Abstract: In this study,the hydrological cycle especially diurnal cycle of the precipitable water vapor derived from GPS( PWV GPS) is investigated based on the ground-based the GPS observation experiment and automatic weather stations in the Chengdu Plain of southwest China during the period of J uly to September 2004.The comparison of the PWV GPS and those from radiosonde observations is given at Chengdu station,with the RMS(Root Mean Square) differences of 3.09 mm.This is surprisingly small considering the distance of up to several tens of kilometers between radiosonde station and closest GPS site.The diurnal variations of water vapor in Chengdu Plain were preliminarily investigated with the PWV GPS and the surface meteorological data.The diurnal variations of the PWV and some surface meteorological elements were composite during the warm days at Chengdu and Pixian.The PWV GPS showed a clear diurnal cycle with the amplitude of about 3 mm and changed obviously in day and little at night.A typical feature of the diurnal variation at Chengdu and Pixian is that the maximum,43.5 mm and 38.0 mm,respectively,appeared in late afternoon,and the minimum,40.5mm and 35.0mm,respectively appeared in early morning.The variation period of PWVGPS is similar to that of surface temperature,they are all controlled by the diurnal variation of solar radiation.The comparison of the PWVGPS and the rainfall shows that there is a good correlation between them in the calm warm season.The precipitating is observed frequently in the evening.The main rainfall process will occur after the PWVGPS reaches its maximum in the late afternoon,which leads to a remarkable decrease of the PWV GPS and a rapid rise of surface water vapor.When the drop of the PWV GPS is stable,the main rainfall process will stop.The accumulating and releasing processes of the PWVGPS is also well related to the rainfall,its increase or decrease continuously suggests the beginning or the end of the rainfall,respectively.