Sara K. McMillan

A review on effectiveness of best management practices in improving hydrology and water quality: Needs and opportunities

Best management practices (BMPs) have been widely used to address hydrology and water quality issues in both agricultural and urban areas. Increasing numbers of BMPs have been studied in research projects and implemented in watershed management projects, but a gap remains in quantifying their effectiveness through time. In this paper, we review the current knowledge about BMP efficiencies, which indicates that most empirical studies have focused on short-term efficiencies, while few have explored long-term efficiencies. Most simulation efforts that consider BMPs assume constant performance irrespective of ages of the practices, generally based on anticipated maintenance activities or the expected performance over the life of the BMP(s). However, efficiencies of BMPs likely change over time irrespective of maintenance due to factors such as degradation of structures and accumulation of pollutants. Generally, the impacts of BMPs implemented in water quality protection programs at watershed levels have not been as rapid or large as expected, possibly due to overly high expectations for practice long-term efficiency, with BMPs even being sources of pollutants under some conditions and during some time periods. The review of available datasets reveals that current data are limited regarding both short-term and long-term BMP efficiency. Based on this review, this paper provides suggestions regarding needs and opportunities. Existing practice efficiency data need to be compiled. New data on BMP efficiencies that consider important factors, such as maintenance activities, also need to be collected. Then, the existing and new data need to be analyzed. Further research is needed to create a framework, as well as modeling approaches built on the framework, to simulate changes in BMP efficiencies with time. The research community needs to work together in addressing these needs and opportunities, which will assist decision makers in formulating better decisions regarding BMP implementation in watershed management projects.

Denitrification of Nitrogen Released from Senescing Algal Biomass in Coastal Agricultural Headwater Streams

Assimilation of inorganic N by photoautotrophs has positive impacts on nutrient retention; however this retention is only temporary. As the biomass senesces, organic and inorganic forms of N are released back to the stream where they can be further transformed (i.e., nitrification, denitrification) or exported downstream. The purpose of this study was to assess the fate of the remineralized N, particularly the potential for removal by denitrification. Experiments were conducted on intact sediment cores from streams in an agricultural watershed. Cores were amended with varying ages of algal leachate and denitrification rates were measured with a membrane inlet mass spectrometer. Results of this study demonstrated that senescing algal biomass stimulated denitrification rates and provided a source of N and labile C to denitrifiers. Regardless of leachate age, addition of leachate to intact cores revealed a net loss of dissolved inorganic N from the water column. Denitrification rates were most strongly related to concentrations of dissolved and particulate C in the overlying water and secondarily to C quality (molar C to N ratio of total dissolved C and N) and NO(3)(-) flux. Using a mass balance approach, the proportion of N from senescing algal biomass that was denitrified accounted for as much as 10% of the total dissolved nitrogen (TDN) and up to 100% of the NO(3)(-) during a 3-h experiment. These results suggest an important link between instream algal uptake and eventual denitrification thereby providing a pathway for permanent removal of watershed-derived N from the stream ecosystem.

Stormwater management network effectiveness and implications for urban watershed function: A critical review

Deleterious effects of urban stormwater are widely recognized. In several countries, regulations have been put into place to improve the conditions of receiving water bodies, but planning and engineering of stormwater control is typically carried out at smaller scales. Quantifying cumulative effectiveness of many stormwater control measures on a watershed scale is critical to understanding how small-scale practices translate to urban river health. We review 100 empirical and modelling studies of stormwater management effectiveness at the watershed scale in diverse physiographic settings. Effects of networks with stormwater control measures (SCMs) that promote infiltration and harvest have been more intensively studied than have detention-based SCM networks. Studies of peak flows and flow volumes are common, whereas baseflow, groundwater recharge, and evapotranspiration have received comparatively little attention. Export of nutrients and suspended sediments have been the primary water quality focus in the United States, whereas metals, particularly those associated with sediments, have received greater attention in Europe and Australia. Often, quantifying cumulative effects of stormwater management is complicated by needing to separate its signal from the signal of urbanization itself, innate watershed characteristics that lead to a range of hydrologic and water quality responses, and the varying functions of multiple types of SCMs. Biases in geographic distribution of study areas, and size and impervious surface cover of watersheds studied also limit our understanding of responses. We propose hysteretic trajectories for how watershed function responds to increasing imperviousness and stormwater management. Even where impervious area is treated with SCMs, watershed function may not be restored to its predevelopment condition because of the lack of treatment of all stormwater generated from impervious surfaces; non-additive effects of individual SCMs; and persistence of urban effects beyond impervious surfaces. In most cases, pollutant load decreases largely result from run-off reductions rather than lowered solute or particulate concentrations. Understanding interactions between natural and built landscapes, including stormwater management strategies, is critical for successfully managing detrimental impacts of stormwater at the watershed scale.

Short-term spatial and temporal variability in greenhouse gas fluxes in riparian zones.

Recent research indicates that riparian zones have the potential to contribute significant amounts of greenhouse gases (GHG: N2O, CO2, CH4) to the atmosphere. Yet, the short-term spatial and temporal variability in GHG emission in these systems is poorly understood. Using two transects of three static chambers at two North Carolina agricultural riparian zones (one restored, one unrestored), we show that estimates of the average GHG flux at the site scale can vary by one order of magnitude depending on whether the mean or the median is used as a measure of central tendency. Because the median tends to mute the effect of outlier points (hot spots and hot moments), we propose that both must be reported or that other more advanced spatial averaging techniques (e.g., kriging, area-weighted average) should be used to estimate GHG fluxes at the site scale. Results also indicate that short-term temporal variability in GHG fluxes (a few days) under seemingly constant temperature and hydrological conditions can be as large as spatial variability at the site scale, suggesting that the scientific community should rethink sampling protocols for GHG at the soil-atmosphere interface to include repeated measures over short periods of time at select chambers to estimate GHG emissions in the field. Although recent advances in technology provide tools to address these challenges, their cost is often too high for widespread implementation. Until technology improves, sampling design strategies will need to be carefully considered to balance cost, time, and spatial and temporal representativeness of measurements.

