Stream Discharge Measurements Research Articles

A drained nutrient-poor peatland forest in boreal Sweden constitutes a net carbon sink after integrating terrestrial and aquatic fluxes.

Northern peatlands provide a globally important carbon (C) store. Since the beginning of the 20th century, however, large areas of natural peatlands have been drained for biomass production across Fennoscandia. Today, drained peatland forests constitute a common feature of the managed boreal landscape, yet their ecosystem C balance and associated climate impact are not well understood, particularly within the nutrient-poor boreal region. In this study, we estimated the net ecosystem carbon balance (NECB) from a nutrient-poor drained peatland forest and an adjacent natural mire in northern Sweden by integrating terrestrial carbon dioxide (CO2 ) and methane (CH4 ) fluxes with aquatic losses of dissolved organic C (DOC) and inorganic C based on eddy covariance and stream discharge measurements, respectively, over two hydrological years. Since the forest included a dense spruce-birch area and a sparse pine area, we were able to further evaluate the effect of contrasting forest structure on the NECB and component fluxes. We found that the drained peatland forest was a net C sink with a 2-year mean NECB of -115 ± 5 g C m-2 year-1 while the adjacent mire was close to C neutral with 14.6 ± 1.7 g C m-2 year-1 . The NECB of the drained peatland forest was dominated by the net CO2 exchange (net ecosystem exchange [NEE]), whereas NEE and DOC export fluxes contributed equally to the mire NECB. We further found that the C sink strength in the sparse pine forest area (-153 ± 8 g C m-2 year-1 ) was about 1.5 times as high as in the dense spruce-birch forest area (-95 ± 8 g C m-2 year-1 ) due to enhanced C uptake by ground vegetation and lower DOC export. Our study suggests that historically drained peatland forests in nutrient-poor boreal regions may provide a significant net ecosystem C sink and associated climate benefits.

Rating curve development and uncertainty analysis in mountainous watersheds for informed hydrology and resource management

Accurate measurement of continuous stream discharge poses both excitement and challenges for hydrologists and water resource planners, particularly in mountainous watersheds. This study centers on the development of rating curves utilizing the power law at three headwaters of the lesser Himalayas—Aglar, Paligaad, and Balganga—through the installation of water level recorders for stage measurement and salt dilution for discharge measurement from 2014 to 2016. The stream stage–discharge relationship, crucially known as the rating curve, is susceptible to numerous factors in mountainous watersheds that are often challenging to comprehend or quantify. Despite significant errors introduced during the rating curve development, such as stemming from observations, modeling, and parameterization, they are frequently overlooked. In this study, acknowledging the inherent uncertainty, we employ the maximum-likelihood method to assess uncertainty in the developed rating curve. Our findings reveal substantial inconsistency in the stage–discharge relationship, particularly during high flows. A novel contribution of this study is introducing a weighing factor concept that correlates uncertainty with the morphological parameters of the watershed. The higher value of the weighting factor in Paligaad (0.37) as compared to Balganga (0.35) and less in the case of Aglar (0.27) will have more uncertainty. The authors contend that precise rating curves and comprehensive uncertainty analyses can mitigate construction costs, foster robust decision-making, and enhance the perceived credibility of decisions in hydrology and water resource management.

The Potential of Isotopic Tracers for Precise and Environmentally Clean Stream Discharge Measurements

Accurate discharge measurement is mandatory for any hydrological study. While the “velocity” measurement method is adapted to laminar flows, the “dilution” method is more appropriate for turbulent streams. As most low-gradient streams worldwide are neither laminar nor turbulent, a methodological gap appears. In this study, we demonstrate that the application of the “dilution” method to a low-gradient small stream gives very satisfactory results in addition to revealing surface/subsurface processes. A variety of chemical and isotopic tracers were injected into the stream (anions, fluorescent dyes, and chloride and hydrogen isotopes). We report the first use of 37Cl for stream discharge measurement and show that 37Cl and 2H can be reliably used as quantitative tracers. Discharge uncertainty calculations show that deuterium is the most accurate tracer method used. We also compare the differences in the tailing part of the restitution curves of tracers and investigate the role of transient surface and hyporheic zones in solute transport in light of a simple transport modelling approach. We conclude that isotopic tracers can be used as “environmentally friendly” tracers for discrete stream discharge measurements and that the application of multi-tracers tests in rivers opens the path to a better understanding of surface–subsurface interaction processes.

