Investigating mechanisms of stormflow generation by natural tracers and hydrometric data: a small catchment study in the Black Forest,Germany

The importance and interaction of various hydrological pathways and identification of runoff source areas involved in solute transport are still under considerable debate in catchment hydrology. To reveal stormflow generating areas and flow paths, hydrometric behaviour of throughfall, soil water from various depths, runoff, and respective concentrations of the environmental tracers ¹⁸O, Si, K, SO₄ and dissolved organic carbon were monitored for a 14‐week period in a steep headwater catchment in the Black Forest Mountains, Germany. Two stormflow hydrographs were selected and, based on ¹⁸O and Si, chemically separated into three flow components. Their sources were defined using mixing diagrams. Additional information about stormflow generating mechanisms was derived from recession analyses of the basin's complete 5‐year hydrograph record. By providing insight into storage properties and residence times of outflowing reservoirs of the basin, recession analysis proved to be a valuable tool in runoff model conceptualization. Its results agreed well with hydrometric and hydrochemical data. Supported by evaluation of 30 hillslope soil profiles a coherent concept of stormflow generation could be derived: whereas in many steeply sloped basins in the temperate region soil water from hillslopes appears to have an immediate effect on the shape of the stormflow hydrograph, its role at this basin is basically restricted to the recharge of the groundwater reservoir in the near‐channel area. Storm hydrograph peaks appear to be derived from a small direct runoff component supplemented by a fast delivery of baseflow from the groundwater reservoir in the valley bottom. Copyright © 2001 John Wiley & Sons, Ltd.     

Effect of snow structure on water flow and solute transport

Water flow through a melting snow pack modifies its structure and stability and affects the release of water and nutrients into soils and surface waters. Field and laboratory observations indicate a large spatial variability on various scales of the liquid water content and flow, a dominant system feature currently not included in numerical models. We investigated experimentally water and dye tracer movement through microstructurally different snow pack horizons and the persistence of preferential flow paths. Naturally rounded snow of varying grain size was artificially packed to obtain well known conditions by sieving it into rectangular bins. Surface melt was induced with infrared lamps. The flow paths were visualized with tracers and liquid water content was monitored with time domain reflectometry probes. Vertical cuts through the snow pack were imaged. The dye tracer patterns allowed the two flow regimes ‘matrix flow’ and ‘preferential flow’ to be distinguished. Matrix flow is apparently dominated by film and capillary flow in the unsaturated snow matrix. The capillary barrier effect at a boundary between a fine over a coarse textured layer on matrix flow in snow was confirmed. In contrast, preferential flow appears as well‐defined flow fingers that advance from 0·1 to 1 cm s^?1. During a melt phase, the advancing flow fingers enlarge and are only partially time invariant. It remains to be shown whether the continuum concept, including the Darcy–Buckingham law is apt to describe the extremely non‐linear nature of water flow and the travel time of solutes in snow under conditions of melt water percolation. Probably, snow packs that include faceted crystals and large variations in bulk density, feature more pronounced capillary barriers and preferential flow triggering, but also stronger impeding of fingers by lateral dispersion. Further, triggering and persistence of preferential flow is complicated by the usually transient infiltration rate. Copyright © 2004 John Wiley & Sons, Ltd.     

Multiple injected and natural conservative tracers quantify mixing in a stream confluence affected by acid mine drainage near Silverton,Colorado

The acidic discharge from Cement Creek, containing elevated concentrations of dissolved metals and sulphate, mixed with the circumneutral‐pH Animas River over a several hundred metre reach (mixing zone) near Silverton, CO, during this study. Differences in concentrations of Ca, Mg, Si, Sr, and SO₄^2? between the creek and the river were sufficiently large for these analytes to be used as natural tracers in the mixing zone. In addition, a sodium chloride (NaCl) tracer was injected into Cement Creek, which provided a Cl^? ‘reference’ tracer in the mixing zone. Conservative transport of the dissolved metals and sulphate through the mixing zone was verified by mass balances and by linear mixing plots relative to the injected reference tracer. At each of seven sites in the mixing zone, five samples were collected at evenly spaced increments of the observed across‐channel gradients, as determined by specific conductance. This created sets of samples that adequately covered the ranges of mixtures (mixing ratios, in terms of the fraction of Animas River water, %AR). Concentratis measured in each mixing zone sample and in the upstream Animas River and Cement Creek were used to compute %AR for the reference and natural tracers. Values of %AR from natural tracers generally showed good agreement with values from the reference tracer, but variability in discharge and end‐member concentrations and analytical errors contributed to unexpected outlier values for both injected and natural tracers. The median value (MV) %AR (calculated from all of the tracers) reduced scatter in the mixing plots for the dissolved metals, indicating that the MV estimate reduced the effects of various potential errors that could affect any tracer. Copyright © 2006 John Wiley & Sons, Ltd.     

