Rainfall in arid regions occurs in large and infrequent events throughout the year. Because the desert environment is a brittle one and because there is so much rain at one time, tremendous erosion occurs. This leads to the creation of scarps and wadis.
Wadi is simply the Arabic word for a riverbed. In an arid region, its water flow is likely to be ephemeral, particularly in headwaters (where permaculturists are most likely to be working). They are also places where a lot of erosion takes place. Looked at from another perspective, wadis are places where a lot of erosion can be stopped.
After teaching a permaculture course in Jordan, designer Geoff Lawton returned to the area a year later and found that someone had built a gabion (an uncemented rock wall, usually held inside a steel cage) across a particular wadi. Although the 8-foot-high gabion was less than a year old, it was already full of settlement and still had water trickling out of the base, despite the fact that there had not been any recent rains.
This natural tendency for gabions in wadis to quickly fill with water-retaining sediment provides us the opportunity to create what I am dubbing the Instant Wadi-Well (for lack of a better name). After determining where the gabion is to be placed, start with a shallow hole about 2 feet (60 cm) deep in a teardrop shape with the tapered end downstream. Place the first row of stones for the well inside the hole and cement them together, leaving the ground unmortared to allow water to seep in. Keep adding and cementing stones until you reach ground level. Once at ground level, continue up but add an extra 2 layers of mortared stone at the tapering end to allow stability in the face of the rushing torrent and sediment that is to come with the first rainfall. Continue this right up the entire height of the well shaft, making the top row at least 3 feet (90 cm) above the height the gabion will be. After all the sediment moves in, it is likely the well casing will only be 2 feet (60 cm) above the sediment.
All that is left is to build the gabion. Wire cages are not absolutely necessary for the gabion to perform, but they are recommended as they make it far less likely that the rock wall will blow out in the face of the torrent of water, sand and silt.
If the well casing is built strong enough, then there will likely be a well within a few major rain event is. The water will have to be tested as deserts tend to have salty soil. Baring salinity problems, this would be a quick and easy way to establish clean wells and combat erosion at the same time!
This technique compliments other systems as well. For example, if there are storage tanks or cisterns at the top of the scarp, a windmill can be employed to pump water from the Instant Wadi Well to this higher storage. From there, it can be gravity fed down to where it is needed.