Summer Storm Events Research Articles

The spatial and temporal dynamics of sediment phosphorus attenuation and release in impacted stream catchments

Sediments can attenuate phosphorus (P) from overlying water and reduce trophic status in zero and first order ditches and streams. These features can be considered as intermediate mitigation features between P mobilised from land, and onward delivery to river systems, if the risk of chemical P release from sediments is minimal. However, risk assessments are rarely based on temporal scale dynamics and especially at fine scale in both sediment and water column environments. In this study, in eutrophic stream catchments, bed sediments were tested fortnightly and spatially over one year for EPC0 (to derive phosphate exchange potential—PEP) and for P across a spectrum from labile to recalcitrant fractions. At the same time stream discharge and P concentrations were measured synchronously at high frequency and resolved to 1-hour intervals and indicated high water quality pressures at all flow rates. PEP indicated spatial and temporal changes most likely caused by periods of source disconnection/reconnection and sediment mobilisation during storm events, moving from periods of high attenuation potential to near saturation. Despite these spatial and temporal changes, PEP did not indicate much potential for chemical P release from the sediments (distributing mostly below or close to zero). However, this may be a misleading risk assessment by itself as physical P release, especially of the labile bicarbonate-dithionite (B-D) P fraction of sediments, was a more dominant process mobilised during storm events reducing by up to 84 % during a succession of summer storm events. The total P and total reactive P loads monitored leaving the catchments were coincident with these changes. The specific downstream trophic effects of this episodic P release will need to be assessed in terms of its bioavailability, in combination with other more noted diffuse and point P source processes.

Assessing land use effects on ecohydrological partitioning in the critical zone through isotope‐aided modelling

AbstractStable water isotopes are naturally occurring conservative tracers that can ‘fingerprint’ water sources and track ecohydrological fluxes across the critical zone (CZ). Parsimonious, tracer‐aided models allow effective quantification of the ecohydrological partitioning of rainfall into different water fluxes. We incorporated stable water isotopes into a one‐dimensional, tracer‐aided model (EcoIsoPlot) to follow the pathway of precipitation through the CZ at a lowland catchment—the long‐term experimental Demnitzer Millcreek Catchment (DMC), Germany—with contrasting vegetation covers (forest, agroforestry, grassland and arable). Precipitation (amount and δ2H), potential evapotranspiration (PET), leaf area index (LAI), air temperature and relative humidity were used as input data for modelling the growing season of 2021. The year had relatively average overall wetness, but a dry, cold spring with snowfall, and an exceptionally large summer storm event (~60 mm precipitation). Multi‐criteria calibration of the model was conducted using depth‐specific soil moisture and soil water δ2H measurements as targets. The novel incorporation of isotopes into model calibration constrained process representation of the estimated water balance with reasonable simulations and uncertainty bounds for water partitioning. Throughout the soil profile, soil moisture dynamics and stable water isotope variations were captured reasonably well. Green water fluxes (evapotranspiration) were highest at the forest site and blue water fluxes (groundwater recharge) highest at the grassland. Comparing simulations with estimated potential evapotranspiration (ET) and measured groundwater table fluctuations added further confidence to the modelling result. Overall, these may suggest a slight underestimation of ET and slight overestimation of recharge, though the results are similar to previous findings. Our study demonstrated the potential of stable water isotope data to enhance relatively simple, transferrable approaches to ecohydrological modelling of water fluxes in the CZ and to help improve model consistency. Such low‐parameterised tracer‐aided models have major potential for evidence‐based applications to aid management and help stakeholder communication.

