Anthropogenic emissions of reactive nitrogen species have significantly increased, largely because of discharges from livestock, agricultural intensification and fertilizer use. Reactive nitrogen affects air quality, sensitive natural ecosystems, and the carbon balance.
The proposed mission aims at providing high spatial resolution measurements (<1 km) of NH3. These data will complement future hyperspectral sounding missions, monitoring emissions over industrial, domestic, and agricultural hotspots. The mission is based on a compact hyperspectral imager in the Thermal Infrared spectral range, deployed on a small satellite platform. We report on the mission design and objectives and address the technical feasibility of retrieving NH3 with a small satellite.
Snow sublimation is an important hydrological process and its spatial and temporal variation remains largely unknown; however, few studies have been conducted to quantify its spatial variability. Our study focuses on the evaluation of two algorithms, Penman–Monteith (P–M) equation and the bulk aerodynamic (BA) parameterization of snow sublimation. The two methods were first evaluated against eddy covariance (EC) measurements of latent heat flux at towers located in the upper reaches of the Heihe River Basin (China). Both methods were in good agreement with the ground observations with high coefficient of determination (R2) and low root mean squared error (RMSE). Next, we estimated subpixel snow sublimation using remote sensing data at a 1-km×1-km spatial resolution. The results based on satellite data were evaluated against ground measurements at the two experimental sites. The P–M equation gave R2=0.75, RMSE=8.4 W m−2 for Dashalong site and R2=0.36, RMSE=9.1 W m−2 for the Dadongshu site and performed better than the BA parameterization, which gave R2=0.65, RMSE=17.5 W m−2 for the Dashalong site and R2=0.06, RMSE=21.2 W m−2 for the Dadongshu site. Overall, the results indicate that P–M is promising for estimating snow sublimation at the regional scale using satellite observations.
Hyperspectral images may be applied to classify objects in a scene. The redundancy in hyperspectral data implies that fewer spectral features might be sufficient for discriminating the objects captured in a scene. The availability of labeled classes of several areas in a scene paves the way for a supervised dimensionality reduction, i.e., using a discrimination measure between the classes in a scene to select spectral features. We show that averaging adjacent spectral channels and using wider spectral regions yield a better class separability than the selection of individual channels from the original hyperspectral dataset. We used a method named spectral region splitting (SRS), which creates a new feature space by averaging neighboring channels. In addition to the common benefits of channel selection methods, the algorithm constructs wider spectral regions when it is useful. Using different class separability measures over various datasets resulted in a better discrimination between the classes than the best-selected channels using the same measure. The reason is that the wider spectral regions led to a reduction in intraclass distances and an improvement in class discrimination. The overall classification accuracy of two hyperspectral scenes gave an increase of about two-percent when using the spectral regions determined by applying SRS.
The regional surface soil heat flux (G0) estimation is very important for the large-scale land surface process modeling. However, most of the regional G0 estimation methods are based on the empirical relationship between G0 and the net radiation flux. A physical model based on harmonic analysis was improved (referred to as “HM model”) and applied over the Heihe River Basin northwest China with multiple remote sensing data, e.g., FY-2C, AMSR-E, and MODIS, and soil map data. The sensitivity analysis of the model was studied as well. The results show that the improved model describes the variation of G0 well. Land surface temperature (LST) and thermal inertia (Γ) are the two key input variables to the HM model. Compared with in situG0, there are some differences, mainly due to the differences between remote-sensed LST and the in situ LST. The sensitivity analysis shows that the errors from −7 to −0.5 K in LST amplitude and from −300 to 300 J m−2 K−1 s−0.5 in Γ will cause about 20% errors, which are acceptable for G0 estimation.
