Our population is growing. We're in a drought. Water supplies are dwindling. If water were plentiful for all people and ecosystems, sustainability would not be the hot issue it is. But we live in an era of limits, and water resources are among the shortest resources in supply. Our groundwater reserves are being mined. Management for sustainable water resources calls for using our water resources to address present needs without compromising future needs. But do we really know when and how the impacts of our activities on water resources today will be felt in the future? Do we know what future needs, values, and supplies will be? How can we best manage for now and the future? Adaptive management may be the best approach we have, but it doesn't solve everything.