Precipitation samples were collected during 45 events at 18 stations in the Kanto Plain, Japan, during June--September 1997. The hydrogen and oxygen isotope ratios were measured for each event and then the deuterium-excess parameter d was determined. Spatial variability (a few per mil) of the deuterium excess was significantly less than its temporal variability (20?). Spatially averaged d was highly correlated with vapor pressure, temperature, and wind vector in the lower atmosphere. The observed d (except for that influenced by evaporation from falling raindrops) was compared to d estimated using meteorological data from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. Estimation of d was based on a simple relationship between the deuterium excess and humidity condition over sea surface. The source area of precipitated water, as indicated by its isotopic footprint, was recognized over seas near Japan as an area where excellent agreement was found between estimated and observed values (e.g., root mean square difference <1). The location of the estimated source area appears to be quite reasonable considering with the direction of the water vapor advection in the lower atmosphere. These results suggest that event-based d values are useful for tracing the hydrological cycle in the atmosphere.