Many hyporheic papers state that the hyporheic zone is a critical component of stream ecosystems, and many of these papers focus on the biogeochemical effects of the hyporheic zone on stream solute ...loads. However, efforts to show such relationships have proven elusive, prompting several questions: Are the effects of the hyporheic zone on stream ecosystems so highly variable in place and time (or among streams) that a consistent relationship should not be expected? Or, is the hyporheic zone less important in stream ecosystems than is commonly expected? These questions were examined using data from existing groundwater modelling studies of hyporheic exchange flow at five sites in a fifth‐order, mountainous stream network. The size of exchange flows, relative to stream discharge (QHEF:Q), was large only in very small streams at low discharge (area ≈ 100 ha; Q < 10 l/s). At higher flows (flow exceedance probability > 0·7) and in all larger streams, QHEF:Q was small. These data show that biogeochemical processes in the hyporheic zone of small streams can substantially influence the stream's solute load, but these processes become hydrologically constrained at high discharge or in larger streams and rivers. The hyporheic zone may influence stream ecosystems in many ways, however, not just through biogeochemical processes that alter stream solute loads. For example, the hyporheic zone represents a unique habitat for some organisms, with patterns and amounts of upwelling and downwelling water determining the underlying physiochemical environment of the hyporheic zone. Similarly, hyporheic exchange creates distinct patches of downwelling and upwelling. Upwelling environments are of special interest, because upwelling water has the potential to be thermally or chemically distinct from stream water. Consequently, micro‐environmental patches created by hyporheic exchange flows are likely to be important to biological and ecosystem processes, even if their impact on stream solute loads is small. Published in 2011 by John Wiley & Sons, Ltd.
Biogeochemical reactions associated with stream nitrogen cycling, such as nitrification and denitrification, can be strongly controlled by water and solute residence times in the hyporheic zone (HZ). ...We used a whole‐stream steady state 15N‐labeled nitrate (15NO3−) and conservative tracer (Cl−) addition to investigate the spatial and temporal physiochemical conditions controlling the denitrification dynamics in the HZ of an upland agricultural stream. We measured solute concentrations (15NO3−, 15N2 (g), as well as NO3−, NH3, DOC, DO, Cl−), and hydraulic transport parameters (head, flow rates, flow paths, and residence time distributions) of the reach and along HZ flow paths of an instrumented gravel bar. HZ exchange was observed across the entire gravel bar (i.e., in all wells) with flow path lengths up to 4.2 m and corresponding median residence times greater than 28.5 h. The HZ transitioned from a net nitrification environment at its head (short residence times) to a net denitrification environment at its tail (long residence times). NO3− increased at short residence times from 0.32 to 0.54 mg‐N L−1 until a threshold of 6.9 h and then consistently decreased from 0.54 to 0.03 mg‐N L−1. Along these same flow paths, declines were seen in DO (from 8.31 to 0.59 mg‐O2 L−1) and DOC (from 3.0 to 1.7 mg‐C L−1). The rates of the DO and DOC removal and net nitrification were greatest during short residence times, while the rate of denitrification was greatest at long residence times. 15NO3− tracing confirmed that a fraction of the NO3− removal was via denitrification as 15N2 was produced across the entire gravel bar HZ. Production of 15N2 across all observed flow paths and residence times indicated that denitrification microsites are present even where nitrification was the net outcome. These findings demonstrate that the HZ is an active nitrogen sink in this system and that the distinction between net nitrification and denitrification in the HZ is a function of residence time and exhibits threshold behavior. Consequently, incorporation of HZ exchange and water residence time characterizations will improve mechanistic predictions of nitrogen cycling in streams.
•Implementation of a mechanistic, dynamic model of network expansion and contraction.•Importance of geologic setting and hydrologic forcing change through a water year.•Network expansion is ...insensitive to hydrologic forcing under wet conditions.•Geologic setting matters most under low and moderate discharge conditions.•Prediction of channel network dynamics may inform management of river corridors.
