Insights into streamflow generation mechanisms using high-frequency analysis of isotopes and water quality in streamflow and precipitation

Studying rapidly changing hydrochemical signals in catchments can help to improve our mechanistic understanding of water flow pathways and travel times. For these purposes, stable water isotopes (δ¹⁸O, δ²H) and other constituents are commonly used as natural tracers. However, high-frequency analyses of liquid water samples are challenging: one must capture highly dynamic behavior with high precision and accuracy, but the lab workload (and sample storage artifacts) involved in collecting and analyzing thousands of bottled samples should also be avoided.

We therefore installed a «lab in the field» at the outlet of the 0.7 km² Erlenbach catchment, a mountainous headwater tributary of the Alp river. The lab consists of Picarro, Inc.'s (Santa Clara, CA, USA) newly developed Continuous Water Sampler Module (CWS) coupled to their L2130-i Cavity Ring-Down Spectrometer, to measure stable water isotopes (δ¹⁸O, δ²H) in precipitation and streamwater at 30 min temporal resolution. Water quality is monitored with a dual-channel ion chomatograph (Metrohm AG, Herisau, Switzerland) for analysis of major cations and anions, as well as with a UV-Vis spectroscopy system and electrochemical probes (s::can Messtechnik GmbH, Vienna, Austria) for characterization of nutrients and basic water quality parameters. Problematic issues, such as sample degradation during storage and transportation that arise in conventional sampling for catchment tracer studies, become irrelevant with our system. At the same time, potential registration errors arising during the collection and handling of a large number of water samples are avoided.

This high-frequency sampling and analysis system enables us to capture the hydrochemical responses of streamwater to low- and high-intensity storm events, as well as subtle isotopic and biogeochemical signals (associated with, e.g., evaporation effects or in-stream biological processes) that would be missed by conventional approaches to sampling and analysis. Together with existing long-term data sets - such as hydro-climatic observations from the Erlenbach meteorological station, as well as water quality data from the Swiss National River Monitoring and Survey Programme (NADUF) - we can draw conclusions the long-and short-term behavior of the catchment. Hydrograph separation of several events show that the catchment responds very quickly to precipitation input and that a large fraction of precipitation is routed directly to the river network by shallow subsurface stormflow or surface runoff. This direct runoff is largest when the catchment is wet, such as during snowmelt in spring. In the future, we will combine our data set with the snowmelt measurements obtained during the PhD project of Andrea Rücker to better characterize groundwater recharge and streamflow generation during spring freshet.