Nadine Bruguier

A simple algorithm to retrieve soil moisture and vegetation biomass using passive microwave measurements over crop fields

Abstract A simple algorithm to retrieve sail moisture and vegetation water content from passive microwave measurements is analyzed in this study. The approach is based on a zeroth-order solution of the radiative transfer equations in a vegetation layer. In this study, the single scattering albedo accounts for scattering effects and two parameters account for the dependence of the optical thickness on polarization, incidence angle, and frequency. The algorithm requires only ancillary information about crop type and surface temperature. Retrievals of the surface parameters from two radiometric data sets acquired over a soybean and a wheat crop have been attempted. The model parameters have been fitted in order to achieve best match between measured and retrieved surface data. The results of the inversion are analyzed for different configurations of the radiometric observations: one or several look angles, L-band, C-band or (L-band and C-band). Sensitivity of the retrievals to the best fit values of the model parameters has also been investigated. The best configurations, requiring simultaneous measurements at L- and C-band, produce retrievals of soil moisture and biomass with a 15% estimated precision (about 0.06 m 3 /m 3 for soil moisture and 0.3 kg/m 2 for biomass) and exhibit a limited sensitivity to the best fit parameters.

Coupling canopy functioning and radiative transfer models for remote sensing data assimilation

Abstract Crop functioning models (CFM) are used in many agricultural and environmental applications. Remote sensing data assimilation appears as a good tool to provide more information about canopy state variables in time and space. It permits a reduction in the uncertainties in crop functioning model predictions. This study presents the first step of the assimilation of optical remote sensing data into a crop functioning model. It consists in defining a coupling strategy between well known and validated crop functioning and radiative transfer models (RTM), applied to wheat crops. The radiative transfer model is first adapted to consistently describe wheat, considering of four layers in the canopy that contain different vegetation organs (soil, yellow leaves and senescent stems, green leaves and stems, green and senescent ears). The coupling is then performed through several state variables: leaf area index, leaf chlorophyll content, organ dry matter and relative water content. The relationships between the CFM outputs (agronomic variables) and RTM inputs (biophysical variables) are defined using experimental data sets corresponding to wheat crops under different climatic and stress conditions. The coupling scheme is then tested on the data set provided by the Alpilles–ReSeDA campaign. Results show a good fitting between the simulated reflectance data at top of canopy and the measured ones provided by SPOT images corrected from atmospheric and geometric effects, with a root mean square error lower than 0.05 for all the wavebands.

Identification and evidence for relationships among geographical isolates of Bursaphelenchus spp. (pinewood nematode) using homologous DNA probes

Homologous DNA probes with a variable degree of repetitivity were used to identify the precise relationships between isolates belonging to the pinewood nematodes. Southern blot hybridization patterns obtained with genomic DNA from 13 geographical isolates confirm the existence of a B. xylophilus group and a B. mucronatus group within these pinewood nematodes. With these probes we were also able to discriminate between geographical isolates. The results clearly indicate the existence of three geographical subgroups in the B. xylophilus species: the United States, Canada and Japan with a close relationship between the USA and Japanese isolates. This supports the hypothesis that the B. xylophilus isolate, which was introduced into Japan, originated from the USA. Furthermore, these probes clarify the taxonomic status of a Minnesota isolate found on Abies balsamea and of the French and Norwegian isolates of pinewood nematodes. The nature of the isolated sequences is also discussed.

Monitoring coniferous forest characteristics using a multifrequency (5–90 GHz) microwave radiometer☆

The objective of this study is to analyze the potential interest of microwave radiometry to monitor coniferous forests. Microwave data were acquired by the airbone multifrequency radiometer PORTOS (5 GHz, 10.6 GHz, 23.8 GHz, 36.5 GHz, and 90 GHz) over a test site in Les Landes Forest (France). The test site consists of large homogeneous stands of maritime pines (Pinus pinaster). The stands are monospecies and even-aged and cover a large range of age and biomass conditions (the above-ground dry biomass ranges from 0 to about 180 tons/ha). The stands were flown over twice in May and August 1994. The microwave response to the coniferous stand characteristics is analyzed. A statistical study is carried out to investigate the potential interest of the PORTOS data to discriminate between the different stand categories. The sensitivity of the stand emissivity to view angle, frequency, measurement date, and stand characteristics is analyzed. It is found that the range of forest stand microwave emissivity is relatively narrow for a vegetation coverage exceeding 95%. Although different stand categories can be distinguished from the PORTOS data, the level of discrimination is not sufficient to allow the development of accurate rules of classification. No relationship could be found between low frequency measurements and the stand biovolume. On the other hand, a linear negative relationship is found between emissivity at 10.6 GHz and average tree variables of the stands (age, height, trunk diameter) for well-developed pine stands. Also, potential interest of PORTOS to estimate forest stand characteristics seems to be significant at 90 GHz. For this frequency channel, good correlation is found between emissivity and tree basal area BA. The results presented in this work contribute to better assess the potential applications of the future ESA/MIMR instrument over coniferous areas.

Relationship between component brightness temperature and geo-structure of a maize canopy

Numerous researchers have observed that the distribution of brightness temperature field over an agricultural canopy strongly depended on canopy biophysical and phenological features coupling with environment conditions. Recently, an in situ experiment was conducted over a maize canopy in Avignon of France to investigate the temporal variation of brightness temperature distributions as a function of canopy geometrical structure. The experiment lasted 3 months throughout maize growth cycle. The results revealed that component brightness temperature (CBT) values are not independent with each other. The correlation of sunlit soil temperature minus vegetation temperature (T sb-Tv) and shaded soil temperature minus vegetation temperature (Tso-Tv) is 0.85 with a RMSE of 1.1degC. In the analysis of maize geometrical structure when it was within its first two of three growth stages, good relations have been found between geometrical parameters with leaf area index LAI. To analyze the relation between CBT and LAI, the differences of CBTs were compared with LAI as a function of date of year, where we found that T sb-Tso has the best relation with LAI. An initial interpretation was given to this result, which is still under further analysis based on more experiment observations. The tie between CBT and LAI is expected to be utilized in the researches on canopy energy balance estimation directly

Error estimation in the acquisition of maize canopy hemispherical directional brightness temperature

A field experiment has been performed from May to August in 1999 over a maize canopy at INRA Avignon branch in south France. Directional brightness temperature (DBT) features were extracted from thermal infrared (TIR) images captured by a TIR camera mounted upon a crane platform. Different from the method with a goniometer, another approach based on the different principles has been developed that the camera can view different part of the field to collect DBT of different angles by means of rotating to different directions instead of moving the camera to make it always focus on a defined sample area. After the description of the measurement method, the evaluation of data acquisition is presented, where the quality and limits of this measurement method are discussed. In this study, a compromise is made in sampling design based on the analysis of canopy heterogeneity and FOV effect. An example of acquiring hemispherical DBT under natural conditions was given at last. In the analysis of the measurement at 13:10 of local time on June 24, 1999, the results showed the spatial heterogeneity of field thermal radiance played as the most important role. Using the new sampling method, the total error for acquiring hemispherical DBT was about 1.4degC in maximum with 95% confidence, when the amplitude of directional variation was larger than 7degC. The errors changed with measurement conditions, which made it different for each measurement