Spatial variation in drainage area — Runoff relationships and implications for bankfull geometry scaling

Fluvial geomorphic analyses frequently require knowledge of bankfull channel geometries, which are thought to be related to characteristic stream discharges. However, relating bankfull geometry to characteristic discharge is challenged by spatially limited stream discharge measurements, which may also lack extensive temporal records. Because of these limitations, discharge is commonly assumed to scale linearly with watershed drainage area. Here we evaluate the assumption of a linear relationship between discharge and drainage area for watersheds across the United States and Canada with limited anthropogenic disturbance. Using machine-learning to objectively cluster hydrologically similar gauges, we find that discharge scales linearly with drainage area for most of North America. However, regions with low average runoff efficiency tend to have non-linear discharge scaling. In regions with non-linear discharge scaling, bankfull channel dimensions increase more rapidly with drainage area than in regions with linear discharge scaling. These results suggest that the recurrence interval of the characteristic discharge that sets channel geometry may be larger in regions where discharge scales non-linearly with drainage area compared to those regions where linear discharge scaling applies.

Hurricane disturbance drives trophic changes in neotropical mountain stream food webs.

Food webs are complex ecological networks that reveal species interactions and energy flow in ecosystems. Prevailing ecological knowledge on forested streams suggests that their food webs are based on allochthonous carbon, driven by a constant supply of organic matter from adjacent vegetation and limited primary production due to low light conditions. Extreme climatic disturbances can disrupt these natural ecosystem dynamics by altering resource availability, which leads to changes in food web structure and functioning. Here, we quantify the response of stream food webs to two major hurricanes (Irma and María, Category 5 and 4, respectively) that struck Puerto Rico in September 2017. Within two tropical forested streams (first and second order), we collected ecosystem and food web data 6 months prior to the hurricanes and 2, 9, and 18 months afterward. We assessed the structural (e.g., canopy) and hydrological (e.g., discharge) characteristics of the ecosystem and monitored changes in basal resources (i.e., algae, biofilm, and leaf litter), consumers (e.g., aquatic invertebrates, riparian consumers), and applied Layman's community-wide metrics using the isotopic composition of 13 C and 15 N. Continuous stream discharge measurements indicated that the hurricanes did not cause an extreme hydrological event. However, the sixfold increase in canopy openness and associated changes in litter input appeared to trigger an increase in primary production. These food webs were primarily based on terrestrially derived carbon before the hurricanes, but most taxa (including Atya and Xiphocaris shrimp, the consumers with highest biomass) shifted their food source to autochthonous carbon within 2 months of the hurricanes. We also found evidence that the hurricanes dramatically altered the structure of the food web, resulting in shorter (i.e., smaller food-chain length), narrower (i.e., lower diversity of carbon sources) food webs, as well as increased trophic species packing. This study demonstrates how hurricane disturbance can alter stream food webs, changing the trophic base from allochthonous to autochthonous resources via changes in the physical environment (i.e., canopy defoliation). As hurricanes become more frequent and severe due to climate change, our findings greatly contribute to our understanding of the mechanisms that maintain forested stream trophic interactions amidst global change.

A random forest approach to improve estimates of tributary nutrient loading

Estimating constituent loads from discrete water quality samples coupled with stream discharge measurements is critical for management of freshwater resources. Nutrient loads calculated based on discharge-concentration relationships form the basis of government nutrient load targets and scientific studies of the response of receiving waters to external loads. In this study, a new model is developed using random forests and applied to estimate concentrations and loads of total phosphorus, dissolved phosphorus, total nitrogen, and chloride, using data from 17 tributaries to Lake Champlain monitored from 1992 to 2021. I benchmark this model against one of the most widespread models currently used to estimate nutrient loads, Weighted Regressions on Time, Discharge, and Season (WRTDS). The random forest model outperformed both the base WRTDS model and an extension of the WRTDS model using Kalman filtering in the great majority of cases, likely due to the inclusion of rate-of-change in discharge and antecedent discharge over different leading windows as predictors, and to the flexibility of the random forest to model predictor-response relationships. The random forest also had useful visualization capabilities which provided important process insights. WRTDS remains a useful model for many applications, but this study represents a promising new approach for load estimation which can be applied easily to existing datasets, and which is easy to customize for different applications.

