Groundwater discharge along streams exerts an important influence on biogeochemistry and thermal regimes of aquatic ecosystems. A common approach for predicting locations of shallow lateral groundwater discharge is to use digital elevation models (DEMs) combined with upslope contributing area algorithms. We evaluated a topography‐based prediction of subsurface discharge zones along a 1500 m headwater stream reach using temperature and water isotope tracers. We deployed fiber‐optic distributed temperature sensing instrumentation to monitor stream temperature at 0.25 m intervals along the reach. We also collected samples of stream water for the analysis of its water isotope composition at 50 m intervals on five occasions representing distinct streamflow conditions before, during, and after a major rain event. The combined tracer evaluation showed that topography‐predicted locations of groundwater discharge were generally accurate; however, predicted magnitude of groundwater inflows estimated from upslope contributing area did not always agree with tracer estimates. At the catchment scale, lateral inflows were an important source of streamflow at base flow and peak flow during a major rain event; however, water from a headwater lake was the dominant water source during the event hydrograph recession. Overall, this study highlights potential utility and limitations of predicting locations and contributions of lateral groundwater discharge zones using topography‐based approaches in humid boreal regions.

Abstract:Stable isotopes of water are one of the most widely used tools to track the pathways of precipitation inputs to streams. In the past, soils have often been treated as black-boxes through which precipitation is routed to streams without much consideration of how, when, and where water is transported along soil and groundwater flow paths. Here, we use time series of stable isotopes ( 18 O) in precipitation, soil/groundwater, and stream water to evaluate how landscape structure and heterogeneity influence seasonal hydrological patterns characteristic of boreal headwater catchments. To do this, we collected water throughout a full year at three adjacent catchments draining forest, mire, and mire/lake ecosystems within the Krycklan Experimental Catchment of northern Sweden. Isotope time series from forest and mire groundwater piezometers showed spatial and temporal heterogeneity in the dominant hydrologic flow pathways connecting precipitation to stream flow at different sites. The isotopic signature of stream water suggested strong connections to the dominant landscape elements within each catchment. These connections translated into greater temporal variability in the isotopic response of streams draining lake and wetland patches, and a much more attenuated pattern in the forest-dominated catchment. Overall, seasonal changes in the isotopic composition of streams and groundwater illustrate how differences in landscape structure result in variable hydrological patterns in the boreal landscape.

Understanding how the sources of surface water change along river networks is an important challenge, with implications for soil-stream interactions, and our ability to predict hydrological and biogeochemical responses to environmental change. Network-scale patterns of stream water reflect distinct hydrological processes among headwater units, as well as variable contributions from deeper groundwater stores, which may vary nonlinearly with drainage basin size. Here we explore the spatial variability of groundwater inputs to streams, and the corresponding implications for surface water chemistry, during winter baseflow in a boreal river network. The relative contribution of recent and older groundwater was determined using stable isotopes of water (δ 18 O) at 78 locations ranging from small headwaters (0.12 km 2 ) to fourth-order streams (68 km 2 ) in combination with 79 precipitation and 10 deep groundwater samples. Results from a two end-member mixing model indicate that deeper groundwater inputs increased nonlinearly with drainage area, ranging from~20% in smaller headwater subcatchments to 70-80% for catchments with a 10.6 km 2 area or larger. Increases in the groundwater contribution were positively correlated to network-scale patterns in surface stream pH and base cation concentrations and negatively correlated to dissolved organic carbon. These trends in chemical variables are consistent with the production of weathering products and the mineralization of organic matter along groundwater flow paths. Together, the use of stable isotopes and biogeochemical markers illustrate how variation in hydrologic routing and groundwater contributions shape network-scale patterns in stream chemistry as well as patchiness in the relative sensitivity of streams to environmental change and perturbation.

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