The Seasonal Origins of Streamwater in Switzerland

Abstract

AbstractQuantifying 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.