This proposal is for a continuation of our existing project, BigFoot.
The overall goal of BigFoot and the proposed BigFoot II project is to
provide validation of MODLand (MODIS Land Science Team) science products,
including land cover, leaf area index (LAI), fraction absorbed
photosynthetic active radiation (fAPAR),
and net primary production (NPP). To do so, we use ground measurements,
remote sensing data, and ecosystem process models at sites representing
different biomes. BigFoot sites are 5 x 5 km in size and surround the
relatively small footprint (.1 km2) of CO2
flux towers. At each site we make multi-year in
situ measurements of ecosystem structure and functional characteristics
that are related to the terrestrial carbon cycle. Our sampling design allows
us to explicitly examine scales of fine-grained spatial pattern in these
properties, and provides for a field-based ecological characterization of
the flux tower footprint. Multi-year measurements ensure that inter-annual
validity of MODLand products can be assessed.
In BigFoot II, for each measurement year we will derive land cover, fAPAR,
and LAI surfaces by linking our in
situ measurements to Landsat ETM+ data. These BigFoot surfaces will be
developed using logic that preserves functionally important fine-grained
information. Errors in these surfaces will be quantified and the surfaces
summarized to provide a characterization of vegetation patterns in the
greater flux tower footprint. Using these land cover and LAI surfaces and
derived climate surfaces, we will model NPP over the 5 x 5 km BigFoot
footprint. Two independent ecosystem process models will be used: Biome-BGC
and IBIS. The ability of the models to capture environmental and ecological
controls on water and carbon cycles will be assessed with the following
comparisons: modeled NPP against in
situ measurements of NPP, modeled GPP to tower-based calculations of GPP,
and modeled daily water vapor and CO2 fluxes to tower estimates.
We will validate MODLand land cover, LAI, fAPAR, and NPP surfaces
by comparing them to BigFoot surfaces derived using field measurement data.
A series of exercises that isolate important scaling factors will be
conducted, so that their effects on NPP model estimates can be better
understood. This will involve rerunning the models after converting
site-specific land cover classes into broad, globally applicable classes,
successive coarsening of land cover and LAI surface grain size, and
generalizing the light use efficiency factor (g)
to coincide with the more generalized land cover classes.
The proposed BigFoot II study will be conducted at nine sites
(several supported by separate funding) that span eight major biomes, from
desert to tundra, to tropical forest. At these sites, in addition to
validation of MODIS products, we will quantify carbon content and NPP,
examine how these variables vary spatially and temporally, and how NPP is
related to climatic variables. Collectively, the standardized NPP data from
the contrasting biomes will elucidate biophysical controls on NPP, and their
sensitivitiy to changing climate and land use. Our standardized data also
allow for direct testing of whether light use efficiency (LUE) differs among
plant functional types, or seasonally for a given type.
A global terrestrial observation system is needed to assist in the
validation of global products such as land cover and NPP from MODIS and
other sensor and modeling programs. A key component of such a system is the
eddy flux tower network, FLUXNET; however, flux sensors measure net
ecosystem productivity (NEP), not NPP. BigFoot is learning how NEP and NPP
are related, and through modeling, how to integrate a wide range of carbon
cycle observations. Another key component of an observing system is the use
of remote sensing and models to scale tower fluxes and field measurements.
Although this may be relatively common at a given site, no other project is
doing so with standardized methods across so many biomes. As such, BigFoot
is a pathfinding activity that will contribute to the development of useful
scaling principles. The project can also serve as a nucleus for the global
terrestrial observing system that is needed to validate global, generalized
products used to monitor the health of the terrestrial biosphere.