SUSTAINABILITY AND THE TIME SCALE OF HUMAN ALTERATIONS TO WATER RESOURCES

Human activities of different types can affect water availability and future sustainability for humans and natural ecosystems. These can include those that change the land surface and the atmosphere as well as direct diversion of surface-water and ground-water flow. The time scales of apparent consequences can range from immediate to centuries. For example, impoundment and (or) diversion of surface water results in an immediate change in the downstream flow characteristics. Land-use changes that affect runoff generally are gradual, and resulting changes to surface-water flow and ground-water recharge may not be detected without careful study of hydrologic data over several decades. Withdrawal of ground water can have the consequence of lowering the water table in the area of pumping that is immediately apparent, of land subsidence that may not be apparent for some decades, and of interception of outflow to springs, streams and wetlands that may not be apparent for decades or even centuries. In managing water resources, society tends to strictly consider short-term consequences, but is more likely to tolerate or not manage long-term consequences that will affect water resources at some time in the future. Long-term monitoring of hydrologic systems is important for understanding natural and human-induced variations; however, adaptive management based on real-time monitoring may be unreliable or insufficient because some long-term changes can persist long after the development of the water resource ceases. Sound long-term management of water resources will require development and institutionalized use of decision tools that incorporate hydrologic data, projections of climate variability and change, population and land-use trends, and societal values.