Why are there oases in the desert?

How it works

When rain falls or snow melts, some of the water soaks into the ground and collects in the tiny spaces between sand grains and in cracks in rock. This is groundwater. Some of it travels a very long way underground, sometimes from distant mountains, before it ends up beneath a desert.

Where the land is high, the groundwater is too deep for plant roots to reach. Where the land dips low, the water is close to the surface, and it may even flow out as a spring or form a pond. Pour water into a sloping tray of sand and the lowest spot gets wet first.

Once there is water, plants such as date palms can take root. Their shade keeps the ground cooler, so less water dries up. Animals, travellers and traders gather there too, which is why many desert villages and towns grew up around oases.

Where to see it in New Zealand

New Zealand has no desert oases, but famous ones exist elsewhere. Siwa Oasis in Egypt's Western Desert has springs and date palm groves, and Crescent Lake near Dunhuang in China lies among tall sand dunes. Groundwater matters here too: Christchurch gets most of its tap water from underground.

Try it together

Fill a shallow tray with dry sand and pile one end higher than the other. Slowly pour water onto the high end, a little at a time, and watch closely. Where does water appear first? That damp, low spot is your mini oasis, where groundwater meets the surface.