Headwater stream networks expand and contract in response to changes in stream discharge. The changes in the extent of the stream network are also controlled by geologic or geomorphic setting – some reaches go dry even under relatively wet conditions, other reaches remain flowing under relatively dry conditions. While such patterns are well recognized, we currently lack tools to predict the extent of the stream network and the times and locations where the network is dry within large river networks. Here, we develop a perceptual model of the river corridor in a headwater mountainous catchment, translate this into a reduced-complexity mechanistic model, and implement the model to examine connectivity and network extent over an entire water year. Our model agreed reasonably well with our observations, showing that the extent and connectivity of the river network was most sensitive to hydrologic forcing under the lowest discharges (Qgauge < 1 L s−1), that at intermediate discharges (1 L s−1 < Qgauge < 10 L s−1) the extent of the network changed dramatically with changes in discharge, and that under wet conditions (Qgauge > 10 L s−1) the extent of the network was relatively insensitive to hydrologic forcing and was instead determined by the network topology. We do not expect that the specific thresholds observed in this study would be transferable to other catchments with different geology, topology, or hydrologic forcing. However, we expect that the general pattern should be robust: the dominant controls will shift from hydrologic forcing to geologic setting as discharge increases. Furthermore, our method is readily transferable as the model can be applied with minimal data requirements (a single stream gauge, a digital terrain model, and estimates of hydrogeologic properties) to estimate flow duration or connectivity along the river corridor in unstudied catchments. As the available information increases, the model could be better calibrated to match site-specific observations of network extent, locations of dry reaches, or solute break through curves as demonstrated in this study. Based on the low initial data requirements and ability to later tune the model to a specific site, we suggest example applications of this parsimonious model that may prove useful to both researchers and managers.
We used an in situ steady state 15N‐labeled nitrate (15NO3−) and acetate (AcO−) well‐to‐wells injection experiment to determine how the availability of labile dissolved organic carbon (DOC) as ...AcO−influences microbial denitrification in the hyporheic zone of an upland (third‐order) agricultural stream. The experimental wells receiving conservative (Cl− and Br) and reactive (15NO3−) solute tracers had hyporheic median residence times of 7.0 to 13.1 h, nominal flowpath lengths of 0.7 to 3.7 m, and hypoxic conditions (<1.5 mg O2 L−1). All receiving wells demonstrated 15N2 production during ambient conditions, indicating that the hyporheic zone was an environment with active denitrification. The subsequent addition of AcO− stimulated more denitrification as evidenced by significant δ15N2 increases by factors of 2.7 to 26.1 in receiving wells and significant decreases of NO3− and DO in the two wells most hydrologically connected to the injection. The rate of nitrate removal in the hyporheic zone increased from 218 kg ha−1 yr−1 to 521 kg ha−1 yr−1 under elevated AcO− conditions. In all receiving wells, increases of bromide and 15N2 occurred without concurrent increases in AcO−, indicating that 100% of AcO− was retained or lost in the hyporheic zone. These results support the hypothesis that denitrification in anaerobic portions of the hyporheic zone is limited by labile DOC supply.
Key Points
The hyporheic zone can retain and remove dissolved organic carbon and nitrate
Conservative and stable isotope (15N) tracers show coupled C and N dynamics
Dissolved organic carbon quality and quantity limit hyporheic denitrification
The fate of biologically available nitrogen (N) and carbon (C) in stream ecosystems is controlled by the coupling of physical transport and biogeochemical reaction kinetics. However, determining the ...relative role of physical and biogeochemical controls at different temporal and spatial scales is difficult. The hyporheic zone (HZ), where groundwater–stream water mix, can be an important location controlling N and C transformations because it creates strong gradients in both the physical and biogeochemical conditions that control redox biogeochemistry. We evaluated the coupling of physical transport and biogeochemical redox reactions by linking an advection, dispersion, and residence time model with a multiple Monod kinetics model simulating the concentrations of oxygen (O2), ammonium (NH4), nitrate (NO3), and dissolved organic carbon (DOC). We used global Monte Carlo sensitivity analyses with a nondimensional form of the model to examine coupled nitrification‐denitrification dynamics across many scales of transport and reaction conditions. Results demonstrated that the residence time of water in the HZ and the uptake rate of O2 from either respiration and/or nitrification determined whether the HZ was a source or a sink of NO3 to the stream. We further show that whether the HZ is a net NO3 source or net NO3 sink is determined by the ratio of the characteristic transport time to the characteristic reaction time of O2 (i.e., the Damköhler number, DaO2), where HZs with DaO2 < 1 will be net nitrification environments and HZs with DaO2 ≪ 1 will be net denitrification environments. Our coupling of the hydrologic and biogeochemical limitations of N transformations across different temporal and spatial scales within the HZ allows us to explain the widely contrasting results of previous investigations of HZ N dynamics which variously identify the HZ as either a net source or sink of NO3. Our model results suggest that only estimates of residence times and O2uptake rates are necessary to predict this nitrification‐denitrification threshold and, ultimately, whether a HZ will be either a net source or sink of NO3.
