Applying microwave radiation response depth to validate soil moisture retrieved from AMSR-E data

Penetrability is one of the greatest advantages of microwave remote sensing over other remote sensing techniques. Estimating the sensing depth of passive microwave remote sensing is meaningful for simulation of satellite signals and validation of land surface parameters. In this paper, microwave radiation response depth (MRRD) was proposed to describe the thickness of the soil layer, within which variations in soil moisture can significantly affect the emitted radiation. Then, a statistical model for estimating MRRD was developed by regression analysis. This model can estimate the MRRD using four parameters, which are soil moisture, soil temperature, frequency, and soil specific surface area. For validation, a controlled field experiment was performed using a truck-mounted multi-frequency microwave radiometer (TMMR) at Baoding, Hebei province, China. The accuracy of the statistical model, in terms of root mean square error (rmse), was approximately 0.54 cm for the available experimental data at the frequency of 6.925, 10.65, and 18.7 GHz. Finally, the statistical model was applied into soil moisture validation. Taking the Global Land Data Assimilation Systems (GLDAS) product as the measurements, we validated the soil moisture retrieved from advanced microwave scanning radiometer-earth observing system (AMSR-E) data. The measured soil moisture obtained from an empirical depth and MRRD were both used to comparison. The rmse between soil moisture retrieved and measured is 0.060 and 0.047 cm³/cm³ for empirical depth and MRRD based method, respectively. These results have shown that it is more reasonable to considered MRRD than an empirical depth in soil moisture validation.