Abstract
Abstract. This study explores how catchment heterogeneity and variability can be summarized in simplified models, representing the dominant hydrological processes. It focuses on Mediterranean catchments, characterized by heterogeneous geology, pedology and land use, as well as steep topography and a rainfall regime in which summer droughts contrast with high-rainfall periods in autumn. The Ardèche catchment (Southeast France), typical of this environment, is chosen to explore the following questions: (1) can such a Mediterranean catchment be adequately characterized by a simple dynamical systems approach and what are the limits of the method under such conditions? (2) what information about dominant predictors of hydrological variability can be retrieved from this analysis in such catchments? In this work we apply the data-driven approach of Kirchner (2009) to estimate discharge sensitivity functions that summarize the behaviour of four sub-catchments of the Ardèche, using low-vegetation periods (November–March) from 9 years of measurements (2000–2008) from operational networks. The relevance of the inferred sensitivity function is assessed through hydrograph simulations, and through estimating precipitation rates from discharge fluctuations. We find that the discharge sensitivity function is downward-curving in double-logarithmic space, thus allowing further simulation of discharge and non-divergence of the model, only during low-vegetation periods. The analysis is complemented by a Monte Carlo sensitivity analysis showing how the parameters summarizing the discharge sensitivity function impact the simulated hydrographs. The resulting discharge simulation results are good for granite catchments, which are likely to be characterized by shallow subsurface flow at the interface between soil and bedrock. The simple dynamical system hypothesis works especially well in wet conditions (peaks and recessions are well modelled). On the other hand, poor model performance is associated with summer and dry periods when evapotranspiration is high and low-flow discharge observations are inaccurate. In the Ardèche catchment, inferred precipitation rates agree well in timing and amount with observed gauging stations and SAFRAN climatic data reanalysis during the low-vegetation periods. The model should further be improved to include a more accurate representation of actual evapotranspiration, but provides a satisfying summary of the catchment functioning during wet and winter periods.
Highlights
Catchments show a high degree of heterogeneity and variability, both in space and time (McDonnell et al, 2007) raising questions about the degree of complexity that must be used to model their behaviour (Sivapalan, 2003a)
Many hydrological models are based on the bottom-up or reductionist approach (Sivapalan, 2003b; Zehe et al, 2006), following the blueprint proposed by Freeze and Harlan (1969)
Adamovic et al.: Assessing the simple dynamical systems approach in a Mediterranean context suitable for point-scale processes (Bloschl and Sivapalan, 1995; Kirchner, 2006). Their use to describe processes at larger scales leads to the calibration of “effective parameters” which are sometimes difficult to link with measurable quantities (Sivapalan, 2003b), recent methods combining the use of small-scale variability and regionalization techniques were shown to be efficient in preserving spatial patterns of variability (Samaniego et al, 2010)
Summary
Catchments show a high degree of heterogeneity and variability, both in space and time (McDonnell et al, 2007) raising questions about the degree of complexity that must be used to model their behaviour (Sivapalan, 2003a). Adamovic et al.: Assessing the simple dynamical systems approach in a Mediterranean context suitable for point-scale processes (Bloschl and Sivapalan, 1995; Kirchner, 2006) Their use to describe processes at larger scales leads to the calibration of “effective parameters” which are sometimes difficult to link with measurable quantities (Sivapalan, 2003b), recent methods combining the use of small-scale variability and regionalization techniques were shown to be efficient in preserving spatial patterns of variability (Samaniego et al, 2010). Klemeš (1983) was one of the first hydrologists proposing the use of alternative modelling concepts He defines the top-down or downward approach as the “route that starts with trying to find a distinct conceptual node directly at the level of interest (or higher) and looks for the steps that could have led to it from a lower level”. Models obtained through this approach are simple, with a limited number of parameters that can be estimated from the available data
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