Towards closure relations in the Representative Elementary Watershed (REW) framework containing observable parameters: Relations for Hortonian overland flow

This study presents the derivation procedure of an integrated closure relation for infiltration and Hortonian overland flow in the Representative Elementary Watershed (REW) framework that contains directly-observable parameters. A physically-based high resolution model is used to simulate the infiltration flux and discharge for 6 × 105 set of synthetic REWs and rainstorms scenarios. This synthetic data set serves as a surrogate of real-world data to deduce the closure relation. The closure relation performance is evaluated against the results from the high resolution model. The results show that the closure relation is capable of predicting accurate hydrological responses for an independent set of synthetic REWs and rainstroms in terms of the Nash–Sutcliffe index, errors in total discharge volume, and peak discharge, especially in cases where a relatively large amount of runoff is produced with fast responses. For the estimation of parameters in the closure relation, a local method using inverse distance weighted interpolation in the parameter space is superior to the global method based on the multiple regression, resulting in a better reproduction of runoff characteristics.