Variable nitrate concentration–discharge relationships in a forested watershed

The relationship between solute concentrations and discharge can inform an integrated understanding of hydrological and biogeochemical processes at watershed scales. Recent work from multiple catchments has shown that there is typically little variation in concentration relative to large variations in discharge. This pattern has been described as chemostatic behavior. Pond Branch, a forested headwater catchment in Maryland, has been monitored for stream nitrate (NO₃^?) concentrations at weekly intervals for 14 years. In the growing season and autumn of 2011 a high‐frequency optical NO₃^? sensor was used to supplement the long‐term weekly data. In this watershed, long‐term weekly data show that NO₃^? concentrations decrease with increasing discharge whereas 6 months of 15‐minute sensor observed concentrations reveal a more chemostatic behavior. High‐frequency NO₃^? concentrations from the sensor collected during different storm events reveal variable concentration–discharge patterns highlighting the importance of high resolution data and ecohydrological drivers in controlling solute export for biologically reactive solutes such as NO₃^?.     

Quantifying aggregation and change in runoff source in accordance with catchment area increase in a forested headwater catchment

There has been a great deal of research interest regarding changes in flow path/runoff source with increases in catchment area. However, there have been very few quantitative studies taking subscale variability and convergence of flow path/runoff source into account, especially in relation to headwater catchments. This study was performed to elucidate how the contributions and discharge rates of subsurface water (water in the soil layer) and groundwater (water in fractured bedrock) aggregate and change with catchment area increase, and to elucidate whether the spatial variability of the discharge rate of groundwater determines the spatial variability of stream discharge or groundwater contribution. The study area was a 5‐km² forested headwater catchment in Japan. We measured stream discharge at 113 points and water chemistry at 159 points under base flow conditions. End‐member mixing analysis was used to separate stream water into subsurface water and groundwater. The contributions of both subsurface water and groundwater had large variability below 1 km². The contribution of subsurface water decreased markedly, while that of groundwater increased markedly, with increases in catchment area. The specific discharge of subsurface water showed a large degree of variability and decreased with catchment area below 0.1 km², becoming almost constant above 0.1 km². The specific discharge of groundwater showed large variability below 1 km² and increased with catchment area. These results indicated that the variabilities of stream discharge and groundwater contribution corresponded well with the variability of the discharge rate of groundwater. However, below 0.1 km², it was necessary to consider variations in the discharge rates of both subsurface water and groundwater. Copyright © 2016 John Wiley & Sons, Ltd.     

A multicatchment analysis of headwater and downstream temperature effects from contemporary forest harvesting

Stream temperature is a key physical water‐quality parameter, controlling many biological, chemical, and physical processes in aquatic ecosystems. Maintenance of cool stream temperatures during summer is critical for high‐quality aquatic habitat. As such, transmission of warm water from small, nonfish‐bearing headwater streams after forest harvesting could cause warming in downstream fish‐bearing stream reaches with negative consequences. In this study, we evaluate (a) the effects of contemporary forest management practices on stream temperature in small, headwater streams, (b) the transmission of thermal signals from headwater reaches after harvesting to downstream fish‐bearing reaches, and (c) the relative role of lithology and forest management practices in influencing differential thermal responses in both the headwater and downstream reaches. We measured summer stream temperatures both preharvest and postharvest at 29 sites—12 upstream sites (4 reference, 8 harvested) and 17 downstream sites (5 reference, 12 harvested)—across 3 paired watershed studies in western Oregon. The 7‐day moving average of daily maximum stream temperature (T_7DAYMAX) was greater during the postharvest period relative to the preharvest period at 7 of the 8 harvested upstream sites. Although the T_7DAYMAX was generally warmer in the downstream direction at most of the stream reaches during both the preharvest and postharvest period, there was no evidence for additional downstream warming related to the harvesting activity. Rather, the T_7DAYMAX cooled rapidly as stream water flowed into forested reaches ~370–1,420 m downstream of harvested areas. Finally, the magnitude of effects of contemporary forest management practices on stream temperature increased with the proportion of catchment underlain by more resistant lithology at both the headwater and downstream sites, reducing the potential for the cooling influence of groundwater.     

