Investigating The Seasonal Variations Of Event, Recent, And Pre-Recent Runoff Components in A Pre-Alpine Catchment Using Stable Isotopes and An Iterative Hydrograph Separation Approach
An analysis of the runoff generation processes in terms of event, recent, and pre-recent runoff components is demonstrated for the Swiss pre-Alpine Alp catchment (46.4 km2) and two smaller tributaries (Erlenbach, 0.7 km2, and Vogelbach, 1.6 km2), whereas, recent water is understood as the contribution of the respective three prior rainfall-runoff events. A four-years time series of daily stable water isotope data in stream water and precipitation is used for the analysis of seasonal variations. In addition, high-frequency data (10 minutely intervals) are used for a detailed visualization of the rapid mobilization of recent water for single events. An iterative extension of the standard two-component hydrograph separation method is applied; this approach can be interpreted as a discretization of the catchment water and tracer mass balance along the event and pre-event time axis. Furthermore, the calculated event, recent, and pre-recent runoff components can be used to estimate time-varying backward travel time distributions. For the Alp catchment and its tributaries, a nivale runoff regime can be shown, in which the event water component generally displays its maximum (>32%) during August. The highest recent water fractions (55-77%) can be expected at the beginning of winter (December, January), whereas the lowest fractions (7-14%) were found at the end of autumn (November). In return, higher pre-recent water fractions can be found in the summer (July, 60-82%) and in the middle of autumn (October, 64-74%). The two tributaries expose an additional peak of pre-recent water in the snowmelt season (April, 47-62%), and at this time also, the lowest event water fractions (6-8%) can be expected. For the Alp catchment, the event component (29-32%) and recent water component (56%) clearly exceeded the pre-recent component (17-21%) in August and September after the month of July, which is characterized by relatively low precipitation, high evapotranspiration, and low soil moisture. This may be an important finding regarding the expected dryer summers in middle Europe in the near future