Paired‐basin comparison of hydrological response in harvested and undisturbed hardwood forests during snowmelt in central Ontario: I. Streamflow,groundwater and flowpath behaviour

Impacts of forest harvesting on groundwater properties, water flowpaths and streamflow response were examined 4 years after the harvest using a paired‐basin approach during the 2001 snowmelt in a northern hardwood landscape in central Ontario. The ability of two metrics of basin topography (Beven and Kirkby's ln(a/tan β) topographic index (TI) and distance to stream channel) to explain intra‐basin variations in groundwater dynamics was also evaluated. Significant relationships between TI and depth to potentiometric surface for shallow groundwater emerged, although the occurrence of these relationships during the melt differed between harvested and control basins, possibly as a result of interbasin differences in upslope area contributing to piezometers used to monitor groundwater behaviour. Transmissivity feedback (rapid streamflow increases as the water table approaches the soil surface) governed streamflow generation in both basins, and the mean threshold depths at which rapid streamflow increases corresponded to small rises in water level were similar for harvested (0·41 ± 0·05 m) and forested (0·38 ± 0·04 m) basins. However, topographic properties provided inconsistent explanations of spatial variations in the relationship between streamflow and depth to water at a given piezometer for both basins. Streamflow from the harvested basin exceeded that from the forested basin during the 2001 melt, and hydrometric and geochemical tracer results indicated greater runoff from the harvested basin via surface and near‐surface pathways. These differences are not solely attributable to harvesting, since the difference in spring runoff from the harvested basin relative to the forested control was not consistently larger than under pre‐harvest conditions. Nevertheless, greater melt rates following harvesting appear to have increased the proportion of water delivery to the stream channel via surface and near‐surface pathways. Copyright © 2005 John Wiley & Sons, Ltd.     

Solute transport in sand and chalk: a probabilistic approach

A probabilistic approach is used to simulate particle tracking for two types of porous medium. The first is sand grains with a single intergranular porosity. Particle tracking is carried out by advection and dispersion. The second is chalk granulates with intergranular and matrix porosities. Sorption can occur with advection and dispersion during particle tracking. Particle tracking is modelled as the sum of elementary steps with independent random variables in the sand medium. An exponential distribution is obtained for each elementary step and shows that the whole process is Markovian. A Gamma distribution or probability density function is then deduced. The relationships between dispersivity and the elementary step are given using the central limit theorem. Particle tracking in the chalky medium is a non‐Markovian process. The probability density function depends on a power of the distance. Experimental simulations by dye tracer tests on a column have been performed for different distances and discharges. The probabilistic approach computations are in good agreement with the experimental data. The probabilistic computation seems an interesting and complementary approach to simulate transfer phenomena in porous media with respect to the traditional numerical methods. Copyright © 2006 John Wiley & Sons, Ltd.     

Measuring gravel transport and dispersion in a mountain river using passive radio tracers

Random walk models of fluvial sediment transport recognize that grains move intermittently, with short duration steps separated by rests that are comparatively long. These models are built upon the probability distributions of the step length and the resting time. Motivated by these models, tracer experiments have attempted to measure directly the steps and rests of sediment grains in natural streams. This paper describes results from a large tracer experiment designed to test stochastic transport models. We used passive integrated transponder (PIT) tags to label 893 coarse gravel clasts and placed them in Halfmoon Creek, a small alpine stream near Leadville, Colorado, USA. The PIT tags allow us to locate and identify tracers without picking them up or digging them out of the streambed. They also enable us to find a very high percentage of our rocks, 98% after three years and 96% after the fourth year. We use the annual tracer displacement to test two stochastic transport models, the Einstein–Hubbell–Sayre (EHS) model and the Yang–Sayre gamma‐exponential model (GEM). We find that the GEM is a better fit to the observations, particularly for slower moving tracers and suggest that the strength of the GEM is that the gamma distribution of step lengths approximates a compound Poisson distribution. Published in 2012. This article is a US Government work and is in the public domain in the USA.     

