Spatial variability in the isotopic composition of rainfall in a small headwater catchment and its effect on hydrograph separation

Abstract

Isotope hydrograph separation (IHS) is a valuable tool to study runoff generation processes. To perform an IHS, samples of baseflow (pre-event water) and streamflow are taken at the catchment outlet. For rainfall (event water) either a bulk sample is collected or it is sampled sequentially during the event. For small headwater catchment studies, event water samples are usually taken at only one sampling location in or near the catchment because the spatial variability in the isotopic composition of rainfall is assumed to be small. However, few studies have tested this assumption. In this study, we investigated the spatiotemporal variability in the isotopic composition of rainfall and its effects on IHS results using detailed measurements from a small pre-alpine headwater catchment in Switzerland. Rainfall was sampled sequentially at eight locations across the 4.3 km2 Zwäckentobel catchment and stream water was collected in three subcatchments (0.15, 0.23, and 0.70 km2) during ten events. The spatial variability in rainfall amount, average and maximum rainfall intensity and the isotopic composition of rainfall was different for each event. There was no significant relation between the isotopic composition of rainfall and total rainfall amount, rainfall intensity or elevation. For eight of the ten studied events the temporal variability in the isotopic composition of rainfall was larger than the spatial variability in the rainfall isotopic composition. The isotope hydrograph separation results, using only one rain sampler, varied considerably depending on which rain sampler was used to represent the isotopic composition of event water. The calculated minimum pre-event water contributions differed up to 60%. The differences were particularly large for events with a large spatial variability in the isotopic composition of rainfall and a small difference between the event and pre-event water isotopic composition. Our results demonstrate that even in small catchments the spatial variability in the rainfall isotopic composition can be significant and has to be considered for IHS studies. Using data from only one rain sampler can result in significant errors in the estimated pre-event water contributions to streamflow.

Predicting Spatial Patterns in Precipitation Isotope (δ2H and δ18O) Seasonality Using Sinusoidal Isoscapes

“…These can sometimes be large; for example, the sinusoidal isoscape underestimated precipitation δ 18 O in December 2015 by an average of 3.8‰ due to isotopically anomalous precipitation (Figure 3a). Interstorm and intrastorm variations can be significant in time (e.g., Celle-Jeanton et al, 2001;Coplen et al, 2015;Munksgaard et al, 2012) and space (Fischer et al, 2017), so isotopically anomalous months are likely to be common, and assuming that individual months follow seasonal patterns could be inadequate for some applications. The choice of which monthly calibrated isoscape to use was unimportant here: all of those methods had similar prediction errors (Figure 3).…”

Spatio‐temporal variability of the isotopic input signal in a partly forested catchment: Implications for hydrograph separation

“…Over the last few decades, isotope‐based hydrograph separation (IHS) has been widely used in hydrology as a useful tool to gain insights into catchment runoff processes (e.g., Fischer, van Meerveld, & Seibert, ; Kubota & Tsuboyama, ; McDonnell, Bonell, Stewart, & Pearce, ; Pearce, Stewart, & Sklash, ; Sklash, Farvolden, & Fritz, ). These, among many other hydrological studies that aim to clarify water origin and movements, relied on the conservative behaviour of water stable isotopes and used D and 18 O as tracers.…”

Runoff generation in a pre-alpine catchment: A discussion between a tracer and a shallow groundwater hydrologist

“…The precipitation in the larger Alptal valley has a distinct northsouth gradient: the mean annual precipitation in the Erlenbach is 2300 mm y -1 (1980-2011 data), which is 30% more than the mean annual precipitation in Einsiedeln (10 km away, 1791 mm y -1 , 1981-2010 data from MeteoSchweiz) and more than twice as much as the mean annual rainfall in Zurich (40 km away, 1134 mm y -1 , 1981-2010 data from MeteoSchweiz). Rainfall measurements by Fischer et al (2017a) across the Zwäckentobel headwater catchments and comparisons of the precipitation data from the Vogelbach, Lümpenenbach and Erlenbach show that event based rainfall amounts vary significantly throughout the study area but that there is no clear relation between event total rainfall and elevation. This is attributed to the high variability in topography and the nearby mountains (Fig.…”

“…1), the differences in peak event water contributions to streamflow for the six sub-catchments were relatively small (Fischer et al, 2017b), and may in part be due to the spatial differences in rainfall and its isotopic composition (Fischer et al, 2017a). For large events with a large spatial variability in the isotopic composition of rainfall and a small difference between the event and pre-event water isotopic composition, the calculated minimum pre-event water fractions differed up to 60% when different rain samplers were used to represent the event water isotopic composition (Fischer et al, 2017a). The isotope hydrograph separation results for long events during which air temperatures decreased towards 0 °C were also highly uncertain (Fischer et al, 2017b).…”

“…2g) that affect the local atmospheric circulation. The dominant moisture sources in the Alptal come from the North Atlantic Ocean and Central Europe (Sodemann and Zubler, 2010) but even if an event has a dominant westerly trajectory from the North Atlantic, the rainfall may approach the catchment from the eastern direction (Fischer et al, 2017a). Summer months are wetter than autumn and winter months (Fig.…”