Every desert gets a certain amount of rainfall. The showers are few and far between, but when the rain falls it usually dashes down in a deluge. It washes off the slopes and floods down into the hollow arroyas. For a short time the desert is flooded with running water.
This flash flood water soon sinks into the soft sand. It seeps down to join a deep reservoir of ground water, In most places the ground water is far below the surface. But here and there it reaches to or near the ground level.Such a spot is an oasis. It may be a spring, a shallow lake or even a little stream which stays watery long after the desert shower. In the region of an oasis the ground water may be fairly near the surface and walls can be dug to irrigate the land.