In this paper, we report the results of an experimental study aimed toward investigating microwave emission from forests. The experiment was carried out in 2006 on two forest stands of poplar (Populus alba) and pine (Pinus italica), using ground-based microwave radiometers at the L-, C-, X-, Ku-, and Ka-bands, in H and V polarizations. Measurements on poplar were performed on different dates and at different incidence and azimuth angles, looking downward (from the top of trees and from below the crown) and upward (from the soil level). Only one downward-looking measurement was carried out over a pine plot with dry soil in April. All the remote sensing measurements were complemented with ?ground-truth? data. The collected experimental data made it possible to quantify the spectral signatures of poplar, as well as the variation of angular trends of brightness temperature in different seasons of the year. The sensitivity of L-band emission to soil properties and leaf biomass was also investigated. Moreover, the measurements on poplar, combined with a simple radiative transfer model (the so-called omega-tau equation), allowed estimating the transmissivity of the canopy with and without leaves. The analysis of data has shown that for the observed forest type, the sensitivity to soil moisture under defoliated trees can be noted at both the L- and C-bands.

Ground-Based Microwave Investigations of Forest Plots in Italy

. Vegetation optical depth (VOD) retrieved from microwave radiometry correlates with the total amount of water in vegetation, based on theoretical and
empirical evidence. Because the total amount of water in vegetation varies with relative water content (as well as with biomass), this correlation
further suggests a possible relationship between VOD and plant water potential, a quantity that drives plant hydraulic behavior. Previous studies
have found evidence for that relationship on the scale of satellite pixels tens of kilometers across, but these comparisons suffer from significant
scaling error. Here we used small-scale remote sensing to test the link between remotely sensed VOD and plant water potential. We placed an L-band
radiometer on a tower above the canopy looking down at red oak forest stand during the 2019 growing season in central Massachusetts, United
States. We measured stem xylem and leaf water potentials of trees within the stand and retrieved VOD with a single-channel algorithm based on
continuous radiometer measurements and measured soil moisture. VOD exhibited a diurnal cycle similar to that of leaf and stem water potential, with
a peak at approximately 05:00 eastern daylight time (UTC − 4). VOD was also positively correlated with both the measured dielectric constant and water potentials of stem xylem
over the growing season. The presence of moisture on the leaves did not affect the observed relationship between VOD and stem water potential. We
used our observed VOD–water-potential relationship to estimate stand-level values for a radiative transfer parameter and a plant hydraulic
parameter, which compared well with the published literature. Our findings support the use of VOD for plant hydraulic studies in temperate forests.

/pdf/l-band-vegetation-optical-depth-as-an-indicator-of-plant-4b1go8fyjh.pdf

L-band vegetation optical depth as an indicator of plant water potential in a temperate deciduous forest stand

Satellite data are the most suitable tools for monitoring time and spatial variations of snow covered areas and for studying snow characteristics on a global scale. Current knowledge of the microwave emission from the deep ice sheet in Antarctica is limited by the lack of low-frequency satellite sensors and by their inadequate knowledge of the physical effects governing microwave emission at wavelengths exceeding 5 cm. On the other hand, in addition to the interest related to climatic changes and to glaciological and hydrological applications, there is growing interest, on the part of the remote sensing community, in using the Antarctic and, in particular, the Dome-C plateau where the Concordia station is located, for calibrating and validating data of satellite-borne microwave and optical radiometers. This is because of the size, structure, spatial homogeneity, and thermal stability of this area. With a view to the future launches of two new low-frequency spaceborne sensors Soil Moisture and Ocean Salinity mission and Aquarius, an experiment was carried out at Dome-C, thanks to financial support from European Space Agency, aimed at evaluating the stability and the absolute value of the L- and C-band brightness temperature Tb. This paper presents a report on the experimental campaign, the characteristics of the radiometric measurements, and on the main results. The C-band Tb data indicated a diurnal cycle amplitude of a few kelvin. It was confirmed that this takes place as a consequence of observed variability in the physical temperature of the top 4 m of the snowpack around the mean surface value of -24degC. In contrast, the L-band data indicated extremely stable Tb values of 192.32 K (1sigma=0.18 K) and 190.77 K (1sigma=0.57 K) at thetas=45deg and thetas=56deg, respectively

DOMEX 2004: An Experimental Campaign at Dome-C Antarctica for the Calibration of Spaceborne Low-Frequency Microwave Radiometers

