FOR 5288: Fast and invisible: Conquering Subsurface Stormflow
through an Interdisciplinary Multi-Site Approach

Abstract:

Headwater streams account for 70% or more of total stream length in most catchments, making it crucial to better understand the processes and controlling factors governing streamflow generation as well as water quality. In this context, stream water chemistry longitudinal profiles can provide valuable insights. This study examines longitudinal stream chemistry profiles across six headwater catchments in three mid-mountain ranges in Germany: The Ore Mountains (catchments OM 1 and OM 2), Black Forest (BF 1 and BF 2), and Sauerland (SL 1 and SL 2). Three to four snapshot sampling campaigns were conducted per catchment across different seasons and catchment wetness conditions. During the campaigns, water samples were collected from 22 stream monitoring points in the Ore Mountains catchments, 14 in the Black Forest, and 14 in Sauerland, and the samples were analyzed for major cations, anions, and dissolved organic carbon. Subsequently, the longitudinal profiles observed were grouped into spatial and temporal patterns. In the Ore Mountains, solute concentrations were generally stable over time. However, the spatial patterns varied between the two neighbouring catchments (OM 1 and OM 2). OM 2 exhibited chemostatic longitudinal profiles for most solutes, while OM 1 showed pronounced spatial variability in solutes such as nitrate, dissolved organic carbon (DOC), chloride, and sodium. This variability is usually linked to monitoring points located near springs, tributaries, and drainage systems. However, some spikes in ion concentrations along the stream were not linked to these obvious inflows, thus potentially indicating hotspots for groundwater inflow. The Sauerland catchments showed elevated concentrations of DOC, magnesium, calcium, and sodium in July 2023, a period associated with lower streamflow. An increase in concentration from upstream to downstream was here seen in both streams for solutes like calcium and sodium, during all snapshot campaigns. However, other solutes, like nitrate and sulfate, showed different longitudinal patterns and notable shifts in solute concentration during the snapshot campaigns in SL 2. The shifts in patterns indicate a dependency on time-variant factors like seasonal changes in water input, and land use practices. BF 1 catchment in the Black Forest showed a decreasing pattern in DOC, from upstream to downstream, while the neighbouring catchment BF 2 showed a chemostatic trend. These trends could be influenced by the land use changes within the catchments. Notable increased nitrate concentrations were seen along reaches adjacent to grassland areas and at sampling points near tile drains in OM 1, BF 1, SL 1, and SL 2. Overall, solute spatial and temporal patterns were stream-specific, with no universal behaviour observed across all catchments. This variability likely results from the interplay of factors such as geology, soils, land use, stream morphology, and climate. High-resolution spatial sampling enabled the identification of point sources and hotspots of groundwater inflow which could be missed by sparse sampling. These findings enhance our understanding of the processes regulating water quality and flow in headwater systems, providing a basis for better management of these systems.

Abstract:

Summary This study investigates how different tree species influence soil hydrological properties that are relevant for the rainfall–runoff response of a given soil type. We hypothesize that for the same soil type, tree species that differ in rooting system, water consumption and associated soil fauna and soil flora lead to different soil moisture dynamics and lateral flow processes during rainfall and hence to different runoff responses. To test this hypothesis, we compare soil moisture patterns and interflow at different soil depths in a Norway spruce (Picea abies (L.) Karst) forest and in a European beech (Fagus sylvatica L.) forest during sprinkling experiments on two 6×10m hillslope segments with the same soil type. Spruce with a shallow rooting system and sinkers that remain very shallow on poorly aerated soils and beech with a heart shaped, often deeper rooting system are two of the most important tree species in Central Europe. At each hillslope, volumetric soil water contents were measured in 6min intervals with 48 TDR waveguides during and after sprinkling with intensities of 100mm/h and 60mm/h (for 1h). The waveguides were installed in 12 soil pits, whereby a single soil pit consisted of four 20cm buriable waveguides installed in 10cm, 30cm, 50cm and 70cm soil depth. Surface and shallow interflow at 10 cm soil depth and interflow at soil depths of 30cm and 60cm was automatically recorded. Despite the high rainfall intensities, no surface flow was observed in any of the experiments and only small amounts of shallow interflow were measured. Soil moisture patterns of lateral cross sections during and after the sprinkling reveal how tree species can alter runoff dynamics: under spruce, coinciding with rooting patterns, a water table develops in approximately 30cm soil depth while the soil water content in 50 and 70cm depth remains low. At the beech site, where coarse roots are found in deeper soil horizons, more water is directed towards deeper, already wetter soil horizons, from where the water table raises into the topsoil with high lateral conductivity. Because the higher water content on top of the stagnic layer allows segments of macropores like old root channels to connect earlier under beech, the beech hillslope exhibits a faster runoff response than the spruce hillslope. A lower water table and a higher macro-porosity makes saturation excess overland flow unlikely under beech. With the shallow water table and a lower available soil volume for preferential flow, a site planted with spruce is prone to saturation excess overland flow under natural rainfall conditions with inflow from the top. The results suggest that different tree species can lead to different rainfall–runoff responses at the same soil type. Though the study site showed minimal variation in soil properties, we cannot exclude that some of the differences in runoff processes we observed are caused by factors other than tree species, because only one large hillslope segment in each forest stand was sprinkled.