All our water comes from rainMost precipitation evaporates, is taken up by plants, or flows to the sea. Some of it ends up as groundwater.
Why is storage limited? The Gulf Islands lack high mountains with winter snow that store water into the summer months. As well, there are few lakes and most are small and the water that can be stored in fractured rock under the islands is limited.
...but little is stored
Winter rains are stored in lakes, cisterns, and underground.