Taking runoff volume of time interval as a vector, the degree and time of con-centration can be resulted by means of vector composition. They are considered as arather new expression describing seasonal distribution of stream, runoff which can beperformed on the basis of different time intervals (day, month, season, etc.) in accor-dance with specific requirements. The resultant vector composition is a graph, takingmonthly runoff as time unit (Fig. 2 and 3).Degree of concentration represents the magnitude of concentration of monthly streamrunoff within twelve months. It is closely related with unequilibrium coefficient ofmonthly stream runoff (Fig. 1) but possesses a higher resolution than that of unequilib-rium coefficient. Time of concentration represents the most regulatable months in termsof accumulation of monthly stream runoff which, in most cases, are comparable to themonths regarding that with practically maximum values occured.Data of the hydrological stations (8,307 station-years in all) in China are selectedin this paper for calculation and illustration. Upon which isopleth maps of degree ofconcentration (Fig. 4) and time of concentration (Fig. 5) of stream-runoff in Chinahave been constructed. Analysis on the law of distribution of the above mentioned twomaps is thus carried out areally. Degree of concentration of stream runoff in Chinais comparatively high, it is especially so in northern part of China. If these runoffvolumes are sufficiently utilized, greater regulating reservoir capacity would be re-quired. Since degree of concentration of rivers in west China occurs in summer monthsit is necessary to prepare regulating reservoir capacity beforehand for flood preven-tion.