AN ISOLATED SAND DUNE'S RESPONSE TO SEVERE DROUGHT IN SOUTHERN BROOKS COUNTY, TEXAS

Conspicuous in satellite images of southwest Brooks County in south Texas, a large (>720 m long) isolated sand dune stretches across the otherwise mostly flat, brushy landscape. The dune is an actively moving feature of the Holocene age South Texas Sand Sheet (STSS), a relict eolian system which covers ~7000 km² north of the Rio Grande. During times of drought leafy vegetation stabilizing these dunes is reduced, exposing sand that is easily transported by wind, leading to periodic reactivation of relict dunes.

In the late 1950s, south Texas experienced the most severe drought in the last 1000 years, leading to the reactivation of parts of the STSS along the Gulf Coast (Barrineau, 2017). To assess changes in this dune's movement and sand thickness, we compared historical aerial photography and recent satellite imagery spanning 61 years, at four time intervals; 1955, 1970, 1996 and 2016. ArcGIS Desktop was used to process and analyze the images. Variations in spectral reflectance were used to evaluate the impact of vegetation growth on dune size and characterize the relative thickness of sand throughout the dune.

Our analysis shows that the dune has changed significantly in size in the years following the historic drought. As vegetation cover on the edges of the dune died, loose permeable sand was exposed, then transported onto the northwest edges of the dune by winds blowing from the southeast. In 1955, the year the severe drought began, the dune was 161,433 m². In a total of 61 years, the dune grew in area over 57% to a size of 254,431 m², with most of that growth occurring in the years between 1955 and 1970. First the dune grew larger in size, mainly in the northwest direction, then it became significantly thinner. The migration and erosion of the dune has exposed relict dunes and associated topographic features. in another 100 years, only traces of its existence will be left.