No abstract

We conducted an integrated groundwater–surface water monitoring programme in a 3.2‐km2 experimental catchment in the Scottish Highlands by sampling all springs, seepages, and wells in six, spatially extensive synoptic surveys over a 2‐year period. The catchment has been glaciated, with steep hillslopes and a flat valley bottom. There is around 70% glacial drift cover in lower areas. The solid geology, which outcrops at higher elevations, is granite and metamorphic schist. The springs and seepages generally occur at the contact between the solid geology and drift or at breaks of slopes in the valley bottom. Samples were analysed for stable isotopes, Gran alkalinity and electrical conductivity. Despite the surveys encompassing markedly different antecedent conditions, the isotopic composition of groundwater at each location exhibited limited temporal variability, resulting in a remarkable persistence of spatial patterns indicating well‐mixed shallow, groundwater stores. Moreover, line‐conditioned excess values derived from the isotope data indicated no evidence of fractionation affecting the groundwater, which suggests that most recharge occurs in winter. The alkalinity and electrical conductivity of groundwater reflected geological differences in the catchment, being highest where more weatherable calcareous rocks outcrop at higher altitudes in the catchment. Springs draining these areas also had the most variable isotope composition, which indicated that they have shorter residence times than the drift covered part of the catchment. The study showed that even in geologically heterogeneous upland catchments, groundwater can be characterized by a consistent isotopic composition, reflecting rapid mixing in the recharge zone. Our work, thus, emphasizes the critical role of groundwater in upland catchments and provides tracer data that can help constrain quantitative groundwater models.

Water movement in hillslopes is determined by the subsurface characteristics that control flow paths connecting precipitation to stream flow generation. The hydrological response of hillslopes is notoriously non-linear and non-stationary; with the relative importance of vertical and lateral flow paths also depending on event characteristics and antecedent conditions. In northern boreal regions, climate change projections indicate that wetter and warmer winter conditions are likely to generate more extreme flood events. Here, we report a study from an upland catchment in northern Scotland where a monitoring year provided an opportunity to contextualise observations during the hillslope response to a winter rainfall event that locally caused the most extreme flooding for over 200 years. Monitoring the hillslope water table, soil moisture and isotopes in precipitation, groundwater and stream flow provided invaluable insight into hillslope-riparian coupling. Groundwater with a shallow water table (<0.05 m deep) in poorly drained valley bottom drift deposits maintained almost fully saturated and stream-connected peat soil profiles in riparian areas. In the wettest periods, the groundwater beneath the peat was artesian. On steeper hillslopes, soils were drier and the water table was generally deeper (0.5 to 1 m below ground level), though the profile could fully saturate and groundwater levels reach the surface during the wettest period. Groundwater in deeper wells typically showed an anticlockwise hysteresis compared to stream flow, and peak levels typically lagged behind the stream by a few hours in the valley bottom and >1 2 day in the upper hillslope. In contrast, shallower wells in the soil profiles in the riparian area showed more a responsive perched groundwater system with transmissivity feedback in the upper soil layers resulting in much more rapid responses which generally peaked before the stream and exhibited clockwise hysteresis. Analysis of stable isotopes in precipitation, groundwater and streamflow, indicated that groundwater was remarkably well mixed with limited fractionation effects, inferring precipitation on the upper, unconfined hillslopes was the dominant source of recharge-particularly during the winter. The study shows that groundwater plays two roles in generating stream flow: a constant baseflow supply to the stream and time varying-exfiltration into the edge of the riparian zone, which contributes to surface runoff during storm events.

Riparian wetlands (RW) are important variable source areas for runoff generation. They are usually characterised by a combination of groundwater exfiltration-which maintains saturated conditions in low-lying organic-rich soils-and direct precipitation. Both processes interact to generate overland flow as a dominant runoff process. The small-scale details of groundwater-surface water (GW-SW) interactions are usually not well understood in RW. Here, we report the results of a study from an experimental catchment in the Scottish Highlands where spatio-temporal runoff processes in RW were investigated using isotopes, alkalinity and hydrometric measurements. We focused on perennial micro-catchments within the RW and ephemeral zero-order channels draining peatland hollows and hummocks to better understand the heterogeneity in GW-SW interactions. The 12-month study period was dominated by the wettest winter (Dec/Jan) period on record. Runoff generation in the RW 2 was strongly controlled by the local groundwater response to direct rainfall, but also the exfiltration of groundwater from upslope. This groundwater drainage is focused in the hollows in ephemeral and perennial drainage channels, but in wet conditions, as exfiltration rates increase, can affect hummocks as well. The hollows provide the dominant areas for mixing groundwater, soil water and direct rainfall to deliver water to the stream network as hollows "fill and spill" to increase connectivity. They also provide wet areas for evaporation which is evident in enriched isotope signatures in summer. Although there is some degree of heterogeneity in the extent to which groundwater influences specific micro-catchments, particularly under low flows, the overall isotopic response is quite similar, especially when the catchment is wet and this responses can explain the isotope signatures observed in the stream. In future, more longitudinal studies of micro-catchments are needed to better explain the heterogeneity observed.

The drought of summer 2018, which affected much of Northern Europe, resulted in low river flows, biodiversity loss and threats to water supplies. In some regions, like the Scottish Highlands, the summer drought followed two consecutive, anomalously dry, winter periods. Here, we examine how the drought, and its antecedent conditions, affected soil moisture, groundwater storage, and low flows in the Bruntland Burn; a sub‐catchment of the Girnock Burn long‐term observatory in the Scottish Cairngorm Mountains. Fifty years of rainfall‐runoff observations and long‐term modelling studies in the Girnock provided unique contextualisation of this extreme event in relation to more usual summer storage dynamics. Whilst summer precipitation in 2018 was only 63% of the long‐term mean, soil moisture storage across much of the catchment were less than half of their summer average and seasonal groundwater levels were 0.5 m lower than normal. Hydrometric and isotopic observations showed that ~100 mm of river flows during the summer (May‐Sept) were sustained almost entirely by groundwater drainage, representing ~30% of evapotranspiration that occurred over the same period. A key reason that the summer drought was so severe was because the preceding two winters were also dry and failed to adequately replenish catchment soil moisture and groundwater stores. As a result, the drought had the biggest catchment storage deficits for over a decade, and likely since 1975–1976. Despite this, recovery was rapid in autumn/winter 2018, with soil and groundwater stores returning to normal winter values, along with stream flows. The study emphasizes how long‐term data from experimental sites are key to understanding the non‐linear flux‐storage interactions in catchments and the “memory effects” that govern the evolution of, and recovery from, droughts. This is invaluable both in terms of (a) giving insights into hydrological behaviours that will become more common water resource management problems in the future under climate change and (b) providing extreme data to challenge hydrological models.