Microwave thermodielectric behavior of soil-water mixtures and potential effects on radar backscatter

Guy Serbin¹, Dani Or² and Dan G. Blumberg^1,3

1	Department of Geological and Environmental Sciences, Faculty of Natural Sciences, Ben Gurion University of the Negev, POB 653, Beer Sheva, 84105, Israel
2	Department of Plants, Soils and Biometerology, Utah State University, Logan, Utah, 84322-4820, United States of America
3	Department of Geography and Environmental Development, Faculty of Humanities and Social Sciences, Ben Gurion University of the Negev, POB 653, Beer Sheva, 84105, Israel

morning poster session

   Recent applications of time domain reflectometry (TDR) and frequency domain spectroscopy in the 200 MHz-6 GHz range to measurement of near-surface soil water content revealed strong temperature effects on measured dielectric permittivity of wet soils (e_b). These effects are attributed to interplay between reduction in the dielectric permittivity of free water and the increase in bulk dielectric permittivity due to release of bound water with increasing temperatures. Soil texture is the major factor in determination of e_b. Low specific surface area soils such as sands do not release large volumes of bound water with increasing temperature whereas high surface area soils such as clays do. A high surface area soil of 250 m²/g, Brocko silt loam with 10% gravimetric water content, showed a 3.5 fold increase in permittivity at 420 MHz and 2.8 fold at 5.3 GHz when measured soil temperature increased from 11°C to 45.5°C. Bound water effects become masked by the dielectric behavior of water at higher water contents such that these effects may not be discernible for wet soils. Furthermore some samples show apparent hysteresis in measured e_b values for the same temperature between heating up and cooling down phases, though this may be due to temperature artifacts. The effects of bound water are greatest low frequency and decrease with increasing frequency, such that P-band (420 MHz) soil water measurements will be more affected by bound water release than C-band (5.3 GHz).
   Simulation results using modeled and measured dielectric permittivities and two semi-empirical radar backscatter models indicate that temperature variations affect the radar backscatter response from different soil textures and contents to the extent that a temperature correction may be required for water content inferences. Significant differences were found between the models for similar parameters. Furthermore the observed hysteresis there may require monitoring of soil temperature prior to and during data acquisition. The strong dependence of measured thermodielectric effects on soil texture offers a promise for development of delineative indices, particularly when using two consecutive measurements obtained at relatively short intervals (<12 h) to maximize temperature differences while reducing evaporation effects.
 

Comparison of Synthetic Aperture Radar (SAR) data with NOAA AVHRR derived NDVI in the Gaza-Negev-Sinai border regions

Guy Serbin¹ and Gil Revivo²

1	Department of Geological and Environmental Sciences, Faculty of Natural Sciences, Ben Gurion University of the Negev, POB 653, Beer Sheva, 84105, Israel
2	Department of Geography and Environmental Development, Faculty of Humanities and Social Sciences, Ben Gurion University of the Negev, POB 653, Beer Sheva, 84105, Israel

afternoon poster session

   The Sinai-Gaza Strip-Negev border region exhibits noticeable differences in both synthetic aperture radar (SAR) and visible and near infrared imagery. This region is comprised of two distinct geomorphic zones- the Mediterranean coastal plain inland of the coastal dunes and the Sinai-Negev sand sea. Spatial variations on either side of borders are due mainly to differences in grazing and agricultural practices.
   Good correlations (r²=0.54~0.86) have been found between mean SAR backscatter coefficients (s⁰) for SIR-C and ERS-2 imagery and NDVI derived from NOAA AVHRR imagery for relatively homogenous regions such as sand dunes and small farms (such as those in Sinai or in the Gaza Strip). This correlation decreased when the large fields of the Israeli coastal plain were added, due partly to random tilling directions and crop rotations.
   The standard deviation of s⁰ was found to also be indicative of land use practices and homogeneity of land cover. The Israeli side of the Sinai-Negev sand sea, with no grazing or farming, exhibits a low standard deviation whereas the Egyptian side, with much grazing and interdunal farming, is higher for all bands and polarizations. In the coastal plain the opposite is the case as the large Israeli farms and higher building concentration in the Gaza Strip result in greater heterogeneity of s⁰ than by the small Egyptian farms. C-band has lower standard deviations than L-band, as its shorter wavelength is reflected more by roughness elements, tilling furrows and plants, resulting in a more homogenous radar backscatter.
   Use of radar imagery may be used to supplement NDVI data for relatively homogenous regions, should acquisition of optical data be difficult or impossible due to weather conditions or geometric view. Furthermore radar imagery and is useful for monitoring of land use and desertification processes.