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CD-15 Abstract

BigFoot II: In situ Measurements, Remote Sensing, and Models to Validate MODIS Products Related to the Terrestrial Carbon Cycle

Warren B. Cohen — USDA Forest Service (US-PI)
Marcos Costa — UFV - Universidade Federal de Viçosa (SA-PI)

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 BigFoot and MODLand 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 sensitivity 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.

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