Vertical motion is an important driver of sunlight exposure in aquatic environments, shaping the growth and fate of materials and organisms. We derive a simple model accounting for turbulent depth fluctuations of particles to predict the depth that contributes the most sunlight exposure (effective depth) as well as the single depth that, if measured at one place over time, produces the same total sunlight exposure as a moving particle (functional depth). Field measurements of light and depth in rivers using neutrally buoyant drifters and buoys validate our model. Effective depth varied from 0.1 to 1.5 m below the water surface and was ~ 30% of the overall water depth on average. Functional depth varied from 0.67 to 2.3 m and was ~ 50% of the overall water depth on average. Functional and effective depth are physically based concepts incorporating turbulent motion, spatial variability, and water clarity offering new approaches to characterize light exposure in aquatic environments.

INTRODUCTION:Army recruits conducting BCT are among the most susceptible population of military personnel to experience exertional heat illness, a concern expected to become increasingly urgent due to steadily rising temperatures. In this study, we provide an empirical analysis of wet bulb globe temperature (WBGT) index trends at U.S. Army BCT installations and quantify the magnitude of these trends. Assuming these warming trends continue, the anticipated effects of increasing temperature trends are discussed in relation to potential impacts on recruit heat illness incidence and training disruption. MATERIALS AND METHODS:We obtained weather data beginning in the early 1960s, including WBGT index measurements derived by the U.S. Air Force 14th Weather Squadron. We apply these datasets to two classifications for high WBGT index days, including one classification accounting for heat illness susceptibility based on prior day heat exposure, to determine when recruits are most at risk of heat illness. The daily likelihood of extreme WBGT index values is described at each installation using a 30-year climatological average. Trends in the WBGT index are evaluated quantitatively during the warm season (May 1-September 30) and full year and compared between decades and by individual BCT classes. RESULTS:Trends in the WBGT index have increased at all four BCT installations. Between January 1960 and October 2022, the mean WBGT index value increased most quickly at Ft Jackson, SC (0.272°C decade-1, CI: 0.255-0.289) and least at Ft Moore, GA (0.190°C decade-1, CI: 0.170-0.210). Ft Moore experiences the greatest heat burden, with the daily likelihood of experiencing a "black flag" event (≥90°F WBGT index) peaking at nearly 50% in late July, while Ft Leonard Wood, MO, experiences the least heat burden. This heat burden is spread unevenly across installations and dependent on BCT class start date. Recruits beginning in mid-June will experience approximately 200 hours of hazardous heat during BCT at Ft Moore, GA; 100 hours at Ft Jackson, SC; 80 hours at Ft Sill, OK; and 61 hours at Ft Leonard Wood, MO. CONCLUSIONS:Temperatures measured on the WBGT index have steadily increased at US Army basic training installations since at least 1960. In the future, adaptation to the BCT program will be required to maintain rigorous standards without incurring unacceptable risk of recruit heat illness. The analysis provided by this study can help inform medical, training, and policy implementations needed to ensure continued BCT in a warming world.

Households that cannot be able to afford their water bills may lose access to drinking water and wastewater services. This study seeks to quantify how many households may struggle to pay for water services across 787 of the largest drinking water providers located within each state of the United States. Household water affordability is the ability for a household to pay for basic water services without undue hardship. Here, we select 6,000 gallons per month (22.7 m 3 /mo) as sufficient to meet basic needs and define undue hardship as spending more than 4.6% of household income (one day of labor each month) to pay for water services. Monthly bills are combined with census income data based on service area boundaries to determine how many households are spending more than 4.6% of their income on water services. We find that basic water services are unaffordable for 17% of the households (28.3 million persons) in this study. The median, or representative community, has one in seven households spending more than 4.6% of their income paying for water services. We developed a data visualization tool to allow users to explore how affordability challenges change across different volumes of water usage and levels of financial hardship ( https://nicholasinstitute.duke.edu/water-affordability/water-affordability-united-states ). This research shows that household water unaffordability is not a localized problem but rather is a challenge experienced by households in communities across the nation.