Modeling stormwater management pond thermal impacts during storm events

Discharges of warm water from shallow stormwater management ponds during summer months into receiving headwater streams pose a risk to aquatic ecosystems in urban watersheds, especially in cool- and cold-water streams. Physically based models such as MINUHET have been used to assess the effects of thermal loading on receiving streams. However, these models require a significant amount of data and are complex to calibrate and use for the optimized designs that ensure protection of vulnerable aquatic life in urban stream ecosystems. This research introduces a user friendly and more accurate machine learning method to predict thermal profiles in stormwater ponds and the pond outlet discharge temperature to the receiving streams during storm events. Monitoring data were collected between 2014 and 2016 from three stormwater ponds in southern Ontario to train and test the new model. The new model is used to demonstrate the degree to which cooling effects can be achieved from remedial actions such as increasing the pond depth by dredging one meter and if a bottom-draw outlet is used instead of a mid-depth draw outlet. The methods presented in this study can be used to support an improved stormwater management pond design guidelines based on the key design objective that the thermal regime of the stormwater pond outlet discharge water must match the thermal regime of the receiving stream. The main objective is to avoid any discernable warming of the stream due to the stormwater pond discharge under a range of summer temperatures and storm event sizes.

Assessment of stormwater discharge contamination and toxicity for a cold-climate urban landscape

BackgroundStormwater is water resulting from precipitation events and snowmelt running off the urban landscape, collecting in storm sewers, and typically being released into receiving water bodies through outfalls with minimal to no treatment. Despite a growing body of evidence observing its deleterious pollution impacts, stormwater management and treatment in cold climates remains limited, partly due to a lack of quality and loading data and modeling parameters. This study examines the quality of stormwater discharging during the summer season in a cold-climate, semi-arid Canadian city (Saskatoon, Saskatchewan).ResultsSeven stormwater outfalls with mixed-land-use urban catchments > 100 km2 were sampled for four summer (June–August 2019) storm events and analyzed for a suite of quality parameters, including total suspended solids (TSS), chemical oxygen demand (COD), dissolved organic carbon (DOC), metals, and targeted polyaromatic hydrocarbons (PAHs). In addition, assessment of stormwater toxicity was done using the two toxicity assays Raphidocelis subcapitata (algae) and Vibrio fischeri (bacteria). Notable single-event, single-outfall contaminant pulses included of arsenic (420 µg/L), cadmium (16.4 µg/L), zinc (924 µg/L), fluorene (4.95 µg/L), benzo[a]pyrene (0.949 µg/L), pyrene (0.934 µg/L), phenanthrene (1.39 µg/L), and anthracene (1.40 µg/L). The IC50 in both R. subcapitata and V. fischeri was observed, if at all, above expected toxicity thresholds for individual contaminant species. Principal component analysis (PCA) showed no clear trends for individual sampling sites or sampling dates. In contrast, parameters were correlated with each other in groups including DOC, COD, TSS, and reduced algal toxicity; and total dissolved solids (TDS), sum of metals, and pH.ConclusionsIn general, stormwater characteristics were similar to those of previous studies, with a bulk of contamination carried by the first volume of runoff, influenced by a combination of rainfall depth, antecedent dry period, land use, and activity within the catchment. Roads, highways, and industrial areas contribute the bulk of estimated contaminant loadings. More intensive sampling strategies are necessary to contextualize stormwater data in the context of contaminant and runoff volume peaks.

Heavy rainfall following a summer drought stimulates soil redox dynamics and facilitates rapid and deep translocation of glyphosate in floodplain soils.

We present field data on the effects of heavy rainfall after drought on the mobility of glyphosate and redox conditions in a clayey floodplain soil. By applying glyphosate together with deuterated water as conservative tracer in combination with time resolved in situ redox potential measurements, the spatial and temporal patterns of water infiltration and pesticide transport as well as the concomitant changes of the redox conditions were revealed. Our findings demonstrate that shrinkage cracks in dry soils can serve as effective transport paths for atmospheric oxygen, water and glyphosate. The rain intensity of a typical summer storm event (approx. 25 mm within one hour) was sufficient to translocate deuterated water and glyphosate to the subsoil (50 cm) within 2 hours. Soil wetting induced partial closure of the shrinkage cracks and stimulated microbial activity resulting in pronounced dynamics of in situ soil redox conditions. Redox potentials in 40 to 50 cm depth dropped permanently to strongly reducing conditions within hours to days but fluctuated between reducing and oxidizing conditions in 10 to 30 cm depth. Our findings highlight the close link between the presence of macropores (shrinkage cracks), heavy rainfall after drought, redox dynamics and pesticide translocation to the subsoil and thus call for further studies addressing the effects of dynamic redox conditions as a limiting factor for glyphosate degradation.

