Detecting seasonal flooding cycles in marshes of the Yucatan Peninsula with SIR-C polarimetric radar imagery

Polarimetric L- and C-band radar imagery from the shuttle imaging radar-C (SIR-C) were acquired over wetlands of the Yucatan Peninsula during the dry (April) and wet (October) seasons of 1994. Field surveys during the flights recorded biophysical data and water depth in 11 marsh sites containing communities of three principal emergent macrophytes: Cladium jamaicense, Typha domingensis, and Eleocharis cellulosa. The only major seasonal change was in flooding. Seasonal changes in polarimetric backscatter magnitude (HH, VV, and CS=(HV+ VH)/2) and phase [\H-V phase difference\ = PD) were extracted for a stable evergreen mangrove forest calibration site, which confirmed that the absolute calibration of the Yucatan imagery exceeded: the SIR-C system calibration. We estimate that seasonal changes of greater than or equal to 2dB in backscatter magnitude and greater than or equal to 10 degrees in phase (PD) are significant in our data. Seasonal changes in L- and C-band magnitude and phase were extracted from the 11 marshes, and significant changes above the calibration limit were noted. Increased flooding in the mac-shes was detected by: 1) an increase in backscatter magnitude in marshes with tall, dense cover; 2) a decrease in backscatter magnitude in marshes with short, sparse cover and 3) an increase in PD in all types of marshes. Magnitude increases result from an increase in double-bounce interactions between the emergent vegetation and water surface, whereas decreases result from an increase in forward scattering off the open water. Average PD values increase owing to an absolute or relative increase in double- compared with single-bounce interaction. Changes from dry or partially flooded to completely flooded as well as increases in water depth, could be detected by most of the polarimetric parameters, but changes from dry to partially flooded could not. C-band PD (CPD) was the radar parameter most sensitive to flooding. CPD changed significantly for all eleven marshes, followed by L-band PD (LPD) and LVV (nine marshes) and LHH, LCS, and CVV (seven marshes). CHH detected significant changes in five marshes but produced changes of +/-1.8-1.9 dB (just below our estimated calibration limit) in. four others. An evaluation of current spaceborne radars indicates that a combination of the European Remote Sensing Satellite (ERS-1,2) and Radarsat radars could detect seasonal flooding in a wide variety of marsh ecosystems, excluding partial flooding and flooding in small patches of short, sparse vegetation. (C) Elsevier Science Inc., 1997.