Remote sensing of runoff variability along the Texas Coast

Coastal Texas contains numerous small rivers, lacustrine areas, and coastal lagoons which drain into local land areas. Seasonal and inter-annual variability in runoff from local land areas have important influences on the estuarine ecology in this region. Typically, such runoff variability is characterized utilizing river flow data from specific gauging stations; however, non-channelized water flow, regulated through variability in precipitation and evaporation may be of equal importance. Using biweekly Normalized Difference Vegetation Index (NDVI) data derived from the Advanced Very High Resolution Radiometer (A VHRR), the spatial and temporal variability of vegetation greeness in this region can be determined. Since NDVI is a synoptic measure that also incorporates many factors of land hydrology, additional correlative information on spatial and temporal variability in hydrology, not captured in the river flow data, may be derived. This pilot study investigated spatial and temporal relationships among precipitation, ground cover vegetation characteristics, NDVI, and river flow in relation to salinity changes in Galveston Bay during 1985 thru 1991. These relationships suggested that a large portion of the freshwater inflow to Galveston Bay must be from unchannelized flows that are related to land hydrology. Crosscorrelation of NDVI and salinity indicated that year-to-year changes in the monthly NDVI preceded variability in monthly salinity of Galveston Bay by several months. In contrast, gauged flow from the Trinity River, which empties directly into Galveston Bay was found to be more of a coincident indicator and it explained less of the observed variance in salinity. From these analyses, we conclude that variance in NDVI provides a correlative measure of non-channelized runoff in this region, and may have utility in other coastal areas