A Novel Instantaneous Unit Hydrograph Derived from the Water-Energy Complementarity Equation

The instantaneous unit hydrograph (IUH), as the fundamental methodological framework of unit hydrograph confluence theory, plays an indispensable role in hydrological simulation and forecasting, marking a significant milestone in the development of hydrology. By reconstructing the proportionality hypothesis equation and the coupled water-energy balance equation, this study reveals their theoretical connections and subsequently proposes a more universally applicable watershed water-energy complementarity constitutive equation, thereby establishing a new theoretical framework for hydrological process modeling. Based on the definition of IUH and its physical assumptions, we developed a water-energy complementary equation for watershed retention capacity, from which a novel IUH - the water-energy complementarity IUH (WIUH) - was derived. The physical meanings of its two key parameters were rigorously interpreted.