ROUGHNESS CONTROLS ON SPATIAL PATTERNS OF SEDIMENT TRANSPORT, VEGETATION AND GROUNDWATER IN A DESERT DUNE FIELD

The spatial pattern of sand flux determines: dune migration rates, size and type; accumulation rates; and grain size sorting. It is commonly observed that sand transport rates are highest at the upwind margin of a dune field, and decline downwind for several kilometers. Although it is well known that abrupt changes in surface roughness (e.g., dunes) trigger growth of an internal boundary layer (IBL), the implications for sand flux have not been quantitatively explored. Here we combine IBL and sediment transport theory, with repeated airborne altimetry surveys and field observations, to demonstrate that IBL development controls the downwind pattern of sand flux at White Sands, New Mexico. Results explain the observed sediment accumulation rate, and also shed light on the origin and evolution of the dune field. Declining sand transport downwind triggers an abrupt change in vegetation density, which in turn drives changes in groundwater depth and salinity by affecting transpiration rates. Thus, roughness from the dunes themselves is responsible for large-scale changes in dune dynamics, vegetation and hydrology. We expect that IBL development controls sand flux patterns in any dune field for which abrupt changes in roughness occur.