Hortonian runoff closure relations for geomorphologic response units: evaluation against field data
This paper presents an evaluation of the closure relation for Hortonian runoff, proposed in Vannametee et al. (2012), that incorporates a scaling component to explicitly account for the process heterogeneity and scale effects in runoff generation for the real-world case studies. We applied the closure relation, which was embedded in an event-based lumped rainfall–runoff model, to a 15 km 2 catchment in the French Alps. The catchment was disaggregated into a number of landform units, referred to as Geomorphologic Response Units (GRUs), to each of which the closure relation was applied. The scaling component in the closure relation was identified using the empirical relations between rainstorm characteristics, geometry, and local-scale measurable properties of the GRUs. Evaluation of the closure relation performance against the observed discharge shows that the hydrograph and discharge volume were quite satisfactorily simulated even without calibration. Performance of the closure relation can be mainly attributed to the use of scaling component, as it is shown that our closure relation outperforms a benchmark closure relation that lacks this scaling component. The discharge prediction is significantly improved when the closure relation is calibrated against the observed discharge, resulting in local-scale GRU-properties optimal for the predictions. Calibration was done by changing one local-scale observable, i.e. hydraulic conductivity ( Ks), using a single pre-factor for the entire catchment. It is shown that the calibrated Ks values are somewhat comparable to the observed Ks values at a local scale in the study catchment. These results suggest that, in the absence of discharge observations, reasonable estimates of catchment-scale runoff responses can possibly be achieved with the observations at the sub-GRU (i.e. plot) scale. Our study provides a platform for the future development of low-dimensional, semi-distributed, physically based discharge models in ungauged catchments.