Microwave remote sensing of soil moisture content over bare and vegetated fields

Abstract

Remote measurements of soil moisture contents over bare fields and fields covered with orchard grass, corn, and soybean were made during October 1979 with 1.4 GHz and 5 GHz microwave radiometers mounted on a truck. Ground truth of soil moisture content, ambient air and soil temperatures was acquired concurrently with the radiometric measurements. The biomass of the vegetation was sampled about once a week. The measured brightness temperatures over bare fields were compared with those of radiative transfer model calculations using as inputs the acquired soil moisture and temperature data with appropriate values of dielectric constants for soil‐water mixtures. Good agreement was found between the calculated and the measured results over 10°‐70° incident angles. The presence of vegetation was found to reduce the sensitivity of soil moisture sensing. At 1.4 GHz the sensitivity reduction ranged from ∼20% for 10‐cm tall grassland to over 60% for the dense soybean field. At 5 GHz the corresponding reduction in sensitivity ranged from ∼70% to ∼90%.

Radiometric measurements over bare and vegetated fields at 1.4-GHz and 5-GHz frequencies

“…This is then followed by discussions of bare and vegetated field measurement results. Conclusions derived from these results are generally supportive of earlier report by Wang et al (1980a).…”

“…Results of different mobile tower measurements on bare soils are generally in agreement with one another and confirm the observations made at aircraft altitudes. On measurements over vegetated fields, there is some discrepancy between the results of Newton and Rouse (1980) and those of Wang et al (1980a) and Kirdiashev et al (19 7 9). At the frequency of 1.4GHz, Newton and Rouse reported a negligible vegetation effect on the radiometric outputs, while Wang et al, and Kirdiashev et al, observed a definite dependence of brightness temperature on vegetation cover.…”

“…The inclusion of the 1980 measurements provides a much broader range in both soil moisture content and vegetation biomass, as compared to that reported by Wang et al (1980a) based on 1979 data alone. The experiment procedure, test site characteristics, ground truth data collection, and microwave sensor specification are first described.…”

“…Based on the data taken in 1979, Wang et al (1980a) showed that the measured brightness temperature can be predicted fairly well by a radiative transfer model (Wilheit, 1978) with two constant parameters characterizing the field surface roughness (Choudhurv, 1978;Choudhury et al, 1979;Wang and Choudhury, 1981). Using the same roughness height h = 0.15 and polarization mixing coefficient Q = 0.13 derived from the 1979 measurements (Wang and Choudhury, 1981), brightness temperatures TB's at 1.4GHz of the bare fields with moisture and temperature profiles measured in 1979 and 1980 were computed with the same radiative transfer theory and the dielectric permittivity model (Wang and Schmugge, 1980).…”

“…Although many passive microwave experiments on the remote estimate of soil moisture content have been conducted at both aircraft and satellite altitudes (Schmugge et al, 1974(Schmugge et al, , 1977Eagleman and Lin, 1976;Blanchard et al, 1981;Schmugge, 1980), measurements at ground level with microwave radiometers mounted on a mobile tower remain a fundame-tal means for studying the effects of soil moisture variation, soil type, surface roughness, and vegetation cover on the microwave radiometric outputs (Newton and Rouse, 1980;Njoku and Kong, 1977;Wang et al, 1980a, b;Schanda et al, 1978;Wang and Choudhury, 1981). Results of different mobile tower measurements on bare soils are generally in agreement with one another and confirm the observations made at aircraft altitudes.…”