Key Takeaways Of 301 analyzed utilities in North Carolina, 51% reported operating revenues less than operating expenditures—operating ratios (ORs) less than 1. Affordability burden and OR were both affected by utilities’ respective population size, population growth or loss rate, and median household income. Annual supplemental funding would be needed to provide all 301 utilities with sufficient revenue to cover operating expenses to ensure ORs are greater than 1. When evaluating consolidation of the utilities into a collective entity, the cumulative revenue of the 301 utilities well exceeded the sum of operating expenses.

Key Takeaways Water affordability is a growing concern, with inflation, aging infrastructure, source water protection, climate change, and other factors pushing up the cost of providing water. Customer assistance program (CAP) rate discounts provide needed assistance but may not be sufficient to ensure that water services are affordable. Rather than relying on one approach, such as CAPs, a combination of approaches might be optimal for addressing water affordability issues.

The cost of providing water services is increasing, placing greater financial burdens on individual households and utilities. Five metrics were calculated at multiple volumes of water usage and were applied to 1791 utilities, estimating bills from 2020 rates data, to gauge financial burdens in four states. More than a fifth of the population in 77% of utilities was experiencing poverty, suggesting widespread poverty is a major contributor to utility financial capability challenges. The Income Dedicated to Water Service metric was developed to understand how many households share a similar financial burden, exploring both the depth and prevalence of affordability challenges. Depending on water use, a tenth to a third of households work more than a day each month to afford water bills. This approach and an interactive visualization tool bring greater transparency to understand the scale of affordability and financial capability challenges ( https://nicholasinstitute.duke.edu/water-affordability/water-affordability-dashboard ).

Stream channels in the American Southwest are misunderstood because they have been understudied and overgeneralized. This misunderstanding has serious consequences for environmental policy, particularly in defining the scope of jurisdiction under the Clean Water Act. While the current status of regulations defining the scope of jurisdictional waters is in flux, the currently-codified regulations require that jurisdictional tributaries must have a physical, chemical, or biological connection to downstream traditional navigable waters. The lack of consistent flow in arid stream channels causes misinterpretations of physio-chemical indicators that are commonly used in perennial streams of humid environments. Here we overview the use of riparian vegetation as a biological indicator of stream channel presence and connectivity in arid environments. Based on a thorough literature review and synthesis of arid region studies, we found key spatiotemporal patterns of arid riparian vegetation that could potentially be used to determine hydrologic connectivity. Much of the vegetation along arid stream channels is well-adapted to water scarcity and varies in response to differences in geomorphology, hydrology, and land use across multiple scales. Riparian vegetation in arid environments ranges from hydroriparian to xeroriparian and can include, or be made up exclusively, of upland species. Regardless of species type, plants near stream channels tend to be denser and larger than upland plants. Access to reliable water results in denser, larger plants and more water-dependent species. To demonstrate the potential of riparian vegetation as an indicator of connectivity, we conclude with a case study of Little Sycamore Wash in northern Arizona. We used the Normalized Difference Vegetation Index to (1) distinguish between riparian and upland vegetation, and (2) assess connectivity of patches of riparian vegetation. By understanding the spatiotemporal variability of riparian vegetation along arid stream channels, we can make better decisions on their regulation and management.

Key TakeawaysWhen people and industries leave a community, water utilities face the potential loss of revenue from departing customers and the cost and issues associated with maintaining excess system capacity.Water systems seek to (1) ensure affordability, (2) maintain high service and quality, and (3) sustain fiscal viability; this creates a trilemma for shrinking cities that can ensure only two of the three.To meet challenges, water service providers in shrinking cities need flexible approaches, such as downsizing, diversifying revenue sources, consolidating with other systems, or privatizing; absent such changes, the federal or state governments must step in.

