In most deserts rain does not fall for several years.
When it does it comes in the form of unexpected and infrequent downpours.
Therefore despite the low rainfall totals (about 250mm per annum), water is still a dominant agent of erosion and deposition.
This is especially true in semi-arid deserts like the Kalahari where rain falls more frequently.
Dew also forms in deserts where temperatures, due to relatively little cover resulting in temperatures falling below dew point, is also found in deserts where it plays an important role in chemical weathering.
The infrequent but violent thunderstorms result in rushing torrents in steep slopes and to sheet floodson gentle slopes.
The run-off is more pronounced due to the relative lack of vegetation in desert landscapes.
The run-off on steep slopes is usually in the form of rills/shallow grooves which link up to form gullies.
These lead in turn towadis/chebkas
During these storms water may flow in these wadis as flash floods and as they progress and carry more and more materials they may turn into mud flows.
The mud is later deposited and forms features known as alluvial fans.
Past pluvial periods
Even though some places in deserts do not receive rain at all for years on end,
There is evidence for example, paleolithic marks on some desert rocks,
This evidence proves that deserts had more pluvial (humid) conditions that current arid conditions in the past.
Some wadis, dry river channels and gullies may have been formed during these pluvial periods.