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 rada r-C (SIR-C) were acquired over wetlands of the Yucatan Peninsula durin g the dry (April) and wet (October) seasons of 1994. Field surveys dur ing 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 o nly major seasonal change was in flooding. Seasonal changes in polarim etric backscatter magnitude (HH, VV, and CS=(HV+ VH)/2) and phase [\H- V phase difference\ = PD) were extracted for a stable evergreen mangro ve forest calibration site, which confirmed that the absolute calibrat ion of the Yucatan imagery exceeded: the SIR-C system calibration. We estimate that seasonal changes of greater than or equal to 2dB in back scatter magnitude and greater than or equal to 10 degrees in phase (PD ) are significant in our data. Seasonal changes in L- and C-band magni tude and phase were extracted from the 11 marshes, and significant cha nges 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 t ypes 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 rel ative increase in double- compared with single-bounce interaction. Cha nges from dry or partially flooded to completely flooded as well as in creases in water depth, could be detected by most of the polarimetric parameters, but changes from dry to partially flooded could not. C-ban d PD (CPD) was the radar parameter most sensitive to flooding. CPD cha nged significantly for all eleven marshes, followed by L-band PD (LPD) and LVV (nine marshes) and LHH, LCS, and CVV (seven marshes). CHH det ected 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 combinat ion of the European Remote Sensing Satellite (ERS-1,2) and Radarsat ra dars could detect seasonal flooding in a wide variety of marsh ecosyst ems, excluding partial flooding and flooding in small patches of short , sparse vegetation. (C) Elsevier Science Inc., 1997.