American Geophysical Union (AGU)
0148-0227
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A semidiscrete model for the scattering of light by vegetation An advanced bidirectional reflectance factor model is developed to account for the architectural effects exhibited by homogeneous vegetation canopies for the first orders of light scattering. The characterization of the canopy allows the simulation of the relevant scattering processes as a function of the number, size, and orientation of the leaves, as well as the total height of the canopy. A turbid medium approach is used to represent the contribution to the total reflectance due to the light scattering at orders higher than 1. This model therefore incorporates two previously separate approaches to the problem of describing light scattering in plant canopies and enhances existing models relying on parameterized formulae to account for the hot spot effect in the extinction coefficient. Simulation results using this model compare quite favorably with those produced with a Monte Carlo ray‐tracing model for a variety of vegetation cases. The semidiscrete model is also inverted against a well‐documented data set of bidirectional reflectance factors taken over a soybean canopy. It is shown that the inversion of the model against a small subset of these measurements leads to reasonable values for the retrieved canopy parameters. These values are used in a direct mode to simulate the bidirectional reflectance factors for solar and viewing conditions significantly different from those available in the subset of soybean data and compared with the full set of actual measurements.
Tập 102 Số D8 - Trang 9431-9446 - 1997
Relationship between gross primary production and chlorophyll content in crops: Implications for the synoptic monitoring of vegetation productivity Accurate estimation of spatially distributed CO2 fluxes is of great importance for regional and global studies of carbon balance. We applied a recently developed technique for remote estimation of crop chlorophyll content to assess gross primary production (GPP). The technique is based on reflectance in two spectral channels: the near‐infrared and either the green or the red‐edge. We have found that in irrigated and rainfed crops (maize and soybean), midday GPP is closely related to total crop chlorophyll content. The technique provided accurate estimations of midday GPP in both crops under rainfed and irrigated conditions with root mean square error of GPP estimation of less than 0.3 mg CO2 /m2 s in maize (GPP ranged from 0 to 3.1 mg CO2 /m2 s) and less than 0.2 mg CO2 /m2 s in soybean (GPP ranged from 0 to 1.8 mg CO2 /m2 s). Validation using an independent data set for irrigated and rainfed maize showed robustness of the technique; RMSE of GPP prediction was less than 0.27 mg CO2 /m2 s.
Tập 111 Số D8 - 2006
Synergistic algorithm for estimating vegetation canopy leaf area index and fraction of absorbed photosynthetically active radiation from MODIS and MISR data A synergistic algorithm for producing global leaf area index and fraction of absorbed photosynthetically active radiation fields from canopy reflectance data measured by MODIS (moderate resolution imaging spectroradiometer) and MISR (multiangle imaging spectroradiometer) instruments aboard the EOS‐AM 1 platform is described here. The proposed algorithm is based on a three‐dimensional formulation of the radiative transfer process in vegetation canopies. It allows the use of information provided by MODIS (single angle and up to 7 shortwave spectral bands) and MISR (nine angles and four shortwave spectral bands) instruments within one algorithm. By accounting features specific to the problem of radiative transfer in plant canopies, powerful techniques developed in reactor theory and atmospheric physics are adapted to split a complicated three‐dimensional radiative transfer problem into two independent, simpler subproblems, the solutions of which are stored in the form of a look‐up table. The theoretical background required for the design of the synergistic algorithm is discussed.
Tập 103 Số D24 - Trang 32257-32275 - 1998
Hemispherical reflectance and albedo estimates from the accumulation of across‐track sun‐synchronous satellite data The estimation of the hemispherical reflectance and the instantaneous albedo of canopies from top of canopy satellite reflectance data was investigated. The study was designed to approximate the specifications of generic sensors aboard satellites like NOAA, VEGETATION, MERIS, MISR, MODIS, and PRISM. These sensors acquire reflectance data in two to six wave bands distributed along the visible, near‐infrared, and middle infrared domains. Five great biomes (grassland, sparse vegetation, tropical forest, boreal forest, and bare soil) were approximated, simulating the corresponding top of canopy reflectances as observed from the satellites using well‐known leaf, soil, and canopy radiative transfer models, including the effect of cloud cover that limits the actual data acquisition scheme. Albedo was accurately derived from the hemispherical reflectance observed in only a few wave bands. When using six wave bands, albedo was estimated within 1% relative accuracy. The MRPV bidirectional reflectance distribution function (BRDF) model was tested to derive the hemispherical reflectance from the top of canopy bidirectional data as sampled by the generic sensors during a 32 day orbit cycle. Results showed that this is the main source of error, with a relative accuracy around 6%. This showed the importance of the directional sampling scheme and possible improvements that may be made to the model and the way it is fitted to the observed data. The algorithm developed produced a relative accuracy less than 7% for the albedo estimation, when using the six wave bands and a ±45° across‐track directional scanning capacity. The results were discussed with particular emphasis on the optimization of sensors and algorithms dedicated to albedo estimation and to the use of hemispherical reflectance as a potential normalized geophysical product for monitoring vegetation.