Sunlight is a critical resource in aquatic systems driving photosynthesis, photodegradation of organic matter and contaminants, animal behavior, and the activity of human pathogens. In rivers, solutes, materials, and organisms are turbulently mixed across the water column during downstream transport and exposed to highly variable sunlight. However, there are no measurements of suspended particles' sunlight exposure during downstream transport to characterize this variability, and it is unclear if current measurement approaches and optical theory capture the light exposure of suspended particles. We deployed neutrally buoyant drifters and stationary buoys in the Upper Mississippi (WI, U.S.A.) and Neuse Rivers (NC, U.S.A.) to measure underwater sunlight from the perspective of suspended particles. In our study sites, underwater sunlight varied more along flowpaths measured by drifters than over time measured by fixed‐site buoys; sunlight exposure along flowpaths was dominated by bursts of light (sunflecks) that accounted for 62–99% of the cumulative sunlight exposure; and modeled sunlight exposure using optical theory was consistently 56–1700% higher than measured sunlight exposure along flowpaths. Our results suggested that suspended particles in the study reaches experienced darker conditions than predicted and have important implications for how to quantify underwater sunlight in rivers.

Most major rivers in the United States are managed by a system of reservoirs; many of which were built more than a half century ago. These reservoirs were designed based on environmental, societal, and regulatory assumptions at the time of construction. Since then, we have learned that climate is not stationary, population growth is being decoupled from energy needs and water demand, and new regulations (such as the Clean Water Act and Endangered Species Act) affect how river systems are managed. This study explores changing environmental, societal, and regulatory conditions relevant to the design and operation of U.S. Army Corps of Engineers reservoirs across the conterminous United States. Results demonstrate large geographic variability in how these conditions have changed over time. In the south-western United States, there is an amplified trend towards drier conditions and less reservoir flexibility with warmer temperatures, less precipitation, high sedimentation rates, and large population growth. In the north-eastern United States, the impacts of increased temperature on reservoirs may be masked by greater precipitation and lower water demand. Environmental, societal, and regulatory changes can reduce the flexibility of reservoir operations and, in some instances, make it challenging for the reservoir to meet its intended purpose as designed decades ago. This study is the first step towards formalizing a process for monitoring broad trends relevant to water resources management for the purpose of moving towards adaptation of infrastructure. An interactive tool was developed for each condition: .

Headwater streams draining urbanized watersheds are subject to frequent and intense storm flows. These floods can disrupt metabolic processes occurring in benthic biofilms via the removal of biomass (i.e., scouring flows, bed mobilization) or light attenuation due to turbidity. Furthermore, channel incision caused by frequent hydraulic disturbance alters the geomorphology of streams, indirectly changing the flow and light regimes experienced by benthic biofilms. We measured dissolved oxygen (DO) and modeled whole-stream metabolism for 18 months in six urban headwater streams in the North Carolina Piedmont, U.S.A. All streams were heterotrophic and had low rates of productivity despite relatively high streamwater nutrient concentrations. Light availability at the channel surface explained more of the day to day variation in gross primary productivity within each stream than did hydrologic disturbance. Yet among streams, the explanatory power of light declined with increasing hydrologic flashiness. We found a surprisingly wide range in DO regimes, which ranged from frequent hypoxia to near constant saturation. Hypoxia was more common in streams with lower channel gradients where bedrock outcroppings and culverts create rapid slope transitions between pools. We hypothesize this geomorphic change increases the susceptibility of benthic biota to perturbation during storms and the mean water residence time during baseflow. Increased water residence times together with elevated organic matter and nutrient inputs can set up ideal conditions for hypoxia at baseflows punctuated by frequent scouring storm flows. As a result, benthic biota are caught between hydrologic and chemical extremes that constrain their productivity.

