Measuring soil moisture with imaging radars

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Abstract
An empirical algorithm for the retrieval of soil moisture content and surface Root Mean Square (RMS) height from remotely sensed radar data was developed using scatterometer data, The algorithm is optimized for bare surfaces and requires two copolarized channels at a frequency between 1.5 and 11 GHz. It gives best results for kh less than or equal to 2.5, mu nu, less than or equal to 35%, and theta greater than or equal to 30 degrees. Omitting the usually weaker hv-polarized returns makes the algorithm less sensitive to system cross-talk and system noise, simplify the calibration process and adds robustness to the algorithm in the presence of vegetation, However, inversion results indicate that significant amounts of vegetation (NDVI > 0.4) cause the algorithm to underestimate soil moisture and overestimate RMS height. A simple criteria based on the sigma(hv)(O)/sigma(vv)(O) ratio is developed to select the areas where the inversion is not impaired by the vegetation. The inversion accuracy is assessed on the original scatterometer data sets but also on several SAR data sets by comparing the derived soil moisture values with in-situ measurements collected over a variety of scenes between 1991 and 1994, Both spaceborne (SIR-C) and airborne (AIRSAR) data are used in the test, Over this large sample of conditions, the RMS error in the soil moisture estimate is found to be less than 4.2% soil moisture.

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