Man and the water cycle: challenges for the 21st century

Water is essential to life. Modern, technological societies use huge quantities of this precious liquid. The hydrologic (water) cycle represents the movement of water through the earth's environmental systems. Humankind primarily makes use of three parts of the hydrologic cycle for its water needs: rivers, lakes, and groundwater. Our use of these components of the water cycle has had a variety of deleterious effects. Many large rivers have had their flow regimes modified and suffered from pollution with attendant negative effects of riverine, estuarial, and oceanic ecosystems. Lakes have suffered most seriously from anthropogenic caused pollution but have been impacted by introduction of exotic biota and regime alteration as well. Groundwater, because of its slow rate of replenishment, has suffered from depletion and accumulating pollutants. Looking toward the next century, we must alter our management of water and put it on an ecologically sustainable footing. This involves three key actions: (1) developing an ecosystems approach, (2) improving the water knowledge base, and (3) promoting improved management and cooperation, both nationally and internationally.