Key Points
Hyporheic (HZ) N is controlled by coupled transport and reaction kinetics
Ratio of HZ residence time to O2 reaction time controls N source‐sink dynamics
We present a process‐based scaling relationship for hyporheic N transformations
Catchment hydrologists have long puzzled over the question: How can catchments rapidly generate storm flows and pulses of solutes in response to storm events? Conceptual models viewing catchments as ...composed of discrete source areas generating flow at unique time scales and with unique chemical characteristics have been used to explain the observed changes in flow and water chemistry. Surprisingly, those conceptual models usually do not treat the stream channel as one of the potential source areas. Here, we propose the channel‐source hypothesis in which the stream itself should be considered as a potential source with the same rigour as other contributing areas. We pose this in the spirit of the scientific use of the word: a hypothesis is not a proven idea but rather a provisional supposition serving as the basis for further study. We suggest that the channel should be considered as a potential source for dissolved organic carbon (DOC). Channels store substantial amounts of organic matter, and stream ecologists have long studied stream carbon cycling. From those studies, we know that leaching and decomposition can generate DOC from particulate organic carbon (POC). Further, POC is stored in channel ‘dead‐zones’—regions of low flow velocity ‐ that can be activated as flow velocity increases, thus releasing accumulated DOC during storms. All catchments are different; there is no reason to assume that channel sources are always important, in every catchment, in every storm. Thus, the channel‐source hypothesis does not replace existing conceptual models. Instead, it adds another potential mechanism that may explain DOC dynamics observed in streams. The channel‐source hypothesis has substantial implications for catchment studies examining sources of DOC in stream water or using DOC as a tracer to determine the locations of, and proportional contributions of, different source areas for streamflow generation.
Organic matter from litter fall or autochthonous production is stored in dead zones within the wetted stream channel under low‐flow conditions. Leaching and microbial processes generate DOC within the organic matter. As water depth and flow velocity increase during the rising leg of the storm hydrograph, organic matter can be scoured out of dead zones, releasing the accumulated DOC into the active stream channel.
The relationship between catchment structure and runoff characteristics is poorly understood. In steep headwater catchments with shallow soils the accumulation of hillslope area (upslope accumulated ...area (UAA)) is a hypothesized first‐order control on the distribution of soil water and groundwater. Hillslope‐riparian water table connectivity represents the linkage between the dominant catchment landscape elements (hillslopes and riparian zones) and the channel network. Hydrologic connectivity between hillslope‐riparian‐stream (HRS) landscape elements is heterogeneous in space and often temporally transient. We sought to test the relationship between UAA and the existence and longevity of HRS shallow groundwater connectivity. We quantified water table connectivity based on 84 recording wells distributed across 24 HRS transects within the Tenderfoot Creek Experimental Forest (U.S. Forest Service), northern Rocky Mountains, Montana. Correlations were observed between the longevity of HRS water table connectivity and the size of each transect's UAA (r2 = 0.91). We applied this relationship to the entire stream network to quantify landscape‐scale connectivity through time and ascertain its relationship to catchment‐scale runoff dynamics. We found that the shape of the estimated annual landscape connectivity duration curve was highly related to the catchment flow duration curve (r2 = 0.95). This research suggests internal catchment landscape structure (topography and topology) as a first‐order control on runoff source area and whole catchment response characteristics.
