An Empirical Relation for the Soil Moisture Measurement Using Emissivity Values at Microwave Frequency Range

The potential of passive microwave radiometry for the soil moisture measurement is a well established one, especially in the long wave length regions due to its high penetrating capacity through vegetation. Radiometric data can be used as a mapping tool for classifying the crops, examining their health and viability and monitoring the farming practices. The radiometric data or the emission of microwave energy commonly referred as microwave brightness temperature TB is proportional to the product of surface temperature and surface emissivity. The surface emissivity of soils is dependent on both water content and its physical characteristics . Hence these data are essential for the retrieval of soil moisture content from the remotely sensed satellite data. In this paper, we present the emissivity data of different Indian soils of different textures at various moisture levels in the frequency range 0.6GHz to 1.2GHz, calculated from the measured values of dielectric constants in the laboratory. The result indicate that the emissivity values are independent of frequency and texture but shows a strong dependence on the moisture content. On the basis of these results, an empirical relation is formulated for calculating the soil moisture content form emissivity values, which shows a good agreement with other reported radiometric measurements. 1 . INTRODUCTION Microwave radiometry from space has been recognized as a powerful tool for the retrieval of several geophysical parameters especially the soil moisture content. ( Newton and Rouse 1980, O’Neill 1985, Ulaby et al 1986, Rao et al 1987, 1990, Jackson 1990).The theory behind the microwave remote sensing of soil moisture is based on the large contrast between the dielectric properties of liquid water (~80 ) and dry soil (~4). Microwave emissivity of soil is dependent on both the water content and physical characteristics of the soil. The water holding capacity of wet soils is determined by its particle size distribution. Hence the emission properties need to be studied to extract the soil moisture information from emissivity data. In the Indian context little ground truth data are available for Indian soils to support the satellite data for retrieving geophysical parameters where different parts of the country have soils of different texture composition . Therefore we calculated the emissivity of soils collected form different parts of India for normal incidence at different moisture conditions in the frequency range 0.6GHz to 1.2GHz from the me asured values of its dielectric constant. An empirical relation also formulated by using the emissivity values to predict the soil moisture content. 2. THEORETICAL BACKGROUND The lumped element approach of Stuchly et al. (1974) was selected for measuring the reflection which is related to the characteristic impedance of the transmission line and that of the sensor probe. The impedance of the sensor is a function of the frequency and relative permittivity of the test sample. Under quasi-static approximation the stationary formula for the aperture admittance of an open ended coaxial line terminated by a semi –infinite medium on a ground plane reduces to (Staebell et al.1990) [ ] [ ] [ ] [ ] ) 1 ( ) / ( 12 ) / ( ) / ( 2 2 5 2 2 2 2 2 3 4