Storm events dominate riverine loads of dissolved organic carbon (DOC) and nitrate and are expected to increase in frequency and intensity in many regions due to climate change. We deployed three high‐frequency (15 min) in situ absorbance spectrophotometers to monitor DOC and nitrate concentration for 126 storms in three watersheds with agricultural, urban, and forested land use/land cover. We examined intrastorm hysteresis and the influences of seasonality, storm size, and dominant land use/land cover on storm DOC and nitrate loads. DOC hysteresis was generally anticlockwise at all sites, indicating distal and plentiful sources for all three streams despite varied DOC character and sources. Nitrate hysteresis was generally clockwise for urban and forested sites, but anticlockwise for the agricultural site, indicating an exhaustible, proximal source of nitrate in the urban and forested sites, and more distal and plentiful sources of nitrate in the agricultural site. The agricultural site had significantly higher storm nitrate yield per water yield and higher storm DOC yield per water yield than the urban or forested sites. Seasonal effects were important for storm nitrate yield in all three watersheds and farm management practices likely caused complex interactions with seasonality at the agricultural site. Hysteresis indices did not improve predictions of storm nitrate yields at any site. We discuss key lessons from using high‐frequency in situ optical sensors.

[1] Runoff mixing patterns for base flow and 42 storm events were investigated for a 3 year period (2008)(2009)(2010) in a 12 ha forested catchment in the mid-Atlantic, Piedmont region of the USA. Eleven distinct runoff sources were sampled independently and included: precipitation, throughfall, stemflow, litter leachate, wetland soil water, tension soil water, shallow groundwater, groundwater seeps, hyporheic water, riparian groundwater, and deep groundwater. A rigorous end-member mixing analysis (EMMA) was implemented and all base flow, storm-flow, and end-member chemistries were evaluated in a two-dimensional mixing space. End-members enclosed stream water chemistry and displayed a systematic continuum in EMMA space. Base-flow chemistry of stream waters was similar to groundwater seeps. Storm-event runoff was attributed to contributions from surficial sources (precipitation, throughfall, stemflow, and litter leachate) on the rising limb of the discharge hydrograph that was followed by soil and shallow groundwater sources on the recession limb of the hydrograph. The shapes of the storm-event hysteresis loops (wide versus tight, linear patterns) varied with hydrologic conditions from wet, hydrologically well-connected conditions to a dry, disconnected state. Detailed temporal data on endmember chemistry allowed us to explain the changes in stream water hysteresis patterns and runoff mixing space to shifts in end-member chemistry that occurred as the catchment became hydrologically disconnected. These results highlight the need to recognize the temporal variation in end-member chemistry as a function of catchment wetness and the need to collect high-frequency data on both--stream water as well as potential runoff endmembers to better characterize catchment flow paths and mixing responses.

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N onpoint source pollution is transported primarily by runoff from urban, agricultural, mining areas, and construction sites. Agricultural activities are being increasingly blamed for deterioration of surface and ground water resources in the United States. Significant progress has been made in developing technologies for controlling point sources, while until recently nonpoint sources of pollution had been relatively neglected. Runoff carries sediment, organic matter, bacteria, pesticides, metals, nutrients, and other chemicals. Nutrients, primarily nitrogen (N) and phosphorus (P), can be a major problem because they can cause eutrophic algae growth which may reduce oxygen availability and increase turbidity in water bodies. Livestock systems, which utilize pastureland for grazing animals and cropland for disposal of manure waste, are one segment of agricultural production for which the extent of nonpoint source pollution is neither clearly defined nor the effectiveness of best management practices (BMPs) adequately demonstrated. In 1983, a study by the U.S. Environmental Protection Agency on the decline in water quality of the Chesapeake Bay indicated that point and nonpoint sources of pollution were among the main causes of the Bay's decline (USEPA, 1983). In particular, the study indicated that nonpoint sources contributed about 67% of the N and 39% of the P entering the Bay. Furthermore, agriculture was estimated to be responsible for 60 and 27% of the N and P loadings from nonpoint sources, respectively. Consequently, in December 1987, the Governors of Pennsylvania, Maryland, and Virginia, the mayor of the District of Columbia, and the Administrator of the EPA pledged to address nonpoint sources as well as other sources of pollution to restore and protect the Chesapeake Bay. This commitment, known as the Chesapeake Bay Agreement of 1987, requires the signatory States to implement cost-sharing programs targeted at reducing nonpoint source pollution of the Bay and its tributaries. Virginia's agricultural cost-sharing program was initiated in response to the Chesapeake Bay program and was designed to encourage voluntary implementation of BMPs such as conservation tillage and installation of animal waste facilities by farmers. Animal waste management involves both storage and proper utilization of waste as fertilizer on agricultural land in order to improve crop production and reduce transport of pollutants by runoff. Timing, method, and rate of application are controllable management factors that influence both the effectiveness of the animal waste as a fertilizer and the degree to which runoff pollution is prevented (Sharpley et al., 1994). While there is documented evidence on the water quality advantages of BMPs on field-size plots, the effectiveness of BMPs, especially animal waste management practices, on large watersheds with varying topography, land use, soils, and geology is relatively unknown. Thus, a comprehensive nonpoint source monitoring program was undertaken in 1986 to quantify