Snížení základního odtoku kvůli evapotranspiraci povrchové a podzemní vody z mokřadu v povodí Liběchovky: porovnání měření a výpočtu Oudinovou metodou

This study is focused on the base flow decrease due to surface water and groundwater evapotranspiration (ET) in wetlands of the Liběchovka catchment. Evapotranspiration in wetlands can significantly affect stream discharge, and its influence will probably still increase in the future due to the global rise of temperatures caused by climate change. The study site is located in the upper part of Liběchovka catchment (Fig. 1). This wetland hosts a small stream supplied by rather steady groundwater inflow, representing the favourable site for detection of the effect of ET. V notch weir and two piezometers were used to monitor stream discharge and wetland water table in 30 minute periods (Fig. 2). Diurnal periodical oscillations of water flow and water table caused by ET were observed (Fig. 3). They occurred in the summer part of the year, especially on sunny days without rain. The amplitude of water table level oscillation was increasing with increasing temperature (Fig. 4). Calculation showed that evapotranspiration reduced the water flow, on average, by 15 % on the warm sunny days. The maximum daily reduction was up to 32 %. Direct measurements were compared with the potential ET calculated using the Oudin’s method (Oudin 2005). The ET from the measurement was 45 % of the potential ET calculated by Oudin’s method. In the next step, the Oudin’s method was used to calculate the potential ET of all wetlands in Liběchovka catchment upstream from the Želízy village, where gauging station of ČHMÚ is situated. The original discharge not reduced by the wetland ET was defined as the sum of the wetland potential ET and the mean annual measured discharge. Due to the ET from wetlands (years 2015–2020), the discharge was reduced on average by 13 %. If only the summer months of July and August were considered, it was reduced, on average, by 26 % and by 39 % in the driest day recorded in this period. To estimate the role of groundwater abstraction on the Liběchovka stream during the period of 2015–2020, the sum of measured stream discharge, groundwater abstraction and wetland ET was calculated. From this sum, 71.5 % represented the directly measured water flow, 18.2 % was the groundwater abstraction, and 10.3 % was the wetland ET (Fig. 8).

Resolving streamflow diel fluctuations in a small agricultural catchment with an integrated surface‐subsurface hydrological model

AbstractDuring dry periods, stream discharge in vegetated catchments can naturally fluctuate up to 10% daily. Despite intensive efforts put in observing and interpreting diel fluctuations of stream discharge across a range of instrumented natural catchments worldwide, the capability of state‐of‐the‐art hydrological models to reproduce and explain such processes has rarely been tested. Here, we used CATHY, a physics‐based integrated surface‐subsurface hydrological model (ISSHM), to simulate the stream discharge in a small tile‐drained agricultural catchment in Switzerland, where streamflow diel fluctuations appeared in dry periods. The model was able to satisfactorily reproduce the measured stream discharge, including the observed diel fluctuations. Next, we designed and simulated a series of modelling scenarios aimed to disentangle the processes contributing to the diel fluctuations in stream discharge. These scenarios revealed the predominant role of evapotranspiration (ET) in the establishment of diel fluctuations in stream discharge. Irrigation sustained the baseflow in dry periods and caused short‐living streamflow peaks, which did not directly cause, but superposed to, diel patterns. Vegetation rooting depths and response to oxygen stress in the root zone slightly affected the amplitude of the streamflow diel fluctuations, while changes in the saturated hydraulic conductivity driven by diel soil temperature fluctuations caused an amplitude of the streamflow diel signal 10 times smaller compared to the one caused by ET. Our study demonstrates that ISSHMs such as CATHY can provide high‐fidelity spatially‐distributed dynamic simulations of evapotranspiration and irrigation fluxes, as well as soil moisture and groundwater flows, enhancing our understanding of the role of these hydrological processes during dry periods and thus making these models useful tools for a more sustainable management of agricultural catchments.

