Variation of the Radar Backscatter of Vegetation through the Growing Season
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In the period between 1975 and l98l the ROVE team (Radar Observation of VEgetation) in the Netherlands collected data on the radar backscatter of crops through the growing season, Using these data general trends in the behaviour of the radar backscatter through the growing season (temporal signatures) can be determined for a number of crops. The results are reported. Comparisons are made with data from the literature and with the vegetation model developed by Attema and Ulaby. This last model can be used also to obtain information on the soil under the vegetation.Keywords:
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Changes and spatial pattern of dry herbaceous biomass was monitored using backscatter coefficients in a time series of ERS-2 SAR images of the semi-arid savanna in Senegal, West Africa. The results were compared to in situ measurements of dry biomass. It has been demonstrated, that the overall spatial and temporal change in SAR backscatter correspond to the changes in biomass, however, the quantitative relation remains to be established.
Herbaceous plant
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Vegetation maps of inaccessible areas in the tropics tend to divide vegetation into broad types.
Vegetation Classification
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Focusing on woody vegetation in Queensland, Australia, the study aimed to establish whether the relationship between Advanced Land Observing Satellite (ALOS) Phased Array L-band SAR (PALSAR) HH and HV backscattering coefficients and above ground biomass (AGB) was consistent within and between structural formations (forests, woodlands and open woodlands, including scrub). Across these formations, 2781 plot-based measurements (from 1139 sites) of tree diameters by species were collated, from which AGB was estimated using generic allometric equations. For Queensland, PALSAR fine beam dual (FBD) 50 m strip data for 2007 were provided through the Japanese Space Exploration Agency's (JAXA) Kyoto and Carbon (K&C) Initiative, with up to 3 acquisitions available for each Reference System for Planning (RSP) paths. When individual strips acquired over Queensland were combined, `banding' was evident within the resulting mosaics, with this attributed to enhanced L-band backscatter following rainfall events in some areas. Reference to Advanced Microwave Scanning Radiometer-EOS (AMSR-E) data indicated that strips with enhanced L-band backscatter corresponded to areas with increased effective vegetation water content (kg m -2 ) and, to a lesser extent, soil moisture (g cm -3 ). Regardless of moisture conditions, L-band HV topographically normalized backscattering intensities backscatter (σ f o ) increased asymptotically with AGB, with the saturation level being greatest for forests and least for open woodlands. However, under conditions of relative maximum surface moisture, L-band HV and HH σ f o was enhanced by as much as 2.5 and 4.0 dB respectively, particularly for forests of lower AGB, with this resulting in an overall reduction in dynamic range. The saturation level also reduced at L-band HH for forests and woodlands but remained similar for open woodlands. Differences in the rate of increase in both L-band HH and HV σ f o with AGB were observed between forests and the woodland categories (for both relatively wet and dry conditions) with these attributed, in part, to differences in the size class distribution and stem density between non-remnant (secondary) forests and remnant woodlands of lower AGB. The study concludes that PALSAR data acquired when surface moisture and rainfall are minimal allow better estimation of the AGB of woody vegetation and that retrieval algorithms ideally need to consider differences in surface moisture conditions and vegetation structure.
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Microwave radar is a potentially useful tool for monitoring the condition of the rangeland.A study was conducted in a mixed prairie community at the Agriculture Canada Research Substation at Onefour, Alberta in 1991 to examine the effects of historical management on synthetic aperture radar (SAR) data obtained from 2 aircraft flights, 24 May 1991 and 1 August 1991.Ground-truthing expeditions were conducted on the same days to obtain estimates of vegetation amounts, species distribution and soil moisture.A former grazing experiment established in 1955 and abandoned 20 years ago enabled comparison of 3 grazing treatments, continuous, rotation and free choice superimposed on native range, crested wheatgrass (Agropyron cristutum (L.) Gaertn.) and Russian wildrye (Hymusjunceus Fish.).The ground data and imagery were integrated in a Geographic Resource Analysis Support System (GRASS).Fields that had been cultivated and seeded to Russian wildrye had higher radar backscatter than native range.The radar bachscatter from crested wheatgrass fields was similar to native range in May but higher than native range in August.Radar backscatter was positively correlated with number of years since seeding with Russian wildrye.Generally there was little difference in radar bachscatter with grazing treatment.Correlation analyses between radar digital number extracted from the ground truth sites and vegetation and soil parameters revealed, dependiig upon swath mode, signifkant relationships between radar bachscatter and the amount of certain grass species, radar bachscatter and canopy moisture, and radar bachscatter and soil moisture in May.A significant negative correlation was observed between radar backscatter from the August images, in both swath modes, and percent ground cover.The results of this study indicated a role for SAR imagery in evaluating range characteristics.
Revegetation
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Land cover effects on ERS scatterometer data are investigated by comparing backscatter parameters with statistics derived from the CORINE land cover classes of the Iberian Peninsula. Although built-up areas, water bodies, and rocky surfaces are important most of the spatial variability of the ERS scatterometer signal can be explained by the percent area of translucent vegetation (grasses, agricultural crops) and non-transparent vegetation (forest, bushes, and shrubs) within the resolution cell of the sensor.
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The use of a non-linear estimation algorithm for retrieving the biomass and structure of vegetation from polarimetric Synthetic Aperture Radar (SAR) data is demonstrated for woody regrowth in Queensland, Australia dominated by Acacia harpophylla (Brigalow). By varying the size and density of trees and associated woody components (branches and trunks), multiple simulations of the backscattering coefficient (σ 0 ) were performed based on the SAR simulation model of. Functions relating σ 0 to these variables were subsequently used to generate spatial estimates from NASA JPL airborne SAR (AIRSAR) data. Above ground biomass was estimated from stem density and size measurements using available allometric relationships. The study demonstrates potential for retrieval of regrowth structure and biomass through nonlinear estimation.
Tree Allometry
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We herewith evaluate the potential of RADARSAT in substituting optical images for early crop identification and early crop surfaces estimation. This paper outlines the operational feasibility of very early RADARSAT based acreage estimation of non-cultivated terrain (set-aside, fallow, bare soils) and economically important crops, already during the winter season. The monitoring of growing crops during spring and early summer has been also carried out to investigate RADARSAT potential for cereals discrimination. Results obtained in the Netherlands during the growing season of 1997, and in the winter 1997/1998, using RADARSAT time-series are presented.
Growing season
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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.
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