Aberystwyth University
Radiative Transfer Modeling of a Coniferous Canopy Characterized by Airborne
Remote Sensing
Essery, Richard; Bunting, Peter John; Rowlands, Aled Prys; Rutter, Nick; Hardy, Janet Hazel; Melloh, Rae; Link,
Tim; Marks, Danny; Pomeroy, John W.
Published in:
Journal of Hydrometeorology
DOI:
10.1175/2007JHM870.1
Publication date:
2008
Citation for published version (APA):
Essery, R., Bunting, P. J., Rowlands, A. P., Rutter, N., Hardy, J. H., Melloh, R., Link, T., Marks, D., & Pomeroy,
J. W. (2008). Radiative Transfer Modeling of a Coniferous Canopy Characterized by Airborne Remote Sensing.
Journal of Hydrometeorology, 9(2), 228-241. https://doi.org/10.1175/2007JHM870.1
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Radiative Transfer Modeling of a Coniferous Canopy Characterized by Airborne
Remote Sensing
RICHARD ESSERY,*
,
** PETER BUNTING,* JANET HARDY,
⫹
TIM LINK,
#
DANNY MARKS,
@
R
AE MELLOH,
⫹
JOHN POMEROY,
&
ALED ROWLANDS,* AND NICK RUTTER*
* Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Aberystwyth, United Kingdom
⫹
Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire
#
University of Idaho, Moscow, Idaho
@
Northwest Watershed Research Center, Boise, Idaho
&
Centre for Hydrology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
(Manuscript received 8 January 2007, in final form 25 June 2007)
ABSTRACT
Solar radiation beneath a forest canopy can have large spatial variations, but this is frequently neglected
in radiative transfer models for large-scale applications. To explicitly model spatial variations in subcanopy
radiation, maps of canopy structure are required. Aerial photography and airborne laser scanning are used
to map tree locations, heights, and crown diameters for a lodgepole pine forest in Colorado as inputs to a
spatially explicit radiative transfer model. Statistics of subcanopy radiation simulated by the model are
compared with measurements from radiometer arrays, and scaling of spatial statistics with temporal aver-
aging and array size is discussed. Efficient parameterizations for spatial averages and standard deviations of
subcanopy radiation are developed using parameters that can be obtained from the model or hemispherical
photography.
1. Introduction
Forest canopies strongly modify radiative fluxes
reaching the underlying surface. This has important im-
plications for hydrological and ecological processes,
such as snowmelt and succession, in forested environ-
ments (Pomeroy and Dion 1996; Battaglia et al. 2002;
Hardy et al. 2004). Conversely, radiation reflected and
emitted from trees complicates the retrieval of forest
snow properties by remote sensing (Chang et al. 1996;
Klein et al. 1998). Land surface models and remote
sensing algorithms, therefore, often include simple rep-
resentations of radiative transfer in canopies. Variants
of Beer’s law or two-stream approximations are gener-
ally used (e.g., Sellers et al. 1986; Verseghy et al. 1993);
these treat canopies as horizontally homogeneous tur-
bid media and only predict the average radiation. The
radiative environment beneath real canopies, however,
is highly heterogeneous because of sun flecks, canopy
gaps, and clearings on wide ranges of length scales. The
significance of spatial heterogeneity at a particular
length scale will depend on the time scale and nonlin-
earity of the process of interest. In snowmelt, for ex-
ample, the spatial variance of shortwave radiation in-
fluences the depletion of snow-covered area (Essery
and Pomeroy 2004) and longwave radiation from sunlit
trunks (Faria et al. 2000; Woo and Giesbrecht 2000).
Geometrical optics and ray-tracing models have been
used to simulate radiative transfer in canopies; the Ra-
diation Transfer Model Intercomparison (Pinty et al.
2004) found that such models now compare well for
homogeneous canopies but still have large discrepan-
cies for complex heterogeneous canopies. These mod-
els are also computationally expensive and require
large amounts of data on canopy structure and the op-
tical properties of canopy elements. A great deal of
work on small-scale radiative transfer through hetero-
geneous canopies has used statistical models that as-
sume random tree distributions or spatially explicit
models with randomly generated tree distributions
** Current affiliation: School of GeoSciences, Grant Institute,
University of Edinburgh, Edinburgh, United Kingdom.
Corresponding author address: Richard Essery, School of Geo-
Sciences, Grant Institute, University of Edinburgh, Edinburgh
EH9 3JW, United Kingdom.
E-mail: richard.essery@ed.ac.uk
228 JOURNAL OF HYDROMETEOROLOGY VOLUME 9
DOI: 10.1175/2007JHM870.1
© 2008 American Meteorological Society
JHM870
(e.g., Satterlund 1983; Li et al. 1995; Yang et al. 2001;
Song and Band 2004). Fewer studies have used real
structural data from natural or managed forests (e.g.,
Courbaud et al. 2003; Stadt et al. 2005). Hemispherical
photography provides a method for relatively rapid
gathering of canopy structure information in the field
(Rich 1990), and several software packages have been
developed for modeling forest light environments from
hemispherical photography, but it is difficult to apply
this to large areas or inaccessible locations. Airborne
remote sensing enables collection of large amounts of
canopy data but has not yet been widely used in radia-
tion modeling.