Tập 104 Số D18 - Trang 22221-22232 - 1999
Vegetation‐modulated landscape evolution: Effects of vegetation on landscape processes, drainage density, and topography Topography acts as a template for numerous landscape processes that include hydrologic, ecologic, and biologic phenomena. These processes not only interact with each other but also contribute to shaping the landscape as they influence geomorphic processes. We have investigated the effects of vegetation on thresholds for channel initiation and landform evolution using both analytical and numerical approaches. Vegetation is assumed to form a uniform ground cover. Runoff erosion is modeled based on a power function of excess shear stress, in which shear stress efficiency is inversely proportional to vegetation cover. This approach is validated using data. Plant effect on slope stability is represented by additional cohesion provided by plant roots. Vegetation cover is assumed to reduce sediment transport rates due to physical creep processes (rainsplash, dry ravel, and expansion and contraction of sediments) according to a negative exponential relationship. Vegetation grows as a function of both available cover and unoccupied space by plants and is killed by geomorphic disturbances (runoff erosion and landsliding) and wildfires. Analytical results suggest that in an equilibrium basin with a fixed vegetation cover, plants may cause a transition in the dominant erosion process at the channel head. A runoff erosion–dominated landscape, under none or poor vegetation cover, may become landslide dominated under a denser vegetation cover. The sign of the predicted relationship between drainage density and vegetation cover depends on the relative influence of vegetation on different erosion phenomena. With model parameter values representative of the Oregon Coast Range (OCR), numerical experiments conducted using the Channel Hillslope Integrated Landscape Development (CHILD) model confirm the findings based on the analytical theory. A highly dissected fluvial landscape emerges when surface is assumed bare. When vegetation cover is modeled, landscape relief increases, resulting in hollow erosion dominated by landsliding. Interestingly, our simulations underscore the importance of vegetation disturbances by geomorphic events and wildfires on the landscape structure. Simulated landscapes resemble real‐world catchments in the OCR when such disturbances are considered.
Tập 110 Số F2 - 2005
Upper mantle structure beneath the Caribbean‐South American plate boundary from surface wave tomography We have measured shear wave velocity structure of the crust and upper mantle of the Caribbean‐South American boundary region by analysis of fundamental mode Rayleigh waves in the 20‐ to 100‐s period band recorded at the BOLIVAR/GEODINOS stations from 2003 to 2005. The model shows lateral variations that primarily correspond to tectonic provinces and boundaries. A clear linear velocity change parallels the plate bounding dextral strike‐slip fault system along the northern coast of Venezuela, illustrating the differences between the South American continental lithosphere, the Venezuelan archipelago, and the Caribbean oceanic lithosphere. At depths up to 120 km beneath the Venezuelan Andes and the Maracaibo block, there is evidence of underthrusting of the Caribbean plate, but there is no other evidence of subduction of the Caribbean plate beneath the South American plate. In eastern Venezuela, linear crustal low velocities are associated with the fold and thrust belts whereas as higher crustal velocities are imaged in the Guayana shield lithosphere. The subducting oceanic part of the South American plate is imaged beneath the Antilles arc. The surface wave images combined with seismicity data suggest shear tearing of the oceanic lithosphere away from the buoyant continental South American plate offshore of northeastern Venezuela. The continental lithosphere south of the slab tear is bent down toward the plate boundary in response to the propagating tear in the lithosphere. We interpret a nearly vertical low‐velocity “column” west of the tear centered beneath the Cariaco Basin, with three‐dimensional asthenospheric flow around the southern edge of the subducting oceanic lithosphere, with the asthenosphere escaping from beneath continental South America and rising into the plate boundary zone. The complex plate boundary structure is best examined in three dimensions. We discuss the new surface wave tomographic inversion in the context of results from other researchers including local seismicity, teleseismic shear wave splits, and interpretations from active source profiling.
