The Curve Number Concept as a Driver for Delineating Hydrological Response Units

Eleni Savvidou,Andreas Efstratiadis,Dimitrios Skarlatos,Antonis Koussis,Antonis Koukouvinos

doi:10.3390/w10020194

Abstract

In this paper, a new methodology for delineating Hydrological Response Units (HRUs), based on the Curve Number (CN) concept, is presented. Initially, a semi-automatic procedure in a GIS environment is used to produce basin maps of distributed CN values as the product of the three classified layers, soil permeability, land use/land cover characteristics and drainage capacity. The map of CN values is used in the context of model parameterization, in order to identify the essential number and spatial extent of HRUs and, consequently, the number of control variables of the calibration problem. The new approach aims at reducing the subjectivity introduced by the definition of HRUs and providing parsimonious modelling schemes. In particular, the CN-based parameterization (1) allows the user to assign as many parameters as can be supported by the available hydrological information, (2) associates the model parameters with anticipated basin responses, as quantified in terms of CN classes across HRUs, and (3) reduces the effort for model calibration, simultaneously ensuring good predictive capacity. The advantages of the proposed approach are demonstrated in the hydrological simulation of the Nedontas River Basin, Greece, where parameterizations of different complexities are employed in a recently improved version of the HYDROGEIOS model. A modelling experiment with a varying number of HRUs, where the parameter estimation problem was handled through automatic optimization, showed that the parameterization with three HRUs, i.e., equal to the number of flow records, ensured the optimal performance. Similarly, tests with alternative HRU configurations confirmed that the optimal scores, both in calibration and validation, were achieved by the CN-based approach, also resulting in parameters values across the HRUs that were in agreement with their physical interpretation.

Highlights

Based on their spatial scale of process representation, hydrological models are generally categorized from lumped to distributed
In the context of distributed hydrological modelling, the configuration of Hydrological Response Units (HRUs) is subject to the conflict between two topics: the accuracy in the representation of process heterogeneity, dictating the required number of HRUs; and model parsimony, associated with the number of parameters to be inferred through calibration
To reduce the subjectivity introduced by the definition of a small number of HRUs in a parsimonious modelling structure, this research provides empirical guidelines for formulating HRUs, based on the widespread known curve number concept

Summary

Introduction

Based on their spatial scale of process representation, hydrological models are generally categorized from lumped to distributed In the former, the watershed is considered as a single unit that responds homogeneously to its spatially-averaged meteorological inputs, while the model parameters are only associated with the macroscopic properties of the watershed as a whole. The fundamental laws of hydraulics and semi-empirical hydrological formulae are applied at each spatial unit, which allows, in theory, to estimate all model parameters from field data. Due to their bottom-up basis, which takes advantage of the significant advances in understanding the hydrological processes at the micro (point, continuum)-scale, fully-distributed models are identified as “physically-based” [1]

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