Investigating flood processes in karst catchments by combining concentration-discharge relationship analysis and lateral flow simulation

Concentration-Discharge (C-Q) relationship analyses allow better understanding catchment water origin and hydrological process variability across time (from storm-event to pluriannual scale). Though, those approaches mostly include integrative methods that consider catchments as a whole. The aim of this article is to assess the potential of a combined approach to investigate the spatio-temporal variability of flood processes within catchments (across river reaches and seasons), focusing on karst areas. The methodology consists in combining C-Q relationship analysis in nested catchments and simulation of lateral exchanges at the reach scale. For that, we analyze high-frequency records of discharge and electrical conductivity (EC) in karst catchments, in which EC informs on water residence time. Contributions of pre-event water (PEW) and event water (EW) during storm events are investigated through hysteresis loop analysis at each gauging station. Combined with lateral EC simulation at the reach scale, they make it possible to infer runoff processes during storms. A unary karst catchment in a temperate/mountainous climate (Loue River) and a binary catchment in a Mediterranean climate (Cèze River) were selected in France, and their conceptual models of water origin and hydrological process variability were drawn. In the Loue catchment, summer and fall storm events are characterized by contributions of PEW through piston-type flows, whereas decreasing EC values in winter and spring storm events indicate the major contribution of EW through surface runoff and fast infiltration. Within the catchment, EW contribution increases downstream, due to the canyon river morphology grading into open valleys. Regarding the Cèze catchment, contributions of EW are higher, indicating that fast infiltration and surface runoff are the dominant processes, associated with a PEW signature in summer and fall. Within the catchment, PEW contribution is found higher in karstified areas. These results show that flood process seasonality is mainly related to karst aquifer saturation rate, while intra-site variability is linked to karst area extension and river morphology. Results are encouraging to extend this approach to a variety of sites, notably those affected by significant surface water-groundwater interaction and groundwater flooding. Such approach, by providing a discretized information on flood processes, could help refining lumped hydrological models, or facilitate the use of semi-distributed ones.

Drought, windthrow and forest operations strongly affect oribatid mite communities in different microhabitats

Climate change is enhancing the annual mean temperature and the risk for droughts and natural disasters. Hot and dry summers not only have a negative impact on forest performance, but also affect fundamental ecosystem processes such as litter decomposition and nutrient cycling and the organisms involved. Oribatid mites are sexually or parthenogenetically reproducing soil-living microarthropods substantially involved in these processes. We compare oribatid mite communities (abundance, species richness, effective Shannon diversity and life-history parameters such as sex ratio, gravidity, number of eggs) in four microhabitats (litter, dead wood, moss and bare soil) before (2016) and after a sequence of disturbance events (2020). These disturbances include the severe drought of 2018/2019 in Germany, a single summer storm event in August 2019, and subsequent forest operations in spring 2020. Abundance and species richness were reduced up to 87% in all microhabitats and so was the effective Shannon diversity in moss (65%). Communities in moss were most affected, while effects were buffered in litter. In litter and moss, sexual species suffered slightly more than parthenogenetic species. Life history parameters were largely unaffected. In bare soil, microarthropods were almost absent. Our study demonstrates that consequences of climate change – drought, windthrow, necessary forest operations – are not restricted to above-ground systems but also strongly affect soil-living microarthropod communities. If natural and human-introduced disturbances remain in the long-term, severe consequences for forest soil arthropods must be expected. Since life-history parameters were unaffected, species probably recover over time if climate becomes more moderate in the short-term.

