Gerald P. Livingston

Microwave backscatter and attenuation dependence on leaf area index for flooded rice fields

As part of an effort to determine whether radar is suitable for wetland vegetation monitoring the authors have studied the dependence of microwave backscatter and attenuation on leaf area index (LAI) for flooded rice fields. They find that the radar return from a flooded rice field does show dependence on LAI. In particular, the C-band VV cross section per unit area decreases with increasing LAI. A simple model for scattering from rice fields is derived and fit to the observed HH and VV data. The model fit provides insight into the relation of backscatter to LAI and is also used to calculate the canopy path attenuation as a function of LAI. >

Differentiating methane source areas in Arctic environments with multitemporal ERS-1 SAR data

An assessment using ERS-1 SAR data to differentiate methane source (wetland) and nonsource (nonwetland) areas was undertaken based on radar backscatter modeling and empirical observations of 24 scenes collected over Barrow, AK, in 1991 and 1992. Differences in backscatter between source and nonsource areas were dependent on surface hydrology and air temperature. Differential freezing of surface materials on daily to seasonal time scales greatly enhanced the separability of wetlands and nonwetlands with ERS-1 SAR. Radar return for nonwetlands decreased dramatically whereas backscatter from wetlands decreased little when freezing air temperatures coincided with the SAR overpass. Maximum separability between wetlands and nonwetlands, as determined from observed and modeled radar backscatter, were the result of changes in the dielectric constant of the plant and surface materials with phase change during freezing. This study has indicated the need to consider air temperature at the time of acquisition in selecting ERS-1 SAR scenes for differentiating methane source and nonsource areas.

Influences of Fire and Climate Change on Patterns of Carbon Emissions in Boreal Peatlands

Projected changes in temperature and precipitation over the next several decades (Houghton et al. 1996) are expected to significantly enhance the release of gaseous carbon from northern peatlands. This enhancement is expected in response to increased rates of microbial decomposition acting on the vast carbon stores underlying these ecosystems (Oechel et al. 1993; Goulden et al. 1998) and to an increase in the frequency, extent, and intensity of peatland wildfires (Zoltai et al. 1998). Over much of the past 10,000 years, northern peatlands have collectively functioned as a globally important sink of atmospheric carbon dioxide (CO2) (Mitsch and Gosselink 1986; Gorham 1991), that is, net primary productivity in these ecosystems has exceeded losses. As a result, nearly one-third of all soil organic matter on earth underlies northern peatlands in the form of partially decomposed organic matter (peat) (Gorham 1991). Evidence is building, however, to suggest that rates of carbon loss from these northern ecosystems due both to microbial decomposition (e.g., Livingston and Morrissey 1991; Carroll and Crill 1997) and fire (Levine et al. 1995) have increased dramatically, even exceeding net primary productivity in some areas (Oechel et al. 1993; Goulden et al. 1998). A dramatic shift in the ecological function of northern peatlands from that of a net carbon sink to a net carbon source could potentially enhance climatic change due to the resultant increased atmospheric loading of radiatively important gases such as CO2 and methane (CH4) (Fung et al. 1991).

Remote sensing of high-latitude wetlands using polarized wide-angle imagery

Representing the areal extent of circumpolar wetlands is a critical step to quantifying the emission of methane, an important greenhouse gas. Present estimates of the areal extent of these wetlands differ nearly seven fold, implying large uncertainties exist in the prediction of circumpolar methane emission rates. Our objective is to use multi- directional and polarization measurement provided by the French POLDER sensor to improve this estimate. The results show that wetlands can be detected, classified and their area quantified using the unique, highly polarized angular signature of the sunglint measured over their water surfaces.