Soil water saturation condition is an essential factor that indicates the possible temporal and spatial hazard of inundations
in floodplains. To monitor wetness conditions over a long period of time and large areas, passive microwave data is used
to study the inundation pattern of large floodplains in Asia, such as the Poyang Lake floodplain. The polarization
difference brightness temperature at 37GHz is sensitive to the water extension even under dense forest. However, the
mixing of signals from open water, bare soil and vegetation makes it difficult to obtain the soil-water saturation
conditions from 37GHz data. That is because 37GHz microwave emission is attenuated by the vegetation canopy, which
shows seasonal changes in Asia floodplains. We developed a linear mixing model to eliminate the signal from vegetation
and derive the soil- water saturation condition from 37GHz data. Vegetation attenuation factors, in terms of vegetation
fractional area and LAI, have been estimated by correlation with the NDVI. Thus the vegetation attenuation function is
built according to the relationship between 37GHz and NDVI data of agricultural areas, with the help of Harmonic
analysis of time series to obtain continuous NDVI time series. Comparing the soil-water saturated area from 37GHz and
water extension area of Poyang Lake from SAR image data at higher spatial resolution, our result shows a good fit with
SAR data but relatively higher values.
Two new drought indicators based on satellite observations of vegetation index and land surface temperature, i.e. the
Normalized Temperature Anomaly Index (NTAI) and the Normalized Vegetation Anomaly Index (NVAI) were applied
to monitor drought events in different regions in China and India. We carried out this analysis for drought events with
distinct duration, intensity and surface condition in 2006 in Sichuan-Chongqing, in 2009 in Inner-Mongolia (China) and
in the Ganga basin (India) using the MODIS LST and NDVI data products and TRMM rainfall data for the period 2001
– 2010. Two newly proposed drought indicators NVAI and NTAI were evaluated against widely accepted indicators
such as Precipitation Anomaly Percentage (PAP), Vegetation Condition Index (VCI) and Temperature Condition Index
(TCI). The results show that NTAI and NVAI responded consistently to climate forcing. Long lasting rainfall anomalies
led to severe drought and anomalies in rainfall, anomalies in NTAI appeared almost simultaneously and followed by
negative anomaly in NVAI. The two new drought indicators NTAI and NVAI can distinguish the stages of drought
evolution. The sensitivity of the indicators and of their anomalies to drought conditions and severity was also evaluated
against drought assessments by operational drought monitoring services, documented how well the indicators meet
expectations on the timely and reliable detection of environmental change.
Developments in sensor technology boost the information content of imagery collected by space- and airborne hyperspectral sensors. The sensors have narrow bands close to each other that may be highly correlated, which leads to data redundancy. This paper first shows a newly developed method to identify the most informative spectral regions of the spectrum with the minimum dependency with each other, and second evaluates the land cover class separability on the given scenes using the constructed spectral bands. The method selects the most informative spectral regions of the spectrum with defined accuracy. It is applied on hyperspectral images collected over three different types of land cover including vegetation, water and bare soil. The method gives different band combinations for each land cover showing the most informative spectral regions; then a discrimination analysis of the available classes in each scene is carried out. Different separability measures based on the distribution of the classes and scatter matrices were calculated. The results show that the produced bands are well-separated for the given classes.
New airborne LiDAR (Light Detection and Ranging) measurement systems, like the FLI-MAP 400 System,
make it possible to obtain high density data containing far more information about single objects, like trees,
than traditional airborne laser systems. Therefore, it becomes feasible to analyze geometric properties of trees
on the individual object level. In this paper a new 3-step strategy is presented to calculate the stem diameter of
individual natural trees at 1.3m height, the so-called breast height diameter, which is an important parameter
for forest inventory and flooding simulations. Currently, breast height diameter estimates are not obtained from
direct measurements, but are derived using species dependent allometric constraints. Our strategy involves three
independent steps: 1. Delineation of the individual trees as represented by the LiDAR data, 2. Skeletonization
of the single trees, and 3. Determination of the breast height diameter computing the distance of a suited subset
of LiDAR points to the local skeleton. The use of a recently developed skeletonization algorithm based on
graph-reduction is the key to the breast height measurement. A set of four relevant test cases is presented and
validated against hand measurements. It is shown that the new 3-step approach automatically derives breast
height diameters deviating only 10% from hand measurements in four test cases. The potential of the introduced
method in practice is demonstrated on the fully automatic analysis of a LiDAR data set representing a patch of
forest consisting of 49 individual trees.
