This is an informal treatment of the instantaneous unit hydrograph IUH/GIUH \cite{Rigon_2015} estimation under the simplifying assumption that rainfall inputs are given as a discrete sequence of rain impulses (that we will call from now, rain record) This, more than being a simplification, is related to the discrete nature of rain at small time scales of aggregation and has a physical basis (e.g. \cite{Lovejoy_1985}).  This manuscript is distributed according to the CC v4 license.