At one level, sustainability, when referring to water resources, means we are not consuming more water than can be renewed.
Sustainability implies a balance between supply and demand.
For example, groundwater is not an unlimited resource. If we use groundwater supplies at a greater rate than the aquifer is recharged, we are violating the principles of sustainability. Groundwater use at that level is not sustainable. Sustainability also has a broader definition, one that takes into account social, economic and environmental values. In this context, a sustained water supply involves more than matching water demand with supply.