Changes in stream flow regime in headwater catchments of the Yellow River basin since the 1950s

The headwater catchments of the Yellow River basin generate over 35% of the basin's total stream flow and play a vital role in meeting downstream water resources requirements. In recent years the Yellow River has experienced significant changes in its hydrological regime, including an increased number of zero‐flow days. These changes have serious implications for water security and basin management. We investigated changes in stream flow regime of four headwater catchments since the 1950s. The rank‐based non‐parametric Mann–Kendall test was used to detect trends in annual stream flow. The results showed no significant trend for the period 1956 to 2000. However, change‐point analysis showed that a significant change in annual stream flow occurred around 1990, and hence the stream‐flow data can be divided into two periods: 1956–1990 and 1991–2000. There was a considerable difference in average annual stream flow between the two periods, with a maximum reduction of 51%. Wet‐season rainfall appears to be the main factor responsible for the decreasing trend in annual stream flow. Reductions in annual stream flow were associated with decreased interannual variability in stream flow. Seasonal stream flow distribution changed from bimodal to unimodal between the two periods, with winter stream flow showing a greater reduction than other seasons. Daily stream flow regime represented by flow duration curves showed that all percentile flows were decreased in the second period. The high flow index (Q₅/Q₅₀) reduced by up to 28%, whereas the reduction in the low flow index (Q₉₅/Q₅₀) is more dramatic, with up to 100% reduction. Copyright © 2006 John Wiley & Sons, Ltd.     

Seasonal mixing from intermittent flow drives concentration-discharge behaviour in a stream affected by coal mine drainage

Abandoned mining operations continue to severely degrade many ecosystems worldwide by releasing acidic water and/or heavy metals into surface and groundwater. Contaminant concentrations in affected streams vary with discharge in patterns that reflect both geochemical reactions and variable mixing of contaminated and non-contaminated waters. However, controls on concentration-discharge (C-Q) patterns remain unclear, particularly for constituents that experience changing solubility across redox and pH gradients. Understanding the C-Q behaviour of contaminants aids in predicting both downstream transport and effects on aquatic life under variable flow. Here, we examined the C-Q behaviours of non-reactive (Na, K, Ca, Mg, Cl⁻) and reactive (Fe, Mn, Al, H⁺, SO₄²⁻) solutes in a stream contaminated with acid mine drainage in northeastern Ohio, USA. Concentration-discharge patterns at the watershed outlet primarily reflected mixing of contaminated baseflow with intermittent inputs of high pH water draining from a passive limestone treatment system into the stream. The treatment system acted as an ephemeral tributary that mitigated contamination in the stream by diluting solutes, raising pH, and driving metal precipitation, but only when flow was present during wet seasons. Consequently, AMD-derived reactive solutes (H⁺, Fe, Mn, Al) decreased with increasing stream discharge while relatively conservative solutes (e.g., Ca, Mg, K, Na) decreased only slightly or were chemostatic. This study highlights both the unique C-Q patterns of reactive solutes when compared to those of non-reactive solutes and the potential for intermittent streams to control C-Q behaviour in headwater catchments.     

Hydrochemical behaviour of dissolved nitrogen and carbon in a headwater stream of the Canadian Shield: relevance of antecedent soil moisture conditions

This paper examines the impact of contrasting antecedent soil moisture conditions on the hydrochemical response, here the changes in dissolved nitrogen (NO₃^?, NH₄⁺ and dissolved organic nitrogen (DON)) and dissolved organic carbon (DOC) concentrations, of a first‐order stream during hydrological events. The study was performed in the Hermine, a 5 ha forested watershed of the Canadian Shield. It focused on a series of eight precipitation events (spring, summer and fall) sampled every 2 or 3 h and showing contrasted antecedent moisture conditions. The partition of the eight events between two groups (dry or wet) of antecedent moisture conditions was conducted using a principal component analysis (PCA). The partition was controlled (first axis explained 86% of the variability) by the antecedent streamflow, the streamflow to precipitation ratio Q/P and by the antecedent groundwater depth. The mean H⁺, NO₃^?, NH₄⁺, total dissolved nitrogen and DOC concentrations and electrical conductivity values in the stream were significantly higher following dry antecedent conditions than after wetter conditions had prevailed in the Hermine, although the temporal variability was high (17 to 138%). At the event scale, a significantly higher proportion of the changes in DON, NO₃^?, and DOC concentrations in the stream was explained by temporal variations in discharge compared with the seasonal and annual scales. Two of the key hydrochemical features of the dry events were the synchronous changes in DOC and flow and the frequent negative relationships between discharge and NO₃^?. The DON concentrations were much less responsive than DOC to changes in discharge, whereas NH was not in phase with streamflow. During wet events, the synchronicity between streamflow and DON or NO₃^? was higher than during dry events and discharge and NO₃^? were generally positively linked. Based on these observations, the hydrological behaviour of the Hermine is conceptually compatible with a two‐component model of shallow (DON and DOC rich; variable NO₃^?) and deep (DON and DOC poor; variable NO₃^?) subsurface flow. The high NO₃^? and DOC levels measured at the early stages of dry events reflected the contribution from NO₃^?‐rich groundwaters. The contribution of rapid surface flow on water‐repellent soil materials located close to the stream channel is hypothesized to explain the DOC levels. An understanding of the complex interactions between antecedent soil moisture conditions, the presence of soil nutrients available for leaching and the dynamics of soil water flow paths during storms is essential to explain the fluxes of dissolved nitrogen and carbon in streams of forested watersheds. Copyright © 2007 John Wiley & Sons, Ltd.     

