North America is a place of hydrologic extremes: it includes some of the wettest, driest, warmest, and coldest places on Earth. This diversity arises because the continent is relatively large, spanning almost 60° of latitude between polar and subtropical regions. As such, virtually every hydrologic phenomenon—from extreme rainfall to extreme aridity—occurs within North America. The challenge thus becomes one of distilling the diverse aspects of North America's hydrology into these few pages. That challenge is made simpler because of the sheer number of published scientific studies and the amount of data available for this continent. In the United States, for example, about 8000 surface-weather stations and 7000 surface-water gauging stations are currently in operation; thousands more are no longer in operation but provide historical data. Several high-quality subsets of these data (numbering more than 1000 stations) have been compiled for use in evaluating long-term trends in temperature, precipitation, and runoff, and for validating results from general circulation models (Karl et al., 1990; Wallis et al., 1991; Slack and Landwehr, 1992). These data are available in digital format, and often can be obtained from government agencies free of charge or at minimal cost (the chapter appendix includes sources of hydrologic data, including Internet addresses). It is safe to say that the job of every hydrologist in North America is made simpler by the abundance and accessibility of information, whether in the form of raw data or published reports and studies.
This chapter summarizes major geographic patterns and long-term temporal trends in the surface-water hydrology of North America. It begins with a brief discussion of the water balance and summarizes previous estimates of the amount of water moved through the North American continent as precipitation, evaporation, and runoff. Subsequent sections examine each of these components, emphasizing important physical processes, regional patterns, and long-term trends; some sections include discussion of extreme events or events of local interest. The chapter attempts a comprehensive treatment of these topics, but it does not include much discussion of the related topics of groundwater hydrology and water quality. Readers interested in groundwater processes and resources should consult volume O-2 of the Decade of North American Geology, edited by Back et al. (1988). Similarly, the article by Hem et al. (1990) and the report compiled by Paulson et al. (1993) summarize important water quality issues in the United States and Canada. Useful companions to this chapter include volume O-1 of the Decade of North American Geology, edited by Wolman and Riggs (1990), and the book Water in Crisis, edited by Gleick (1993).