Abstract: The isotopic composition(T,180)of rainfall asd various runoff components including interflow from unsaturated zone and groundwater flow from saturated zone was monitored during the rainfall events of 1986-1989 in a special designed eaperimeatal catchment with all the runoff components are measurable. The contribution of prevenient water (old water) to surface runoff is identified and the portion of old water ranged from 0% to 50.5% is estimated. It implies that the current concept which considers surface runoff is always orginated from event rainfall is very likely a misconception. The different contributions of old water in interfiow and groundwater flow are identifled and estimated, too. The ezistance of temporal isotopic variability of runoff components during rainfall events demonstrates the temporal variations of old water composition in these components. For the four patterns of hydrograph with different combinations of runoff components, different compositions of old water are found ranged from 0% to 77%.As a comparison, several natural basins were sampled. From these results, it is shown that the current method of hydrograph separation up to now used widely in this country is, unrealistic in generally. These results also pose questions about the concepts related to rainfall-runoff relationships and to the UH analyses.