In this paper we reviewed the use of microwave remote sensing methods for characterizing crop canopies and vegetation water stress related phenomena. Our analysis includes both active and passive systems that are ground-based, airborne, or spaceborne. Most of the published results that have examined crop canopy characterization and water stress have used active microwave systems. In general, quantifying the effect of dynamic vegetation properties, and water stress related processes in particular, on the measured microwave signals can still benefit from improved models and more observational data. Integrated data sets providing information on both soil status and plant status are lacking, which has hampered the development and validation of mathematical models. There is a need to link three-dimensional functional, structural crop models with radiative transfer models to better understand the effect of environmental and related physiological processes on microwave signals and to better quantify the impact of water stress on microwave signals. Such modeling approaches should incorporate both passive and active microwave methods. Studies that combine different sensor technologies that cover the full spectral range from optical to microwave have the potential to move forward our knowledge of the status of crop canopies and particularly water related stress phenomena. Assimilation of remotely sensed properties, such as backscattering coefficient or brightness temperature, in terms of estimating biophysical crop properties using mathematical models is also an unexplored avenue.

Characterization of Crop Canopies and Water Stress Related Phenomena using Microwave Remote Sensing Methods: A Review

Microwave remote sensing observations provide all weather, day/night monitoring of the earth's surface and make it possible to probe forest vegetation at various depths by operating at different frequencies. Significant progress in microwave radiometry of land surfaces has been made by using advanced airborne and spaceborne instruments and by developing physical and statistical models needed for interpreting the data. At present, a new multi-frequency scanning radiometer, launched in 2002 is providing global observations of the earth's surface at a relatively high resolution, and collected data are currently under study. This paper provides a review of experimental and theoretical investigations carried out in recent years to study the relationships between microwave emission and forest features at regional and global scale. It is shown that, despite the relatively small amount of experimental data currently available, microwave radiometry has proved to be an efficient technique in monitoring for...

Microwave radiometry of forests

Multifrequency microwave radiometric measurements with airborne instruments were carried out over six forest stands in Tuscany, Italy. The collected data pointed out the sensitivity of microwave emission to forest type and biomass.

Airborne multi-frequency L- to Ka-band radiometric measurements over forests

Experimental and theoretical investigations carried out over the past years at CNR in Florence and by other scientists worldwide, have pointed out that microwave emission is sensitive to vegetation biomass. The potential of microwave radiometry in the detection of vegetation biomass on different spatial scales has been further investigated by analyzing multitemporal data collected with the Special Sensor Microwave Imager (SMM/I) on several European test areas and in field experiments with a ground based radiometer. The results are reported and discussed. >

Passive microwave remote sensing of vegetation biomass

This paper presents the results obtained during the airborne IROE-STAAARTE campaign in 1997 on the agricultural site "Les Alpilles" (France). The radiometric measurements were carried out using the French airplane ARAT equipped with two microwave IROE radiometers at C- and X- bands (6.8 and 10 GHz) and one infrared sensor (8-14 micron). Using two frequency data it was possible to separate six vegetation classes of the whole area. The soil moisture of agricultural fields was estimated using the C-band normalized temperature, whilst the biomass of wheat was retrieved through the polarisation index at X-band.

Monitoring crop biomass and soil moisture with airborne microwave radiometers

Spatial modeling of hydrologic processes like evapotranspiration or runoff requires a large number of land surface parameters. One of the key parameters is albedo which determines the available solar energy at the surface. Until now NOAA-AVHRR with several repetitions a day was the preferred sensor to obtain surface albedo although its spatial resolution is only 1 km. Since the launch of the IRS-P3 satellite in March 1996 there are two new data sets available: MOS-B with a repetition rate of 19 days and WIFS with a temporal coverage of 5 to 19 days. They have a better spatial resolution than NOAA-AVHRR (MOS-B with 520x520 m and WIFS with 188x188 m). The radiometric resolution of WIFS is comparable to NOAA-AVHRR CHI and CH2. MOS-B provides data in 13 channels between 0.4 and 1.1 /spl mu/m comparable to the upcoming MERIS sensor on ENVISAT.

Comparison of the albedo derived from MOS-B and WIFS with NOAA-AVHRR

The major microwave characteristics of various types of land surfaces obtained from their emissivity spectra and polarization state are investigated and discussed. >

http://ieeexplore.ieee.org/iel2/1064/7705/00322481.pdf

The use of the Special Sensor Microwave Imager (SSM/I) for detecting land surface features in Europe

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