Effects of catastrophic flooding on stream biogeochemistry in a headwater stream in Shenandoah National Park,USA

This study examined the stream chemistry changes in Staunton River (a second‐order headwater stream with an average annual discharge 704 m³ ha^?1 yr^?1, Shenandoah National Park, Virginia) resulting from a catastrophic flood in June 1995. This flood, which followed after 800 mm of rain in a 4‐day period, caused large‐scale debris flows and complete scouring of riparian soils down to bedrock in the lower 2 km of the stream, and has been estimated to be a 1000‐year flood. The flood affected stream chemistry on both short‐ and long‐term time scales. The primary short‐term response was elevations in stream concentration of Ca²⁺, Mg²⁺, and K⁺ by 59%, 87%, and 49%, respectively, for 6 months immediately following the flood. The long‐term impact of decreased concentration of all base cations and SiO₂ during summer months (8% average) lasted about 2 years. At the episodic time scale, Ca²⁺, Mg²⁺, and K⁺ flushed from soil sources during pre‐flood storms while Na⁺ and SiO₂ diluted; these trends generally reversed during post‐flood storms for 2 years. Short‐term effects are attributed to the leaching of unconsolidated soil and upturned organic matter that clogged the streambed after the flood. The long‐term and superimposed episodic impacts may have resulted from the loss of riparian soils and vegetation in the flood. Copyright © 2008 John Wiley & Sons, Ltd.     

Suspended sediment dynamics in a steep,glacier‐fed mountain stream,Place Creek,Canada

This study examined suspended sediment concentration (SSC) during the ablation seasons of 2000 and 2001 in Place Creek, Canada, a steep, glacier‐fed mountain stream. Comparison of stream flow in Place Creek with that in an adjacent, almost unglacierized catchment provided a rational basis for separating the ablation seasons into nival, nival–glacial, glacial and autumn recession subseasons. Distinct groupings of points in plots of electrical conductivity against discharge supported the validity of the subseasonal divisions in terms of varying hydrological conditions. Relationships between SSC and discharge (Q) varied between the two study seasons, and between subseasons. Hysteresis in the SSC–Q relationship was evident at both event and weekly time‐scales. Some suspended sediment released from pro‐glacial Place Lake (the source of Place Creek) appeared to be lost to channel storage at low flows, especially early in the ablation season, with re‐entrainment at higher flows. Multiple regression models were derived for the subseasons using predictor variables including Q, Q², the change in Q over the previous 3 h, cumulative discharge over the ablation season, total precipitation over the previous 24 h and SSC measured at 1500 hours as an index value for each day. The models produced adjusted R² values ranging from 0·71 to 0·91, and provided tentative insights into the differences in SSC dynamics amongst subseasons. Introduction of the index value of SSC significantly improved the model fit during the nival–glacial and glacial subseasons for both years, as it adjusts the model to the current condition of sediment supply. Copyright © 2003 John Wiley & Sons, Ltd.     

