Boreal landscape modelling of hydrological processes and carbon and nitrogen exchange requires an adequate spatial database of topography, edaphic conditions and vegetation cover. Spatial variability in boreal vegetation types and edaphic factors depends largely on topography and geomorphology. One aspect of the authors' work is the development of an a priori landscape classification scheme which is based on (a) topographic information extracted from interferometric and conventional digital elevation models (DEM) and (b) knowledge of ecophysiological requirements of major vegetation types. Based on this information classes of soil types and correlated potential vegetation will be defined. This a priori landscape classification is then checked by a SAR remote sensing classification based on radar backscatter and coherence which detects structural discrepancies between predicted and current land cover (e.g. open areas vs. forest due to fire disturbance or non-climax vegetation). Both, ERS-Tandem data and JERS data are used. Enhancement of the SAR based vegetation classification is expected from matching the SAR classification with the a priori landscape classification. As a next step the enhanced functional-structural landscape and vegetation classification may be used for upscaling fluxes and assessing pool sizes on a regional and continental scale.
In 1990 the NASA/JPL airborne synthetic aperture radar DC-8 (Airsar) was flown over an area in northern Belize and the surrounding countries of Guatemala and Mexico. The three-frequency polarimetric radar signatures of a variety of natural areas have been extracted, and many have a unique radar signature. Scattering mechanisms which may explain these signatures and results of an image classification technique are presented.
Automated recording stations have been installed at the Bonanza Creek Experimental Forest, a Long Term Ecological Research (LTER) site located near Fairbanks, Alaska, in a forest stand of the Tanana River floodplain underlain by discontinuous permafrost. These stations provide a continuous record of dielectric constant and temperature of tree trunks, and soil moisture and temperature profiles down to the root zone. Along with the weather stations deployed at the same location, these measurements provide a continuous record of the environmental and phenologic conditions of the forest during a complete seasonal cycle. At the same time, ERS-1 SAR imaged the study site repeatedly from space to provide radar backscatter measurements of the forest approximately three times a month. Here, we examine the temporal dynamic of ERS-1 SAR measurements in relation with the changing environmental and phenologic state of the forest canopy and of the forest ground layers during the winter/spring and fall/winter transitions of 1992 and 1993. During these transitions, we examine whether changes in radar backscatter observed by ERS-1 may be related to freezing or thawing of the soil and vegetation in order to determine the start and end of the growing season for the forest. The results of this analysis are used in turn to determine whether similar changes are observed over larger regions. Mosaics of SAR data generated along three different North-South Alaskan ERS-1 transects that intercept with our study site are used in combination with hourly air temperature and daily precipitation rates gathered at airport weather stations by the National Weather Service. Results obtained using ERS-1 data collected from January 1992 to mid-1993 will be discussed.
The occurrence and magnitude of temporal and spatial tree water status changes in the boreal environment were studied in a floodplain forest in Alaska and in four forest types of Central Canada. Under limited water supply conditions from the rooted soil zone in early spring (freeze/thaw transition) and during summer, trees show declining water potentials. Coincidental change in tree water potential, tree transpiration and tree dielectric constant had been observed in previous studies performed in Mediterranean ecotones. If radar is sensitive to chances in tree water status as reflected through changes in dielectric constant, then radar remote sensing could be used to monitor the water status of forests. The SAR imagery is examined to determine the response of the radar backscatter to the ground based observations of the water status of forest canopies. Comparisons are made between stands and also along the large North-South gradient between sites. Data from SAR are used to examine the radar response to canopy physiological state as related to vegetation freeze/thaw and growing season length.
New research is finding that satellite‐based radar remote sensing techniques are particularly well‐suited for quantifying the transition of remote boreal regions from a frozen to a thawed condition. The implications for studying global warming are far reaching. If the timing or areal extent of this freeze/thaw transition were to change significantly, measurable changes in boreal climate, hydrology, and biogeochemistry would result. Abrupt transition from frozen to thawed conditions occurs each year over roughly 50 million km 2 of the Earth's remote terrestrial surface at latitudes above 40°N. Radar remote sensing works well to capture this transition because of the way electromagnetic radiation at radar wavelengths interacts with polar water molecules in solid and liquid states. Also, radar has the substantial advantages at high latitudes of both penetrating through clouds and not requiring solar illumination of the land surface.
The scanning multichannel microwave radiometer (SMMR) launched aboard the Nimbus-7 satellite provided global data at five frequencies (6.6, 10.7, 18, and 37 GHz) and two polarizations. The global data over land surfaces from this instrument are available for the period of January 1979 to December of 1985. The 6.6 GHz data set, being the lowest frequency of SMMR is especially important for studying the vegetation canopies and the moisture variations of the underlying soil surface. In this study, a microwave emission model for vegetation canopies has been developed to simulate the 6.6 GHz channel of SMMR. The canopy model consists of three layers of crown, trunk and underlying soil. The emissivity from the canopy is obtained by first computing the bistatic radar cross section of the three layer canopy using the distorted Born approximation and then integrating the radar cross sections over the hemispherical scattering angles according to the conservation of energy. In this formulation, each layer of the canopy is modeled as a random distribution of canonical shape dielectric scatterers (discs and cylinders). The dielectric constant of the scatterers are determined according to the available moisture in various components of the canopy. The model is then verified over homogeneous agricultural canopies using a ground stationed radiometer system. In particular, the properties of the polarization ratio with respect to the available vegetation biomass and soil moisture have been analyzed and it is found that this ratio becomes less sensitive to soil moisture as the vegetation biomass increases. The model has then been modified to take into account the large spatial resolution of the SMMR data by introducing a distribution of gaps in each resolution cell. The model simulations are then used in conjunction with monthly averaged 6.6 GHz SMMR data over two areas of Amazon forest and North-West US to study the effect of the moisture and vegetation changes.< >
The spherical reflector at the Arecibo Observatory (AO) offers great advantages for the design of simple and inexpensive high performance steerable antennas at VHF. Light and small feeds have the added benefit that they can be quickly installed in the Arecibo platform. It is important to evaluate the performance of any given feed including the effects of the spherical reflector. The optimization is emphasized of two parameters, namely, the distance below the focal point of the reflector and the beam width of a point feed. For the design of the feed at 46.8 MHz at the AO there were other requirements independent of MST (mesosphere stratosphere troposphere) work. The design of the primary array is discussed along with its performance with the AO spherical reflector.
