Leonid Vulfson

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.

Microwave remote sensing of physically buried objects in the Negev Desert: implications for environmental research

We report remote detections of physically buried specularly reflecting objects using microwave radar at two sites: Ashalim and Tseelim in the northern region of the Negev Desert, Israel. These detections provide confirmation that microwave subsurface remote sensing is a genuine phenomenon. At Ashalim, a scatterometer operating in the P-band (441 MHz, 68 cm) was mounted on a cherry picker truck at a height of 8 m and used to detect two triangular aluminum mesh reflectors (forming a 1-m square area reflector) buried down to a depth of 8 cm in dry sand. At Tseelim, the same scatterometer was mounted on an airplane flying at an altitude of 70 m and used to detect 1-m square aluminum reflectors (each one submerged at a different location along the airplane flight path) buried down to a depth of 20 cm. The experimental results compare favorably with a theoretical model that incorporates radar absorption effects arising in the sandy subsurface layer and radar interference effects arising from phase differences between reflections from the surface and buried reflector. The theoretical modeling also predicts the detection of a subsurface reflector down to a depth of about 4.4 m. This experiment and the associated modeling approach is the first of a series of planned experiments, which we outline for the detection and the theoretical evaluation of buried reflectors using remote microwave and VHF radar. We identify potential subject areas for environmental research.

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.