Forest fires are one of the major environmental hazards in Mediterranean Europe. Biomass burning reduces
carbon fixation in terrestrial vegetation, while soil erosion increases in burned areas. For these reasons, more
sophisticated prevention tools are needed by local authorities to forecast fire danger, allowing a sound allocation
of intervention resources. Various factors contribute to the quantification of fire hazard, and among them
vegetation moisture is the one that dictates vegetation susceptibility to fire ignition and propagation. Many
authors have demonstrated the role of remote sensing in the assessment of vegetation equivalent water thickness
(EWT), which is defined as the weight of liquid water per unit of leaf surface. However, fire models rely on the
fuel moisture content (FMC) as a measure of vegetation moisture. FMC is defined as the ratio of the weight
of the liquid water in a leaf over the weight of dry matter, and its retrieval from remote sensing measurements
might be problematic, since it is calculated from two biophysical properties that independently affect vegetation
reflectance spectrum.
The aim of this research is to evaluate the potential of the Moderate Resolution Imaging Spectrometer
(MODIS) in retrieving both EWT and FMC from top of the canopy reflectance. The PROSPECT radiative
transfer code was used to simulate leaf reflectance and transmittance as a function of leaf properties, and the
SAILH model was adopted to simulate the top of the canopy reflectance. A number of moisture spectral indexes
have been calculated, based on MODIS bands, and their performance in predicting EWT and FMC has been
evaluated. Results showed that traditional moisture spectral indexes can accurately predict EWT but not FMC.
However, it has been found that it is possible to take advantage of the multiple MODIS short-wave infrared
(SWIR) channels to improve the retrieval accuracy of FMC (r2 = 0.73). The effects of canopy structural
properties on MODIS estimates of FMC have been evaluated, and it has been found that the limiting factor is
leaf area index (LAI). The best results are recorded for LAI>2 (r2 = 0.83), while acceptable results (r2 = 0.58)
can still be achieved for lower vegetation cover density.
Forest fires are one of the major environmental issues in large areas of Southern Italy, and more generally in Mediterranean Europe. Biomass burning reduces carbon fixation in terrestrial vegetation, while risk of soil erosion increases in burned areas. The premier action against fires is prevention, and in this context fire risk mapping is an invaluable tool.
Various factors, either static or dynamic, contribute to the definition of fire risk. Among them, vegetation moisture plays a key role, since forests susceptibility to fire increases with increasing plant water stress and biomass dryness. A tool is needed to allow a timely detection of such forest conditions, and space-borne and airborne remote sensing can be very effective to this end.
Many authors have demonstrated the role of remote sensing in the assessment of vegetation moisture. Various multi-spectral systems have been reported to be useful, such as Landsat TM, SPOT or NOAA AVHRR. We have recently started a research to evaluate fire risk in the rural environment of Southern Italy using the Moderate Resolution Imaging Spectrometer (MODIS), carried on board of EOS Terra and Aqua satellites. The MODIS systems have 20 spectral wavebands covering the visible, the near infrared and the shortwave infrared with a spectral resolution of 10-50 nm.
This paper describes the results of a preliminary experiment to identify the most useful bands or band combinations (spectral indexes) for the detection of biological indicators of plant water stress. PROSPECT radiative transfer code has been adopted to simulate leaf reflectance as a function of leaf properties. Results highlighted the potential of single and combined simulated MODIS bands in the retrieval of vegetation moisture indicators related to fire risk.