The Influence of Wind Direction during Storms on Sea Temperature in the Coastal Water of Muping, China

Sea temperature structures are important for water stratification and marine ecosystems. In the coastal water of Muping, China, stationary measurements of sea temperature captured temporal temperature changes during two summer storm events. The north component of the wind during the two storms was opposite. The temperature responded differently to wind directions in the two storm events. A well-validated numerical ocean model was used to investigate the mechanism of sea temperature variation of the coast of Muping. The model revealed that the southerly and easterly wind was upwelling-favorable in the study area. They generated the shoreward transport of bottom cold water, which induced bottom water cooling, enhanced stratification, and weakened vertical mixing. On the other hand, the northerly and westerly wind was downwelling-favorable and enhanced turbulent mixing. The alongshore upwelling-favorable wind caused more cross-shore transport than cross-shore upwelling-favorable wind, which resulted in stronger bottom cooling. Similarly, alongshore downwelling-favorable wind generated lower temperature than cross-shore wind. A surface cold-water band was formed in the second storm. Although it was formed during upwelling-favorable wind, the temperature balance analysis indicated that vertical mixing and westward horizontal advection were the two dominant processes compared to upwelling.

Managing Stormwater by Accident: A Conceptual Study

Stormwater-driven road salt is a chronic and acute issue for streams in cold, urban environments. One promising approach for reducing the impact of road salt contamination in streams and adjacent aquifers is to allow “accidental wetlands” to flourish in urban areas. These wetlands form naturally as a byproduct of human activities. In this study, we quantified the ability of an accidental wetland in northwestern North Carolina, USA, to reduce the timing and peak concentration of road salt in a stream. Monitoring suggests that flow and transport processes through the wetland reduce peak concentrations and delay their arrival at the adjacent stream. We expand these findings with numerical simulations that model multiple meltwater and summer storm event scenarios. The model output demonstrates that small accidental wetland systems can reduce peak salinities by 94% and delay the arrival of saltwater pulses by 45 days. Our findings indicate that accidental wetlands improve stream water quality and they may also reduce peak temperatures during temperature surges in urban streams. Furthermore, because they find their own niche, accidental wetlands may be more effective than some intentionally constructed wetlands, and provide opportunities to explore managing stormwater by letting nature take its course.

Nitrogen dynamics and nitrogen-to-phosphorus stoichiometry in cold region agricultural streams.

Cold agricultural regions are getting warmer and experiencing shifts in precipitation patterns, which affect hydrological transport of nutrients through reduced snowpack and higher annual proportions of summer rainfall. Previous work has demonstrated that the timing of phosphorus (P) concentrations is regionally coherent in streams of the northern Great Plains, suggesting a common climatic driver. There has been less investigation into patterns of stream nitrogen (N), despite its importance for water quality. Using high-frequency water quality data collected over 6 yr from three southern Manitoba agricultural streams, the goal of this research was to investigate seasonal patterns in N and P concentrations and the resultant impacts of these patterns on N/P stoichiometry. In the spring, high concentrations of inorganic N were associated with snowmelt runoff, while summer N was dominated by organic forms; inorganic N concentrations remained consistently low in the summer, suggesting increased biological N transformation and N removal. Relationships between N concentration and discharge showed generally weak model fits (r2 values for significant relationships ranging from .33 to .48), and the strength and direction of model fits differed among streams, seasons, and forms of N. Dissolved organic N concentrations were strongly associated with dissolved organic carbon. Nitrogen-to-phosphorus ratios varied among streams but were significantly lower during summer storm events (p<.0001). These results suggest that climate-driven shifts in temperature and precipitation may negatively affect downstream water quality in this region.

