eau dans oued essendilene sahara algérie

Terres Touareg

What’s the difference between a wadi and an oasis?

We often wonder what the difference is between a wadi and an oasis. The two terms can lead to confusion, but they are in fact two very
different sources of water. Here’s an explanation.

A wadi, a natural watercourse

Oued, from the Arabic Wādī, which translates as valley, is a river that often carries water only temporarily, after the rainy season. The climate of wadis remains temperate, hot and dry in summer, humid during winter. Wadis are found only in arid regions. Most often dry, wadis can experience spectacular flooding, carrying huge quantities of mud and sometimes changing their bed. This is why a wadi is said to roll more than it flows. Wadis appear to be often dry, but in reality they are not.

Rivers flow below ground in the underground rock and rise to the surface when there is a large amount of rainfall. Palms and trees are indicators of this underground water. Wadis vary in size. Some are very large, resembling huge rivers that stretch for several kilometres through rocky valleys that are sometimes impassable, while others are very small, resembling streams.

An oasis, a man-made watering place

An oasis is not just a watering hole in the desert where nomads stop to water their herds. In fact, an oasis is an area of vegetation created by man for agricultural purposes, thanks to the proximity of an existing natural water source (deep water, groundwater, wadi, river flowing into the desert, etc.). An oasis can therefore be fed by a wadi, but not only that. Oases can be found right in the middle of the desert, but to find water, you have to dig and draw from the water table or underground rivers, which are themselves fed by melting snow and rainwater.

Oases are also sometimes large gardens where men and women live permanently. They are called oasis dwellers. Perfectly organised, they have built canals, shared the land and above all shared the precious water. In traditional oases, they use irrigation techniques that existed thousands of years ago. The fields are served by seguias, canals dug into the ground to water a plot. The town of Djanet is a large oasis.

The role of the palm tree in an oasis

In oases, large trees are used to provide shade and protect crops. Date palms tower over vegetable gardens (carrots, tomatoes, chillies, olives, etc.) and small fields of maize and barley. The trick inside an oasis is to spread the planting out over different levels. At the top are the palm trees. Below the palms are the fruit trees, and below the fruit trees on the ground are the cereals and vegetable gardens.

All these plants grow in the shade of each other, but the most resistant is the palm tree, which has to withstand all the heat. To withstand the sun and the heat, the palm tree has far fewer leaves than other trees, so its evaporation surface is much smaller. What’s more, the trunk is covered with a dead fibre that creates a kind of insulation, preventing the temperature from rising too quickly. Despite all these adaptations, a palm tree consumes 500 litres of water a day.

To feed the palm tree, it has roots that fetch water from depths of up to 15 metres, enabling it to live in the desert. Thanks to the palm trees, the Tuaregs manage to create a microclimate in the desert without which all the oases and crops would not exist. The palm trees provide protection from the heat and the hot winds, and also retain the humidity inside the oases, as if under a bell.

When exposed to heat, trees sweat, as do all plants (cereals, fruit trees). All transpire and release water in the form of vapour which, instead of evaporating into the atmosphere, remains trapped under the canopy of the palm trees. This moisture is reused by the plants and at night, when the temperature drops, the water condenses.

What’s the difference between a wadi and an oasis?
What’s the difference between a wadi and an oasis?
What’s the difference between a wadi and an oasis?

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