Evaluation of bedload yield in gravel-bed rivers using scour chains and painted tracers: the case of the Esconavette Torrent (Southern French Prealps)

The event-based bedload yields of a small gravel-bed river (the Esconavette Torrent) have been concomitantly determined by surveying coarse sediment deposition in a trap and by monitoring the active layer of the bed and the displacement of painted tracers. The geometry of the active layer was obtained by means of scour chains and topographic resurveys. The cumulative bedload yield of 4 flow events measured in the trap and by the chain and tracer approach was respectively 174 and 153 m³. The consistency between those two field-based estimates confirms that the deployment of scour chains and tracers in gravel-bed rivers have the potential to provide a robust assessment of bedload transport. This potential theoretically depends on the spatial density of scour chains and the ability of the tracing technique to fit the grain size distribution of the active layer. The results demonstrate that a distance between scour chains that represents 10-15% of the active channel width is sufficient for a rather accurate determination of event bedload transport rate by reconstruction of scour and fill throughout a cross-section.     

Comparison of ground-based and UAV a-UHF artificial tracer mobility monitoring methods on a braided river

Radio frequency identification (RFID) technologies, which allow wireless detection of individual buried or immersed tracers, represent a step forward in sediment tracking, especially passive integrated transponders (PIT tags) that have been widely used. Despite their widespread adoption in the scientific community, they typically have low efficiency when deployed in river systems with active bedload transport or deep wet channels, attributed to their technical specifications. A recent evaluation of active ultra-high frequency transponders (a-UHF tags) assessed their larger detection range and provided a methodology for their geopositioning. In this study, we test five different survey methods (one including an unmanned aerial vehicle [UAV]) in a sediment tracking study, and compare them in terms of recovery rate, field effort, geopositioning error, and efficiency. We then tested the method on a larger reach following a Q₅ flood and performed cross-comparisons between active and passive RFIDs. The results confirmed that the a-UHF RFID technology allowed rapid (1.5 h ha⁻¹) survey of a large area (<34 ha) of emerged bars and shallow water channels with recovery of a high percentage of tracers (72%) that had travelled large distances (mean ≈ 1000 m; max ≈ 3400 m). Moreover, the tracers were identified with low geopositioning error (mean ≈ 7.1 m, ≤1% of their travel distance). We also showed that a UAV-based survey was fast (0.38 h ha⁻¹), efficient (recovery rate = 84%), and low error (mean ≈ 4.2 m). Thus, a-UHF RFID technology permits the development of a variety of survey methods, depending on the study objectives and the human and financial resources available. This allows field efforts to be optimized by determining an appropriate balance between the high equipment cost of a-UHF tracers and the resulting reduced survey costs. © 2019 John Wiley & Sons, Ltd.     

The influence of local hydrogeologic forcings on near‐stream event water recharge and retention (Upper San Pedro River,Arizona)

The rise in stream stage during high flow events (floods) can induce losing stream conditions, even along stream reaches that are gaining during baseflow conditions. The aquifer response to flood events can affect the geochemical composition of both near‐stream groundwater and post‐event streamflow, but the amount and persistence of recharged floodwater may differ as a function of local hydrogeologic forcings. As a result, this study focuses on how vertical flood recharge varies under different hydrogeologic forcings and the significance that recharge processes can have on groundwater and streamflow composition after floods. River and shallow groundwater samples were collected along three reaches of the Upper San Pedro River (Arizona, USA) before, during and after the 2009 and 2010 summer monsoon seasons. Tracer data from these samples indicate that subsurface floodwater propagation and residence times are strongly controlled by the direction and magnitude of the dominant stream–aquifer gradient. A reach that is typically strongly gaining shows minimal floodwater retention shortly after large events, whereas the moderately gaining and losing reaches can retain recharged floodwater from smaller events for longer periods. The moderately gaining reach likely returned flood recharge to the river as flow declined. These results indicate that reach‐scale differences in hydrogeologic forcing can control (i) the amount of local flood recharge during events and (ii) the duration of its subsurface retention and possible return to the stream during low‐flow periods. Our observations also suggest that the presence of floodwater in year‐round baseflow is not due to long‐term storage beneath the streambed along predominantly gaining reaches, so three alternative mechanisms are suggested: (i) repeated flooding that drives lateral redistribution of previously recharged floodwater, (ii) vertical recharge on the floodplain during overbank flow events and (iii) temporal variability in the stream–aquifer gradient due to seasonally varying water demands of riparian vegetation. Copyright © 2011 John Wiley & Sons, Ltd.