Characterizing the Effects of Stormwater Mitigation on Nutrient Export and Stream Concentrations

Urbanization increases nutrient loading and lowers residence times for processing of reactive solutes, including nitrate, total dissolved nitrogen, orthophosphate, and dissolved organic carbon), which leads to increased stream concentrations and mass export. Stormwater control measures mitigate the impacts of urbanization, and have the potential to improve stream water quality, however the net effect instream is not well understood. We monitored two urban and two suburban watersheds in Charlotte, NC to determine if mitigation controlled the fraction of total mass export during storm, if development classification as either urban or suburban (defined by the age, density and distribution of urban development) controlled storm nutrient and carbon dynamics, and if stormwater control measures were able to change stream water chemistry. While average concentrations during stormflow were generally greater than baseflow, indicating that storms are important times of solute export, the fraction of storm-derived export was unrelated to mitigation by stormwater control measures. Development classification was generally not an important control on export of N and dissolved organic carbon. However, event mean concentrations of orthophosphate were higher at the suburban sites, possibly from greater fertilizer application. Stormwater control measures influenced instream water chemistry at only one site, which also had the greatest mitigated area, but differences between stormwater control measure outflow and stream water suggest the potential for water quality improvements. Together, results suggest stormwater control measures have the potential to decrease solute concentrations from urban runoff, but the type, location, and extent of urban development in the watershed may influence the magnitude of this effect.

A model of hydrology and water quality for stormwater control measures

Abstract Stormwater control measures (SCMs) are a management strategy that can reduce runoff and nitrogen (N) export from urban watersheds by creating an ecosystem that stores water and promotes N retention and removal. This work develops and explores a computer model that simulates the dynamic hydrologic and water quality processes of SCMs. Data from an SCM in Charlotte, NC was used for model calibration and validation. The hydrologic routines successfully simulated distributions of observed storm event outflow volumes and duration without calibration. Through calibration, the model reproduced observed distributions of event mean concentrations of both nitrate and ammonium in SCM outflow. Model sensitivity to varied air temperature, inflow N concentrations, and inflow volumes was determined. Scenario testing showed that deeper SCMs have greater inorganic N removal efficiencies because they have more stored volume of relatively N-deplete water, and therefore have a greater capacity to dilute relatively N-rich inflow.

Denitrification along the Stream-Riparian Continuum in Restored and Unrestored Agricultural Streams

Although floodplain restoration may improve stream-riparian connectivity and increase channel stability through cross-vane installation, stream bank regrading, and buffer revegetation, the impact of these geomorphic alterations on denitrification across the aquatic-terrestrial interface is poorly understood. Seasonal denitrification enzyme activity (DEA) was compared in stream-riparian ecotones of four streams with varying hydrologic connection, riparian vegetation composition, and agricultural influence. Riparian and stream DEA was generally higher in sites adjacent to agricultural fields due to proximity to nitrate source. Mean DEA was higher in the dormant season (riparian: 928 ± 116 ng N g dry mass [DM] h, stream: 108 ± 149 ng N g DM h) than in the growing season (riparian: 355 ± 55 ng N g DM h, stream: 45 ± 40 ng N g DM h) and was influenced by antecedent precipitation, soil texture, and landscape characteristics (land use, vegetation type, topography) in multivariate models. Hot spots of denitrification occurred at the confluence of fine soil textures with high organic matter and moisture, often at low-lying areas of the landscape. At the restored site, we observed enhanced denitrification in the reconnected floodplain but decreased stream denitrification capacity in pools. Across all sites, streams had lower DEA than riparian zones, stressing the importance of restoration approaches designed to conserve riparian areas and maintain hydrologic connectivity. Easily obtainable data including precipitation, adjacent land use, vegetation, and stream and floodplain geomorphology reasonably predicted denitrification potential compared with more complex models, showing potential for application in water resources and nitrogen management.

Before the storm: Antecedent conditions as regulators of hydrologic and biogeochemical response to extreme climate events

While the influence of antecedent conditions on watershed function is widely recognized under typical hydrologic regimes, gaps remain in the context of extreme climate events (ECEs). ECEs are those events that far exceed seasonal norms of intensity, duration, or impact upon the physical environment or ecosystem. In this synthesis, we discuss the role of source availability and hydrologic connectivity on antecedent conditions and propose a conceptual framework to characterize system response to ECEs at the watershed scale. We present four case studies in detail that span a range of types of antecedent conditions and type of ECE to highlight important controls and feedbacks. Because ECEs have the potential to export large amounts of water and materials, their occurrence in sequence can disproportionately amplify the response. In fact, multiple events may not be considered extreme in isolation, but when they occur in close sequence they may lead to extreme responses in terms of both supply and transport capacity. Therefore, to advance our understanding of these complexities, we need continued development of a mechanistic understanding of how antecedent conditions set the stage for ECE response across multiple regions and climates, particularly since monitoring of these rare events is costly and difficult to obtain. Through focused monitoring of critical ecosystems during rare events we will also be able to extend and validate modeling studies. Cross-regional comparisons are also needed to define characteristics of resilient systems. These monitoring, modeling, and synthesis efforts are more critical than ever in light of changing climate regimes, intensification of human modifications of the landscape, and the disproportionate impact of ECEs in highly populated regions.