Using geophysical surveys to test tracer‐based storage estimates in headwater catchments

Abstract

Hydrogeophysical surveys were carried out in a 3.2km2 Scottish catchment where previous isotope studies inferred significant groundwater storage that makes important contributions to streamflow.We used electrical resistivity tomography (ERT) to characterise the architecture of glacial drifts and make an approximation of catchment-scale storage. Four ERT lines (360-535m in length) revealed extensive 5-10m deep drift cover on steeper slopes, which extends up to 20-40m in valley bottom areas. Assuming low clay fractions, we interpret variable resistivity as correlating with variations in porosity and water content. Using Archie's Law as a first approximation, we compute likely bounds for storage along the ERT transects. Areas of highest groundwater storage occur in valley bottom peat soils (up to 4m deep) and underlying drift where up to 10,000mm of precipitation equivalent may be stored. This is consistent with groundwater levels which indicate saturation to within 0.2m of the surface. However, significant slow groundwater flow paths occur in the shallower drifts on steeper hillslopes, where point storage varies between ~1,000mm-5,000mm. These fluxes maintain saturated conditions in the valley bottom and are recharged from drift-free areas on the catchment interfluves. The surveys indicate that catchment scale storage is >2,000mm which is consistent with tracer-based estimates.

Testing the maximum entropy production approach for estimating evapotranspiration from closed canopy shrubland in a low‐energy humid environment

“…Over the entire study period, the residual rainfall (46.7%) after ET loss percolated into the deeper soils, because surface runoff is negligible at the study plot due to the flat terrain and permeable podzolic soils (Tetzlaff et al, ). Based on previous studies using geophysical surveys (Soulsby et al, ) and a tracer‐aided model (Birkel, Soulsby, & Tetzlaff, ), most of the excess rainfall beyond ET becomes storage in soils; then may later recharge groundwater (Tetzlaff et al, ), and contribute to downslope riparian zones and streams through subsurface lateral flow (Blumstock, Tetzlaff, Dick, Nuetzmann, & Soulsby, ). The MEP‐derived water balance highlights the qualitative and quantitative effects of vegetation on water partitioning and storage in heather dominated areas such as the study site.…”

Spatial organization of groundwater dynamics and streamflow response from different hydropedological units in a montane catchment

“…Bedrock outcrops are found at higher elevation at the northern edge of the catchment. Electrical resistivity tomography surveys in 2013 revealed drift depths of 7 to 40 m in the valley bottom (Soulsby et al ., in review).…”

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

“…Such incomplete mixing within the unsaturated zone could have an impact on catchment transit time estimates (van der Velde et al, 2015). Given the high storage volumes of the periglacial deposits (Soulsby et al, 2016b) and the high water-mixing volume in the riparian zone in the Bruntland Burn, it is yet unclear, which role the relatively slow moving and partly mixed soil waters in the soil matrix at the hillslopes could play regarding long tails of transit times in the Scottish Highlands, as reported by Kirchner et al (2010).…”

“…The underlying geology of the BB is characterized by granitic and metamorphic rocks (Soulsby et al, 2007). About 60 % of the catchment is covered by up to 40 m of glacial drift deposits which maintain a high groundwater storage (Soulsby et al, 2016b). The climate is temperate-boreal oceanic with mean daily air temperatures ranging between 2 • C in January and 13 • C in July.…”