1. What is the young water fraction (Fyw) in streamflow?
The young water fraction (Fyw) is the fraction of streamflow that is younger than a certain age, typically 2-3 months. It is used as an alternative metric to the commonly used mean transit time (mTT) to understand how catchments store and release water and solutes of different ages. The age threshold (tyw) separating young from old water is not pre-defined in the traditional procedure, but it is important to use the same pre-defined tyw for inter-catchment comparison of Fyw. In this study, an alternative procedure for estimating Fyw with any pre-defined tyw is proposed, allowing for comparison of effects of data sampling frequencies on Fyw estimation results. The study applied the traditional and proposed procedures using daily oxygen isotope (d 18 O) data in the Alp and Erlenbach catchments, Switzerland, and found that the estimated Fyw significantly increases when the sampling frequency changes from sub-monthly to monthly time steps. This highlights the importance of selecting the tyw and sampling frequency in Fyw estimation.
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2. How can Fyw be estimated with different tracer sampling frequencies?
Fyw can be estimated with different tracer sampling frequencies by using the procedure proposed by Kirchner (2016). This procedure allows Fyw to be estimated with any user-defined tyw, which can be adjusted based on the hydrological/water quality process of interest. However, the effect of tracer sampling frequencies on the estimated Fyw varies. Studies have shown that Fyw increases almost two-fold with high sampling frequencies (daily and sub-daily) compared to weekly sampling frequency (Stockinger et al., 2016). Similarly, Gallart et al. (2020) found that Fyw estimated with weekly sampling frequency is significantly lower than that of dynamic sampling (sub-weekly with variable time steps). Therefore, it is crucial to consider the role of isotope sampling frequency in inter-catchment comparisons based on Fyw (Stockinger et al., 2016). To evaluate the effect of tracer sampling frequencies on the estimated Fyw with a fixed tyw, a study was conducted using high-frequency tracer data from the Alp catchment and one of its tributaries (Freyberg et al., 2022).
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3. What is the elevation range of the Alp and Erlenbach catchments?
The elevation of the Alp catchment ranges from 840 to 1896 meters above sea level (a.s.l), while the Erlenbach catchment spans from 1,111 to 1,654 meters a.s.l. These elevations indicate the varying altitudes within the catchment areas, which can influence the hydrological and climatic conditions experienced in these regions. The Alp catchment, with its lower elevation range, may experience different weather patterns and precipitation levels compared to the higher elevation Erlenbach catchment. Understanding the elevation range is crucial for studying the hydro-climatic regime and water resources in these catchments, as it can impact factors such as snowmelt, runoff, and water availability. Additionally, the elevation range can provide insights into the potential for glacier and snowpack dynamics within the catchments, which are important components of the hydrological cycle in mountainous regions. Overall, the elevation range of the Alp and Erlenbach catchments plays a significant role in shaping the hydrological and climatic characteristics of these areas, making it an essential factor to consider in research and water resource management studies.
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4. How does the proposed procedure compare to the traditional method?
In the proposed procedure, the 100 best-fit gamma functions are used to calculate Fyw, accounting for parameter uncertainty. The traditional procedure only uses the best-fit sine wave function for observed d 18 O composition in streamflow. The proposed procedure considers snowpack storage as part of the catchment storage, while the traditional method does not differentiate between liquid and snow precipitation. Both methods have been used in previous studies to estimate the young water fraction (Fyw).
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