Exploring Response Time Influence on Rising Limb of a Hydrograph With Emphasis on Raindrop Characterisation: An Experimental Study

ABSTRACT Rainfall‐runoff modelling is pivotal in hydrology, yet reliance on natural rainfall poses significant challenges since it is so erratic. Rainfall simulators provide a controlled environment; however, replicating natural rainfall accurately in simulators remains challenging due to factors like raindrop velocity, terminal speed, spray dynamics, kinetic energy and droplet size. In this study, these parameters were comprehensively evaluated to ensure the rainfall simulator effectively replicates natural rainfall conditions. A series of laboratory experiments were conducted using this simulator, involving 12 distinct rainfall intensities and three rainfall durations. Importantly, the study extends beyond constant rainfall events by examining moving rainfall patterns, categorised into upstream (u/s), downstream (d/s), extreme upstream (Ex u/s) and extreme downstream (Ex d/s) rainfall. The influence of these dynamic patterns on surface flow and hydrograph components was systematically analysed. Special emphasis was placed on the rising limb of the hydrograph, which captures the initial response of runoff to rainfall within a catchment and its relationship with rainfall characteristics. The regression‐based model effectively captures rainfall‐runoff interactions, with Nash‐Sutcliffe Efficiency (NSE) and coefficient of determination ( R 2 ) exceeding 0.8 and Percent Bias (PBIAS) below 10% in both calibration and validation. Nonlinear regression proved to be the most reliable prediction approach. The study elucidates the relationship between rainfall characteristics and runoff dynamics under moving rainfall conditions. It provides critical insights into the relationship between rainfall patterns and runoff behaviour, contributing to advancements in hydrological modelling and water resource management.