Diana L. Karwan

Trees are found ubiquitously in urban environments and are appreciated for a range of ecosystem services. However, their ability to reduce stormwater runoff volumes – and any tradeoffs involved with nutrients and other pollutants typically carried by stormwater runoff – are largely overlooked in stormwater management practices due to lack of robust data. We undertook a three-year project to monitor tree-scale water quantity and quality fluxes and examine how these local measurements and patterns related to the watershed observations across the city of Saint Paul, Minnesota. In this conference paper, we report on our multi-year effort to quantify the effects of trees on the urban hydrologic cycle, which included measuring the stormwater interception capacity of urban trees and their contributions to coarse organic matter, nitrogen, and phosphorus fluxes, using a series of watersheds in the Saint Paul, Minnesota, USA. At the tree scale, we quantified patterns in canopy throughfall amount, transpiration, and nutrient fluxes in canopy throughfall, which relates directly to stormwater runoff generated under deciduous trees. Under most species at most sites, canopy throughfall was statistically lower than open precipitation. Transpiration rates, determined by the sapflux method, differed across individual trees, with tree health and canopy defoliation explaining some of the differences between individual trees. Canopy interception also altered throughfall nutrient concentrations and fluxes relative to open precipitation. In spring and summer seasons between 2023-2024 mean soluble reactive phosphorus (SRP) and total organic carbon (TOC) fluxes were significantly higher under ash and maple trees than in open precipitation despite lower overall water fluxes under the canopy. With this process, deciduous trees potentially contribute increased phosphorus fluxes to stormwater while leaves remain on the tree canopy. Beyond the individual tree scale, stormwater fluxes, as monitored at the watershed level, showed variation with landcover. The presence of deciduous street tree canopy in watersheds corresponded to patterns in nutrient concentrations in stormwater at the storm-event and seasonal time scales.

Chronic wasting disease (CWD) is a contagious prion disorder affecting cervids such as deer, elk, caribou, and moose, causing progressive and severe neurological degeneration followed by eventual death. As CWD prions (PrP^Sc) accumulate in the body, they are shed through excreta and secreta, as well as through decomposing carcasses. Prions can persist in the environment for years, posing significant concerns for ongoing transmission to susceptible cervids and pose an unknown risk to sympatric species. We used a validated protocol for real-time quaking-induced conversion (RT-QuIC) in vitro prion amplification assay to detect prions in soil collected within and around an illegal white-tailed deer (Odocoileus virginianus, WTD) carcass disposal site and associated captive WTD farm in Beltrami County, Minnesota. We detected PrP^Sc in 26 of 201 soil samples across 15 locations within the illegal disposal site and one on the farm that housed the cervids. Importantly, a subset of RT-QuIC positive soil samples was collected from soils where carcasses were recovered, providing direct evidence that environmental contamination resulted from this illegal activity. These findings reveal that improper cervid carcass disposal practices may have important implications for ongoing CWD transmission through the environment.

Objective: We described source and phosphorus (P) retention potential of soft, fine-grained, streambed sediment and associated phosphorus (sed-P) during summer low-flow conditions. Combining in-channel, sed-P storage with relative age provided context on relevance to western Lake Erie Basin management goals. Methods: In 2019, rapid geomorphic assessment (30 reaches) compared streambed-sediment storage (S) to streambank erosion (E), providing annual sediment budgets (S:E). Streambed sediment (13 reaches) was fingerprinted and analyzed for sed-P. The P saturation ratio (PSR; four reaches) quantified potential sorption/desorption of dissolved P (DP) between the water column and streambed sediment. Analyses were supplemented with data from 2017 and 2021. The ratio of two fallout radionuclides, beryllium-7 (54-day half-life) and excess lead-210 (22.3 years), apportioned “new” sediment based on time since rainfall contact. Results: Streambed sediment was mostly streambank (54–96%) for contributing areas > 2.7 km²; for upstream reaches, a larger percentage was apportioned as upland (cropland, pasture, forest, and road), with < 30% streambank. Streambank erosion correlated with contributing area; however, soil type (ecoregion), stream characteristics, and land use combined to drive streambed-sediment storage. Individual-reach S:E (accumulation of 0.01–35 years of streambank erosion) differentiated erosional and depositional in-channel environments. Most reaches indicated that 17–57% of sediment had recent contact with rainfall. Streambed-sediment PSR indicated a low potential for further sorption of DP from the water column; one reach was a P source when sampled. Conclusion: Sed-P was higher in streambed sediment than in source samples, which varied by land use and ecoregion. This indicates homogenization resulting from in-stream sorption of DP during sediment transport that occurs over multiple events.

