Calculate the residence time of a sand grain in a barchan dune. The footprint of the dune is 75 m long in the wind direction, and repeat photos have shown that the dune is moving at 15 m/year. For the same dunefield, calculate the mean speed of a sand grain over the desert floor if the spacing between barchan dunes in the wind direction, as deduced from air photo patterns, is 500 m. Recall that it is safe to say that there is no storage of sand between the dunes on these rocky desert floors.