Development of a soil moisture model for use with passive microwave remote sensors

Abstract

Soil moisture profiles were simulated for a hypothetical loamlike soil with a water and heat balance model. Expected X-band and L-band radiometer response to these conditions were simulated by a radiative transfer model. From these simulations, a model was developed to estimate soil water content in two layers of a 1.5 m soil profile. Soil water content in the top 21 cm of the hypothetical soil was related to L-band emissivity over a wide range of soil moisture conditions. Inverted soil moisture profiles which result from small rains were classified by use of the rate of change in L-band emissivity one day after the rain. The amount of water added to the soil profile below the 21 cm depth due to percolation was related to a ratio of the rate of change in X-band and L-band emissivities one day after the rain. These relationships were combined into a comprehensive model that predicts soil moisture in two zones of the soil profile. This model was tested with measurements of soil water content and soil temperature collected during the four seasons of the year in a sandy loam soil contained in an array of lysimeters. X-band and L-band emissivities required in the predictor equations were calculated by the radiative transfer model from measured soil moisture and soil temperature data. The technique developed from simulated results to classify inverted soil moisture profiles was found to be seasonally dependent. It was also found that the second layer algorithm showed seasonal dependence. Predictions of soil water content in the top 21 cm of the soil profile from L-band emissivity calculated by the radiative transfer model occasionally conformed with measured soil water content. Since the equation to estimate soil water content for inverted soil moisture profiles did not fit the measured data, soil water content on such occurrences was overpredicted. Applications of small amounts of water produced the most disagreement between predicted and measured soil water content. ...