Effect of weir´s theoretical discharge coefficient on discharge measurements in small Andean streams

Andean ecosystems provide important hydrological services for downstream communities. Due to this importance, several hydrological studies have been carried out in recent years, with emphasis on hydrological processes identification and land use change impacts. In several studies, but also for the operation of small-scale irrigation and drinking water projects, small streams have been equipped with compound, sharp-crested weirs for discharge estimation. To transform the water level (stage) into a discharge (water rate), weir equations use theoretical discharge coefficients, which do not necessarily apply under the actual field conditions, mainly site fluviomorphology and weir construction aspects, introducing uncertainty in their measurements. Therefore, this study analyzes the effect of using theoretical coefficients instead of adjusted coefficients in field. The study was conducted on 9 micro-catchments (0.2 – 7.53 km2) located in the Zhurucay Ecohydrological Observatory in the páramo of southern Ecuador. To calibrate the coefficients, discharge curves were generated by mechanical and salt-dilution gauging methods. Results revealed that the discharge coefficients differed from their theoretical value by up to 15% for triangular (V-notch) weir section (DCvn) and by up to 41% for rectangular weir section (DCr). The DCvn affects 4 times more in low and medium discharges estimation than DCvn in high discharges. On the other hand, salt-dilution method is more precise for medium and high discharges, but at very low discharges, it overestimates discharge up to 10%. Overall, results suggest that it is essential to calibrate the discharge coefficients in the field to avoid errors in hydrological studies.

Enhancing urban and sub-urban riparian areas through ecosystem services and ecotourism activities

Abstract Riparian ecosystems in urban environments are usually degraded. Their sustainable management can reduce water pollution, mitigate climate change while providing recreation opportunities. This study assessed the current condition of the streams/torrents and their riparian areas in the city of Drama in Greece. In addition, practices to improve their functionality and enhance community awareness were recommended. A spatial database of the wider urban setting including suburbs and rural areas (weather, land cover, soils, roads, protected areas, riparian areas, etc.) was developed within GIS. The datasets were then inputted in a hydrologic model to simulate the water balance and stream discharge in the main urban streams and torrents. Field measurements of stream water discharge, and vegetation and stream channel conditions were undertaken using two visual protocols. The results of the protocols indicate that most streams/torrents and their riparian areas are of low ecological quality. Nature-based solutions were recommended to improve their ecological quality. In addition, the implementation of Eco-Routes was recommended to enhance the awareness of their importance and to promote their sustainable management. Overall, the urban streams/torrents and riparian ecosystems of Drama are degraded and innovative management practices should be implemented to conserve them and feature their ecosystem services.

Groundwater contributions to surface water in the Assiniboine Delta Aquifer (ADA): A water quantity and quality perspective

In the Lake Winnipeg Basin (LWB), at both basin and regional scales, there are currently gaps regarding the significance of groundwater as a mediator of nutrient and contaminant fluxes to Lake Winnipeg. During 2018, surface water and groundwater samples were collected from various locations across the largest sand and gravel aquifer (Assiniboine Delta Aquifer [ADA]; 3800 km2) in the LWB as well as from surface water courses flowing above this aquifer. The samples collected during spring and fall were analyzed for anions and cations as well as for a series of isotopic and geochemical tracers (e.g. water isotopes, carbon 14, artificial sweeteners, pesticides, etc.). The results reveal that groundwater and the small watercourses flowing above the ADA have a similar chemical composition, which is in contrast with the chemical composition of the main watercourse flowing above the aquifer (Assiniboine River [AR]). When corroborated with stream discharge measurements this indicates that groundwater plays a significant role at local scale in controlling both the flow and the chemical composition of the AR tributaries. Nitrate showed low to non-detectable concentrations in both groundwater and surface water. With respect to groundwater, this could be related to the reducing conditions in the deeper aquifer in conjunction with relatively large groundwater travel times of up to 2400 years.