Reservoirs are critical infrastructure typically built to function as designed for 50 to 100 years. The majority of U.S. Army Corps of Engineers reservoirs are more than 50 years old. The environmental, societal, and regulatory conditions surrounding the reservoir, that is, the reservoir's expected conditions, shaped its design. Many of these expectations assumed a future similar to the past. However, recent decades have experienced warming climates, cyclical changes in precipitation, the introduction of new regulations, and populations concentrating in urban environments. The design documents for nine U.S. Army Corps of Engineers were obtained to compare the expected conditions when reservoirs were authorized with the conditions experienced since the reservoir began operating. In some instances, we found large differences between expectations and reality. Average precipitation at Philpott, North Carolina was 15% less than expected whereas the sedimentation rate at Redmond, Kansas was twice the expected rate. Reservoirs can adapt to changing conditions by updating water control plans, which has occurred at five of these reservoirs in the last decade. Reallocations are sometimes needed to address more significant changes. For example, Redmond has reallocated storage space due to higher than expected sedimentation, and Falls, North Carolina is seeking reallocation due to higher than expected population growth and water demand. As conditions change, controversies and litigation around Corps reservoir management will likely continue. This highlights the importance of clearly documenting changing conditions through consistent and ongoing data collection and analysis to facilitate adapting reservoir operations in a timely manner, thereby minimizing controversy.

Reservoir operations must respond to changing conditions, such as climate, water demand, regulations, and sedimentation. The U.S. Army Corps of Engineers (Corps) can reallocate reservoir storage to respond to such changes. We assembled and analyzed a database of reservoir reallocations implemented and proposed by the Corps. While only a small portion of total reservoir storage nationwide has been reallocated, there are substantial differences in reallocation frequency and magnitude across the nation: some Corps Districts and Divisions use reallocation while others do not, relying more on discretion and small-scale adaptation of operations. This difference illustrates how water resource agencies like the Corps decentralize management decisions to allow responding to disparate conditions. Decentralized decision-making provides a responsive approach to water management, while centralized and hierarchical decision-making is a slower, more deliberative approach. Decentralized decision-making may lead to the accumulation of short-term, local decisions over time to the point that the system is managed differently than anticipated. Reallocation, which is a form of planned adaptive management, can be accommodating of multiple competing demands and different stakeholders, yet expensive and less temporally responsive. The challenge for any large water resource management agency is to balance between local-level, responsive discretion vs. centralized, planned decision-making.

Western water infrastructure was funded in the early and mid‐20th Century through federal financing through the Bureau of Reclamation. Over the past 30 years, federal financing has been less forthcoming, which has been commensurate with an increase in the need for financing rehabilitation and replacement of western irrigation infrastructure. As federal appropriations have declined, there has been increased interest in alternative approaches to infrastructure including public–private partnerships (P3s), loan guarantees, or title transfer of federal infrastructure. However, two of these approaches — P3s and loan guarantees — are precluded by existing federal budgetary policies, particularly Office of Management and Budget (OMB) scoring practices. If the OMB changed its policies for P3s or loan guarantees, private capital could play an important role in recapitalizing aging Reclamation infrastructure.

We examined macroinvertebrate drift at 4 sites downstream of Abanakee Dam on the Indian River, NY, on separate days at base-flow conditions and following days during recreational releases (rapid releases supporting white-water rafting enterprises). Macroinvertebrate drift rates were highest near the dam due to high numbers of drifting Simuliidae at both base flow and during a release. At the other 3 sites, Simuliidae were less abundant in the drift, and Chironomidae and Sphaeriidae had especially high drift densities during a release, suggesting a greater vulnerability to catastrophic drift. Macroinvertebrate drift was not affected by differences in stream gradients or shear forces that did differ between sites. Our drift densities during the recreational releases were higher than observations from other studies during natural floods, suggesting greater drift vulnerability to rapid increases (similar to 15 min) in discharge when flood gates are opened.

Aquatic ecosystems have two distinct zones: the water column and benthic zone. Although the benthic zone has received considerable attention, recent studies have found the water column capable of accounting for a majority of whole ecosystem processes in rivers. The relative role of these zones inevitably varies across a size continuum of rivers, from headwaters to large transcontinental systems. A fundamental question in aquatic science is where along this size continuum do ecosystem processes potentially shift from occurring largely in the benthic zone to largely in the water column? Sediment structures the physical template of the benthic and water column zones of rivers and the contact area between water and sediment mediates ecological, geochemical, and physical processes. High concentrations of suspended sediments are hypothesized to cause a shift from benthic to water column dominance in rivers. We developed an analytical model for the contact area between surface water and all sediment particles in benthic and water column volumes. The model was implemented with empirical data along the main stem of major US rivers. The ratio of water column to benthic sediment contact area scaled as a power function of watershed area. There was more sediment–water contact area in the water column than the benthic zone in rivers equal to or greater than 5th to 9th order depending on the river basin. This suggests material processing could be occurring largely in the water column in rivers greater than 5th order. However, dams and variation in discharge caused rivers to oscillate between water column and benthic dominance over time and space.

