No abstract

[1] Hyporheic flow can be extremely variable in space and time, and our understanding of complicated flow systems, such as exchange around small dams, has generally been limited to reach-averaged parameters or discrete point measurements. Emerging techniques are starting to fill the void between these disparate scales, increasing the utility of hyporheic research. When ambient diurnal temperature patterns are collected at high spatial resolution across vertical profiles in the streambed, the data can be applied to one-dimensional conduction-advection-dispersion models to quantitatively describe the vertical component of hyporheic flux at the same high spatial resolution. We have built on recent work by constructing custom fiber-optic distributed temperature sensors with 0.014 m spatial resolution that are robust enough to be installed by hand into the streambed, maintain high signal strength, and permit several sensors to be run in series off a single distributed temperature sensing unit. Data were collected continuously for 1 month above two beaver dams in a Wyoming stream to determine the spatial and temporal nature of vertical flux induced by the dams. Flux was organized by streambed morphology with strong, variable gradients with depth indicating a transition to horizontal flow across a spectrum of hyporheic flow paths. Several profiles showed contrasting temporal trends as discharge decreased by 45%. The high-resolution thermal sensors, combined with powerful analytical techniques, allowed a distributed quantitative description of the morphology-driven hyporheic system not previously possible.

The rapid retreat of the glaciers of the Cordillera Blanca is having a noticeable impact on the downstream hydrology. Although groundwater is a critical hydrologic component that sustains stream flows during the dry season, its characteristics and its contribution to downstream hydrology remain poorly understood. In this study, we analyse the hydrochemical and isotopic properties of potential hydrologic sources mixing in surface streams to characterize the proglacial hydrology in four glacially fed watersheds within the Cordillera Blanca, Peru. Water samples from streams, glacial melt and groundwater were collected in 2008 and 2009 and analysed for major ions and stable isotopes (δ18O and δ2H). Multivariate analysis of variance was used first to identify the hydrochemical and isotopic characteristics (tracers) of the water samples that depend primarily on the water source. Then several analyses, including hierarchical cluster analysis and mixing diagrams, were performed using these source‐dependent tracers, enabling a qualitative description of the key hydrological mechanisms that characterize the study watersheds. Finally, we applied a multi‐component spatial mixing model, the hydrochemical basin characterization method, to quantify the contributions of different water sources to the outflow from the four watersheds. The hydrochemical basin characterization method results show that groundwater is a major component of the discharge during the dry season and that the groundwater contribution to outflow is greater than 24% in all of the valleys. The results are used to develop a conceptual proglacial hydrological model of the Cordillera Blanca valleys. Talus and avalanche cones are identified as key components of the hydrology of the valleys. The talus deposits collect precipitation and runoff from higher elevations (approximately 400 m above the valley floor) and have a residence time that is long enough to actively release substantial volumes of water throughout the dry season. Copyright © 2014 John Wiley & Sons, Ltd.

Vertical fluid exchange between surface water and groundwater can be estimated using diurnal signals from temperature time series methods based on amplitude ratios (A r), phase shifts (Δϕ), or combined use of both (A r Δϕ). The A r , Δϕ, and A r Δϕ methods are typically applied in conditions where one or more of their underlying assumptions are violated, and the reliability of the various methods in response to non-ideal conditions is unclear. Additionally, A r Δϕ methods offer the ability to estimate thermal diffusivity (κ e) without assuming any thermal parameters, although the value of such output has not been broadly tested The A r , Δϕ, and A r Δϕ methods are tested under non-steady, 1D flows in sand column experiments, and multi-dimensional flows in heterogeneous media in numerical modeling experiments. Results show that, in non-steady flow conditions, estimated κ e values outside of a plausible range for streambed materials (0.028 to 0.180 m 2 d-1) coincide with time periods with erroneous flux estimates. In heterogeneous media, sudden changes of κ e with depth also coincide with erroneous flux estimates. When (known) fluxes are variable in time, poor identification of Δϕ leads to poor flux estimates from Δϕ and A r Δϕ methods. However, when fluxes are steady, or near zero, A r Δϕ methods provide the most accurate flux estimates. This comparison of A r , Δϕ and A r Δϕ methods under non-ideal conditions provides guidance on their use. In this study, A r Δϕ methods have been coded into a new version of VFLUX, allowing users easy access to recent advances in heat tracing.

[1] Natural channel design restoration projects in streams often include the construction of cross-vanes, which are stone, dam-like structures that span the active channel. Vertical hyporheic exchange flux (HEF) and redox-sensitive solutes were measured in the streambed around four cross-vanes with different morphologies. Observed patterns of HEF and redox conditions are not dominated by a single, downstream-directed hyporheic flow cell beneath cross-vanes. Instead, spatial patterns of moderate (<0.4 m d À1 ) upwelling and downwelling are distributed in smaller cells around pool and riffle bed forms upstream and downstream of structures. Patterns of biogeochemical cycling are controlled by dissolved oxygen concentrations and resulting redox conditions, and are also oriented around secondary bed forms. Strong downwelling into the hyporheic zone (0.5-3.5 m d À1 ) was observed immediately upstream of structures, but was limited to an area 1-2 m from the cross-vane; these hyporheic flow paths likely rejoin the stream at the base of cross-vanes after residence times too short to alter nitrate concentrations or accumulate reaction products. Total hyporheic exchange volumes are $0.4% of stream discharge in restored reaches of 45-55 m. Results show that shallow hyporheic flow and associated biogeochemical cycling near cross-vanes is primarily controlled by secondary bed forms created or augmented by the cross-vane, rather than by the cross-vane itself. This study suggests that cross-vane restoration structures benefit the stream ecosystem by creating heterogeneous patches of varying HEF and redox conditions in the hyporheic zone, rather than by processing large amounts of nutrients to alter in-stream water chemistry.Citation: Gordon, R. P., L. K. Lautz, and T. L. Daniluk (2013), Spatial patterns of hyporheic exchange and biogeochemical cycling around cross-vane restoration structures: Implications for stream restoration design, Water Resour.