Estimating stream solute loads from fixed frequency sampling regimes: the importance of considering multiple solutes and seasonal fluxes in the design of long‐term stream monitoring networks

Reduced sampling frequency is known to increase the error associated with estimates of stream solute load. However, the extent to which the magnitude of error differs among commonly measured solutes and across seasons is unclear. In this study, a high sampling frequency data set from two forested streams (one upland‐draining and one wetland‐draining stream) in south‐central Ontario was systematically sub‐sampled to simulate weekly, fortnightly and monthly fixed frequency sampling regimes for 12 stream solutes. We found that solutes which had a higher degree of temporal variation in concentration (i.e. higher %RSD) had poorer precision (C_v) in estimates of annual load relative to solutes with a lower %RSD. In addition, the magnitude and direction of bias varied considerably among solutes and were related to differences in spring concentration‐discharge relationships (m_{[spring Q vs C]}) among the 12 solutes. Solutes which decreased in concentration with increases in spring flow (i.e. m_{[spring Q vs C]} <0) exhibited positive bias in annual load while solutes which increased in concentration with increases in spring flow (i.e. m_{[spring Q vs C]} >0) were negatively biased. In terms of differences between seasonal and annual load errors, precision was generally lower for estimates of seasonal load relative to annual load while bias varied in both magnitude and direction among seasons. When the root mean square error (RMSE) of load estimates was compared to a threshold of acceptable error (<15%), the proportion of solutes attaining acceptable levels of uncertainty ranged from 11/12 for annual load estimates at a weekly sampling frequency to only 4/12 at a monthly frequency when both annual and seasonal loads were considered. Our results demonstrate that commonly measured solutes do not behave uniformly in response to changes in sampling frequency and that estimates of seasonal loads are often less accurate than estimates of annual load. These findings provide important insights into the design of stream monitoring programs and the evaluation of existing long‐term data sets. Copyright © 2015 John Wiley & Sons, Ltd.     

Carbon isotope fractionation of dissolved inorganic carbon (DIC) due to outgassing of carbon dioxide from a headwater stream

The stable isotopic composition of dissolved inorganic carbon (δ¹³C‐DIC) was investigated as a potential tracer of streamflow generation processes at the Sleepers River Research Watershed, Vermont, USA. Downstream sampling showed δ¹³C‐DIC increased between 3–5‰ from the stream source to the outlet weir approximately 0·5 km downstream, concomitant with increasing pH and decreasing PCO₂. An increase in δ¹³C‐DIC of 2·4 ± 0·1‰ per log unit decrease of excess PCO₂ (stream PCO₂ normalized to atmospheric PCO₂) was observed from downstream transect data collected during snowmelt. Isotopic fractionation of DIC due to CO₂ outgassing rather than exchange with atmospheric CO₂ may be the primary cause of increased δ¹³C‐DIC values downstream when PCO₂ of surface freshwater exceeds twice the atmospheric CO₂ concentration. Although CO₂ outgassing caused a general increase in stream δ¹³C‐DIC values, points of localized groundwater seepage into the stream were identified by decreases in δ¹³C‐DIC and increases in DIC concentration of the stream water superimposed upon the general downstream trend. In addition, comparison between snowmelt, early spring and summer seasons showed that DIC is flushed from shallow groundwater flowpaths during snowmelt and is replaced by a greater proportion of DIC derived from soil CO₂ during the early spring growing season. Thus, in spite of effects from CO₂ outgassing, δ¹³C of DIC can be a useful indicator of groundwater additions to headwater streams and a tracer of carbon dynamics in catchments. Copyright © 2007 John Wiley & Sons, Ltd.     

Insights on stream temperature processes through development of a coupled hydrologic and stream temperature model for forested coastal headwater catchments

Stream temperature controls a number of biological, chemical, and physical processes occurring in aquatic environments. Transient snow cover and advection associated with lateral throughflow inputs can have a dominant influence on stream thermal regimes for headwater catchments in the rain‐on‐snow zone. Most existing stream temperature models lack the ability to properly simulate these processes. We developed and evaluated a conceptual‐parametric catchment‐scale stream temperature model that includes the role of transient snow cover and lateral advection associated with throughflow. The model consists of routines for simulating canopy interception, snow accumulation and melt, hillslope throughflow runoff and temperature, and stream channel energy exchange processes. The model was used to predict discharge and stream temperature for a small forested headwater catchment near Vancouver, Canada, using long‐term (1963–2013) weather data to compute model forcing variables. The model was evaluated against 4 years of observed stream temperature. The model generally predicted daily mean stream temperature accurately (annual RMSE between 0.57 and 1.24 °C) although it overpredicted daily summer stream temperatures by up to 3 °C during extended low streamflow conditions. Model development and testing provided insights on the roles of advection associated with lateral throughflow, channel interception of snow, and surface–subsurface water interactions on stream thermal regimes. This study shows that a relatively simple but process‐based model can provide reasonable stream temperature predictions for forested headwater catchments located in the rain‐on‐snow zone.     

