Quantifying uncertainties in tracer-based hydrograph separations: a case study for two-, three- and five-component hydrograph separations in a mountainous catchment

The hydrograph separation technique using natural tracers, in which different runoff components are quantified according to their chemical signature, is a widely used method for investigating runoff generation processes at tile catchment scale. The first objective of this study is to demonstrate a modified methodology for separating three and five runoff components using O-18 and dissolved silica as tracers. The second is to evaluate, with an uncertainty propagation technique using Gaussian error estimators, the hydrograph separation uncertainties that arise due to different error effects. During four summer storm events, an interaction among three main runoff components having distinct dissolved silica concentrations was demonstrated for the mountainous Zastler catchinent (18.4 km(2), southern Black Forest Mountains, southwest Germany). The three main runoff components are Surface storage (low silica, saturated and impermeable areas), shallow ground water (medium silica, periglacial and glacial drift cover), and deep ground water (high silica, crystalline detritus and hard rock aquifer). Together with the event and pre-event water fractions of surface runoff and shallow ground water runoff, five runoff components are considered in all. Pre-event water from shallow ground water storage dominated the total discharge during floods and was also important during low flows. Event water from shallow ground water was detectable only during the falling limb of a larger flood with high antecedent moisture conditions and during the peaks of three events with low antecedent moisture conditions. Runoff from surface storage is only significant during floods and can be composed of event and pre-event water. The latter reacts later and is important only during the peak of the large event with high antecedent Moisture conditions. Runoff from the deeper ground water behaves quite consistently (pure pre-event water). It is demonstrated that large relative uncertainties must be considered for the quantification of runoff components. Uncertainties are caused by: (i) tracer analysis and discharge measurement; (ii) intra-storm variability of O-18; (iii) elevation effect of O-18 and silica; (N) solution of minerals during runoff formation; and (v) general spatial heterogeneity of tracer concentrations. The last source of error was the most significant. The error structure was analysed in detail and showed varying error significance within an event and for different events. It is shown that, for the meso-scale catchment investigated, only qualitative results of the contribution of a runoff component can be obtained by the hydrograph separation technique. The importance of reducing errors that have the largest impact is clearly demonstrated; therefore, a targeted sampling strategy is required. Copyright (C) 2003 John Wiley Sons, Ltd.