On the bias of instantaneous FAPAR estimates in open-canopy forests
Abstract
The fraction of absorbed photosynthetically active radiation (FAPAR) has been identified by the Global Climate Observing System (GCOS) as an essential climate variable required to support the works of both the UNFCCC and the IPCC. The validation of operationally re-trieved space-borne FAPAR products is thus a critical step in efforts to ensure the availability of accurate and reliable terrestrial surface information. Field validation activities so far tend to focus on agricultural areas that lack the heterogeneity and structural complexity of for-est environments. In this presentation in situ FAPAR estimation strategies are evaluated for open-canopy forests under a large variety of architectural, spectral and illumination related conditions. Particular emphasis is given to the role of the background brightness, the ratio of direct to total incident radiation and the size of the domain within which the sampling is carried out. A parametric relationship is developed that allows for improved instantaneous FAPAR estimates, in particular, when the background is bright or snow-covered. Finally, the bias introduced by a given choice of sample numbers and their spatial locations (i.e., random versus grid or transect-based schemes) is evaluated for clear and overcast sky conditions as well as for spatially variable background albedos.
- Publication:
-
38th COSPAR Scientific Assembly
- Pub Date:
- 2010
- Bibcode:
- 2010cosp...38..299W