Synoptic water isotope surveys to understand the hydrology of large intensively managed catchments

Precise knowledge of hydrological processes across large scale catchments is crucial to sustainably meet the growing water demand and improve water management strategies in cities. However, it has been always a major challenge to comprehensively understand the hydrology of a large catchment due to the spatial heterogeneity of climate, topography and anthropogenic activities. Combining tracers with hydroclimatic records, this study used seasonal synoptic surveys in 2021 to better understand the water cycling, storage and losses of the intensively managed 10,000 km² catchment of the River Spree in Berlin, Germany. Apart from the upper headwaters, the hydrology of the Spree is heavily regulated by reservoir releases, pumped minewater discharges, engineered flows in wetlands and lakes, water abstractions and urban drainage. Moreover, the catchment is drought-sensitive with potential evapotranspiration often exceeding annual rainfall. This is reflected in the spatial and temporal variability of the isotopic composition of river water. In the steeper, upper headwater areas, the river is dominated by groundwater sources but showing evident influence by rainfall in winter, with a “flashy” rainfall-runoff response. However, flows in the middle part of the catchment have enhanced baseflows and attenuated high flows from extensive reservoir and pumped minewater releases. The reservoir waters are isotopically heavier and reflect the effects of open water evaporation. Fractionation effects strengthen downstream as managed wetland areas and natural lakes further enhance evaporation and attenuate flows. Our estimations on evaporation losses also show that the mine water pumping, water abstractions and wastewater additions largely contribute to the catchment water balance and therefore have pronounced impacts on water evaporations. Seasonally, the effects of evaporation on the isotopic composition of the lower river network are strongest in summer and autumn, though they remain in winter and spring, indicating a large memory effect due to long mean travel times within the river system. Tritium variability along the river reflects inputs of younger and older water in different parts of the river system; though the influence of pumped groundwater means that the mean age of stream water in the lower river is likely to be >50 years. Isotope studies at large scales play a valuable role to in better understanding the hydrology of this complex, heavily modified river system and provide an evidence base for more sustainable management of the potentially fragile water resource situation in the future.