1 SOIL MOISTURE ESTIMATION USING MULTI-INCIDENCE AND MULTIPOLARIZATION ASAR DATA

The potential of Advanced Synthetic Aperture Radar (ASAR) for the retrieval of surface soil moisture over bare soils was evaluated for several ASAR acquisition configurations: (1) one date/single channel (one incidence and one polarization), (2) one date/two channels (one incidence and two polarizations), (3) two dates/two channels (two incidences and one polarization), and (4) two dates/four channels (two incidences and two polarizations). The retrieval of soil moisture from backscattering measurements is discussed, using empirical inversion approaches. When compared with the results obtained with a single polarization (HH or HV), the use of two polarizations (HH and HV) does not enable a significant improvement in estimating soil moisture. For the best estimates of soil moisture, ASAR data should be acquired at both low and high incidence angles. ASAR proves to be a good remote sensing tool for measuring surface soil moisture, with accuracy for the retrieved soil moisture that can reach 3.5% (RMSE). INTRODUCTION Soil moisture and surface roughness are significant indicators for hydrologic studies and the monitoring of agricultural environments. These parameters play an important role in the distribution of precipitation between runoff and infiltration. The possibility of retrieving these soil parameters has been investigated by using scatterometers, satellites, space shuttles, and airborne synthetic aperture radars [1,2,3,4,6,7,8,9,12]. The launch of the new European Environmental Satellite (ENVISAT) in March 2002, carrying the C-band Advanced Synthetic Aperture Radar (ASAR), should enable the scientific community to improve and increase its ability to retrieve physical parameters, based on ENVISAT’s capability of providing images in HH, HV, and VV polarizations (two polarizations are possible simultaneously) and at various incidence angles between 15° and 45°. The objective of the present study is to investigate empirical inversion approaches in order to retrieve volumetric soil moistures for bare soil from ASAR images acquired at various incidence angles and in HH and HV polarizations. This work will enable us to evaluate the potential of the new ASAR sensor for extracting surface soil moisture. DATA SET The image data used in this study were acquired by the ASAR SAR between 9 February 2003 and 20 April 2004 over two study sites (16 images, HH and HV polarizations, incidence angle between 20° to 43°). The first lies to the west of Paris, near Villamblain, France (latitude 48° 00` N, longitude 01° 34` E). The second is located near Toulouse in the Touch catchment basin (latitude 43° 27` N, longitude 01° 02` E). The sites are composed mainly of agricultural fields intended for growing wheat and corn. Simultaneously with the radar acquisition, ground truth measurements including soil moisture, surface roughness, and bulk density were performed on several bare soil test fields. Soil moisture content was measured using the gravimetric method (upper 0-5 cm soil layer, 10 locations within each test field). The volumetric soil moistures range from 5.4% to 47.3% with a standard deviation of about ±1.7%. The soil bulk density ranges from 0.86 to 1.66 with a standard deviation of about 0.06. Most of the in situ ground measurements of soil moisture were made within ± 2 h of the ASAR overpasses. Soil roughness measurements were also carried out, using a 2 meter-long needle profilometer with a 1-cm sampling interval (10 roughness profiles for each test field : 5 parallel and 5 perpendicular to the row direction). On the basis of these measurements the roughness parameters, such as the root mean square (rms) surface height and the correlation length (L) were calculated using the mean of all experimental auto-correlation functions, both parallel and perpendicular. The rms values fluctuate between 0.5 cm and 3.56 cm; the