Development of an extended routing scheme in reference to consideration of multi-dimensional flow relations between hydrological model entities

Abstract

Unlike to so called lumped models in which the quantification of several processes results from a statistical relation between system input and system output for the whole catchment, distributed models enable the differentiation of single modelling entities (e.g. Hydrological Response Units - HRU) which can be parameterized and modelled independently. Furthermore spatially distributed hydrological modelling requires a topological linkage of several entities in order to reproduce relevant attenuation and translation processes within the stream but also during the transport of water in form of lateral surface or subsurface flow. Most often such linkage is considered by a one dimensional (1D) approach which links one modelling entity to only one receiver that follows in flow direction. The comparison with actual lateral water movement in catchments show that such a 1D routing scheme is often too simple which can lead to an overestimation of the runoff concentration along the 1D flow paths. On the other hand an underestimation of runoff in flow cascades that do not reside next to the main 1D flow paths can occur as the affected HRUs don't receive realistic inflow from their source entities above. As a catchment-wide consequence the 1D routing scheme can result in a significant over- or underestimation of the contributing area for specific parts of a catchment which can have important implications on the spatial distribution of accompanying processes such as spatial variation of soil moisture, soil erosion or solute transport. For example by modelling soil erosion, the one dimensional approaches show functional weaknesses because of the overestimation of rill erosion along the 1D flow paths. To address the problems outlined above a new two-level approach has been developed that allows a multi- dimensional linkage of model entities in such a way that each entity can have various receivers to which the water is passed. On the first level the neighbourhood relations between the model entities are quantified. The area of a HRU which contributes to the runoff to one of its neighbours is compared to the overall contribution area of the same HRU. As a result a fraction of the entire runoff can be assigned to every flow relation. In a second step, an exact analysis of the flow cascades is conducted to identify so-called circle flows, in which the runoff of a HRU flows back to the same HRU. Throughout the model run, the runoff of the HRUs involved in a circle flow is not routed to catchments outlet. The improved routing method introduces a new approach for the elimination of circle flows by modifying certain flow relations without altering the whole topology abundantly. This extended routing scheme was implemented in the hydrological modelling system J2000 and was used for the simulation of the hydrological processes of a number of meso-scaled catchments in Thuringia, Germany. The paper will present the most important facts of the extended routing scheme, the first simulation results along with the comparison of those obtained with the 1D linkage and will highlight the impacts on the hydrological process dynamics.