During a trip through a desert, few news are as good as spotting an oasis among the dunes. Both in film fiction —or jokes— and in the real world, finding that pool of water, surrounded by dense bushes and tall palm trees, implies having the precious liquid, and, if lucky, some edible fruit or root that take to the mouth
An oasis is an unusual landscape form, a rarity that dots nonpolar deserts—Antarctica is the largest desert, but it has no oases. Due to its extension, it can be from a small lagoon with a couple of palm trees, to a huge surface of water , with abundant vegetation on the riverbank, a refuge for fauna, and even with human settlements and crop fields.
The secret is in the water table
Rain, snow, and other forms of precipitation are rare in deserts. In fact, that is the parameter that mainly defines deserts. The rare times it rains, the water quickly seeps through the sand and rocks and is lost underground. The little water that is retained on the surface is quickly evaporated by the heat of the ground and the insolation.
However, even if there is little precipitation, every desert has limits, and beyond those limits there are areas with other climatic trends, where it can rain or snow. Where rivers of enormous proportions can be formed; the Nile , for example, is one of the largest rivers in the world and is adjacent to the Sahara desert , the second largest desert in the world, after Antarctica, and the largest of the hot deserts.
These bodies of water do not only exist on the surface. Through the subsoil they can extend for miles, infiltrating between rock layers, even under tons and tons of desert sand. This totally flooded subsoil layer is the water table .
When the dunes move
Desert dunes are mobile geomorphological structures. Without plants to support the soil with their roots, the sand travels at the mercy of the winds, forming new dunes and disappearing the previous ones in short periods of time.
At some point, a gully may form deep enough for the water table to be exposed to the outside. Then a spring is produced, from which the water begins to flow. And there, fed by those subsoil waters, a lagoon will form. When this happens, the seeds of plants carried by the winds or by animals, and mixed with the sand, can germinate and flourish . Dry plants in a state of dormancy, also brought by the winds, will roll down the slope and, when they come into contact with the water, they will rehydrate and turn green.
If the plants manage to take root and establish themselves, before the lagoon dries up or is covered with sand again, they will begin to form soil and fix it with their roots, favoring the preservation of the new oasis. On many occasions, the very nature of the sand, rich in fossilized exoskeletons of diatoms, acts as a fertilizer, providing great support to the oasis.
The destinations of an oasis
Most of the oases that are formed represent small lagoons that last for a short period of time, just enough for the plants to complete their life cycle and disperse their seeds to the wind, before drying up by evaporation and transpiration, or being buried. under tons of sand. Some do not have emerged water, and are only evident by the presence of vegetation, which collects water from the subsoil thanks to the roots.
However, when an oasis has enough vegetation, it warms the environment thanks to evapotranspiration , favoring more suitable temperatures. As the surface of the oasis is colder than that of the surrounding desert, downward air currents are generated on the oasis, and upward currents outside of it. This phenomenon reduces the presence of hot and dry air around it, while helping the oasis expand by displacing the surrounding dunes, and at the same time increasing atmospheric stability over the oasis , reducing water loss and creating a benign microclimate. .
The larger the oasis, the greater this feedback effect. If the water table is well supported and water is abundant enough, it can form large lakes, which, thanks to feedback, remain in equilibrium . These oases can last for years, centuries or even millennia, and in some cases, have their own toponym .
In fact, some have been known since ancient times. Like the oasis of El Kharga , in Egypt, with a current population of more than 180,000 inhabitants. It has remains of an ancient Egyptian temple, a Roman temple converted into a Coptic church. Other examples of great ancient oases are that of Huacachina , in Peru; that of Dunhuang , in China; or that of Tamerza , in Tunisia, the scene of great films such as The English Patient or Indiana Jones in Search of the Lost Ark .
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Li, C. et al. 2019. Accumulation of organic carbon and its association with macro-aggregates during 100 years of oasis formation. CATENA, 172, 770-780. DOI: 10.1016/j.catena.2018.09.044
Zhuang, Y. et al. 2014. Dew variability in three habitats of a sand dune transect in a desert oasis ecotone, Northwestern China. Hydrological Processes, 28(3), 1399-1408. DOI: 10.1002/hyp.9675