A. Kotlyar

Soil moisture (water-content) assessment by an airborne scatterometer: the Chernobyl disaster area and the Negev desert.

Abstract Soil water content is an important component that influences meso- and microscale processes. The agricultural capacity of a site is directly affected by soil water content and it is especially important in arid regions. In temperate and humid regions, soil water content is important in determining flood risks. Environmentally, soil water content will influence the risk of carrying pollutants through the soil. For these reasons, a scatterometer was developed as a remote sensor for mapping soil water content. The scatterometer is frequency modulated using a continuous wave. This scatterometer operates at nadir with a wide antenna diagram of 10°. Measurements were conducted in two environments with different implications. The first was in the Chernobyl nuclear disaster area and the second in the Negev desert. Results show a good correlation between soil water content and the amplitude of the returned signal measured by the scatterometer. Thus, the scatterometer provides an efficient tool for mapping soil water content. The long wavelength (P-band) of λ=68 cm makes this scatterometer more sensitive to soil water content and less affected by surface roughness than scatterometers operating at shorter wavelengths.

Retrieving parameters of bare soil surface roughness and soil water content under arid environment from ERS-1, -2 SAR data

Evaluation of the environmental and agricultural potential of arid lands is often limited by the lack of information on soil surface roughness and water content. The current study proposes an efficient method to retrieve these parameters of bare soil from single-channel ERS-1, -2 synthetic aperture radar (SAR) data. New equations were derived by combining the model for vertically co-polarized mode backscattering coefficient the model for the real part of dielectric constant and the empirical equation interrelating parameters of roughness. These equations allowed for calculation of the root mean square (RMS) height h of small surface roughness (h ≤ 1 cm) for naturally sandy, flat areas of the Negev desert (Israel) during dry periods when is extremely low and generally known. As soil roughness was found to be sufficiently constant under the arid environment, this study showed that calculated h could be reliably used to retrieve during the wet period. Statistical analysis of the relative errors of retrieved h and showed their high independence on the absolute values. Retrieved values of h and obtained from ERS-2 SAR data showed acceptable correlation with the direct ground measurements. Therefore, the effectiveness of the proposed methodology for h and retrieval was proved.

Microwave subsurface remote sensing in the Negev Desert: monitoring of soil water-content

We report the use of the microwave remote sensing as a technique with great potential for the mapping of subsurface properties including the monitoring of soil water conditions. Remote sensing experiments with microwave instrumentation were conducted in the Negev desert in Israel. The remote sensors used were a P-band (68 cm, 441 MHz) scatterometer and an ERS-2 C-band (5.3 cm, 5.7 GHz) SAR (synthetic aperture radar) along with the collection of ground truth data such as volumetric and gravimetric soil water-content, surface roughness and dielectric measurements. Corner reflectors in the field were used for calibration and geo-rectification of the SAR data. The results of the microwave experiments are in a good agreement with the developed theoretical models that take into account the effects of the random surface roughness. The optical modelling of microwave processes is presented as a tool for developing the physical basis for empirical studies. This practice simplifies testing theoretical predictions and reduces the immense cost of running field and laboratory measurements in the microwave range.

Remote sensing in microwave and gamma ranges for the monitoring of soil water content of the root zone

Knowledge of vertical distribution of the soil water content in the root zone W plays a key role in the optimization of irrigation and, hence, for water saving. Therefore, in this study, synergistic use of the frequency modulated continuous-wave P-band (λ = 68 cm) scatterometer (SC) and the gamma-ray radiometer (GR) for the range of 50–3000 keV, both low in sensitivity to soil surface roughness and vegetation, was proposed. This combination allowed the determination of W at the three depth ranges: 0–5, 0–30, and 5–30 cm. This considerably specified the vertical distribution of W. Both instruments were developed as an integrated remote-sensing system that was installed onboard a light aircraft and tested on an irrigated agricultural region of the Negev desert, Israel. For processing and analysing the results, analytical models for both reflection coefficient ( ) and intensity of the natural gamma radiation ( ) were developed for various types of soils and vertical distributions of W. Using these models, the problem of the inverse retrieval of W was solved, and the depth of sampling z was evaluated. SC allowed the measurement of average W for the depth of 0.5–5.0 cm depending on W and dW/dz at the soil surface. For GR, the sampling depth varied from 20 to 30 cm depending on the mean value of W in the top soil layer. Field experiments showed that W retrieved by the developed system was in good agreement (r2 ≥ 0.9) with ground measurements, thus indicating sufficient accuracy of both instruments.

Soil moisture assessment by an airborne scatterometer in Chernobyl disaster area and Negev Desert

The presented results demonstrate the use of airborne radar scatterometers flown aboard a light aircraft for environmental monitoring and precision agriculture. A good agreement between values measured in the field and estimated by the scatterometers has been found with some overestimation for the lower moisture sites. Furthermore, for a more temperate site in Ukraine the scatterometer was found to be very useful also in estimating the depth of the groundwater table. Alongside the scatterometer a gamma-ray radiometer was flown at the Chernobyl nuclear power plant disaster site. The ability to map soil moisture and water table depth in conjunction with gamma-ray radiometry is extremely useful in monitoring the after affects and the rehabilitation of the Chernobyl NPP disaster area. Due to deforestation caused by the Chernobyl disaster high levels of soil moisture can be hazardous themselves in creating swampland. This phenomenon can be monitored and mapped by airborne scatterometers.

Light scattering from particles located under a rough dielectric layer

A discussion is presented of the effect of roughness on the detectability of subsurface particles by means of the light-scattering method. We have studied the scattering of light by calibrated spheres located under a slightly rough dielectric surface both experimentally and theoretically. In our experiments, the scattering from slightly rough layers with nonresonant particles was dominated by the roughness, and the scattering diagram did not bear any discernible indications of the spheres. However, at resonance, the subsurface particles manifested themselves by an increase in the total scattered intensity and by well-pronounced maxima in the angular dependence of both the scattering diagram and the backscattered intensity. Theoretical calculations show that the angular positions of the maxima in the scattering diagram are essentially determined by the interference of fields scattered by the particles and by the surface, whereas the contribution of the multiple interparticle scattering is negligible. By contrast, the oscillations in the angular dependence of the backscattered intensity are due primarily to the scattering between neighboring spheres.

Backscattering of light from a dielectric layer on a reflecting substrate

Scattering of light from rough dielectric films is studied experimentally. It is shown that the interference pattern of the scattered field depends critically on the power spectrum of the roughness, especially on its long-scale component. When the height of roughness is small compared with the wavelength, the backscattering peak (if it exists) is due to the interference of the singly scattered fields; hence the properties of the backscattered peak are rather unusual. In particular, the width of the peak is determined by the thickness of the film and is independent of the parameters of disorder. The intensity of the peak increases with an increase of the rms height of the surface roughness and becomes independent of the rms when the roughness is of the order of the wavelength.