Air photographs of 2 large semi-desert plains in the Haud and the Sawl Haud show rhythmic patterns of crescentic vegetation arcs in many cases and water lanes in rarer cases. Field investigation showed that the arcs were made by rhythmical arrangement of concentrations of small trees, shrubs, herbs and grasses. Rainfall of less than 3-4 inches per yr. results in the general absence of defined drainage channels. The vegetation arcs indicate precisely the direction of the very low slopes, and define the watersheds in these featureless plains. The tiers of arcs are invariably oriented so that the chords of the arcs are at right angles to the direction of drainage and the arcs are convex upslope. An hypothesis to account for origin of the arcs is presented. Sheets of water flowing over this nearly flat surface after the very rare rainstorms sweep along the loose organic material lying on the ground and deposit it in the form of strand lines, which are on the contour and hence convex upslope. The deposit contains both animal excreta in high proportion and seeds which will germinate to produce an arc of vegetation. Subsequent rainstorms, occurring at the rate of not more than 1 or 2 every few years, would probably repeat the process in the same rhythm, since an arc once established would afterward act as a sieve to enmesh more debris. Vegetation arcs are well-established, containing acacia trees about 50 yrs. old, and indicate the stability of the semi-desert surface and the absence of appreciable soil erosion at present. Water lanes appear on air photographs as straight tracks fairly clear of brush, often multiple or parallel, on terrain of very low slope where normal incised drainage lines are absent. Water lanes generally occur among vegetation arcs, cutting them at right angles to their chords, and thus run down the maximum slope. They may arise from and end in unpatterned brush, or a single lane may continue an incised drainage line; they seem to form an intermediate link between vegetation arc areas and regions with incised drainage lines. Water lanes generally are characterized by exact parallelism when multiple, the great width of each separate channel, and the lack of noticeable erosive effect from water flow.