Soil moisture estimation in a semiarid watershed using 3 RADARSAT - 1 satellite imagery and genetic programming

6 [1] Soil moisture is a critical element in the hydrological cycle especially in a semiarid 7 or arid region. Point measurement to comprehend the soil moisture distribution 8 contiguously in a vast watershed is difficult because the soil moisture patterns might 9 greatly vary temporally and spatially. Space-borne radar imaging satellites have been 10 popular for they may exhibit all-weather observation capability. Yet the estimation 11 methods of soil moisture based on the active or passive satellite imageries remain 12 uncertain. This study aims at presenting a systematic soil moisture estimation method for 13 the Choke Canyon Reservoir Watershed (CCRW), a semiarid watershed with an area of 14 over 14,200 km in south Texas. With the aid of five corner reflectors, the RADARSAT-1 15 Synthetic Aperture Radar (SAR) imageries of the study area acquired in April and 16 September 2004 were processed by both radiometric and geometric calibrations at first. 17 New soil moisture estimation models derived by genetic programming (GP) technique 18 were then developed and applied to support the soil moisture distribution analysis. The 19 GP-based nonlinear function derived in the evolutionary process uniquely links a series of 20 crucial topographic and geographic features, including slope, aspect, vegetation cover, 21 and soil permeability, with the well-calibrated SAR data. Research findings indicate 22 that the novel application of GP was proved useful for generating a highly nonlinear 23 structure in regression regime, which exhibits very strong correlations statistically 24 between the model estimates and the ground truth measurements (volumetric water 25 content) on the basis of the unseen data sets. In an effort to produce the soil moisture 26 distributions over seasons, it eventually leads to characterizing localto regional-scale 27 soil moisture variability and performing the possible estimation of water storages of the 28 terrestrial hydrosphere.