http://www.lpi.usra.edu/publications/slidesets/geology/sgeo/slide_21.html

The Republic of South Yemen lies on the edge of one of the world's great sand seas, the Rubh-al-Khali, but even this dry desert region bears the unmistakable imprint of flowing streams and rivers. The branching pattern in the photograph could only have been produced by running water, draining off the surrounding land. These filigree patterns are termed “dendritic drainages” because of their similarity to the way in which trees branch out into progressively finer twigs.

The dry gullies or wadis appear to pose something of a paradox in an area that is apparently exceptionally arid desert, with no vestige of plant life.