Relating surface backscatter response from TRMM precipitation radar to soil moisture: results over a semi-arid region

Abstract. The Tropical Rainfall Measuring Mission (TRMM) carries aboard the Precipitation Radar (TRMMPR) that measures the backscatter (σ°) of the surface. σ° is sensitive to surface soil moisture and vegetation conditions. Due to sparse vegetation in arid and semi-arid regions, TRMMPR σ° primarily depends on the soil water content. In this study we relate TRMMPR σ° measurements to soil water content (m_s) in the Lower Colorado River Basin (LCRB). σ° dependence on m_s is studied for different vegetation greenness values determined through Normalized Difference Vegetation Index (NDVI). A new model of σ° that couples incidence angle, m_s, and NDVI is used to derive parameters and retrieve soil water content. The calibration and validation of this model are performed using simulated and measured m_s data. Simulated m_s is estimated using the Variable Infiltration Capacity (VIC) model and measured m_s is acquired from ground measuring stations in Walnut Gulch Experimental Watershed (WGEW).

σ° model is calibrated using VIC and WGEW m_s data during 1998 and the calibrated model is used to derive m_s during later years. The temporal trends of derived m_s are consistent with VIC and WGEW m_s data with a correlation coefficient (R) of 0.89 and 0.74, respectively. Derived m_s is also consistent with the measured precipitation data with R=0.76. The gridded VIC data is used to calibrate the model at each grid point in LCRB and spatial maps of the model parameters are prepared. The model parameters are spatially coherent with the general regional topography in LCRB. TRMMPR σ° derived soil moisture maps during May (dry) and August (wet) 1999 are spatially similar to VIC estimates with correlation 0.67 and 0.76, respectively. This research provides new insights into Ku-band σ° dependence on soil water content in the arid regions.