Field studies of hyporheic exchange in mountain systems are often conducted using short study reaches and a limited number of observations. It is common practice to assume these study reaches ...represent hyporheic exchange at larger scales or different sites and to infer general relationships among potential causal mechanisms from the limited number of observations. However, these assumptions of representativeness are rarely tested. In this study, we develop numerical models from four segments of mountain streams in different geomorphologic settings and extract shorter reaches to test how representative exchange metrics are in shorter reaches compared to their reference segments. We also map the locations of the representative reaches to determine if a pattern exists based on location. Finally, we compare variance of these shorter within‐site reaches to 29 additional reaches across the same basin to understand the impacts of inferring causal mechanisms, for example, the expectation that wide and narrow valley bottoms will yield different hyporheic exchange patterns. Our results show that the location and length strategy of the study reach must be considered before assuming an exchange metric to be representative of anything other than the exact segment studied. Further, it is necessary to quantify within and between site variations before making causal inferences based on observable characteristics, such as valley width or stream morphology. Our findings have implications for future field practices and how those practices are translated into models.
Plain Language Summary
Hydrology models are used to generalize processes and make predictions about how water moves through the environment. For example, models are used to study how water moves in and out of streambeds because processes occurring in the streambed can significantly impact the water quality. These models assume that data and observations from short sections of streams can represent longer portions of a stream or similar streams in different places. However, short sections of the stream may not accurately represent the conditions of the whole site that is being modeled, meaning that the assumptions and ultimately the models are misrepresenting the process. To test this, we compared how water was exchanged between a long segment of a stream and its subsurface to exchanges occurring in a shorter section of the stream and its subsurface. We found that only some of the shorter sections of the stream we modeled accurately represented the longer segment of the stream from which it was taken. These findings are important for future field studies as they demonstrate how scientists need to consider several aspects of the stream before they choose a section of a stream as the basis of their assumption for a model.
Key Points
Reach length strategy and location both determine how representative a reach is compared to the larger stream segment in which it is located
Researchers must clarify, test, and quantify assumptions of within‐ and between‐reach variation
Assumed behavior cannot be inferred from visual inspection and iteration between field and modeling studies minimize uncertainties in hyporheic exchange
Surface topography can influence flow pathways and the location of runoff source areas and water transport in steep headwater catchments. However, the influence of topography on spatial patterns of ...residual soil moisture is less well understood. We measured soil volumetric water content (VWC) on 14 dates at 0–30 and 30–60 cm depth at 54 sites on a steep, 10 ha north‐facing forested slope in the west‐central Cascades Mountains of Oregon, USA. Spatial patterns in VWC were persistent over time, and contrary to expectations VWC at 30–60 cm depth was greater on divergent than convergent slopes, especially during wet periods (R2 = 0.27, p < 0.001). Vegetation characteristics were assessed for all VWC monitoring locations and soil properties were determined for 13 locations as local factors that affect spatial patterns in VWC. Mean VWC over all dates was negatively correlated to gravimetric rock content (R2 = 0.28, p = 0.03) and positively correlated to water storage at field capacity (R2 = 0.56, p < 0.01). The variability in rock content in quick‐draining soils influenced soil‐water retention, and by extension, created spatially heterogenous but temporally persistent patterns in VWC. While spatial patterns were persistent, they were not easily explained by surficial topography in a steep, mountainous landscape with rocky, well‐drained soils. Further research is needed to understand if combined soil‐terrain metrics would be a more useful proxy for VWC than terrain‐based wetness metrics alone.
Plain Language Summary
Surface topography can direct the movement of water through the subsurface in ways that affect water availability for plants. When soils saturate, excess water may move downslope, leading to more water in hillslope hollows. However, in places where soils do not become saturated, the factors controlling the spatial patterns in water availability are less well understood. We designed our study to test if the redistribution of water, following surface topography, controlled the spatial patterns of soil moisture across a steep forested hillslope with well‐drained soils. We found that spatial patterns of soil moisture were similar throughout the year, but the movement of water downslope was not responsible for these patterns because soils were drier in hillslope hollows and wetter on the intervening ribs. Additionally, we found that the amount of water in the soil was strongly related to rock content. Thus, the presence of rocks may contribute to the persistence in spatial patterns of soil moisture. We recommend future research focuses on understanding how spatial patterns in forest soil properties affect soil water storage. This will allow us to improve computer models that predict water stress in forest ecosystems, and thereby, help forest managers develop plans to mitigate water stress.
Key Points
Terrain‐based metrics were poor predictors of shallow soil water content
Soil properties were strong predictors of shallow soil water content
Spatial patterns in soil water content were persistent across seasonal variations in catchment wetness state
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