The Sahara Desert is not encroaching to the South, quite the opposite. This is a result of increasing rainfall in the region (as a result of natural climate cycles, the Sahara was not always dry) and also the "global greening" effect of increased atmospheric CO2, allowing plants to survive in more arid climates than they can at lower CO2 levels. The success of this strategy may be substantially assisted by 20th century increases in CO2. That said, the micro swales, or "half moons" that they are using to capture and utilize the still scarce water resources are a great idea. They're not something that's new and many variations have been used throughout the world for millennia. My local climate has almost the opposite problem. I get close to 1m of rain per year (I'm not in a tropical zone either), so my biggest problems are waterlogging, rather than aridity. However, the Summers can also be very hot and dry. So, my garden beds are surrounded by a moat of sorts, with a small dam that controls the maximum water level on the downslope side of them. The beds are raised a little (about 6") inside this. This allows excess rainfall to be directed around the beds, but the first bit is captured and stored in the upper part of the moat, where it soaks in and drains through the soil over the following days. There are different strategies that can be used, based on the local environment and topology and the scale. For instance, in larger scale arid regions, long swales (ditches dug along the topological lines of the land) can be used to slow the run-off of water and allow it to soak deep into the soil. In smaller scales and damper environments, methods like Hügelkultur can work very well (I use this in my greenhouse).