MULTI-TEMPORAL REMOTE SENSING ASSESSMENT OF SOIL MOISTURE AND VECTOR-BORNE DISEASE POTENTIAL IN THE PLEISTOCENE BEAUMONT AND LOWER RIO GRANDE DELTA, SOUTHERN TEXAS AND NORTHERN TAUMALIPAS, US-MEXICO BORDER REGION

A dengue fever outbreak near Matamoros, Mexico during the Fall, 2004 became an epidemic during the Summer, 2005 as a result of heavy rainfall and flooding associated with the landfalls of Hurricane Emily and Tropical Storm Gert. Dengue fever is a vector-borne disease spread by mosquitoes, which breed in areas of high soil moisture often containing pools of stagnant water. In this study, normalized difference vegetation index (NDVI) combined with surface radiant temperatures was used to model soil moisture availability (Mo), which measures relative amounts of moisture in the soils not available for evapo-transpiration by vegetation. A previous study applying this method found mosquito infection rates throughout the Nile Delta, Egypt to be highest in areas where modeled Mo values range between 20 – 60%, with 0% Mo defining dry areas and 100% defining water-saturated areas, wetlands and perennial bodies of water.

For the Rio Grande Delta area, both Landsat-7 ETM+ and ASTER measured NDVI images (15 – 30 meter spatial resolution) and surface radiant temperature images (60 – 90 meter spatial resolution) were used to map the distribution of Mo values throughout wet and drought periods between 06/00 and 09/05. The results show that the most persistent areas of high (> 60%) Mo corresponds with “resacas”, or abandoned meanders, along the Rio Grande floodplain and delta, which often persists even through drought periods. In particular, pre- and post- Hurricane Emily ASTER images show an increase in flooded areas around ephemeral lakes and resacas along the Rio Grande floodplain. Within the Pleistocene Beaumont formation, areas of moderate to high Mo within the 20 – 60% range of high mosquito-breeding probability correspond with impermeable muds from remnant interdistributary, abandoned channel-fill, and overbank flood deposits. One such area near Raymondville, TX produced Mo values near 100% during a prolonged wet period in November, 2003. Mineral maps produced using drought period ASTER imagery (March, 2001) and x-ray diffraction results from Beaumont soil samples suggest that the wetter, impermeable areas may have relatively higher smectite contents than the surrounding sandier, moderately permeable areas. Satellite-derived soil moisture data could be useful for targeted mosquito control measures in the future.