Water
Desert greening is more or less a function of water availability. If sufficient water for irrigation is at hand any hot, cold, sandy or rocky desert can be greened. Water can be made available through saving, reuse, rainwater harvesting, desalination, or direct use of seawater for salt-loving plants. These different paths have unique features, i.e.: conserving water is a cheap solution. Reuse of treated water and the closing of cycles is the most efficient because closed cycles stand for unlimited and sustainable supply - rainwater management is a decentralized solution and applicable for inland areas - desalination is very secure as long as the primary energy for the operation of the desalination plant is available - Direct use of seawater for seawater agriculture is the most potent, only limited by the need for pumping up the water from sea-level.
Desalination is for example done in project like the Sahara Forest Project. This project uses solar stills for the generation of the freshwater.
There are theoretical water sources, too. Like the generation of artificial rain through cloud seeding of various kinds. An available technology called "atmospheric water generation" or air to water is used by the military and is available as micro-solution for drinking-water fountains. But this technology uses 200 times more energy than modern desalination plants and cannot be considered for desert greening.
Once the (fresh or sea)water has been attained, it needs to be distributed. This can be done using dug canals or in some instances aqueducts (which are both the least attractive option since they allow much water to be evaporated), troughs (as used in the Keita Project), earthenware piping (semi-open or closed) or even underground systems i.e. Qanat.
Depending on the method of distribution of the water, it can then be provided on different methods to the plants. A costly solution (used only on pipes) is drip irrigation. Other methods are the use of wadis (basically V-shaped ponds dug in the earth) or by simply planting the trees in holes inside/over the water pipe itself. The tree's roots can then suck the water straight from the water pipe (used in Qanat, hydroponics, ...) A similar technique can be done with semi-open pipes (i.e. dug throughs in the Keita Project).
The use of water is, however, not always without problems. Desert greening by the Helmand and Arghandab Valley Authority irrigation scheme in Afghanistan significantly reduced the water flowing from the Helmand River into Lake Hamun and this, together with drought, was cited as a key reason for the severe damage to the ecology of Lake Hamun, much of which has degenerated since 1999 from a wetland of international importance into salt flats.
Read more about this topic: Greening Deserts
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