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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

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

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

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.



  • Using multi-year data sets,

    characterize ecological processes and fine-grained spatial patterns at

    each site.

  • Develop and validate

    site-specific land cover, LAI and fAPAR surfaces at each site

    by linking in situ observations and remotely sensed imagery.

  • Model NPP at each site over

    time using two independent process-based models; compare and validate the


  • Validate MODLand land cover,

    LAI, fAPAR, and NPP surfaces and examine the contribution of

    several scaling factors to differences between MODLand and BigFoot


  • mso-bidi-font-family:"Times New Roman";mso-ansi-language:EN-US;mso-fareast-language:

    EN-US;mso-bidi-language:AR-SA">Facilitate the achievement of the GTOS goal to

    develop a network of sites to serve as long-term global NPP monitoring


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