In the present work we show the potential of multiangular hyperspectral PROBA-CHRIS data to estimate aerosol optical properties over dense dark vegetation. Data acquired over San Rossore test site (Pisa, Italy) have been used together with simultaneous ground measurements. Additionally, spectral measurement over the canopy have been performed to describe the directional behavior of a Pinus pinaster canopy. Determination of aerosol properties from optical remote sensing images over land is an under-determined problem, and some assumptions have to be made on both the aerosol and the surface being imaged. Radiance measured on multiple directions add extra information that help in reducing retrieval ambiguity. Nevertheless, multiangular observations don't allow to ignore directional spectral properties of vegetation canopies. Since surface reflectivity is the parameter we wish to determine with remote sensing after atmospheric correction, at least the shape of the bi-directional reflectance factor has to be assumed. We have adopted a Rahman BRF, and have estimated its geometrical parameters from ground spectral measurements. The inversion of measured radiance to obtain aerosol optical properties has been performed, allowing simultaneous retrieval of aerosol model and optical thickness together with the vegetation reflectivity parameter of the Rahman model.
Measurements of spectro-directional radiances done with the imaging spectrometer CHRIS on-board the agile platform PROBA are being used to determine key properties of terrestrial vegetation at the appropriate spatial resolution. These data on vegetation properties can then be used to improve the accuracy and the parameterizations of models describing biosphere processes, i.e. photosynthesis and water use by irrigated crops and trees.
The vegetation properties considered are: albedo, Leaf Area Index (LAI), fractional cover, fraction of absorbed photosynthetically active radiation (fAPAR) and canopy chlorophyll content.
The Natural Park of San Rossore (Pisa, Central Italy) is a primary test site for several national and international research projects dealing with forest ecosystem monitoring. In particular, since 1999 measurements of transpiration and ecosystem gas-exchange have been regularly taken in the park pine forest to characterize its main water and carbon fluxes. In the same period, several aerial flights have been carried out with onboard hyper-spectral sensors (MIVIS, VIRS, AISA), while a series of satellite images have been acquired using both conventional (NOAAAVHRR, Landsat-TM/ETM+) and advanced sensors (CHRIS-PROBA).
The final objective of these activities is to calibrate and validate methodologies which integrate remotely sensed and ancillary data for monitoring forest ecosystem. More specifically, a major research effort has been focused on evaluating the additional information content provided by advanced hyper-spectral multi-angular sensors about the main parameters needed for forest characterization (species, LAI, pigment content, etc.). These activities are part of
projects which are financed by the Italian and European Space Agencies (ASI and ESA, respectively) within the framework of the CHRIS-PROBA and SPECTRA missions.
During 2002 and 2003 nine complete multi-angular acquisitions were successfully performed over the San Rossore site. This paper summarizes first results of the evaluation of data acquired so far, particularly forward modeling of Top Of Canopy (TOC) reflectances. The models KUUSK, SAIL and GeoSAIL were used to simulate spectro-directional reflectance of different stands in the forest and compared with PROBA - CHRIS and airborne hyperspectral observations. Deviations of simulated from observed reflectances were significant.
The monitoring of the carbon stock in terrestrial environments, as well as the improved understanding of the surface-atmosphere interactions controlling the exchange of matter, energy and momentum, is of immediate interest for an improved assessment of the various components of the global carbon cycle. Studies of the Earth System processes at the global scale rely on models that require an advanced understanding and proper characterization of processes at smaller scales. The prime objective of the Surface Processes and Ecosystem Changes Through Response Analysis (SPECTRA) Mission is to determine the amount, assess the conditions and understand the response of terrestrial vegetation to climate variability and its role in the coupled cycles of energy, water and carbon. The amount and state of vegetation will be determined by the combination of observed vegetation properties and data assimilation. Many vegetation properties are related to features of reflectance spectra in the region 400 nm - 2500 nm. Detailed observations of spectral reflectance reveal subtle features related to biochemical components of leaves such as chlorophyll and water. The architecture of vegetation canopies determines complex changes of observed reflectance spectra with view and illumination angle. Quantitative analysis of reflectance spectra requires, therefore, an accurate characterization of the anisotropy of reflected radiance. This can be achieved with nearly - simultaneous observations at different view angles. Exchange of energy between the biosphere and the atmosphere is an important mechanism determining the response of vegetation to climate variability. This requires measurements of the component temperature of foliage and soil. The latter are closely related to the angular variation in thermal infrared emittance. Scientific preparations for SPECTRA are pursued along two avenues: a) the nature of the expected data and candidate algorithms are evaluated by generating and using synthetic hyper - spectral multi - angular\radiometric data; algorithms are evaluated with actual hyper -spectral data collected with a variety of airborne systems and concurrent ground measurements;
