Isotopic hydrograph separation in two small mountain catchments during multiple events

Abstract

Two-component isotopic hydrograph separation (IHS) was developed to determine the event- and pre-event components of a single storm event. Its application for several sucessive events requires repeated determination of isotopic signatures of end-members (precipitation, pre-event component) for each event. The existence of several possible alternative signatures results in differences in calculated contributions of event-/pre- event components. This article addresses the question of how big the differences could be in small mountain catchments with different methods for detemining the end member signatures. We analyzed data on isotopic composition of daily/event precipitation at different elevations in two catchments located in the highest part of the Carpathians in July 2014.The isotopic composition of streamflow sampled every 4-6 hours was analyzed as well. Elevational gradients of δ18O and δ2H in precipitation in the study period were -0.18 ‰ 100 m-1 and -1.1 ‰ 100 m-1, respectively. An elevation gradient in deuterium excess (0.29 ‰ 100 m-1) was also found. Precipitation on the windward side of the mountains was isotopically lighter than expected for a given rain gauge elevation. Five large rainfall-runoff events occurred in the study period in the meso-scale catchment of the Jalovecký creek (Western Tatra Mountains, area 22.2 km2) and in the headwater catchment of the Škaredý creek (High Tatra Mountains, area 1.4 km2). Isotopic hydrograph separation was conducted using eight options for the isotopic signatures of event and pre-event water. The isotopic signature of the event water (rainfall) was alternatively represented by data from high or low elevations. Pre-event water was represented either by the streamflow before the event or by the value taken from the statistics of the long-term data on isotopic composition of the stream. Both isotopes (18O and 2H) were used to calculate event water fractions during peak flows of individual events. Calculated peak flow event water fractions were below 0.2-0.3 for most events. However, the differences in calculated event water fractions for alternative isotopic composition of end-members were significant even if we did not take into account changes in isotopic composition during individual rainfalls. Coefficients of variation for event water fractions calculated for various options varied during individual events from 0.14 to 0.36. It is therefore perhaps better to use a range of possible values instead of a single accurate number to interpret the IHS results. Hydrograph separations based on 18O and 2H provided similar results.