Multi-criteria assessment of the Representative Elementary Watershed approach on the Donga catchment (Benin) using a downward approach of model complexity

100

103

Abstract. According to Dooge (1986) intermediate-scale catchments are systems of organized complexity, being too organized and yet too small to be characterized on a statistical/conceptual basis, but too large and too heterogeneous to be characterized in a deterministic manner. A key requirement for building structurally adequate models precisely for this intermediate scale is a better understanding of how different forms of spatial organization affect storage and release of water and energy. Here, we propose that a combination of the concept of hydrological response units (HRUs) and thermodynamics offers several helpful and partly novel perspectives for gaining this improved understanding. Our key idea is to define functional similarity based on similarity of the terrestrial controls of gradients and resistance terms controlling the land surface energy balance, rainfall runoff transformation, and groundwater storage and release. This might imply that functional similarity with respect to these specific forms of water release emerges at different scales, namely the small field scale, the hillslope, and the catchment scale. We thus propose three different types of "functional units" -specialized HRUs, so to speak -which behave similarly with respect to one specific form of water release and with a characteristic extent equal to one of those three scale levels. We furthermore discuss an experimental strategy based on exemplary learning and replicate experiments to identify and delineate these functional units, and as a promising strategy for characterizing the interplay and organization of water and energy fluxes across scales. We believe the thermodynamic perspective to be well suited to unmask equifinality as inherent in the equations governing water, momentum, and energy fluxes: this is because several combinations of gradients and resistance terms yield the same mass or energy flux and the terrestrial controls of gradients and resistance terms are largely independent. We propose that structurally adequate models at this scale should consequently disentangle driving gradients and resistance terms, because this optionally allows Published by Copernicus Publications on behalf of the European Geosciences Union. E. Zehe et al.: HESS Opinions: Thermodynamic reinterpretation of the HRU conceptequifinality to be partly reduced by including available observations, e.g., on driving gradients. Most importantly, the thermodynamic perspective yields an energy-centered perspective on rainfall-runoff transformation and evapotranspiration, including fundamental limits for energy fluxes associated with these processes. This might additionally reduce equifinality and opens up opportunities for testing thermodynamic optimality principles within independent predictions of rainfall-runoff or land surface energy exchange. This is pivotal to finding out whether or not spatial organization in catchments is in accordance with a fundamental organizing principle.

Section: Are Hrus and Landscape Organization Resulting From Co-evolutmentioning

confidence: 99%

Section: Pioneering Research and Models To Balance Necessary Compleximentioning

confidence: 99%

HESS Opinions: From response units to functional units: a thermodynamic reinterpretation of the HRU concept to link spatial organization and functioning of intermediate scale catchments

Zehe

Ehret

Pfister³

et al. 2014

100

103

“…It discretizes the landscape into irregular hydro-landscapes . Infiltration and water redistribution are modelled using an efficient solution of the Richards equation (Ross, 2003;Varado et al, 2006b) with hydraulic properties described using standard pedo-transfer functions (Rawls and Brakensiek, 1985). The model takes into account the vertical heterogeneity of soil hydraulic properties as described in the available soil databases.…”

Section: Small Catchment Modellingmentioning

confidence: 99%

“…As much as possible, hydrological signatures (Gupta et al, 2008;Willems, 2009;Clark et al, 2011), as derived from the data analysis will be used. A multi-site and multi-variable evaluation (Varado et al, 2006a;Moussa et al, 2007) will be performed. In addition, the use of uncertain observed data to evaluate and compare several modelling scenarios raises significant methodological challenges because model evaluation entails comparing two time series of distributions, as opposed to two times series of values.…”

Section: Regional-scale Modellingmentioning

confidence: 99%

Multi-scale hydrometeorological observation and modelling for flash flood understanding

Braud¹,

Ayral

Bouvier

et al. 2014

Self Cite

Abstract. This paper presents a coupled observation and modelling strategy aiming at improving the understanding of processes triggering flash floods. This strategy is illustrated for the Mediterranean area using two French catchments (Gard and Ardèche) larger than 2000 km 2 . The approach is based on the monitoring of nested spatial scales: (1) the hillslope scale, where processes influencing the runoff generation and its concentration can be tackled; (2) the small to medium catchment scale (1-100 km 2 ), where the impact of the network structure and of the spatial variability of rainfall, landscape and initial soil moisture can be quantified; (3) the larger scale (100-1000 km 2 ), where the river routing and flooding processes become important. These observations are part of the HyMeX (HYdrological cycle in the Mediterranean EXperiment) enhanced observation period (EOP), which will last 4 years (2012)(2013)(2014)(2015). In terms of hydrological modelling, the objective is to set up regional-scale models, while addressing small and generally ungauged catchments, which represent the scale of interest for flood risk assessment. Topdown and bottom-up approaches are combined and the models are used as "hypothesis testing" tools by coupling model development with data analyses in order to incrementally evaluate the validity of model hypotheses. The paper first presents the rationale behind the experimental set-up and the instrumentation itself. Second, we discuss the associated modelling strategy. Results illustrate the potential of the approach in advancing our understanding of flash flood processes on various scales.

“…The existing closure schemes require parameter values that are representative for the region that is lumped by the model, which typically has a size above 10 5 m 2 (e.g. Fenicia et al, 2005;Varado et al, 2006). Direct measurement of lumped parameter values representative to this scale is notably difficult or even almost not possible (Mileham et al, 2008;Molnár and Julien, 2000;van Schaik et al, 2010), although remote sensing techniques might provide this possibility in the near future (Lakshmi, 2013;Vereecken et al, 2008).…”

Section: Introductionmentioning

confidence: 99%

Hortonian runoff closure relations for geomorphologic response units: evaluation against field data

Vannametee

Karssenberg

Hendriks

et al. 2013

Abstract. 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 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 GRUproperties optimal for the predictions. Calibration was done by changing one local-scale observable, i.e. hydraulic conductivity (K s ), using a single pre-factor for the entire catchment. It is shown that the calibrated K s values are somewhat comparable to the observed K s 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.