Variability of stream extents controlled by flow regime and network hydraulic scaling

Stream networks expand and contract through time, impacting chemical export, aquatic habitat, and water quality. Although recent advances improve prediction of the extent of the wetted channel network (L ) based on discharge at the catchment outlet (Q ), controls on the temporal variability of L remain poorly understood and unquantified. Here we develop a quantitative, conceptual framework to explore how flow regime and stream network hydraulic scaling factors co-determine the relative temporal variability in L (denoted here as the total wetted channel drainage density). Network hydraulic scaling determines how much L changes for a change in Q , while the flow regime describes how Q changes in time. We compiled datasets of co-located dynamic stream extent mapping and discharge to analyze all globally available empirical data using the presented framework. We found that although variability in L is universally damped relative to variability in Q (i.e., streamflow is relatively more variable in time than network extent), the relationship is elastic, meaning that for a given increase in the variability in Q , headwater catchments will experience greater-than-proportional increases in the variability of L . Thus, under anticipated climatic shifts towards more volatile precipitation, relative variability in headwater stream network extents can be expected to increase even more than the relative variability of discharge itself. Comparison between network extents inferred from the L -Q relationship and blue lines on USGS topographic maps shows widespread underestimation of the wetted channel network by the blue line network.     

The role of the hyporheic zone across stream networks

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 (Q_HEF: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, Q_HEF: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.     

Trend analyses with river sediment rating curves

Sediment rating curves, which are fitted relationships between river discharge (Q) and suspended‐sediment concentration (C), are commonly used to assess patterns and trends in river water quality. In many of these studies, it is assumed that rating curves have a power‐law form (i.e. C = aQ^b, where a and b are fitted parameters). Two fundamental questions about the utility of these techniques are assessed in this paper: (i) how well to the parameters, a and b, characterize trends in the data, and (ii) are trends in rating curves diagnostic of changes to river water or sediment discharge? As noted in previous research, the offset parameter, a, is not an independent variable for most rivers but rather strongly dependent on b and Q. Here, it is shown that a is a poor metric for trends in the vertical offset of a rating curve, and a new parameter, â, as determined by the discharge‐normalized power function [C = â (Q/Q_GM)^b], where Q_GM is the geometric mean of the Q‐values sampled, provides a better characterization of trends. However, these techniques must be applied carefully, because curvature in the relationship between log(Q) and log(C), which exists for many rivers, can produce false trends in â and b. Also, it is shown that trends in â and b are not uniquely diagnostic of river water or sediment supply conditions. For example, an increase in â can be caused by an increase in sediment supply, a decrease in water supply or a combination of these conditions. Large changes in water and sediment supplies can occur without any change in the parameters, â and b. Thus, trend analyses using sediment rating curves must include additional assessments of the time‐dependent rates and trends of river water, sediment concentrations and sediment discharge. Published 2014. This article is a U.S. Government work and is in the public domain in the USA. Hydrological Processes published by John Wiley & Sons Ltd.     

Catchment chemostasis revisited: Water quality responds differently to variations in weather and climate

Solute concentrations in streamflow typically vary systematically with stream discharge, and the resulting concentration–discharge relationships are important signatures of catchment biogeochemical processes. Solutes derived from mineral weathering often exhibit decreasing concentrations with increasing flows, suggesting dilution of a kinetically limited weathering flux by a variable flux of water. However, previous work showed that concentration–discharge relationships of weathering‐derived solutes in 59 headwater catchments were much weaker than this simple dilution model would predict. Instead, catchments behaved as chemostats, with rates of solute production and/or mobilization that were nearly proportional to water fluxes, on both event and interannual timescales. Here, we re‐examine these findings using data for a wider range of solutes from 2,186 catchments, ranging from ~10 to >1,000,000 km² in drainage area and spanning a wide range of lithologic and climatic settings. Concentration–discharge relationships among this much larger set of larger catchments are broadly consistent with the previously described chemostatic behaviour, at least on event and interannual timescales for weathering‐derived solutes. Among these same catchments, however, site‐to‐site variations in mean concentrations of weathering‐derived solutes exhibit strong negative correlations with long‐term average precipitation and discharge, reflecting strong climatic control on long‐term leaching of the critical zone. We use multiple regression of site characteristics including discharge to identify potential controls on long‐term mean concentrations and find that lithologic and land cover controls are significant predictors for many analytes. The picture that emerges is one in which, on event and interannual timescales, weathering‐derived stream solute concentrations are chemostatically buffered by groundwater storage and fast chemical reactions, but each catchment's chemostatic “set point” reflects site‐to‐site differences in climatically driven evolution of the critical zone. In contrast to these weathering products, some nutrients and particulates are often near‐chemostatic across all timescales, and their long‐term mean concentrations correlate more strongly with land use than climatic characteristics.     