Tracking the evolution of particulate organic matter sources during summer storm events via end-member mixing analysis based on spectroscopic proxies

Despite the growing attention to the effects of hydrological precipitation on organic carbon export along the continuum land-river-ocean, limited effort has been made to understand the export and the reactivity of particulate organic carbon (POC) compared to those of dissolved organic carbon (DOC). Yet, further understanding of the controlling mechanisms on the export of particulate organic matter (POM) from terrestrial systems is fundamental. In this study, we assessed the temporal changes of the source contributions to riverine POM in two adjacent rivers of the same watershed during two summer storm events, which included the early and the late events, using end-member mixing analysis (EMMA) based on spectroscopic proxies. The EMMA showed relatively high contributions of terrestrial materials to the riverine POM for both rivers during the early summer storm event. However, this trend did not persist until the late summer storm event, which presented the decreased contributions of the terrestrial sources and less dynamic changes in the source distributions compared to those observed in the early summer rain event. These results demonstrate the combined impacts of the land use/surrounding area, the hydrology, and the intra-seasonal variations on the dominant riverine POM sources. This study provides an interesting insight into the importance of the intense hydrological events on the export of the terrestrial OM and further on the potential modification of the existing carbon mass balance along the continuum land-river-ocean.

The Influence of Climate Change on the Restoration Trajectory of a Nutrient-Rich Deep Lake

Nutrient reduction in impacted lowland freshwater systems is ecologically and culturally important. Gaining a greater insight into how lakes respond to lowering nutrient loads and how climate-driven physical limnology affects present and future cycling of available nutrients is important for ecosystem resource management. This study examines the nutrient decline in a hypereutrophic freshwater lake (Rostherne Mere, Cheshire, UK) 25 years after sewage effluent diversion, a uniquely long-term analysis of a recovering nutrient-rich deep lake. Using nutrient, phytoplankton, climate and catchment hydrological monitoring, the contemporary lake system is compared to previous studies from 1990 to 2002. Nutrient change since point source load diversion showed annual average and maximum phosphorus (P) concentrations decreased significantly for the first 10 years (1992: ~ 600 µg P L−1; 2002: ~ 200 µg P L−1), but have since stabilised due to a substantial legacy sediment P internal load. Dissolved inorganic nitrogen (DIN) concentrations have not substantially changed since diversion, resulting in the alteration of the DIN/SRP ratio from a system characterised by N limitation (N:P ~ 5), to one predominantly P limited (N:P > 20). Nutrient changes over this time are shown to drive ecological change, especially in the cyanobacterial and algal communities. Furthermore, very high-resolution monitoring of lake inflow and outflow (every 5 min during 2016) shows that water residence time at this lake is significantly shorter than previously estimated (~ 0.8 years compared to previous estimates of ~ 1.6–2.4 years). Together with long-term data demonstrating that the stratification period at Rostherne Mere has increased by 40 days over the last ~ 50 years (due to later autumnal mixing), we show that a rapid rate of epilimnetic flushing together with a long stratification period substantially reduces the available epilimnetic P during the summer cyanobacterial bloom. This is of growing importance for many such lakes, given widespread climate-driven lengthening of stratification and a national trend of decreasing summer rainfall (decreasing seasonal flushing) but more intense summer storm events (resulting in short-term flushing events).

