The National Research Program (NRP) conducts basic and problem oriented hydrologic research in support of the mission of the U.S. Geological Survey (USGS). Relevant hydrologic information provided by the USGS is available today to assist the Nation in solving its water problems because of a conscious decision made in years past to invest in research. The NRP is designed to encourage pursuit of a diverse agenda of research topics aimed at providing new knowledge and insights into varied and complex hydrologic processes that are not well understood. The emphasis of these research activities changes through time, reflecting the emergence of promising new areas of inquiry and the demand for new tools and techniques with which to address water-resources issues. Knowledge gained and methodologies developed in this program apply to all of the hydrologic investigations of the USGS, to the water-oriented investigations and operations of other agencies, and to the general scientific community. Through the years, many of the Geological Survey's major research and resource assessment initiatives related to existing and emerging national water-resources problems had their origins in the NRP.