The Seasonal Origins of Streamwater in Switzerland

Abstract

Quantifying the relative contributions of winter versus summer precipitation to streamflow may be important for understanding water‐resource sensitivity to precipitation variability. Here we compare volume‐weighted mean δ18O values in precipitation and streamflow for 12 catchments in Switzerland, to determine whether summer or winter precipitation is overrepresented in streamflow, relative to its proportion of total precipitation. Similarities between precipitation and streamflow weighted‐mean δ18O values indicate that roughly equal fractions of summer and winter precipitation supply streams in Switzerland. These results, together with mass conservation, suggest that similar fractions of summer and winter precipitation supply evapotranspiration. These findings contrast with the assumption that because summer precipitation falls when transpiration rates and evaporative demand are high, it should be underrepresented in streamflow and overrepresented in evapotranspiration. This contrast between seasonal water‐balance variations and the partitioning of seasonal precipitation into runoff and evapotranspiration demonstrates substantial interseasonal carryover of precipitation in storages that supply evapotranspiration.

Plant and root‐zone water isotopes are difficult to measure, explain, and predict: Some practical recommendations for determining plant water sources

“…Conventionally, it has been assumed that these isotope profiles are near‐monotonic, with heavier isotope ratios in shallower depths and lighter isotope ratios in greater depths (solid red line in Figure 1). This characteristic isotope‐depth profile often occurs when shallow soil waters become enriched as isotopically lighter water is preferentially evaporated (Barnes & Allison, 1984; Zimmermann et al, 1967) or when isotopically heavier growing‐season precipitation recharges the soils that supply evapotranspiration (however, not always the most‐recent precipitation is evapotranspired; Allen, von Freyberg, Weiler, Goldsmith, & Kirchner, 2019). Beneath the maximum evaporation penetration depth, infiltrated precipitation water mixes with previously stored water over seasons and years so that the isotopic signature of soil water represents the long‐term average of previous precipitation events that have recharged these soils; thus, deeper soil waters are usually isotopically lighter than shallow soil waters.…”

Seasonal partitioning of precipitation between streamflow and evapotranspiration, inferred from end-member splitting analysis

“…Watershed 3 is a small (42.4 ha) headwater basin that has served as a hydrologic reference watershed for manipulation experiments conducted in several other nearby watersheds (Bailey et al, 2003). Its soils are well-drained Spodosols with a 3-15 cm thick, highly permeable organic layer at the surface, underlain by glacial drift of highly variable thickness (averaging roughly 0.5 m, Bailey et al, 2014), which in turn overlies schist and granulite bedrock that is believed to be highly impermeable (Likens, 2013).…”

“…(Green et al, 2015), with a growing season extending from June through September (Fahey et al, 2005). Watershed 3 has a humid continental climate, with average monthly temperatures ranging from −8 • C in January to 18 • C in July (Bailey et al, 2003). Annual average precipitation was 136 cm yr −1 from 1958 through 2014, distributed relatively evenly throughout the year, and annual average streamflow was about 87 cm yr −1 , implying evapotranspiration losses of roughly 49 cm yr −1 , or about one-third of average precipitation (USDA Forest Service Northern Research Station, 2016a, b).…”

The CH-IRP data set: a decade of fortnightly data on <i>δ</i><sup>2</sup>H and <i>δ</i><sup>18</sup>O in streamflow and precipitation in Switzerland

“…From this data set, young water fractions can also be calculated; for instance, using the data presented here, Von Freyberg et al (2018b) assessed how sensitive the young water fraction is to both hydro-climatic forcing and catchment properties. Using the δ 18 O values in precipitation and streamflow for 12 catchments of the presented data set, Allen et al (2019) assessed whether summer or winter precipitation is overrepresented in streamflow, relative to its proportion of total precipitation. Parts of this data set (composition of isotopes in precipitation) were also used to re-investigate the relationship between transit times and catchment topography (Seeger and Weiler, 2014).…”