||CHANNELING AND CONCENTRATION OF RAIN ALONG THE SLOPES
Definition characters description and diffusion
Since the most ancient times, in the Hadramaut Valley, in Yemen, the communities sharing an interest in irrigation have been using the highlands and the steep slopes, opportunely shaped and insulated as an impluvium to harvest rainfall in a system of basins and cisterns. Thanks to this constant work and to knowledge handed down by specific organizations, rushing torrents that come down the slopes after rainfall are controlled. Since no other water source is available but this, the regions of the slope exist thanks to the preservation of valuable flows and to the careful terracing system that creates the plots of land among which water is carefully shared out.
General characters description and diffusion
The town of Aden, in Yemen, located on an isle, is made up of the cone of a huge extinct volcano and is now linked to the mainland. Aden is built on the breaking point of the edge of the big crater that opens with a depression towards the sea. The volcanic cone was used as a huge basin to harvest the rainfall and humidity particles that were conveyed to the depression from the lower slopes that had skillfully been made impermeable. Harvesting reservoirs had been built against the sides of the sloping walls in order to catch the smallest runoff. Tunnels and channels fed a series of 50 big open-air cisterns having a capacity of 1,400,000 hl of water and connected with eachother, along the slope towards the sea. Today, the archaeological site of Qana is a neglected place and the ancient cisterns of Aden are empty, thus the town is supplied with water by long aqueducts and expensive desalination plants. Maintenance of those cisterns is no longer carried out since their ancient working has been forgotten and water is not conveyed through them anymore. Therefore, the previous abundance is now ascribed to the magic power of the mythical Queen of Sheba to whom those systems were dedicated. The most imposing rock in Petra, in Jordan, is Umm al-Biyara, the Arabian name of which means 'mother of cisterns'. Archaeological evidence and an environmental analysis demonstrate that the mountain basin surrounding the wadi Musa valley has been wholly arranged over time in order to control water resources and convey them to the Nabatean town. Petra, which supplies all the waters, organizes the desert through every kind of water and drainage system: the arrangement of tanks, cisterns and gutters for the harvesting of rainwater and microflows dripping down the sandstone walls; the protection and the support along both slopes and the wadi water courses in order to produce soil and farming land; the diversion of the rare but rushing and devastating floods and the canalization of the spring water. In the classic age, from the 3rd century AD, the development of important towns in the inland areas of Yucatan was organized around natural depressions called aguada, into which the water collected by the dams and cisterns along the slopes flowed. The surfaces of the aguada were paved with flat stones whose joins were waterproofed by means of red and brown clay. Wells and chultun were dug at the bottom to keep the water when the aguada dried up. The system is very similar to the technique of the cisternali typical of the Apulian karst areas in the south of Italy. At the end of the classic age, around the 9th century AD, driven by defensive purposes or under the pressure of the need to irrigate by the force of gravity larger and larger areas of farmland the Maya hydraulic technology developed so much that it exploited the highest peaks as harvesting systems.
Advantages and sustainability
These methods are both sustainable and beneficial because they control pontentially damaging torrents of rain water and use the steep slopes to collect water then used for irrigation and other vital needs.