Microwave Measurements of Moisture Distributions in the Upper Soil Profile

Abstract

Laboratory and field experiments were conducted to investigate the ability of microwave remote sensing systems to detect the moisture status of a silt loam soil exhibiting abrupt changes in moisture content near the surface. Laboratory soil profiles were prepared with a discontinuous moisture boundary in the subsurface. Reflectivity measurements of these profiles were made with a bistatic reflectometer operating over the frequency ranges of 1–2 and 4–8 GHz (wavelength ranges of 30–15 and 7.5–3.75 cm, respectively). These measurements exhibited a well‐developed coherent interference pattern in good agreement with a simple two‐layer reflectivity model. Field measurements of bare soil surfaces were conducted for initially saturated profiles and continued for extended periods of drying. During drying, coherent interference patterns similar to those observed in the laboratory were detected. These appear to be due to steep moisture gradients occurring between drying layers near the surface. The field results were modeled by a five‐segment linear moisture profile with one or two steep segments and a multilayer reflectivity program. Agreement between model and field response over the frequency range was used to estimate the depth of drying layers within the soil. These depths were monitored over the second and third drying cycles. Formation of the drying layers under field conditions appears to be influenced by drying time, tillage, and evaporative demand. In any case, it appears that the coherent effects caused by nonuniform moisture profiles may substantially affect the reflectivity of even rough soil surfaces.

Estimation of Soil Moisture from Optical and Thermal Remote Sensing: A Review

“…Wavelengths of 0.4–2.5 μm represent the radiation of the sun reflected from the Earth’s surface [83,84]. These approaches mainly use reflectance to estimate the SM in visible infrared remote sensing techniques.…”

“…Although the signal penetration capabilities of optical and thermal methods are worse than those of thermal infrared and microwave methods, the reflectance domain is the most operational because the images are easily available over a broad range of ground resolutions [ 77 , 78 , 79 , 80 , 81 , 82 ]. Wavelengths of 0.4–2.5 μm represent the radiation of the sun reflected from the Earth’s surface [ 83 , 84 ]. These approaches mainly use reflectance to estimate the SM in visible infrared remote sensing techniques.…”

Could operational hydrological models be made compatible with satellite soil moisture observations?

“…Only a brief description of them is summarized here, and interested readers are encouraged to read further details from the references provided. Optical : it measures the reflected radiation of the Sun from the Earth's surface, known as the reflectance (Sadeghi et al ., ). Its correlation with the soil moisture has long been recognized (Ångström, ).…”

“…• Optical: it measures the reflected radiation of the Sun from the Earth's surface, known as the reflectance (Sadeghi et al, 1984). Its correlation with the soil moisture has long been recognized (Ångström, 1925).…”

Satellite remote sensing applications for surface soil moisture monitoring: A review

“…2 Optical remote sensing for soil moisture estimation Remote sensing of soil moisture content using the solar domain with wavelengths between 0.4 and 2.5 μm measures the reflected radiation of the sun from the Earth's surface, known as reflectance (Sadeghi et al, 1984). Compared with microwave and thermal infrared domains that have been most commonly used for soil moisture estimation (Price, 1980;Wuthrich, 1994, Engman and Chauhan 1995, Jackson et al, 1995, little attention has been paid to the use of the solar domain (Liu et al, 2003).…”