The aims of this paper are to demonstrate the use of
remote sensing in mapping of forest stand characteris-
tics for radiative transfer modeling, to develop a model
that uses stand maps to simulate patterns of solar ra-
diation beneath the canopy, to evaluate the model in
comparison with ground-based measurements, to inves-
tigate the spatial and temporal scaling of subcanopy
radiation statistics, and to develop parameterizations of
those statistics. Data from aerial photography and lidar
(light detection and ranging) scanning of a coniferous
forest with areas of varying canopy density and unifor-
mity are used, as described in the next section. Crowns
are delineated in the photograph to map the location
and crown diameter of each tree, and tree heights are
assigned from lidar elevation returns. A spatially ex-
plicit model that uses these data to simulate transmis-
sion of solar radiation to the forest floor is developed in
section 3; the complex pattern of transmission through
canopy gaps is explicitly represented, but, for numerical
efficiency and to minimize the number of parameters
required, a very simple scheme is used for transmission
through crowns. Model results are compared with
ground-based hemispherical photography and distrib-
uted measurements of radiation under the canopy on
clear and cloudy days in section 4. Because the spatially
explicit model is too computationally expensive to run
for large areas or long times, efficient parameteriza-
tions of radiation statistics are sought in section 5. Fi-
nally, section 6 presents conclusions and discusses fu-
ture plans.
2. Site and data description
The Cold Land Processes Experiment (CLPX) was
conducted in Colorado during the winters of 2002 and
2003. Data used in this study were obtained at the Local
Scale Observation Site (LSOS; Hardy et al. 2004, 2007,
manuscript submitted to J. Hydrometeor.), which was
within the U.S. Forest Service Fraser Experimental
Forest [39.9°N, 105.9°W, 2780 m above sea level
(ASL)] and is shown by the color infrared orthophoto-
graph in Fig. 1a. A clearing separates two stands of
contrasting structure. To the south of the clearing, there
is a uniform lodgepole pine plantation; from manual
FIG. 1. (a) Aerial color infrared orthophotograph (shown in
black and white) of the study site. The area shown is 100 m ⫻ 100
m, with north at the top. (b) Normalized difference between near-
infrared and red bands of the photograph. Boxes show the areas
in which radiometer arrays were deployed, and crosses show the
locations at which the hemispherical photographs in Fig. 4 were
taken.
A
PRIL 2008 E S S E R Y E T A L . 229
surveying, the trees have an average height of 12.6 m
with standard deviation 2.4 m. To the north of the clear-
ing, there is a discontinuous mixed-age stand, predomi-
nantly of lodgepole pine but with some subalpine fir
and Engelmann spruce; the average tree height is 9.3 m
and the standard deviation is 4.9 m. The histograms in
Fig. 2 show that the tree heights have an approximately
normal distribution for the uniform stand but a bimodal
distribution of short and tall trees for the discontinuous
stand.
Solar radiation was measured beneath the uniform
and discontinuous canopies within the areas shown by
boxes in Fig. 1b with two radiometer arrays (Link et al.
2004; Pomeroy et al. 2008, hereafter POM), each con-
sisting of 10 Eppley pyranometers (0.3–3-
m wave-
length, 160° sky view, 5% accuracy). Figure 3 shows
5-min averages of 10-s measurements from each array
for days 84 and 85 (24 and 25 March) in 2003. Also
shown is the above-canopy radiation measured by a
radiometer mounted at a height of 18 m on a mast in a
clearing 200 m to the southwest of the study area. Day
84 was clear, with strong direct illumination in open
FIG. 3. Solar radiation measured by 10-radiometer arrays in the (a) uniform and (b) discontinuous stands. Dotted
lines show incoming radiation measured above the canopy, and gray lines show diffuse radiation estimated by the
method described in section 3.
FIG. 2. Histograms of tree heights from ground surveys (solid lines) and lidar elevations (dashed lines) for the
(a) uniform and (b) discontinuous stands.
230 JOURNAL OF HYDROMETEOROLOGY VOLUME 9
areas, whereas day 85 was overcast. Averages and stan-
dard deviations across the arrays are larger for the dis-
continuous stand than the uniform stand and larger for
the clear day than the overcast day.
Hemispherical photographs were taken at points on a
staggered 20-m grid (Melloh et al. 2003); example pho-
tographs from the discontinuous stand, the clearing,
and the uniform stand at the points marked by crosses
in Fig. 1b are shown in Fig. 4, overlain with the sun
track for day 84. These photographs were used by Mel-
loh et al. (2003) to study gap fraction distributions and
by Hardy et al. (2004) to model transmission of solar
radiation through the canopy with the Gap Light Ana-
lyzer software package (Frazer et al. 1999).
FIG. 4. Observed and simulated hemispherical sky view for the points marked by crosses in Fig. 1b. White lines on the photographs
show the sun track for 24 Mar. Images on the right show differences between sky masks obtained from the photographs and the model,
with white pixels where the model is more open than observed, black pixels where it is more closed, and gray pixels where the two agree.
A
PRIL 2008 E S S E R Y E T A L . 231
