Noah M. Schmadel

Spectral scaling of heat fluxes in streambed sediments

Advancing our predictive capabilities of heat fluxes in streams and rivers is important because of the effects on ecology and the general use of heat fluxes as analogues for solute transport. Along ...

Hydrogeomorphic controls on hyporheic and riparian transport in two headwater mountain streams during base flow recession

Solute transport along riparian and hyporheic flow paths is broadly expected to respond to dynamic hydrologic forcing by streams, aquifers, and hillslopes. However, direct observation of these dynamic responses is lacking, as is the relative control of geologic setting as a control on responses to dynamic hydrologic forcing. We conducted a series of four stream solute tracer injections through base flow recession in each of two watersheds with contrasting valley morphology in the H.J. Andrews Experimental Forest, monitoring tracer concentrations in the stream and in a network of shallow riparian wells in each watershed. We found hyporheic mean arrival time, temporal variance, and fraction of stream water in the bedrock-constrained valley bottom and near large roughness elements in the wider valley bottom were not variable with discharge, suggesting minimal control by hydrologic forcing. Conversely, we observed increases in mean arrival time and temporal variance and decreasing fraction stream water with decreasing discharge near the hillslopes in the wider valley bottom. This may indicate changes in stream discharge and valley bottom hydrology control transport in less constrained locations. We detail five hydrogeomorphic responses to base flow recession to explain observed spatial and temporal patterns in the interactions between streams and their valley bottoms. Models able to account for the transition from geologically dominated processes in the near-stream subsurface to hydrologically dominated processes near the hillslope will be required to predict solute transport and fate in valley bottoms of headwater mountain streams.

Stream solute tracer timescales changing with discharge and reach length confound process interpretation

Improved understanding of stream solute transport requires meaningful comparison of processes across a wide range of discharge conditions and spatial scales. At reach scales where solute tracer tests are commonly used to assess transport behavior, such comparison is still confounded due to the challenge of separating dispersive and transient storage processes from the influence of the advective timescale that varies with discharge and reach length. To better resolve interpretation of these processes from field-based tracer observations, we conducted recurrent conservative solute tracer tests along a 1 km study reach during a storm discharge period and further discretized the study reach into six segments of similar length but different channel morphologies. The resulting suite of data, spanning an order of magnitude in advective timescales, enabled us to (1) characterize relationships between tracer response and discharge in individual segments and (2) determine how combining the segments into longer reaches influences interpretation of dispersion and transient storage from tracer tests. We found that the advective timescale was the primary control on the shape of the observed tracer response. Most segments responded similarly to discharge, implying that the influence of morphologic heterogeneity was muted relative to advection. Comparison of tracer data across combined segments demonstrated that increased advective timescales could be misinterpreted as a change in dispersion or transient storage. Taken together, our results stress the importance of characterizing the influence of changing advective timescales on solute tracer responses before such reach-scale observations can be used to infer solute transport at larger network scales.

Dynamic hyporheic and riparian flow path geometry through base flow recession in two headwater mountain stream corridors

The hydrologic connectivity between streams and their valley bottoms (stream corridor) is a critical determinant of their ecological function. Ecological functions are known to be spatially and temporally variable, but spatial dimensions of the problem are not easily quantified and thus they are usually overlooked. To estimate the spatial patterns of connectivity, and how connectivity varies with changes in discharge, we developed the hyporheic potential model. We used the model to interpret a series of solute tracer injections in two headwater mountain streams with contrasting valley bottom morphologies to estimate connectivity in the stream corridor. The distributions of flow path origination locations and the lengths of hyporheic flow paths appear to vary with base flow recession, even in cases where transport timescales are apparently unchanged. The modeled distribution of origination locations further allowed us to define a spatial analog to the temporal window of detection associated with solute tracer studies, and enables assessment of connectivity dynamics between streams and their corridors. Altogether, the reduced complexity hyporheic potential model provides an easy way to anticipate the spatial distribution and origination locations of hyporheic flow paths from a basic understanding of the valley bottom characteristics and solute transport timescales. Plain Language Summary The manuscript details a simple method to assess the spatial connectivity of streams and their riparian zones. While the timescales of exchange in the river corridor have been broadly studied, the complimentary spatial dimension (i.e., the geometry of exchange flowpaths) remains largely unknown. The major challenge in assessing the spatial dimensions of exchange is the limited information available in the subsurface. Here, we develop a reduced complexity model of valley bottom transport to overcome these information limitations. With this model, relatively simple field site characterization and solute tracer data are combined to assess the spatial distribution of downwelling along a headwater mountain stream. We validate the model with a numerical experiment, and demonstrate its application in two watersheds of contrasting geology, repeated through baseflow recession.

The influence of spatially variable stream hydraulics on reach scale transient storage modeling

Within the context of reach scale transient storage modeling, there is limited understanding of how best to establish reach segment lengths that represent the effects of spatially variable hydraulic and geomorphic channel properties. In this paper, we progress this understanding through the use of channel property distributions derived from high-resolution imagery that are fundamental for hydraulic routing. We vary the resolution of reach segments used in the model representation and investigate the minimum number necessary to capture spatially variable influences on downstream predictions of solute residence time probability density functions while sufficiently representing the observed channel property distributions. We also test if the corresponding statistical moments of the predictions provide comparable results and, therefore, a method for establishing appropriate reach segment lengths. We find that the predictions and the moment estimates begin to represent the majority of the variability at reach segment lengths coinciding with distances where observed channel properties are spatially correlated. With this approach, reach scales where the channel properties no longer significantly change predictions can be established, which provides a foundation for more focused transient storage modeling efforts.

