CWP and Earth Observation (EO)
Earth Observation (EO) is when remote sensing (RS) techniques are used from satellites to measure Earth's state. There are various types of Earth parameters that can be measured (reflectivity, emissivity, gravitational potential, etc.) and used to monitor the state of various Earth spheres (atmosphere, hydrosphere, biosphere etc.). In this course, we are interested in reflected radiances in the visible and near-infrared domains from vegetated surfaces.
At the onset of EO history (the 1970s), it was found that healthy vegetation could be easily identified on satellite images and separated from clouds, water and soil with the help of the Normalized Difference Vegetation Index (NDVI). NDVI enhances the differences between strong absorption of red light by plants (plants use red light to drive photosynthesis) and high reflectance in near-infrared (plants do not use this light and try to avoid overheating due to excess absorption). NDVI is used to compute yield and evapotranspiration for CWP computations.
Photosynthesis in WaPOR
Photosynthesis with EO data is estimated with the Light Use Efficiency (LUE) model. The LUE model is based on a positive linear relationship between absorbed light and photosynthesis: more light, more photosynthesis. Unfavorable conditions (too high temperature, too dry air, too much light, too dry soil) limit this linear relationship as multiplicative stress factors. As was mentioned above, not all light is used in photosynthesis but its part from 400 to 700 nm, which is called photosynthetically active radiation (PAR). NDVI is translated into the fraction of absorbed PAR with a simple linear equation.
Evapotranspiration in WaPOR
Evapotranspiration with EO data is estimated with the Penman-Monteith (PM) equation. Similarly to photosynthesis, the equation is driven by the absorbed energy (net radiation) and adjusted to atmospheric conditions through a set of aerodynamic resistances. NDVI, once translated into vegetation fractional cover, is used to partition net radiation between soil (evaporation) and vegetation (transpiration). Another EO-derived component required for the Penman-Monteith equation is surface albedo, required for net radiation computation.
Weather data also comes from EO
Both photosynthesis (LUE) and evapotranspiration (PM) estimation require meteorological data: incoming radiation in shortwave (solar) and longwave (atmosphere) domain, air temperature, air humidity, wind speed, and surface pressure. Those meteorological parameters often come from EO meteorological satellites as well.
Reading task:
Please, go through sub-sections 2.2.1 NDVI (raw pdf pp 60-65, document pp 48-53) and 2.2.5 Weather data (raw pdf pp 77-81, document pp 65-69) of the WaPOR v2 methodology.
Answer the following question:
- What type of pre-processing does the NDVI undergo?
- Which data level (1, 2 or 3) has the NDVI quality flag?
- What type of pre-processing do the weather data undergo?
- Which weather parameter is discussed in the separate from other weather parameters subsection?
- why do you think this is the case?
After that, please, go through Annex 1 Summary tables of sensors used in WaPOR v2.0, L1, L2, L3 (raw pdf pp 82-85, document pp 70-73) of the WaPOR v2 methodology.
Answer the following question:
- What extra EO datasets are used to make the resolution sharper (L1 250 m -> L3 30 m)?