GLOBAL CHANGE AND WATER RESOURCES IN THE NEXT 100 YEARS
			
			ABSTRACT
The economic, political, and scientific decision-making entities of U.S. 
society are in the midst of a continental-scale, multi-year experiment 
in the United States, in which we have not defined our testable 
hypotheses or set the duration and scope of the experiment, which poses 
major water-resources challenges for the 21st century. What are we 
doing? We are expanding population at three times the national growth 
rate in our most water-scarce region, the southwestern United States, 
where water stress is already great and modeling predicts decreased 
streamflow by the middle of this century. We are expanding irrigated 
agriculture from the west into the east, particularly to the 
southeastern states, where increased competition for ground and surface 
water has urban, agricultural, and environmental interests at odds, and 
increasingly, in court. We are expanding our consumption of 
pharmaceutical and personal care products to historic high levels and 
disposing them in surface and groundwater, through sewage treatment 
plants and individual septic systems. These substances are now 
detectable at very low concentrations and scientists have documented 
substantial effects on aquatic species, particularly on fish 
reproduction function. These are a few examples of our national-scale 
experiment.
In addition to these water resources challenges, over which we have some 
control, precipitation and streamflow patterns have been changing, and 
are predicted to change in coming decades, with western mid-latitude 
North America generally drier. Based on the instrumented record, 
hydrologists have already documented trends in more rain and less snow 
in western mountains. Hydrologists have documented earlier snowmelt peak 
spring runoff in northeastern and northwestern  states, and western 
montane regions. Peak runoff is now about 2 weeks earlier than it was in 
the first half of the 20th century.
Decision makers are now required to include fish and other aquatic 
species in negotiation over how much water to leave in the river, rather 
than, as in the past, how much water humans could remove from a river. 
Additionally, decision makers must pay attention to the quality of that 
water, including its temperature.
  Sea level rise presents challenges for fresh water extraction from 
coastal aquifers as they are compromised by increased saline intrusion. 
A related problem faces users of ‘run-of-the-river’ water-supply intakes 
that are threatened by a salt front that migrates further upstream 
because of higher sea level.
Global change and water resources challenges that we face this century 
include a combination of local and national management problems and 
evolving changes in climate that are already upon us. This set of 
challenges will continue and likely intensify as the non-climatic and 
climatic factors (such as predicted rising temperature and changes in 
the distribution of precipitation in time and space) continue to develop.