Abstract. The young water fraction F yw , defined as the proportion of catchment outflow younger than approximately 2-3 months, can be estimated directly from the amplitudes of seasonal cycles of stable water isotopes in precipitation and streamflow. Thus, F yw may be a useful metric in catchment inter-comparison studies that investigate landscape and hydro-climatic controls on streamflow generation. Here, we explore how F yw varies with catchment characteristics and climatic forcing, using an extensive isotope data set from 22 small-to medium-sized (0.7-351 km 2 ) Swiss catchments. We find that flow-weighting the tracer concentrations in streamwater resulted in roughly 26 % larger young water fractions compared to the corresponding unweighted values, reflecting the fact that young water fractions tend to be larger when catchments are wet and discharge is correspondingly higher. However, flow-weighted and unweighted young water fractions are strongly correlated with each other among the catchments. They also correlate with terrain, soil, and land-use indices, as well as with mean precipitation and measures of hydrologic response. Within individual catchments, young water fractions increase with discharge, indicating an increase in the proportional contribution of faster flow paths at higher flows. We present a new method to quantify the discharge sensitivity of F yw , which we estimate as the linear slope of the relationship between the young water fraction and flow. Among the 22 catchments, discharge sensitivities of F yw are highly variable and only weakly correlated with F yw itself, implying that these two measures reflect catchment behaviour differently. Based on strong correlations between the discharge sensitivity of F yw and several catchment characteristics, we suggest that low discharge sensitivities imply greater persistence in the proportions of fast and slow runoff flow paths as catchment wetness changes. In contrast, high discharge sensitivities imply the activation of different dominant flow paths during precipitation events, such as when subsurface water tables rise into more permeable layers and/or the river network expands further into the landscape.

One of the most important functions of catchments is the storage of water. Catchment storage buffers meteorological extremes and interannual streamflow variability, controls the partitioning between evaporation and runoff, and influences transit times of water. Hydrogeological data to estimate storage are usually scarce and seldom available for a larger set of catchments. This study focused on storage in prealpine and alpine catchments, using a set of 21 Swiss catchments comprising different elevation ranges. Catchment storage comparisons depend on storage definitions.This study defines different types of storage including definitions of dynamic and mobile catchment storage. We then estimated dynamic storage using four methods, water balance analysis, streamflow recession analysis, calibration of a bucket-type hydrological model Hydrologiska Byråns Vattenbalansavdelning model (HBV), and calibration of a transfer function hydrograph separation model using stable isotope observations. The HBV model allowed quantifying the contributions of snow, soil and groundwater storages compared to the dynamic catchment storage.With the transfer function hydrograph separation model both dynamic and mobile storage was estimated. Dynamic storage of one catchment estimated by the four methods differed up to one order of magnitude. Nevertheless, the storage estimates ranked similarly among the 21 catchments. The largest dynamic and mobile storage estimates were found in high-elevation catchments. Besides snow, groundwater contributed considerably to this larger storage.Generally, we found that with increasing elevation the relative contribution to the dynamic catchment storage increased for snow, decreased for soil, but remained similar for groundwater storage.

Abstract. The transit time of water is a fundamental property of catchments, revealing information about the flow pathways, source of water and storage in a single integrated measure. While several studies have investigated the relationship between catchment topography and transit times, few studies expanded the analysis to a wide range of catchment properties and assessed the influence of the selected transfer function (TF) model. We used stable water isotopes from mostly baseflow samples with lumped convolution models of time-invariant TFs to estimate the transit time distributions of 24 meso-scale catchments covering different geomorphic and geologic regions in Switzerland. The sparse network of 13 precipitation isotope sampling sites required the development of a new spatial interpolation method for the monthly isotopic composition of precipitation. A pointenergy-balance based snow model was adapted to account for the seasonal water isotope storage in snow dominated catchments. Transit time distributions were estimated with three established TFs (exponential, gamma distribution and two parallel linear reservoirs). While the exponential TF proved to be less suitable to simulate the isotopic signal in most of the catchments, the gamma distribution and the two parallel linear reservoirs transfer function reached similarly good model fits to the fortnightly observed isotopic compositions in discharge, although in many catchments the transit time distributions implied by equally well fitted models differed markedly from each other and in extreme cases, the resulting mean transit time (MTT) differed by orders of magnitude. A more thorough comparison showed that equally suited models corresponded to agreeing values of cumulated transit time distributions only between 3 and 6 months. The short-term ( < 30 days) component of the transit time distributions did not play a role because of the limited temporal resolution of the available input data. The long-term component (> 3 years) could hardly be assessed by means of stable water isotopes, resulting in ambiguous MTT and hence questioning the relevance of an MTT determined with stable isotopes. Finally we investigated the relation between MTT estimates based on the three different TF types as well as other transit time properties and a range of topographical catchment characteristics. Depending on the selected transfer model, we found a weak correlation between transit time properties and the ratio between flow path length over the flow gradient, drainage density and the mean discharge. The catchment storage derived from MTTs and mean discharge did not show a clear relation to any catchment properties, indicating that in many studies the mean annual discharge may bias the MTT estimates.

Water travel times reflect hydrological processes, yet we know little about how travel times in the unsaturated zone vary with time. Using the soil physical model HYDRUS-1D, we derived time variable travel time distributions for 35 study sites within the Attert catchment in Luxembourg. While all sites experience similar climatic forcing, they differ with regard to soil types (16 Cambisols, 12 Arenosols, and 7 Stagnosols) and the vegetation cover (29 forest and 6 grassland). We estimated site specific water flow and transport parameters by fitting the model simulations to observed soil moisture time series and depth profiles of pore water stable isotopes. With the calibrated model, we tracked the water parcels introduced with each rainfall event over a period of several years. Our results show that the median travel time of water from the soil surface to depths down to 200 cm is mainly driven by the subsequent rainfall amounts. The median time until precipitation is taken up by roots is governed by the seasonality of evapotranspiration rates. The ratio between the amount of water that leaves the soil profile by on the one hand and evaporation and transpiration on the other hand also shows an annual cycle. This time variable response due to climatic forcing is furthermore visible in the multimodal nature of the site specific master transit time distribution representing the flow-averaged probability density for rainwater to become recharge. The spatial variability of travel times is mainly driven by soil texture and structure, with significant longer travel times for the clayey Stagnosols than for the loamy to sandy Cambisols and Arenosols.

We assessed how the seasonal variability of precipitation δ H and δ O is propagated into soil and xylem waters of temperate trees, applied a hydrological model to estimate the residence time distribution of precipitation in the soil, and identified the temporal origin of water taken up by Picea abies and Fagus sylvatica over 4 yr. Residence times of precipitation in the soil varied between a few days and several months and increased with soil depth. On average, 50% of water consumed by trees throughout a year had precipitated during the growing season, while 40% had precipitated in the preceding winter or even earlier. Importantly, we detected subtle differences with respect to the temporal origin of water used by the two species. We conclude that both current precipitation and winter precipitation are important for the water supply of temperate trees and that winter precipitation could buffer negative impacts of spring or summer droughts. Our study additionally provides the means to obtain realistic estimates of source water δ H and δ O values for trees from precipitation isotope data, which is essential for improving model-based interpretations of δ O and δ H values in plants.