New water fractions, transit time distributions, and solute concentrations at Plynlimon, Wales.

Transit time distributions estimated from stable water isotopes (deuterium and oxygen-18) are frequently used to assess transport and storage of water in catchments. We analyzed 2.25 years of 7‑hourly and 4.5 years of weekly measurements of stable water isotopes in precipitation and streamwater at the Plynlimon catchments in Wales, UK using the ensemble hydrograph separation technique. We thereby quantified new water fractions – the average contribution of recent precipitation to streamflow – in the different subcatchments, and determined transit time distributions as the contribution of precipitation to streamflow over a range of lag times.

We found that on average only 3 % of streamwater was made up of precipitation that fell within the last 7 hours, and 13-15 % of streamwater was made up of precipitation that fell within the previous week. However, these new water fractions increased with discharge, indicating that more recent precipitation reached the stream when the catchment was wet, and the contributions of recent precipitation to streamflow were highest during large events. 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 the drier spring and summer compared to fall and winter.

A comparison between changes in solute concentrations and new water fractions with discharge provides additional insight into the storage and release of water and solutes from the catchments. Our analysis demonstrates that changes in solute concentrations primarily reflect changes in flowpaths between dry and wet conditions, rather than changes in the fraction of recent precipitation in streamflow.