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 eventbased
lumped rainfall–runoff model, to a 15 km2
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 GRUproperties
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 lowdimensional,
semi-distributed, physically based discharge
models in ungauged catchments.