Spatial considerations of stream hydraulics in reach scale temperature modeling

While a myriad of processes control water temperature, the most significant in streams without notable shading or groundwater inputs are surface heat fluxes at the air-water interface. These fluxes are particularly sensitive to parameters representing the water surface area to volume ratio. Channel geometry dictates this ratio; however, it is currently unclear how spatial variability in stream hydraulics influences temperature predictions or how the contribution of the boundary condition influences interpretation of processes most sensitive to this variability. To investigate these influences over long reach scales, we used high-resolution spatial observations collected over a 25 km reach within a Laplace-domain solution to a two-zone temperature transient storage model. We found that for the study reach and flow condition, changes in the surface area to volume ratio did not generally coincide with changes in stream temperature. Though, notable changes in cumulative mean residence time corresponded with changes in the temperature extremes throughout the study reach. The surface heat fluxes were clearly the most sensitive to spatially variable hydraulics that translated into high residence times once the contribution of the boundary condition decayed. Consistent with solute transport, reach segment lengths that reflect the spatial correlation in observations were necessary to capture the spatial influences of hydraulics on temperature predictions. This approach provides a fundamental step for determining whether spatial detail related to stream hydraulics is important to support accurate temperature predictions and how best to represent that detail.

Woody debris is related to reach-scale hotspots of lowland stream ecosystem respiration under baseflow conditions

Stream metabolism is a fundamental, integrative indicator of aquatic ecosystem functioning. However, it is not well understood how heterogeneity in physical channel form, particularly in relation to and caused by in‐stream woody debris, regulates stream metabolism in lowland streams. We combined conservative and reactive stream tracers to investigate relationships between patterns in stream channel morphology and hydrological transport (form) and metabolic processes as characterized by ecosystem respiration (function) in a forested lowland stream at baseflow. Stream reach‐scale ecosystem respiration was related to locations (“hotspots”) with a high abundance of woody debris. In contrast, nearly all other measured hydrological and geomorphic variables previously documented or hypothesized to influence stream metabolism did not significantly explain ecosystem respiration. Our results suggest the existence of key differences in physical controls on ecosystem respiration between lowland stream systems (this study) and smaller upland streams (most previous studies) under baseflow conditions. As such, these findings have implications for reactive transport models that predict biogeochemical transformation rates from hydraulic transport parameters, for upscaling frameworks that represent biological stream processes at larger network scales, and for the effective management and restoration of aquatic ecosystems.

Hydrologic controls on hyporheic exchange in a headwater mountain stream

The spatial and temporal scales of hyporheic exchange within the stream corridor are controlled by stream discharge and groundwater inflow interacting with streambed morphology. While decades of study have resulted in a clear understanding of how morphologic form controls hyporheic exchange at the feature scale, we lack comparable predictive power related to stream discharge and the spatial structure of groundwater inflows at the reach scale, where spatial heterogeneity in both geomorphic setting and hydrologic forcing are present. In this study, we simulated vertical hyporheic exchange along a 600 m mountain stream reach under high, medium, and low stream discharge while considering groundwater inflow as negligible, spatially uniform, or proportional to upslope accumulated area. Most changes to hyporheic flow path residence time or length in response to stream discharge were small (< 5%), suggesting that discharge is a secondary control relative to morphologically-driven hyporheic exchange. Groundwater inflow was a primary control and mostly caused decreases in hyporheic flow path residence time and length. This finding generally agrees with expectations from the literature; however, flow path response was not consistent across the study reach. Instead, we found that flow paths driven by large hydraulic gradients coinciding with large morphologic features were less sensitive to changes in groundwater inflow than those driven by hydraulic gradients similar to the valley gradient. Our results indicate that consideration of heterogeneous arrangement of morphologic features is necessary to differentiate between hyporheic flow paths that persist in time and those that are sensitive to changing hydrologic conditions.

Thresholds of lake and reservoir connectivity in river networks control nitrogen removal

Lakes, reservoirs, and other ponded waters are ubiquitous features of the aquatic landscape, yet their cumulative role in nitrogen removal in large river basins is often unclear. Here we use predictive modeling, together with comprehensive river water quality, land use, and hydrography datasets, to examine and explain the influences of more than 18,000 ponded waters on nitrogen removal through river networks of the Northeastern United States. Thresholds in pond density where ponded waters become important features to regional nitrogen removal are identified and shown to vary according to a ponded waters’ relative size, network position, and degree of connectivity to the river network, which suggests worldwide importance of these new metrics. Consideration of the interacting physical and biological factors, along with thresholds in connectivity, reveal where, why, and how much ponded waters function differently than streams in removing nitrogen, what regional water quality outcomes may result, and in what capacity management strategies could most effectively achieve desired nitrogen loading reduction.Lakes, reservoirs, and other ponded waters are common in large river basins yet their influence on nitrogen budgets is often indistinct. Here, the authors show how a ponded waters’ relative size, shape, and degree of connectivity to the river network control nitrogen removal.

How Hydrologic Connectivity Regulates Water Quality in River Corridors

U.S. Geological SurveyUnited States Geological Survey; National Science Foundation Hydrologic Sciences ProgramNational Science Foundation (NSF)NSF - Directorate for Geosciences (GEO); USGS National Water Quality Program; DOE Office of Biological and Environmental Research (BER) in the Subsurface Biogeochemistry Program (SBR) as part of SBRs Scientific Focus Area at the Pacific Northwest National Laboratory (PNNL)