Spatial patterns of soil nitrate in Japanese forested watersheds: importance of the near-stream zone as a source of nitrate in stream water

Spatial patterns of N dynamics in soil were evaluated within two small forested watersheds in Japan. These two watersheds were characterized by steep slopes (>30°) and high stream NO⁻₃ drainage rates (8·4 to 25·1 kg N ha⁻¹ yr⁻¹) that were greater than bulk precipitation N input rates (7·5 to 13·5 kg N ha⁻¹ yr⁻¹). Higher rates of nitrification potential at near-stream zones were reflected in greater NO⁻₃ contents for soil at the near-stream zones compared with ridge zones. Both stream discharge rates and NO⁻₃ concentrations in deep unsaturated soil at the near-stream zones were positively correlated to NO⁻₃ concentrations in stream water. These relationships, together with high soil NO⁻₃ contents at the near-stream zones, suggest that the near-stream zone was an important source of NO⁻₃ to stream water. Nitrate flux from these near-stream zones was also related to the drainage of cations (K⁺, Ca²⁺ and Mg²⁺). The steep slope of the watersheds resulted in small saturated areas that contributed to the high NO⁻₃ production (high nitrification rates) in the near-stream zone. © 1998 John Wiley & Sons, Ltd.     

A rational sediment transport scaling relation based on dimensionless stream power

The concept of stream channel grade – according to which a stream channel reach will adjust its gradient, S, in order to transport the imposed sediment load having magnitude Q_b and characteristic grain size D_b, with the available discharge Q (Mackin, 1948 , Geological Society of America Bulletin 59 : 463–512; Lane, 1955 , American Society of Civil Engineers, Proceedings 81 : 1–17) is one of the most influential ideas in fluvial geomorphology. Herein, we derive a scaling relation that describes how externally imposed changes in either Q_b or Q can be accommodated by changes in the channel configuration, described by the energy gradient, mean flow depth, characteristic grain size and a parameter describing the effect of bed surface structures on grain entrainment. One version of this scaling relation is based on the dimensionless bed material transport parameter (W*) presented by Parker and Klingeman ( 1982 , Water Resources Research 18 : 1409–1423). An equivalent version is based on a new dimensionless transport parameter (E*) using dimensionless unit stream power. This version is nearly identical to the relation based on W*, except that it is independent of flow resistance. Both versions of the scaling relation are directly comparable to Lane's original relation. In order to generate this stream power‐based scaling relation, we derived an empirical transport function relation relating E* to dimensionless stream power using data from a wide range of stable, bed load‐dominated channels: the form of that transport function is based on the understanding that, while grain entrainment is related to the forces acting on the bed (described by dimensionless shear stress), sediment transport rate is related to the transfer of momentum from the fluid to the bed material (described by dimensionless stream power). Copyright © 2010 John Wiley & Sons, Ltd.     

Acidification recovery in a changing climate: Observations from thirty-five years of stream chemistry monitoring in forested headwater catchments at the Turkey Lakes watershed,Ontario

