New water fractions and transit time distributions at Plynlimon, Wales, estimated from stable water isotopes in precipitation and streamflow

Abstract

Abstract. Long-term, high-frequency time series of passive tracers in precipitation and streamflow are essential for quantifying catchment transport and storage processes, but few such data sets are publicly available. Here we describe, present, and make available to the public two extensive data sets of stable water isotopes in streamflow and precipitation at the Plynlimon experimental catchments in central Wales. Stable isotope data are available at 7-hourly intervals for 17 months, and at weekly intervals for 4.25 years. Precipitation isotope values were highly variable in both data sets, and the high temporal resolution of the 7-hourly streamwater samples revealed rich isotopic dynamics that were not captured by the weekly sampling. We used ensemble hydrograph separation to calculate new water fractions and transit time distributions from both data sets. Transit time distributions estimated by ensemble hydrograph separation were broadly consistent with those estimated by spectral fitting methods, suggesting that they can reliably quantify the contributions of recent precipitation to streamflow. We found that on average, roughly 3 % of streamwater was made up of precipitation that fell within the previous 7 h, and 13 %–15 % of streamwater was made up of precipitation that fell within the previous week. The contributions of recent precipitation to streamflow were highest during large events, as illustrated by comparing new water fractions for different discharges and precipitation rates. This dependence of new water fractions on water fluxes was also reflected in their seasonal variations, with lower new water fractions and more damped catchment transit time distributions in spring and summer compared to fall and winter. We also compared new water fractions obtained from stable water isotopes against those obtained from concentrations of chloride, a solute frequently used as a passive tracer of catchment transport processes. After filtering the chloride data for dry deposition effects, we found broadly similar new water fractions using chloride and stable water isotopes, indicating that these different tracers may yield similar inferences about catchment storage and transport, if potentially confounding factors are eliminated. These stable isotope time series comprise some of the longest and most detailed publicly available catchment isotope data sets. They complement extensive solute data sets that are already publicly available for Plynlimon, enabling a wide range of future analyses of catchment behavior.

The variation and controls of mean transit times in Australian headwater catchments

“…MTT estimates from 3 H activities combined with those from other techniques would potentially yield complementary information. For example, in catchments where MTTs are months to decades the variations in stable isotopes or Cl may allow a better understanding of the temporal variations in inputs of the very young (<1 year) water components (Botter, 2012; Hrachowitz et al, 2013; Knapp et al, 2019; Visser et al, 2019) that are difficult to study using 3 H. By contrast, 3 H allows the input of water that is more than a few years old at both baseflow and higher streamflows to be assessed. Duvert et al (2016) interpreted variations in δ 18 O values over high flow events in a catchment with 3 H activities significant below those of modern rainfall as reflecting mixing between water with MTTs of a few weeks and baseflow that had MTTs of a few decades.…”

“…StorAge Selection Functions consider how waters with different residence times are mobilized relative to the proportion of that water in catchment storage (Benettin, Kirchner, Rinaldo, & Botter, 2015; Benettin, Bailey et al, 2015; Harman, 2015; van der Velde, Torfs, van der Zee, & Uijlenhoet, 2012). Ensemble hydrographs allows the relative contribution of recent precipitation to streamflow over time intervals of hours to days to be made (Kirchner, 2019; Knapp, Neal, Schlumpf, Neal, & Kirchner, 2019). These approaches also require time‐series data, albeit over shorter timescales (individual events or a few months).…”

Technical note: Calculation scripts for ensemble hydrograph separation

“…For example, if and are sampled daily, "new water" means water that fell within the previous day 90 (and thus is expressed in units of day -1 ), and if they are sampled weekly, "new water" means water that fell within the previous week (and thus is expressed in units of week -1 ). Because the meaning and dimensions of new water fractions depend on the sampling interval, so do the numerical values, as illustrated by Knapp et al (2019). In our example, the weekly event new water fraction, calculated from weekly sampling of the daily values in Fig.…”

“…Ensemble hydrograph separation (EHS) has recently been proposed as a new tool for quantifying catchment transit times, using time series of passive tracers like stable water isotopes or chloride. Benchmark tests using synthetic data have shown that this method should yield quantitatively accurate answers to the questions posed above (Kirchner, 2019), and initial applications to real-world data sets (e.g., Knapp et al, 2019) have demonstrated the potential of this technique. 25…”

Technical Note: Evaluation of a low-cost evaporation protection method for portable water samplers

“…The above methods and modifications were originally designed for single-sample collection using a precipitation totalizer (e.g., IAEA, 2014). For many hydrological questions, however, higher-frequency measurements of stable water isotopes are of interest, requiring daily or even sub-daily sampling of precipitation or streamwater (e.g., Knapp et al, 2019;Rücker et al, 2019;von Freyberg et al, 2018;Wang et al, 2019). This can be achieved with field-deployable automatic water samplers with programmable pump-and-distribution systems that fill and store a series of empty open bottles.…”