Water availability refers to how much water is physically accessible in terms of its quantity and whether that water is safe to use in terms of its quality. The differences in water quantity and quality across the landscape, and how they change through time, determine whether there is enough water for human and ecosystem needs.

The amount and quality of available water is influenced by human activities, like using water for agriculture, drinking water supplies, and thermoelectric power generation, and by natural events like fires, tropical cyclones, and drought. Understanding water supply and demand will support better and more efficient water management for economic growth, protection of aquatic ecosystems, agriculture and energy production, and human population growth.

The concept of water availability is broad, many groups have made more specific definitions of water availability. Most include aspects of water quantity, water quality, and water timing (when water is available throughout the year).

Some definitions of water availability focus on human water use, while others recognize the tradeoffs between human and ecological water needs. .