Arid landscapes present unique challenges in water management, because these are generally places with low precipitation and high evaporation. So the main goal of water management in these landscapes is to minimize evaporation. Evaporation is a big problem, not only because it robs you of water, but because of what the water leaves behind: salt. In a landscape with severe evaporation, salt levels can build up in the soil to the point where nothing will grow. So simply adding more water could even make the problem worse, and stopping evaporation has to be our highest priority. Evaporation has two main causes: sunlight and wind. Most strategies that minimize evaporation from sunlight also address wind-driven evaporation, so these can, for the most part, be treated as one issue. Basically, everything should be done to hide water from both sunlight and wind. The best place for water in these areas is underground. So this is the place where swales have the most importance. In these places, although there is often very little precipitation, when it comes, it usually comes all at once. All seventeen centimeters of annual rainfall can come in 12 hours. So the swales you dig in these landscapes can be very large, as many as ten meters across, in order to catch all of the rainwater that can land on the property in one day, and soak it into the ground, out of the sun and wind.
In these landscapes, large above-ground water reservoirs can be disastrous, because they are exposed to the sun and wind. So when you need a reservoir of water that’s clean, safe from evaporation and easily accessible in these landscapes, you can sometimes build an underground pond. First you build a standard pond with a compacted clay dam, and a pipe with a spigot under the dam, then you fill it with boulders. On top of the boulders you put smaller rocks, then you put down a layer of gravel and then a layer of sand. Dirt can’t get into it, and it keeps the water safe from the sun. Plant a shallow-rooted, sun-hardy aggressively spreading groundcover on top of it, and don’t let trees grow on it.
On steep slopes in dry lands, gabions on contour or with a 1-2% grade towards ridges and away from valleys are a good method for slowing the flow of water and equalizing the concentrations of water at any given point.
In arid landscapes, canyons and steep valleys are quite common. These areas can be vast, and they can be almost 100% rocky surfaces, which let water flow away without being captured. It is vital to slow the erosive floods of water that normally occur after a rain. These floods are often a limiting factor Putting a gabion across the valley is good for that, but because of the huge amounts of flow involved, it needs to be built specially. Instead of a metal cage filled with rocks, these gabions take the form of piles of boulders and rocks stretched across the valley. It is important to include a lot of large rocks in the gabion to keep it stable. These gabions will build up sediment behind them in the floods. This sediment will be soaked by water in each seasonal flood, and will generally give a slow flow of water down the valley for about a month after that. So if you add another gabion further up or down the valley, you will get about two months of flow. These valleys can be kilometers long. In many of them you could fit hundreds of gabions. In each flood, these gabions together could soak up enough water to feed a stream for years. In the plains that these canyons open into, you would never lack for water again, in landscapes that historically have been so dry that only certain extremely specialized species could survive.
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