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Model for a dune field with exposed water table

Aeolian transport in coastal areas can be significantly affected by the presence of an exposed water table. In some dune fields, such as in Lençóis Maranhenses, northeastern Brazil, the water table oscillates in response to seasonal changes of rainfall and rates of evapotranspiration, rising above the ground during the wet season and sinking below in the dry period. A quantitative understanding of dune mobility in an environment with varying groundwater level is essential for coastal management as well as for the study of long-term evolution of many dune fields. Here we apply a model for aeolian dunes to study the genesis of coastal dune fields in presence of an oscillating water table. We find that the morphology of the field depends on the time cycle, $T_{\mathrm{w}}$, of the water table and the maximum height, $H_{\mathrm{w}}$, of its oscillation. Our calculations show that long chains of barchanoids alternating with interdune ponds such as found at Lençóis Maranhenses arise when $T_{\mathrm{w}}$ is of the order of the dune turnover time, whereas $H_{\mathrm{w}}$ dictates the growth rate of dune height with distance downwind. We reproduce quantitatively the morphology and size of dunes at Lençóis Maranhenses, as well as the total relative area between dunes.