Nitrate in Streams During Winter Low‐Flow Conditions as an Indicator of Legacy Nitrate

AbstractWinter low‐flow (LF) conditions in streams provide a potential opportunity to evaluate the importance of legacy nitrate in catchments due to the dominance of slow‐flow transport pathways and lowered biotic activity. In this study, the concentration, flux, and trend of nitrate in streams during winter low‐flow conditions were analyzed at 320 sites in the conterminous United States. LF flow‐normalized nitrate concentrations varied from <0.1 to >20 mg‐N L−1 and LF conditions contributed between 2% and 98% of the winter nitrate flux. LF nitrate concentrations generally exceeded 2.5 mg‐N L−1 in the upper Midwest, with smaller regions of high LF nitrate concentrations in eastern Texas and along the northern mid‐Atlantic coast. Groundwater was inferred to be the primary or sole contributor of nitrate to streams during winter LF conditions at 140 of our 320 sites. Among these 140 sites, nitrate from groundwater comprised 45% or more of the winter nitrate flux at a quarter of the sites. Among the same 140 sites, concentrations of nitrate in streams during winter LF conditions generally increased between 2002 and 2012 at sites where 40% or more of the winter flux was from groundwater, suggesting that concentrations of nitrate in the contributing groundwater system were increasing. Using metrics developed herein, we characterize the potential importance of legacy nitrate at sites in this study and discuss methods to characterize sites with fewer samples than required by our models or at sites without continuous stream discharge measurements.

Hydrologic Response of Sierra Nevada Mixed-Conifer Headwater Catchments to Vegetation Treatments and Wildfire in a Warming Climate

We used a hydro-ecologic model (RHESSys) constrained by measurements of stream discharge, and spatially distributed snow and soil moisture, to simulate the impacts of operational forest treatments, historical wildfire and climate warming on productive mixed-conifer forests. We compared the response of two headwater catchments at the rain-snow-transition elevation in the wetter central Sierra and more water-limited southern Sierra. The variability of precipitation exerted a greater influence on annual evapotranspiration and runoff than vegetation changes from operational fuels treatment or historical wildfire. The short-term impacts of vegetation changes associated with wildfire, however, did have a greater effect on evapotranspiration and runoff than temperature increases in a warming climate. The average central-Sierra headwater response of evapotranspiration and runoff to fuels treatments (-12%, +12%, respectively) and wildfire (-43%, +46%) were greater than the projected responses to a 4.5oC temperature increase (+2% and -7%). The response in the southern Sierra was limited by lower annual precipitation and showed no response to fuels treatments; but the catchment showed respective changes of -11% and +17% in evapotranspiration and runoff for wildfire, versus +9% and -3% to a 4.5oC temperature increase. These results suggest that in the central Sierra, reductions in vegetation from either fuels treatments or historical wildfire can, temporarily, offset reductions in streamflow from a warming climate. In the southern Sierra, impacts of fuels treatments were small, and only more-extensive vegetation removal as would occur with wildfire, results in significant changes in hydrologic fluxes. Further research is needed to investigate how initial hydrologic changes and climate effects evolve as vegetation adapts and regrows following disturbance.

Dissolved Inorganic Geogenic Phosphorus Load to a Groundwater-Fed Lake: Implications of Terrestrial Phosphorus Cycling by Groundwater

The general perception has long been that lake eutrophication is driven by anthropogenic sources of phosphorus (P) and that P is immobile in the subsurface and in aquifers. Combined investigation of the current water and P budgets of a 70 ha lake (Nørresø, Fyn, Denmark) in a clayey till-dominated landscape and of the lake’s Holocene trophic history demonstrates a potential significance of geogenic (natural) groundwater-borne P. Nørresø receives water from nine streams, a groundwater-fed spring located on a small island, and precipitation. The lake loses water by evaporation and via a single outlet. Monthly measurements of stream, spring, and outlet discharge, and of tracers in the form of temperature, δ18O and δ2H of water, and water chemistry were conducted. The tracers indicated that the lake receives groundwater from an underlying regional confined glaciofluvial sand aquifer via the spring and one of the streams. In addition, the lake receives a direct groundwater input (estimated as the water balance residual) via the lake bed, as supported by the artesian conditions of underlying strata observed in piezometers installed along the lake shore and in wells tapping the regional confined aquifer. The groundwater in the regional confined aquifer was anoxic, ferrous, and contained 4–5 µmol/L dissolved inorganic orthophosphate (DIP). Altogether, the data indicated that groundwater contributes from 64% of the water-borne external DIP loading to the lake, and up to 90% if the DIP concentration of the spring, as representative for the average DIP of the regional confined aquifer, is assigned to the estimated groundwater input. In support, paleolimnological data retrieved from sediment cores indicated that Nørresø was never P-poor, even before the introduction of agriculture at 6000 years before present. Accordingly, groundwater-borne geogenic phosphorus can have an important influence on the trophic state of recipient surface water ecosystems, and groundwater-borne P can be a potentially important component of the terrestrial P cycle.

