Toward more accurate determination of the water cycle in association with climate variability and change as well as baseline data on the impacts of this variability on water resources, the was launched on July 1, 2001, is now in the buildup phase after the preliminary data period from July to September, 2001 and will have the two annual cycle periods from October, 2002 to September, 2004.

CEOP is seeking to achieve a database of common measurements from both in situ and satellite remote sensing, model output, and four-dimensional data analyses (4DDA; including global and regional reanalyses) for a specified period. In this context, a number of carefully selected reference stations are linked closely with the existing network of observing sites involved in the GEWEX Continental Scale Experiments, which are distributed across the world. The initial step of CEOP is to develop a pilot global hydro-climatological dataset with global consistency under the climate variability that can be used to help validate satellite hydrology products and evaluate, develop and eventually predict water and energy cycle processes in global and regional models. Based on the dataset, we will address the studies on the inter-comparison and inter-connectivity of the monsoon systems and regional water and energy budget, and a path to down-scaling from the global climate to local water resources, as the second step.

The CEOP was originally proposed by the Global Energy and Water Cycle Experiment (GEWEX) Hydrometeorology Panel (GHP) and initiated and managed within GEWEX of the World Climate Research Programme (WCRP). It is being developed and implemented within the WCRP; in particular, the Climate Variability and Predictability (CLIVAR) study, the emerging Climate and Cryosphere (CliC) project, and the Working Group on Numerical Experimentation (WGNE), which is the joint activity between the World Meteorology Organization (WMO) Commission for Atmospheric Sciences and the WCRP Joint Scientific Committee (JSC). The Committee on Earth Observation Satellite (CEOS) agencies use their best efforts to contribute to CEOP and in particular, to cooperate the satellite data integration and the field campaigns at reference sites. It has also been endorsed by the Integrated Global Observing Strategy Partnership (IGOS-P) as the first element of the IGOS Water Cycle Theme.

CEOP will enable us to address the local to global water cycle issues through a 'hands-on', focused examination of the water cycle over the particular time period.