Lecture 12 Flashcards
Why do we need quantitive information
What is the quantitative information for vegetation
To understand regional to global scale environmental phenomena ▪ Environmental degradation and climatic change are significant issues.
▪ Aim is to identify the nature, extent and severity of degradation processes to provide information to better manage the global environment.
▪ This can be input to a model (e.g., Earth System model, atmospheric transport model) to produce a better prediction of future events.
Vegetation:
▪ Type
▪ Amount (e.g., percentage cover)
▪ Quality (e.g. biomass) how much carbon is stored
▪ Canopy height (aerodynamic surface roughness)
Describe the feedback of loop for thinning or removal of the forest canopy.
Thinning or removal of the forest canopy. Greater insolation at the soil surface. Increases the air temperature and decreases relative humidity near the soil surface. increase fire risk. reduces tree cover and prevents tree regeneration.
Why estimate vegetation properties?
Inter- and intra-annual global vegetation monitoring on a periodic basis
2. Global biogeochemical, climate and hydrological modelling
3. Net primary production (NPP) and carbon balance
4. Detecting anthropogenic and climate change
5. Agricultural activities (plant stress, harvest yields, precision agriculture) – information on yield important for food security.
What could we measure? for vegetation properties
- Quantity of vegetation
- Structure figures out shape and height of canopy – important for biomass
- Chemical composition (e.g., pigment content - chlorophyll etc.) important for photosynthesis – productivity.
Vegetation indices (VI) describe them
Vegetation indices can be used to manage, monitor and understand our environment –> identify areas of vegetation growth or reduction. ▪ Better sensitivity than individual spectral bands for detection of biomass Ideal VI “the index should be particularly sensitive to vegetative covers, insensitive to soil background, little affected by atmospheric effects, environmental effects and solar illumination geometry and sensor viewing conditions (Jackson et al., 1983)
▪ There are a lot of vegetation indices (more than 50)
Give some examples of VI
Most widely used VI is : ▪ Normalised Difference Vegetation Index (NDVI)
Some are designed for specific purposes/sensors or wavebands :
▪ Enhanced Vegetation index (EVI)
▪ MERIS Terrestrial Chlorophyll Index (MTCI) – sensitive to chlorophyll in the leaves
How do NDVI values vary
NDVI varies between -1 and 1. 0.1 and 0 = soil, less than 0 = water or clouds. Healthy, dense vegetation has high NDVI Stressed, or sparse vegetation produces lower NDVI. Bare rock, soil have NDVI near zero. Snow produces negative values of NDVI. Clouds produce low to negative values of NDVI
Advantages of NDVI
NDVI – can track how the growing season changes, phenological cycle, growing season is lengthening and shortening. track changes in seasonal cycle.
Disadvantages of NDVI
- Reduced sensitivity at high biomass or over dense vegetation
- Any factor that unevenly influences the red and NIR reflectance will influence the NDVI such as atmospheric path radiance, soil wetness
- Sensitive to background variation - underlying soil brightness (dark/bright soil) NDVI decreases when soil surface is bright, not as accurate in arid regions, bright sandy soil = less accurate.
- Therefore, many different indices developed:
to account for specific limitations or for specific sensors / wavebands
what is Soil Adjusted Vegetation Index (SAVI)
Designed to remove the dependency of the NDVI on the brightness of canopy background reflectance.
What wavebands are used in NDVI and why?
NIR and Red. Healthy, dense vegetation has high NDVI (High NIR and low red) stressed, or sparse vegetation produces lower NDVI (higher red and lower NDVI) so a good indicator of vegetation green ness.
What factors effect the NDVI magnitude?
Dense vegetation influences how visible the soil is below the canopy (saturates over dense canopies, absorption reaches a limit, signal doesn’t change over forests of high density vegetation )/Atmospheric contamination (atmospheric influences the red more than NIR so red is more effected than NIR if not removed./ soil background/ percentage vegetation cover
What NDVI value is normally soil
0.1 / 0.15
