Abstract: The basic hypotheses of the Geomorphologic Instantaneous Unit Hydrograph (GIUH), the waiting time probability density function(WPDF), and the relationship between the GIUH and conceptual models of concentrated flow in a watershed (CFW) are reviewed. It is indicated that handling of CFW by means of statistical mechanics is theoretically not reasonable because of non-weak interaction between the water particles. Although determination of the waiting time probability density fu nction is a key to the GIUH, the existing methods involve some unproved hypotheses about it. If the area-time curve and exponential type function are used as the waiting time probability density function, then the GIUH is equivalent tojthe conceptual models of CFW. Because the GIUH depends only on the parameters of geomorphology and flow velocity of channel network, it may provide a new parameter esti mation method for conceptual models of CFW.