Water is not only a human resource but a basic biological resource, essential to sustaining the community of life on Earth. Changing physical, climatic, biological, social. and economic conditions entail changes in the distribution, quality, and availability of water, affecting biodiversity as well as human welfare. Water resources used inefficiently are too often depleted and degraded. Agricultural production must be confined to where it can be implemented efficiently and sustainably while protecting the larger environment. In view of portending climate change, the issue is no longer merely local or regional, but indeed global.