation through tree canopies. Tree crowns are depicted as
three-dimensional solids: ellipsoids, cones, cylinders, or spheres.
Each crown is assigned a leaf area density which determines its light transmission
characteristics. A ray-tracing approach is used to estimate the probability
of light transmission to any point at any time. A digital elevation
model (DEM) of the terrain can be incorporated so that estimates incorporate
topographic effects (i.e., shading, slope orientation). A separate
module can produce a hemispheric ("fisheye") view from any point.
This permits the estimation of diffuse sky radiation penetration to any
point (still under construction). SolTran was derived from CrnSol
and PJSol (see Intro and Results from CrnSol).
Application at the WRCCRF
Goal: Estimate direct beam photosynthetically active radiation (PAR) at the ground surface throughout the year.
Parameterization: Used data of Jiquan Chen for crown dimensions and locations of 3610 trees. All crowns were depicted as ellipsoids and assigned a leaf area density of 0.25 m2/m3 (see previous work). A DEM for the site was constructed by Jiquan Chen at 1m postings. Values of PAR computed at each point are for a horizontal surface (i.e., these results do not include a correction for slope orientation).
Simulation target area: The tree-mapped area was 400 m x 200 m, but only the central 300m (east-west) by 100m (north-south) was simulated using 30000 points (1m spacing). That provided a 50 m buffer around the target area to minimize edge effects on simulated PAR.
Results:
Daily
course at summer solstice (contains animated GIF: 426,874
bytes)
Yearly
course (contains animated GIF: 680,098 bytes)
Fish-eye
Views along a Vertical Transect (contains animated GIF:
150,243 bytes)
Return to Simulations of below-canopy ....
Last Updated on February 2, 1998
By Scott N. Martens
Email: scott@cstars.ucdavis.edu