Large runoff, sediment, and nutrient exports from watersheds could occur due to individual extreme climate events or a combination of multiple hydrologic and meteorological conditions. Using high-frequency hydrologic, sediment, and turbidity data we show that freeze-thaw episodes followed by intense winter (February) rainstorms can export very high concentrations and loads of suspended sediment and particulate organic carbon (POC) and nitrogen (PN) from mid-Atlantic watersheds in the US. Peak suspended sediment ([ 5000 mg L -1 ), POC ([ 250 mg L -1 ) and PN ([ 15 mg L -1 ) concentrations at our 12 and 79 ha forested watersheds for the February rainfall-runoff events were highest on record and the fluxes were comparable to those measured for tropical storms. Similar responses were observed for turbidity values ([ 400 FNU) at larger USGS-monitored watersheds. Much of the sediments and particulate nutrients likely originated from erosion of stream bank sediments and/or channel storage. Currently, there is considerable uncertainty about the contribution of these sources to nonpoint source pollution, particularly, in watersheds with large legacy sediment deposits. Future climate projections indicate increased intensification of storm events and increased variability of winter temperatures. Freeze-thaw cycles coupled with winter rain events could increase erosion and transport of streambank sediments with detrimental consequences for water quality and health of downstream aquatic ecosystems.

Watershed export of excessive reactive nitrogen (N) is a major environmental concern globally, as eutrophication of coastal waters causes hypoxia, habitat degradation, and loss of biodiversity (Howarth, 2008;Schaefer et al., 2009). Functioning riparian zones that are hydrologically connected to streams and rivers help mitigate downstream N transport through N removal by numerous biogeochemical pathways (Vidon et al., 2010;Zhao et al., 2021). Complete denitrification is a particularly important removal pathway by which facultative anaerobic bacteria reduce nitrate and nitrite to N 2 gas, permanently removing N from reactive pools (Groffman et al., 1992;Hill, 2019;Lutz et al., 2020). Denitrification occurs in areas with available organic carbon (C), high concentrations of nitrate, and wet, low oxygen sediments (Zhao et al., 2021). These conditions are often present in riparian

Large storm events possess significant erosive energy capable of mobilizing large amounts of sediment and particulate organic matter (POM) into fluvial systems. This study investigated how stream POM composition varied as a function of the watershed POM source, particle size, storm event magnitude, and seasonal timing. POM composition was characterized for multiple watershed sources and for stream POM following storms in a second‐order forested stream. Carbon (C) and nitrogen (N) amount, C:N ratio and isotopic content (13C and 15N) were determined for solid phase POM, whereas dissolved organic C, total N concentrations, and fluorescence characteristics were determined for solution/extracted POM. Key findings from this study were the following: (1) Composition of POM varied greatly with watershed sources with forest floor litter being C and N rich and labile, while stream banks and bed were C and N poor and recalcitrant. (2) Summer storms mobilized more carbon and nitrogen‐rich labile sources, while winter events mobilized more carbon‐ and nitrogen‐poor refractory material from near‐stream sources. (3) POM composition varied by size class, with the coarse POM showing more C and N rich and labile properties, while the fine POM displayed more degraded and refractory properties. If climate variability increases the magnitude and intensity of large storm events, our observations suggest that this will not only increase the inputs of POM to aquatic systems but also result in the delivery of coarser, C and N rich, and more bioavailable POM to the stream drainage network.

Dams are increasingly being removed across the United States (US) (Bellmore et al., 2017;Foley et al., 2017). Since 1912, more than 1,490 dams have been removed across the US and Pennsylvania leads the nation in the number of milldams and their removals (American Rivers, 2020). Most (>90%) of these milldams are classified as low-head dams (height < 7 m) and are typically a relic of colonial and post-colonial era milling activities (Merritts et al., 2011;Walter & Merritts, 2008). Dam removal numbers could be higher since not all dam removals are recorded. This could particularly be true for the Mid-Atlantic Piedmont region, where thousands of small mill dams existed since the late 1600s (Walter & Merritts, 2008).Low-head dam removals are primarily being driven by needs for public safety, reduction in financial liability, recreational access, aesthetics, and/or improvement in fish habitat (