Sediment and fluvial particulate carbon flux from an eroding peatland catchment

AbstractErosion and the associated loss of carbon is a major environmental concern in many peatlands and remains difficult to accurately quantify beyond the plot scale. Erosion was measured in an upland blanket peatland catchment (0.017 km2) in northern England using structure‐from‐motion (SfM) photogrammetry, sediment traps and stream sediment sampling at different spatial scales. A net median topographic change of –27 mm yr–1 was recorded by SfM over the 12‐month monitoring period for the entire surveyed area (598 m2). Within the entire surveyed area there were six nested catchments where both SfM and sediment traps were used to measure erosion. Substantial amounts of peat were captured in sediment traps during summer storm events after two months of dry weather where desiccation of the peat surface occurred. The magnitude of topographic change for the six nested catchments determined by SfM (mean value: 5.3 mm, standard deviation: 5.2 mm) was very different to the areal average derived from sediment traps (mean value: –0.3 mm, standard deviation: 0.1 mm). Thus, direct interpolation of peat erosion from local net topographic change into sediment yield at the catchment outlet appears problematic. Peat loss measured at the hillslope scale was not representative of that at the catchment scale. Stream sediment sampling at the outlet of the research catchment (0.017 km2) suggested that the yields of suspended sediment and particulate organic carbon were 926.3 t km–2 yr–1 and 340.9 t km–2 yr–1, respectively, with highest losses occurring during the autumn. Both freeze–thaw during winter and desiccation during long periods of dry weather in spring and summer were identified as important peat weathering processes during the study. Such weathering was a key enabler of subsequent fluvial peat loss from the catchment. © 2019 John Wiley &amp; Sons, Ltd.

A River Temperature Model to Assist Managers in Identifying Thermal Pollution Causes and Solutions

Thermal pollution of rivers degrades water quality and ecosystem health, and cities can protect rivers by decreasing warmer impervious surface stormwater inflows and increasing cooler subsurface inflows and shading from riparian vegetation. This study develops the mechanistic i-Tree Cool River Model and tests if it can be used to identify likely causes and mitigation of thermal pollution. The model represents the impacts of external loads including solar radiation in the absence of riparian shade, multiple lateral storm sewer inflows, tributaries draining reservoirs, groundwater flow, and hyporheic exchange flow in dry weather steady flows and wet weather unsteady flows. The i-Tree Cool River Model estimates the shading effects of the riparian vegetation and other features as a function of heights and distances as well as solar geometry. The model was tested along 1500 m of a New York mountain river with a riparian forest and urban areas during 30 h with two summer storm events in 2007. The simulations were sensitive to the inflows of storm sewers, subsurface inflows, as well as riparian shading, and upstream boundary temperature inflows for steady and unsteady conditions. The model simulated hourly river temperature with an R2 of 0.98; when shading was removed from the simulation the R2 decreased 0.88, indicating the importance of riparian shading in river thermal modeling. When stormwater inflows were removed from the simulation, the R2 decreased from 0.98 to 0.92, and when subsurface inflows were removed, the R2 decreased to 0.94. The simulation of thermal loading is important to manage against pollution of rivers.

Production of sediment from the running surface of unpaved legacy roads in southeast Ohio, USA

ABSTRACTUnpaved, or unbound aggregate, roads are thought to be one of the largest anthropogenic sources of fine sediments in the stream channels of small watersheds. Sedimentation can reduce water quality in streams negatively impacting aquatic habitat as well as being a detriment to municipal drinking water sources. Even during periods of low-volume use roads can be a chronic source of fine sediment. Legacy roads may degrade stream water quality more than newly built roads as they were constructed before modern best management practices. This study measured the production of sediment from the road surface of a legacy road system within a small watershed using controlled precipitation experiments to determine the mass of sediment that roads could produce during one storm event. Sediment production was not related to road slope or strength as measured by Clegg hammer. The production of sediment from the road surface increased 95% on average with wet traffic use. Using an estimated road and stream connectivity of 50%, surface runoff from the roads in Middle Creek watershed could account for 16% of the total sediment yield from the watershed during a small summer storm event. Compacted tire tracks were utilized as flow paths during runoff experiments, emphasizing the importance of continual maintenance and best management practices. This study highlights the need to disconnect unbound aggregate roads from stream channels in order to prevent negative water quality impacts associated with sedimentation.

