LAI/ fAPAR

definitions:

see: MOD15 Running et al.

• •

LAI

- one-sided leaf area per unit area of ground - dimensionless

fAPAR

- fraction of PAR (SW radiation waveband used by vegetation) absorbed proportion

LAI

 direct quantification of amount of (green) vegetation   structural quantity uses:  radiation interception (fAPAR)   evapo-transpiration (H 2 0) photosynthesis (CO 2 )   respiration leaf litter-fall

LAI

 full canopy structural definition (e.g. for RS) requires  leaf angle distribution (LAD)   clumping canopy height  macrostructure shape

LAI

 preferable to fAPAR/NPP (fixed CO 2 ) as LAI relates to standing biomass  includes standing biomass (e.g. evergreen forest)   can relate to NPP can relate to site H 2 0 availability (link to ET)

fAPAR

 radiometric quantity  more directly related to remote sensing  e.g. relationship to RVI, NDVI  uses:  estimation of primary production / photosynthetic activity  e.g. radiation interception in crop models  monitoring, yield  e.g. carbon studies  close relationship with LAI  LAI more physically-meaningful measure

Issues

 empirical relationship to VIs can be formed  but depends on LAD, leaf properties (Chl., structure)   need to make relationship depend on land cover relationship with VIs can vary with external factors, tho’ effects of many can be minimised

Estimation of LAI/fAPAR

 initial field experiments on crops/grass   correlation of VIs - LAI developed to airborne and satellite  global scale - complexity of natural structures

Estimation of LAI/fAPAR

 canopies with different LAI can have same VI    effects of clumping/structure can attempt different relationships dept. on cover class can use fuller range of spectral/directional information in BRDF model  fAPAR related to LAI     varies with structure can define through clump leaf area ground cover

Estimation of LAI/fAPAR

 fAPAR relationship to VIs typically simpler  linear with assymptote at LAI 6 • need to define different relationships for different cover types

MODIS LAI/fAPAR algorithm

  RT model-based define 6 cover types (biomes) based on RT (structure) considerations      

grasses & cereals shrubs broadleaf crops savanna broadleaf forest needle forest

MODIS LAI/fAPAR algorithm

  have different VI-parameter relationships can make assumptions within cover types   e.g., erectophile LAD for grasses/cereals e.g., layered canopy for savanna  use 1-D and 3D numerical RT models (Myneni) to forward-model for range of LAI  result in LUT of reflectance as fn. of view/illumination angles and wavelength  LUT ~ 64MB for 6 biomes

Method

  preselect cover types (algorithm) minimise RMSE as fn. of LAI between observations and appropriate models  if RMSE small enough, fAPAR / LAI output  backup algorithm if RMSE high - VI-based