Habitat fragmentation restricts the movement of individuals across a landscape. In terrestrial and aquatic systems, barriers to movement can modify population and community dynamics at local or regional scales. This study contrasted life history traits related to lifespan with habitat fragmentation to determine impacts on species population genetic structure in the Neuse River Basin, USA. For this, we simulated gene flow among evenly-spaced populations in a river network and tracked individual and population genetics for 200 years. The modeled scenarios represent a full cross between five life history strategies and four riverscapes representing varying degrees of fragmentation. The five life history strategies include species (based on freshwater mussels) with average lifespans ranging from 10 to 50 years and age at maturity from 2 to 6 years. The movement landscapes included a (1) panmictic, (2) stepping-stone landscape allowing movement to only neighboring populations during each dispersal event, (3) partially-fragmented landscape divided by dams currently in the network, and (4) fully-fragmented landscape. Results suggest species with shorter lifespans have higher population genetic structure in fragmented landscapes than species with longer lifespans. Furthermore, species with shorter lifespans in highly fragmented landscapes may be harboring genetic degradation or decline as allele fixation and loss. Although anthropogenic fragmentation of many river systems is only 100–200 years old, the simulation indicates that species can respond genetically in that period of time. Additionally, the time frame of the simulation suggests that genetic impacts of habitat fragmentation in some species present in the Neuse River Basin may not yet be manifesting and restoration activities could be successful.

The United States (U.S.) Army Corps of Engineers operates reservoirs across the U.S. with 89% of reservoirs constructed prior to 1980. Many reservoirs have experienced changes in environmental conditions (e.g., climate and sediment yield) and societal conditions (e.g., water/energy demand and ecological flows) since construction. These changes may challenge the potential for reservoirs to meet their operational targets (OTs) (management goals). Historic daily reservoir data and OTs were collected for 233 reservoirs. Analyses were developed to identify when and where reservoirs may be systematically departing from OTs in terms of the frequency and magnitude of departure. Fifty‐six percent of reservoirs consistently met operating targets, 30% were borderline, and 13% experienced frequent and large magnitude departures. Fifty‐two percent of reservoirs with large departures were due to shortages and were located in the South Pacific and Southwestern divisions. This work provides a framework to identify reservoir performance in relation to management goals, a necessary step for moving toward adaptive management under changing conditions. All individual reservoir analyses are provided via an interactive data visualization tool: https://nicholasinstitute.duke.edu/reservoir-data.

A central goal in limnology is measurement of physical, biogeochemical, and biological process rates. We can measure process rates from the temporal and spatial patterns they create in a measured variable, and we use 3 approaches for making those measurements: the fixed-site approach for detecting temporal pattern at a location, the snapshot approach for detecting spatial pattern at an instant in time, and the flow path approach for detecting temporal pattern as it changes through space. To compare and contrast these approaches, we present patterns in temperature collected simultaneously based on all 3 approaches. Translating these patterns into process rates requires different assumptions for each approach, and these assumptions lead to uncertainty in process rates. We propose that these assumptions and related uncertainty can be reduced by making simultaneous measurements based on all 3 approaches. Each approach fills gaps in the spatial and temporal patterns measured by the others, and these patterns can be combined to derive a process rate. We develop a conceptual theory to support this strategy for measuring process rate based on 2 criteria: the mixing time of a water body and the analytical limitations of the measurement. This new strategy for measuring process rates in aquatic environments has the potential to increase the resolution of rate measurements, reduce their uncertainty, and enhance limnologists' ability to resolve process rates from an increasing flow of environmental data.