Improving Urban Runoff in Multi-Basin Hydrological Simulation by the HYPE Model Using EEA Urban Atlas: A Case Study in the Sege River Basin, Sweden

In this study, the high-resolution polygonal land cover data of EEA Urban Atlas was applied for land-use characterization in the dynamic multi-basin hydrological model, HYPE. The objective of the study was to compare this dedicated urban land cover data in semi-distributed hydrological modelling with the widely used but less detailed EEA CORINE. The model was set up for a basin including a small town named Svedala in southern Sweden. In order to verify the ability of the HYPE model to reproduce the observed flow rate, the simulated flow rate was evaluated based on river flow time series, statistical indicators and flow duration curves. Flow rate simulated by the model based on Urban Atlas generally agreed better with observations of summer storm events than the CORINE-based model, especially when the daily rainfall amount was 10 mm/day or more, or the flow exceedance probability was 0.02 to 0.5. It suggests that the added value of the Urban Atlas model is higher for heavy-to-medium storm events dominated by direct runoff. To conclude, the effectiveness of the proposed approach, which aims at improving the accuracy of hydrological simulations in urbanized basins, was supported.

Microbial Pollution Characterization at a TMDL Site in Michigan: Effect of Hydrological Conditions on Pollution Loading

Abstract Communities throughout the United States are developing and implementing watershed management plans to address nonpoint sources of pollution and meet Total Maximum Daily Load (TMDL) requirements. Once a TMDL is established, a watershed management plan is developed and implemented to reduce contaminant sources and attain TMDL goals. Developing an effective TMDL and remediation plan should take into account fluctuation of pollution loadings and the timing of first-flush events. The objective of this study is to investigate the effect of hydrological conditions on microbial pollutant levels at a TMDL site during spring and summer storm events. A total of 64 water samples were collected from Sloan Creek in mid-Michigan in the spring/summer of 2015. All samples were analyzed to quantify concentrations of E. coli, bovine-associated Bacteroides (BoBac) gene markers, and human-associated Bacteroides (HuBac) gene markers. Discharge was the driving force of microbial contaminant loading in the studied water body. E. coli concentrations had significant strong correlation with precipitation and discharge, and BoBac concentrations were positively related to discharge. E. coli, BoBac and HuBac patterns suggested first-flush phenomena occurred during summer storms. E. coli permit exceedance rates increased from 31% before first-flush, to 100% during and after first-flush in the summer. The resulting information may help develop a plan for restoring impaired waters and establish the maximum amount of pollutants that the body of water can receive during different hydrological conditions.

Assessing Intra-Event Phosphorus Dynamics in Drainage Water Using Phosphate Stable Oxygen Isotopes

Abstract. Quantifying fluxes and pathways of dissolved reactive phosphorus (DRP) in tile-drained landscapes has been hampered by a lack of measurements that are sensitive to P fate and transport processes. One potential tool to help understand these dynamics is the oxygen isotope signature of phosphate (d18OPO4); however, its potential benefits and limitations are not well understood for intra-event dynamics at the field scale. The objectives of this study were to quantify intra-event variability of d18OPO4 signatures in tile drainage water and assess the efficacy of d18OPO4 to elucidate mechanisms and flow pathways controlling DRP transport to tile drains. We collected water samples during a summer storm event from a subsurface (tile)-drained field located in west-central Ohio and analyzed for d18OPO4 of DRP. Supplementary water quality measurements, hydrologic modeling, and soil temperature data were used to help understand intra-event d18OPO4 dynamics. Results of the soil extraction analysis from our study site highlight that the soil water-extractable P (WEP) pool was not in equilibrium with long-term, temperature-dependent water isotope values. This result suggests that P-rich soils may, at least partially, retain their original source signature, which has significant implications for identifying hotspots of P delivery in watershed-scale applications. Results of the storm event analysis highlight that equilibration of leached DRP in soil water creates a gradient between isotopic compositions of pre-event shallow subsurface sources, pre-event deep subsurface sources, and the WEP tied up in surface soils. The current study represents the first intra-event analysis of d18OPO4 and highlights the potential for phosphate oxygen isotopes as a novel tool to improve understanding of P fate and transport in artificially drained agroecosystems. Keywords: Agriculture, Edge-of-field, Macropores, Phosphate oxygen isotopes, Tile-drainage.