Long-term ecosystem studies are valuable for understanding integrated ecosystem response to global changes in atmospheric deposition and climate. We examined trends for a 35-year period (1982/83–2017/18) in concentrations of a range of solutes in precipitation and stream water from nine headwater catchments spanning elevation and surficial geology gradients at the Turkey Lakes watershed (TLW) in northeastern Ontario, Canada. Average annual water year (WY, October to September) concentrations in precipitation significantly declined over the period for sulphate (SO₄²⁻), nitrate (NO₃⁻) and chloride (Cl⁻), while calcium (Ca²⁺) and potassium (K⁺) concentrations increased, resulting in a significant pH increase from 4.2 to 5.7. Trends in stream chemistry through time are generally consistent with expectations associated with acidification recovery. Concentration of many stream water solutes (SO₄²⁻, Cl⁻, calcium [Ca²⁺], magnesium [Mg²⁺] and NH₄⁺ generally decreased, while others (silica [SiO₂] and dissolved organic carbon [DOC]) generally increased. Increases were also observed for alkalinity (six of nine catchments), acid neutralizing capacity ([ANC]; six of nine catchments) and pH (eight of nine catchments), while conductivity declined (six of nine catchments). Variability in trends among catchments are associated with differences in surficial geology and wetland cover. While absolute solute concentrations were generally lower at bedrock dominated high-elevation catchments compared to till dominated lower elevation catchments, the rate of change of concentration was often greater for high elevation catchments. This study confirms continued, but non-linear stream chemistry recovery from acidification, particularly at the less buffered high and moderate elevation sites. The heterogeneity of responses among catchments highlights our incomplete understanding of the relative importance of different mechanisms influencing stream chemistry and the consequences for downstream ecosystems.     

The role of wetland coverage within the near‐stream zone in predicting of seasonal stream export chemistry from forested headwater catchments

Stream chemistry is often used to infer catchment‐scale biogeochemical processes. However, biogeochemical cycling in the near‐stream zone or hydrologically connected areas may exert a stronger influence on stream chemistry compared with cycling processes occurring in more distal parts of the catchment, particularly in dry seasons and in dry years. In this study, we tested the hypotheses that near‐stream wetland proportion is a better predictor of seasonal (winter, spring, summer, and fall) stream chemistry compared with whole‐catchment averages and that these relationships are stronger in dryer periods with lower hydrologic connectivity. We evaluated relationships between catchment wetland proportion and 16‐year average seasonal flow‐weighted concentrations of both biogeochemically active nutrients, dissolved organic carbon (DOC), nitrate (NO₃‐N), total phosphorus (TP), as well as weathering products, calcium (Ca), magnesium (Mg), at ten headwater (<200 ha) forested catchments in south‐central Ontario, Canada. Wetland proportion across the entire catchment was the best predictor of DOC and TP in all seasons and years, whereas predictions of NO₃‐N concentrations improved when only the proportion of wetland within the near‐stream zone was considered. This was particularly the case during dry years and dry seasons such as summer. In contrast, Ca and Mg showed no relationship with catchment wetland proportion at any scale or in any season. In forested headwater catchments, variable hydrologic connectivity of source areas to streams alters the role of the near‐stream zone environment, particularly during dry periods. The results also suggest that extent of riparian zone control may vary under changing patterns of hydrological connectivity. Predictions of biogeochemically active nutrients, particularly NO₃‐N, can be improved by including near‐stream zone catchment morphology in landscape models.     

The effect of riparian land use on transport hydraulics in agricultural headwater streams located in northeast Ohio,USA

This study examined if riparian land use (forested vs agricultural) affects hydraulic transport in headwater streams located in an agriculturally fragmented watershed. We identified paired 50‐m reaches (one reach in agricultural land use and the other in forested land use) along three headwater streams in the Upper Sugar Creek Watershed in northeast Ohio, USA (40° 51′42″N, 81° 50′29″W). Using breakthrough curves obtained by Rhodamine WT slug injections and the one‐dimensional transport with inflow and storage model (OTIS), hydraulic transport parameters were obtained for each reach on six different occasions (n = 36). Relative transient storage (A_S:A) was similar between both reach types (As: A = 0·3 ± 0·1 for both agricultural and forested reaches). Comparing values of F_med²⁰⁰ to those in the literature indicates that the effect of transient storage was moderately high in the study streams in the Upper Sugar Creek Watershed. Examining travel times revealed that overall residence time (HRT) and residence time in transient storage (T_STO) were both longer in forested reaches (forested HRT = 19·1 ± 11·5 min and T_STO = 4·0 ± 3·8 min; agricultural HRT = 9·3 ± 5·3 min and T_STO = 1·7 ± 1·4 min). We concluded that the effect of transient storage on solute transport was similar between the forested and agricultural reaches but the forested reaches had a greater potential to retain solutes as a result of longer travel times. Copyright © 2009 John Wiley & Sons, Ltd.