The Effects of Urbanization and Retention‐Based Stormwater Management on Coastal Plain Stream Nutrient Export

AbstractStormwater nutrient pollution can be more effectively managed if there is a predictable link between urbanization and pollutant export. The goal of this study was to determine the effects of increased watershed impervious surface cover (ISC) and retention‐based stormwater management on stream discharge and nutrient export from coastal plain streams in the southeastern United States. To quantify coastal plain stream nutrient export, measurements of stream discharge and concentrations of dissolved nutrients, particulate nitrogen, and algal biomass (as chlorophyll a) were collected during baseflow and stormflow for four years from five streams on Marine Corps Base Camp Lejeune near Jacksonville, North Carolina. The study streams had watersheds that spanned a range of ISC (1–38%) and included an urban watershed drained extensively by stormwater ponds. Urban streams had higher rates of annual discharge than less impacted streams due to elevated discharge at all rates of flow, more cumulative discharge at high flows, and dampened seasonal patterns. Streams with higher watershed ISC had higher rates of annual export of all measured nutrients due to increased stream discharge and concentrations of inorganic and particulate nitrogen. The relative importance of dissolved organic nitrogen decreased with watershed ISC, but it was still the dominant form of nitrogen export in every study stream except the stream that was dominated by particulate nitrogen export from stormwater pond algal production. Based on these findings, this study suggests that stormwater management emphasizing stormwater harvesting and evapotranspiration, increased wetland area, and decreased anthropogenic nutrient sources could reduce nutrient export from urban coastal plain streams.

A global hotspot for dissolved organic carbon in hypermaritime watersheds of coastal British Columbia

Abstract. The perhumid region of the coastal temperate rainforest (CTR) of Pacific North America is one of the wettest places on Earth and contains numerous small catchments that discharge freshwater and high concentrations of dissolved organic carbon (DOC) directly to the coastal ocean. However, empirical data on the flux and composition of DOC exported from these watersheds are scarce. We established monitoring stations at the outlets of seven catchments on Calvert and Hecate islands, British Columbia, which represent the rain-dominated hypermaritime region of the perhumid CTR. Over several years, we measured stream discharge, stream water DOC concentration, and stream water dissolved organic-matter (DOM) composition. Discharge and DOC concentrations were used to calculate DOC fluxes and yields, and DOM composition was characterized using absorbance and fluorescence spectroscopy with parallel factor analysis (PARAFAC). The areal estimate of annual DOC yield in water year 2015 was 33.3 Mg C km−2 yr−1, with individual watersheds ranging from an average of 24.1 to 37.7 Mg C km−2 yr−1. This represents some of the highest DOC yields to be measured at the coastal margin. We observed seasonality in the quantity and composition of exports, with the majority of DOC export occurring during the extended wet period (September–April). Stream flow from catchments reacted quickly to rain inputs, resulting in rapid export of relatively fresh, highly terrestrial-like DOM. DOC concentration and measures of DOM composition were related to stream discharge and stream temperature and correlated with watershed attributes, including the extent of lakes and wetlands, and the thickness of organic and mineral soil horizons. Our discovery of high DOC yields from these small catchments in the CTR is especially compelling as they deliver relatively fresh, highly terrestrial organic matter directly to the coastal ocean. Hypermaritime landscapes are common on the British Columbia coast, suggesting that this coastal margin may play an important role in the regional processing of carbon and in linking terrestrial carbon to marine ecosystems.