Quantifying Subsurface Lateral Flow along Sloping Horizon Boundaries in Soil Profiles of a Hummocky Ground Moraine

Core Ideas Hydropedologic factors control subsurface lateral flow in eroded hillslope soils. Depth to C horizon, hydraulic properties, and initial conditions are most sensitive. Virtual 2D model experiment predicts effect of tracer injection on lateral movement. Lateral flow at sloping C horizon surface locally separated from tracer plume. Erosion‐induced feedback on subsurface lateral flow by varying depths to C horizon. Subsurface lateral flow in hillslope soils depends on lower permeability or texture‐contrasting soil horizons. In the arable hummocky soil landscape, erosion processes caused glacial till appearance closer to the soil surface at upslope positions. The objective of this work was to quantify the potential for subsurface lateral flow depending on the erosion‐affected spatial hydropedological complexity. The eroded Haplic Luvisol profile was studied due to the presence of a relatively dense C horizon that varied in depth, thickness, and sloping angle. A two‐dimensional numerical modeling and sensitivity analysis for the saturated hydraulic conductivity (Ks) of the C horizon and the depth to C horizon (i.e., soil solum thickness) was performed for rainstorms in 2011 and 2012 using HYDRUS‐2D. A Ks value of ≤2.5 cm d−1 for the C horizon was required for lateral flow initiation. Lateral flow was (i) increasing with decreasing solum thickness, indicating an erosion‐induced feedback on subsurface lateral flow, and (ii) dependent on the soil moisture regime prior to rainstorms. The effect of lateral flow on the movement of a conservative tracer was simulated in the form of a “virtual experiment”. Simulation scenarios revealed only a relatively small lateral shift of the tracer plume caused by a local decoupling of water (already lateral) from subsequent tracer movement (still vertical). Longer term simulations suggested that, for the present conditions, lateral flow was limited mostly to occasional summer storm events. Even without considering preferential flow contribution to lateral flow, highly complex hydropedologic interactions are present in erosion‐affected heterogeneous soil profiles.

Capturing temporal and spatial variability in the chemistry of shallow permafrost ponds

Abstract. Across the circumpolar north, the fate of small freshwater ponds and lakes (&lt; 1 km2) has been the subject of scientific interest due to their ubiquity in the landscape, capacity to exchange carbon and energy with the atmosphere, and their potential to inform researchers about past climates through sediment records. A changing climate has implications for the capacity of ponds and lakes to support organisms and store carbon, which in turn has important feedbacks to climate change. Thus, an improved understanding of pond biogeochemistry is needed. To characterize spatial and temporal patterns in water column chemistry, a suite of tundra ponds were examined to answer the following research questions: (1) does temporal variability exceed spatial variability? (2) If temporal variability exists, do all ponds (or groups of ponds) behave in a similar temporal pattern, linked to seasonal hydrologic drivers or precipitation events? Six shallow ponds located in the Hudson Bay Lowlands region were monitored between May and October 2015 (inclusive, spanning the entire open-water period). The ponds span a range of biophysical conditions including pond area, perimeter, depth, and shoreline development. Water samples were collected regularly, both bimonthly over the ice-free season and intensively during and following a large summer storm event. Samples were analysed for nitrogen speciation (NO3−, NH4+, dissolved organic nitrogen) and major ions (Cl−, SO42−, K+, Ca2+, Mg2+, Na+). Across all ponds, temporal variability (across the season and within a single rain event) exceeded spatial variability (variation among ponds) in concentrations of several major species (Cl−, SO42−, K+, Ca2+, Na+). Evapoconcentration and dilution of pond water with precipitation and runoff inputs were the dominant processes influencing a set of chemical species which are hydrologically driven (Cl−, Na+, K+, Mg2+, dissolved organic nitrogen), whereas the dissolved inorganic nitrogen species were likely mediated by processes within ponds. This work demonstrates the importance of understanding hydrologically driven chemodynamics in permafrost ponds on multiple scales (seasonal and event scale).