Upload
templeton-luka
View
108
Download
15
Embed Size (px)
DESCRIPTION
Plant Canopy Analysis. Colin S. Campbell, Ph.D. Decagon Devices and Washington State University. Radiant energy and plant canopy analysis. Why do we care about the radiation environment of plant canopies? Calculate Leaf Area Index (LAI) Crop growth stage Ecosystem health - PowerPoint PPT Presentation
Citation preview
Radiant energy and plant canopy analysis
Why do we care about the radiation environment of plant canopies?
Calculate Leaf Area Index (LAI) Crop growth stage Ecosystem health Radiation use efficiency
Radiant energy and plant canopy analysis Must have detailed knowledge of light
environment to use photosynthesis models
Partitioning ET into E and T Need to know fraction of energy intercepted
by canopy and fraction transmitted to soil
Intercepted PAR and Biomass Production Cumulative intercepted
photosynthetically active radiation (PAR) linearly related to total biomass production
Tools for detailed analysis
Radiation budget & view factors Fisheye analysis
Plant canopy light environment Fisheye analysis Ceptometer – measures light interCEPTed
by canopy
Definitions Zenith angle (Ψ) – angle between sun and the
zenith (vertical)
Transmission coefficient (τ) – fraction of sunlight transmitted through canopy to ground
Direct beam radiation – sunlight coming directly from sun (leaves a shadow)
Diffuse radiation – sunlight that has been scattered
Ψ
Plant canopy light environment -leaf area index (LAI)
Leaf Area Index (LAI) – One-sided surface area of leaves/surface area of soil
Unit area basis: m2 m-2
How do we measure Destructive sampling Light attenuation
Ceptometer Fisheye photograph
LAI – destructive sampling Harvest leaves from 1
m2 canopy area Physically measure
surface area with optical meter
Advantages: direct measurement
Disadvantages: destructive, time consuming, wilting
LAI-light attenuation theory
A
anexp
A
a
a
LAI is transmission and n is the number of leavesThis is only true if all of the leaves are horizontal
Unit ground Area
a
Leaf with area = a
Real canopies (leaves aren’t all horizontal)
Leaf angle distribution parameter (χ) Describes the orientation of the leaves
vertical canopyχ = 0
(onions < 1)
spherical canopyχ = 1
(most canopies)
horizontal canopyχ = ∞
(strawberries χ = 3)
Extinction coefficient We can use the leaf angle distribution to
determine an extinction coefficient (G) at a particular zenith angle
773.0
222
182.1774.1
sincos
G
cos
*exp
LAIGWhat does G do for us?
- allows us to relate τ to LAI for any canopy, given
What parameters do we need to calculate LAI?
Zenith angle (ψ) Time and location
latitude and longitude
Canopy extinction coefficient (G) Calculate from leaf angle distribution (χ) = 1
for most canopies
Canopy transmission coefficient (τ) Estimate from fisheye image Use ceptometer to measure directly
LAI from Ceptometers
Measure above-canopy radiation
Measure below-canopy radiation
Meter calculates τ
LAI from Ceptometer
What else does a meter need to calculate LAI? Time and location (zenith angle) Estimate of leaf angle distribution
parameter (χ)
Note: LAI measurements are best without beam radiation (only diffuse radiation) broken clouds are worst (changing
radiation conditions)
Decagon Accupar LP-80
LI-COR LAI-2000
What is a fisheye analysis? Simply a projection of a hemisphere onto a
plane In our case, a picture is taken through a special lens
that projects a full 180 degree hemisphere onto the film
What does this image allow us to do?
Determine view factors of surrounding objects
View factor determines radiative influence of one object on another
Determine light transmission coefficients through a canopy – diffuse and direct
What does this image allow us to do?
Determine when a particular location will be in direct sunlight
Determine what percentage of time a location will be sunlit
Determine τ, LAI, and χ values
Determining view factors
Simplest analysis
total
objectobject A
AF
Aobject is the area of the picture taken up by the
object of interest
Atotal is the total area of the photograph
Determining transmission coefficients
2. Visually estimate the fraction of sky visible in each grid sector (1 = full sky)
3. Average the value for each zenith angle band = τψ (we will do this for our lab picture for 15, 45, and 75°)
754515 25.05.025.005.1 d
Calculating average understory radiant fluxes
τd is the transmission coefficient for diffuse radiation But, if we average over a day or longer, it
approximates the total radiation transmission coefficient so:
avgTdbc S
Φbc = average radiant flux density below canopyST avg = average total radiation above canopy
Calculating times and duration of direct beam
Plot sun path on grid Segment into time steps (hours)
E
S
W
N
LAI from fisheye photo First method
G58 = 0.5 for all leaf angle distributions! Determine τ at ψ = 58° from fisheye photo
5.0
58cos*)ln( 58LAI
1. Determine τ at ψ=15, 45, and 75° from fisheye photo
2. Calculate G at each zenith angle with an arbitrary value of χ
3. Calculate new values of τ for each zenith angle using G from step 2 and an arbitrary LAI value
4. Calculate sum of squared errors between τfisheye and τarbitrary
5. Use solver to minimize SSE by adjusting arbitrary LAI and χ values
6. Results in decent estimate for both LAI and χ!
LAI from fisheye photo(method 2: use solver in excel)