There is a wide set of digital images, where the problem of detecting specific structures is filtering between multiple and complex lines and secondary elements. The real problem is extracting relevant information from images, discarding uninteresting information previously, during and after the segmentation process. In this work, we resume the advantages and disadvantages of each approach, concluding a basic preference of filtering as soon as possible. In this sense, we present a method of filtering during segmentation, which mixes the mobile windows and the seeded regions approaches. Main steps are: 1) The whole image is divided in windows with a size related with the searched structures; 2) Previous knowledge about the location of the searched elements is applied to reduce the number of windows; 3) The number of windows is reduced using distribution and compacity conditions; 4) The population of each work windows is analyzed to fix one threshold; 5) Filtered work pairs are segmented using simple two populations criteria; 6) Analyzing the detected segments, the list of work window-threshold pairs is extended to include new windows. Most relevant result is the definition of a new
border based segmentation approach, which gives good results searching specific objects in complex images.
Global Models of the Earth - Atmosphere System describe the role of the terrestrial biosphere using increasingly complex Land Surface Models (LSM). These models mimic the exchange of energy, water and carbon between the land and the atmosphere, with emphasis on the role of terrestrial vegetation. Literature shows a clear trend towards fully interactive LSM-s, i.e. accounting for the dynamic response of vegetation to weather and climate. The latter may not be limited to biomass accumulation and address slower changes in vegetation type and composition. Improving the performance of such models require addressing two broad questions: Can we measure vegetation properties with the accuracy required by model sensitivity? How do we measure vegetation properties over the grid size of Global Models and are we able to incorporate the inherent spatial heterogeneity of terrestrial vegetation? The role of terrestrial vegetation in the land - atmosphere exchanges of energy, water and carbon is determined by properties, such as albedo, fAPAR, LAI and chlorophyll, related to spectro - directional radiance in the range 0.4 im - 14 im.. These variables and their spatial patterns can, therefore, be determined with accurate observations of spectro - directional radiance at selected view - angles and wavelengths. The paper summarizes results of several field experiments and airborne campaigns in Spain and France dedicated to these scientific objectives during the period 1998 - 2000. Examples are presented of the use of multi- angular hyper-spectral measurements to determine LAI, fAPAR, cholorophyll and heat fluxes with both field and airborne measurements. Particular attention is dedicated to illustrate the need for multi-angular observations in the entire spectral range 0.4im - 14 im..
It is well known that reflectance of Earth surface largely depends upon amount of biomass, crop type, development stage, ground coverage. The knowledge of these parameters -- together with groundbased meteorological data -- allows for the estimate of crop water requirements and their spatial distribution. Recent research has shown the possibility of using multispectral satellite images in combination with other information for mapping crop coefficients in irrigated areas. This approach is based on the assumption that crop coefficients (Kc) are greatly influenced by canopy development and vegetation fractional ground cover; since these parameters directly affect the reflectance of cropped areas, it is possible to establish a correlation between multispectral measurements of canopies reflectance and the corresponding Kc values. Within this frame, two different approaches may be applied: (1) definition of spectral classes corresponding to different crop coefficient values and successive supervised classification for the derivation of crop coefficients maps; (2) use of analytical relationships between the surface reflectance and the corresponding values of vegetation parameters, i.e., the leaf area index, the albedo and the surface roughness, needed for the calculation of the potential evapotranspiration according to the combination type equation. The two different techniques are discussed with reference to the results of their application to specific case-studies. The aim of this report is to illustrate the suitability of remote sensing techniques as an operational tool for assessing crop water demand at regional scale.
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