Sediment connectivity is a conceptualization for the transfer and storage of sediment among different geomorphic compartments across upland landscapes and channel networks. Sediment connectivity and dysconnectivity are linked to the water cycle and hydrologic systems with the associated multiscale interactions with climate, soil, topography, ecology, and landuse/landcover under natural variability and human intervention. We review current sediment connectivity and modeling approaches evaluating and quantifying water and sediment transfer in catchment systems. Many studies highlight the interaction between sediment and water in defining landscape connectivity, but many efforts to quantify and/or simulate sediment connectivity rely on the topographic/structural controls on sediment erosion and delivery. More recent modeling efforts integrate functional and structural connectivity to capture hydrologic properties influencing sediment delivery. Though the recent modeling development is encouraging, a comprehensive sediment connectivity framework, which integrates geomorphic and hydrologic processes across spatiotemporal scales, has not yet been accomplished. Such an effort requires understanding the hydrologic and geomorphic processes that control sediment source, storage, and transport at different spatiotemporal scales and across various geophysical conditions. We propose a path for developing this new understanding through an integrated hydrologic and sediment connectivity conceptual model that broadly categorizes dominant processes and patterns relevant to understanding sediment flux dynamics. The conceptual model describes hydrologic–sediment connectivity regimes through spatial-temporal feedback between hydrologic processes and geomorphic drivers. We propose that in combining hydrologic and sediment connectivity into a single conceptual model, patterns emerge such that catchments will exist in a single characteristic behavior at a particular instance, which would shift with space and time, and with landscape disturbances. Using the conceptual model as a “thinking” tool, we extract case studies from a multidisciplinary literature review—from hydrology, geomorphology, biogeochemistry, and watershed modeling to remote-sensing technology—that correspond to each of the dominant hydrologic–sediment connectivity regimes. Sediment and water interactions in real-world examples through various observational and modeling techniques illustrate the advancements in the spatial and temporal scales of landscape connectivity observations and simulations. The conceptual model and case studies provide a foundation for advancing the understanding and predictive capability of watershed sediment processes at multiple spatiotemporal scales. Plain language summary: Soil erosion and movement across the landscape are closely linked to rain events and flow pathways. Landscape connectivity is a way to consider how soil erosion from different parts of the landscape is connected to the streams. We explore where soil erosion occurs and how eroded soil moves across the landscape through the interaction with rainfall and drainage. The comprehensive understanding of sediment connectivity and its dependence on rainfall characteristics and watershed hydrology may help to inform the effective distribution of conservation funds and management actions to address water pollution from excess sediment.

Agricultural headwater streams and ditches commonly host dense stands of aquatic vegetation that grow and decay over seasons and exert physical and biological controls on the transport of nutrients from cropland to larger rivers. This study examined changes in the transport of phosphorus (P) in an agricultural drainage ditch in the Maumee River Basin (Ohio, USA) by conducting constant rate injections of a novel tracer mixture (conservative salt [Cl as NaCl], dissolved P [KH₂PO₄], and a fluorescent fine particle) in spring, summer, and fall. We quantified transport behavior for solutes and particles using a traditional transient storage modeling framework consisting of mobile and immobile storage zones connected by a first‐order exchange rate constant. Transient storage was greatest during the spring, when thicker vegetation caused more pooling and flow stagnation, and decreased through fall, as vegetation thinned. Soluble P uptake lengths were 8.7 times longer in fall than spring, likely due to declines in biological uptake rates with colder temperatures and immobile zone storage with thinning vegetation. Particle capture lengths also decreased by a factor of 4.3 from fall to spring. With the increasing eutrophication of Lake Erie and waterbodies around the world that lie downstream from agricultural landscapes, it is beneficial to understand nutrient transport across watersheds, including small agricultural streams. This study highlights the physical and biological roles that aquatic vegetation plays in small agricultural streams by creating seasonally variable immobile zones that slow the flow of nutrients, providing surface area for biofilms, and capturing particles that bind nutrients.