A global meta-analysis of exotic versus native leaf decay in stream ecosystems

Streams are reliant on streamside plants for allochthonous inputs that form the base of the stream food web. The impact of exotic leaves on stream ecosystems has been investigated regionally, but not assessed globally. In this study, we conducted a global meta-analysis contrasting native and exotic aquatic litter decay. We used a linear mixed effects model to identify how the site-specific factors and experimental treatments of latitude, temperature, pH, conductivity and the exclusion of invertebrates (via mesh size) cause relative differences in exotic and native decay rates. Subsequently, we assessed whether differences in native and exotic litter decay rates were driven by the intrinsic factors of leaf litter quality. We found that overall leaf litter decay rates were similar for native and exotic leaves on a global scale. However, we also found that exotic leaves decayed faster than native leaves at higher stream temperatures and in larger mesh sizes (allowing macroinvertebrate access); yet, those differences disappeared at lower temperatures and in finer mesh sizes which excluded macroinvertebrates. Post hoc analysis of leaf quality suggested that differences between higher native and lower exotic leaf C:N ratios might explain differences in decomposition rates observed at high temperature, and differences between lower native and higher exotic leaf C:P ratios associated with differences in decomposition rates observed at coarse mesh sizes. Our study suggests that exotic leaf litter is likely to have the most impact in high-temperature stream ecosystems and that the nature of the impact is potentially related to the C:N ratio differences between native and exotic leaves. This might indicate that higher temperature stream ecosystems are more vulnerable to changes in litter decay rates due to invasion by exotic plant species than lower temperature stream ecosystems, and should be monitored accordingly. We recommend that future leaf litter decay studies include stream discharge measurements and leaf litter quality information so that both intrinsic and extrinsic factors that impact litter decay can be fully accounted for when assessing the factors that drive differences in leaf litter decay rates.

Stream phosphorus dynamics of minimally impacted coastal plain watersheds

AbstractUnderstanding natural variation in stream phosphorus (P) concentrations over space and time is critical for understanding natural drivers of catchment behavior and establishing regulatory standards. Across minimally impacted benchmark streams (n = 81) in Florida, spatial variation in mean total P concentrations was large, indicating the importance of geologic controls on catchment solute dynamics. While this variation was significantly predicted by geographic regions, within regions we observed nearly comparable cross‐site variation, suggesting important finer‐scale heterogeneity in baseline catchment chemistry. Within‐site residual variation (unexplained by region or site) was as large as spatial variation, suggesting temporal variation in response to drivers such as flow may be critically important. To further explore timescales of P export variation, we collected long‐term, high‐frequency (subdaily) measurements of stream discharge (Q) and soluble reactive P (SRP) in 2 forested watersheds. We observed significant variation at annual, event, and diel timescales, all of which arise primarily from corresponding Q‐variation. Over the entire period of record, we generally observed a strong dilution signal, with SRP concentrations declining with increased Q. Despite significant SRP variation, flow variation was far larger and, thus, dominated temporal control on downstream flux. Within‐storm events, we observed strong and consistent clockwise SRP versus Q hysteresis, suggesting mobilization of proximal SRP stores. Diel variation exhibited mid‐afternoon concentration minima, Q‐controlled amplitude, and pronounced seasonal shifts in both magnitude and timing consistent with riparian evapotranspiration‐regulating lateral inputs of P‐rich groundwater. Such high‐resolution temporal signals allow identification of solute sources and provide insights into geologic and hydrologic drivers of solute variation.

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

AbstractThere 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.

Development and testing of a modified transparent velocity-head rod for stream discharge measurements

Accurate measurements of streamflow velocity and discharge can be challenging due to the technical nature of the instruments and often-associated high cost. Velocity-head rods measure the head difference in water flowing around an obstacle (i.e. the head rod) and use hydraulic theory to estimate water velocity. This article presents the construction and testing of a modified transparent velocity-head rod (mTVHR). Approximately 2400 pairs of mTVHR and SonTek FlowTracker (FT, acoustic Doppler) measurements were made on 31 dates, by seven operators at 14 sites. This data set allowed for the development of a calibrated relationship for velocity, for the investigation of accuracy among multiple users and for the definition of stream conditions where mTVHR use is appropriate. While there was evidence of differences between users for measurements of velocity-head and water depth, these differences were small and did not greatly influence discharge estimates derived using the instrument. While statistical analysis determined a minimum measureable velocity-head value of 1 mm, field experience with the instrument indicates the mTVHR should be used to measure velocity-head values greater than 2 mm. The maximum water depth recommended is approximately 0.75 m due to the 1-m height of the instrument and the difficulty of safe operation at moderately fast streamflow velocities. Turbulent streams and/or streams with large bed materials are not appropriate for use of the mTVHR due to rapid velocity-head fluctuations. The mTVHR is an accurate instrument for measuring streamflow velocity, depth and discharge. In addition, its ease of use, low cost (< CAN $100), and durability make it an attractive option for streamflow measurement.