A key step in applying land surface parameterization schemes is to estimate model parameters that vary spatially and are unique to each computational element. Improved methods for parameter estimation (especially for parameters important to runoff response) are needed and require data from a wide range of climate regimes throughout the world. Accordingly, the GEWEX Hydrometeorology Panel (GHP) endorsed the concept of an international Model Parameter Estimation Project (MOPEX) at its Toronto meeting, August 1996. Phase I of MOPEX was funded by NOAA in financial year (FY) 1997, Phase II in FY 2000 and Phase III in FY 2003. MOPEX was adopted as a project of the IAHS/WMO Working Group on GEWEX and of the WMO Commission on Hydrology (CHy), and is now a contributor to the Combined Enhanced Observing Period (CEOP) of the World Climate Research Program (WCRP). In 2004, MOPEX became a Working Group of the IAHS Prediction in Ungauged Basins (PUB) Initiative. MOPEX is also expected to contribute to the work of the Hydrologic Ensemble Prediction Experiment (HEPEX). The primary goal of MOPEX is to develop techniques for the a priori estimation of the parameters used in land surface parameterization schemes of atmospheric models and in hydrological models. A major early effort of MOPEX has been to assemble a large number of high-quality historical hydrometeorological and river basin characteristics data sets for a wide range of river basins (500-10 000 km 2 ) throughout the world. MOPEX data sets are available via the Internet (ftp://hydrology.nws.noaa.gov/pub/gcip/mopex/US_Data/). This paper documents the development of data sets for river basins in the USA. Several highly successful parameter estimation workshops have been organized by MOPEX. The first was held as part of the IAHS General Assembly in Birmingham, UK, in July 1999. The second workshop was hosted in April 2002 in Tucson, Arizona, USA, by the SAHRA/University of Arizona. The third MOPEX workshop was held as part of the IAHS General Assembly in Sapporo, Japan, July 2003. The fourth, in Paris, France, July 2005 was organized by the CEMAGREF in collaboration with the ENGREF, Meteo France, National Weather Service and the SAHRA/University of Arizona. The fifth workshop was held as part of the IAHS Scientific Assembly, February 2005, Foz do Iguacu, Brazil. The purpose of the future phases of the project is to: (a) continue collecting additional international data sets; update data from the USA by adding recent years, including data for elevation zones in mountainous areas and refining energy forcing data; (b) continue to conduct international MOPEX workshops; (c) provide leadership to develop a better scientific understanding of how to improve procedures for a priori parameter estimation; (d) make a significant hydrological contribution to CEOP and PUB; and (e) demonstrate transferability of MOPEX results. The basic data collection strategy being used in MOPEX is to seek the most readily available and highest quality data first. During the next three years, analyses of the available MOPEX data sets by the international scientific community will be emphasized.
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