Empirical assessment of how forest disturbance affects streamflow has been traditionally limited to small, experimental catchments. However, larger catchments where landscape management occurs have emergent drivers of streamflow at scale, and thus may exhibit novel responses to land cover disturbance. We used statistical models of water yield and annual maximum peak streamflow for multiple large (> 50 km²) forested catchments in the low‐relief glaciated region of central North America to investigate how forest disturbance and climatic variability affect water yield and peak flows in similar landscapes. We used linear models, linear mixed effects models, and probabilistic flood‐frequency analysis with Bayesian parameter estimation in two case studies. These included: (1) a wildfire that burned ~30% of the 650 km² wilderness Upper Kawishiwi catchment, and (2) 11 catchments within the St. Louis River Basin ranging from 56 to 8880 km² with a patchwork disturbance regime wherein ~0.25% to 1% of the catchment is harvested or converted to non‐forest land use each year. We also assessed the most likely hydrological recovery year after forest disturbance, and the relative importance of stationary and nonstationary drivers of streamflow. We found forest disturbance correlated with declines in water yield for low‐level disturbance regimes in some catchments, but that water yield increased in response to the large‐scale wildfire. Positive and negative associations of forest disturbance with peak flows were observed. Hydrologic recovery time ranged from 5 to 13 years for water yield and peak flows following disturbance. Despite these effects of forest disturbance on streamflow, effects of climatic variability and stationary catchment size factors were more prominent streamflow drivers. Basins larger than ~50 km² in low‐relief glaciated regions can be impacted by forest cover change even on <30% of basin area, but climatic variability and catchment spatial scale has a larger effect than forest disturbance.

Purpose: Nutrient and sediment pollution of surface waters remains a critical challenge for improving water quality. This study takes a user-friendly field-scale tool and assesses its ability to model at both the field and watershed scale within the Fox River Watershed (FRW), Wisconsin, USA, along with assessing how targeted vegetation implementation could attenuate nutrient and sediment exports. Methods: To assess potential load reductions, the nutrient tracking tool (NTT) was used with a scoring system to identify areas where vegetation mitigation could be implemented within three selected FRW sub-watersheds. A corn soybean rotation, an implementation of a 10-m-vegetated buffer, a full forest conversion, and tiling were modeled and assessed. The corn–soybean results were aggregated and compared to watershed level gauge data in two sub-watersheds. Edge-of-field data was compared to modeled results using multiple parameterization schemes. Results: The agricultural areas that scored higher and were untiled showed greater potential nutrient and sediment export reduction (up to 80 to 95%) when vegetation mitigation was implemented in the model. Field-scale results aggregated to the watershed scale showed disparities between modeled and measured phosphorus exports but modeled sediment exports fell within observed gauge data ranges. Field-specific parameter adjustments resulted in more accurate modeled results compared to measured edge-of-field export data but needed further refinement. Conclusion: Targeted mitigation using a vegetation-based scoring system with the NTT model was shown to be a helpful tool for predicting nutrient and sediment reductions. Using a field-scale model aggregated to the watershed scale presents tradeoffs regarding processes found beyond the edge of field.

The catchment approach has been traditionally limited to small, experimental catchments where water fluxes can be determined with high accuracy. However, larger catchments where landscape management occurs have emergent drivers of streamflow at scale, and thus may exhibit novel responses to land cover disturbance. We used statistical models of water yield and annual maximum peak streamflow for multiple forested catchments in the low-relief glaciated region of central North America to investigate how forest disturbance may affect water yield and peak flows in similar landscapes. We utilized linear models, linear mixed effects models, and probabilistic flood-frequency analysis, with Bayesian parameter estimation in two case studies in Minnesota, USA: 1) a wildfire comprising ~30% of a 650km ² wilderness Upper Kawishiwi catchment, and 2) 11 catchments within the St. Louis River Basin ranging from 56 to 8,880 km ² with a patchwork disturbance regime wherein ~0.25% to 1% of the catchment is harvested or converted to non-forest land use each year. We also assessed for the most likely hydrological recovery year after forest disturbance, and the relative importance of stationary and nonstationary drivers of streamflow. We found forest disturbance correlated with declines in water yield for low-level disturbance regimes, but that water yield increased in response to the large-scale wildfire. Positive and negative associations of forest disturbance with peak flows were observed, generally with low confidence. Hydrologic recovery time ranged from 5 to 12 years for water yield and peak flows following disturbance. Despite these effects of forest disturbance on streamflow, effects of climate variability and stationary catchment size factors were more prominent drivers of streamflow. Basins larger than ~50 km ² in low-relief glaciated regions were resilient to forest cover change when it comprised <30% of basin area, but climate change may have a larger effect than could be mitigated by land management.