Tập 114 Số B1 - 2009
Aseismic continuation of the Lesser Antilles slab beneath continental South America We present results of travel time inversions of teleseismic P and S waves recorded at the SECaSA92 (Southeast Caribbean South America 1992) temporary broadband array in northeastern Venezuela and Trinidad. The inversions reveal the unusual structure of the southern termination of the Lesser Antilles subduction zone: A minimum 2% relatively high‐velocity anomaly trends WSW from the seismically defined Lesser Antilles slab beneath and NW of the Paria Peninsula to a point below the Venezuelan Serranía del Interior, well south of the Caribbean coast. Resolution tests utilizing actual ray geometries and densities of the source data indicate that the regional‐scale structure beneath the study area is reasonably well resolved. Thus a detached and detaching subducted South American slab appears to lie beneath continental South America. We infer that oceanic South American lithosphere has been overridden to a significant degree by continental South America. The detached slab now lying beneath continental South America was driven into its current position after detaching from the former eastward striking Mesozoic ocean‐continent passive margin as this margin entered the subduction zone. Because oceanic and continental South America are still attached without apparent relative motion between them along the Atlantic passive margin southeast of our study region, the slab must be the actively moving element during continental overriding. Thus the slab and its surrounding mantle (both Caribbean and South America) beneath northeastern South America are mobile and have moved ESE relative to the stable Guyana Shield craton.
Tập 108 Số B1 - 2003
Seismicity and tectonics of the southeastern Caribbean We present 33 new focal mechanisms for SE Caribbean earthquakes (1963–1988). We use these mechanisms, in conjunction with 28 previously available mechanisms, to distinguish between two models of plate boundary zone interaction in the SE Caribbean: the trench‐trench transform and hinge faulting model, and the right oblique collision model. Shallow (0–70 km) and intermediate (70–200 km) depth earthquakes occur in the study region; we focus on the tectonic causes of these events and the motions they delineate. The shallow earthquakes are in a broad linear zone which trends NE from the Paria Peninsula of Venezuela towards Barbados. Intermediate depth earthquakes cluster beneath and NW of the Peninsula, and deepen to the NW, perpendicular to the NE‐trending shallow events. The vertical distribution of the earthquakes suggests a slab with steep NW dip. Shallow, dextral strike slip on E‐striking faults is restricted to a 60‐km‐wide linear zone between the Gulf of Cariaco and the western margin of the Gulf of Paria. Dextral strike slip is active only as far east as the Gulf of Paria, and not within or east of Trinidad. Shallow thrust events with ENE‐striking planes, distributed between the Araya Peninsula and the Gulf of Paria, indicate collision at crustal levels between South America and Caribbean, and that folding and thrusting are still active over a 60‐km interval south of the Araya‐Paria isthmus. Active thrusting in Venezuela corroborates predictions of transpression between Caribbean and South America and discounts transtensional motions between the two plates in the SE Caribbean. The conjunction of shallow thrust, strike slip, and normal earthquakes in the Gulf of Paria at around 62.3° may be the expression of unpartitioned oblique compressive deformation in the plate boundary zone. Intermediate (165 km > h > 70 km) depth thrust and dip slip events within the NW‐dipping slab indicate that oceanic lithosphere, probably originally attached to South America, subducts to the NW beneath the Caribbean plate. Shallow normal faulting events E and NE of Trinidad are expressions of plate bending about near‐horizontal axes parallel to the Lesser Antilles subduction zone. We conclude that the earthquake mechanisms provide strong support for the right oblique collision model of Caribbean‐South American plate interaction.
Tập 98 Số B8 - Trang 14299-14319 - 1993
Air pressure and cosmogenic isotope production The cosmic ray flux increases at higher altitude as air pressure and the shielding effect of the atmosphere decrease. Altitude‐dependent scaling factors are required to compensate for this effect in calculating cosmic ray exposure ages. Scaling factors in current use assume a uniform relationship between altitude and atmospheric pressure over the Earth's surface. This masks regional differences in mean annual pressure and spatial variation in cosmogenic isotope production rates. Outside Antarctica, air pressures over land depart from the standard atmosphere by ±4.4 hPa (1σ) near sea level, corresponding to offsets of ±3–4% in isotope production rates. Greater offsets occur in regions of persistent high and low pressure such as Siberia and Iceland, where conventional scaling factors predict production rates in error by ±10%. The largest deviations occur over Antarctica where ground level pressures are 20–40 hPa lower than the standard atmosphere at all altitudes. Isotope production rates in Antarctica are therefore 25–30% higher than values calculated by scaling Northern Hemisphere production rates with conventional scaling factors. Exposure ages of old Antarctic surfaces, especially those based on cosmogenic radionuclides at levels close to saturation, may be millions of years younger than published estimates.
Tập 105 Số B10 - Trang 23753-23759 - 2000