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Concentration‐discharge (c‐Q) relations have been used to infer watershed‐scale processes governing solute fluxes. Prior studies have documented inconsistent concentration‐discharge patterns at the storm‐event scale driven by changes in end‐member concentrations. Other studies have evaluated c‐Q data from all periods in a composite fashion to quantify chemostasis (relatively invariant changes in concentration over several orders of magnitude variation in streamflow). Here we examine 3 years of high‐frequency nitrate and discharge data (49,861 data points) to complement 14 years of weekly data (699 data points) for an urban stream in Baltimore, MD, U.S. to quantify c‐Q relationships. We show that these relationships are variable through time and depend on the temporal scale at which they are investigated. On a storm‐event scale, the sensor data exhibit a watershed‐specific dQ/Q threshold when storms switch from counter‐clockwise to clockwise c‐Q behavior. On a seasonal scale, we show the influence of hydrologic variability and in‐stream metabolism as controls on stream nitrate concentrations and fluxes. On a composite scale, we evaluate the c‐Q data for chemostasis using analysis of both c‐Q slopes and CVc/CVQ, as a function of time. The slopes of c‐Q data for both long‐term weekly and high‐frequency data sets are in close agreement on an annual basis and vary between dry and wet years; the CVc/CVQ analysis is less sensitive to hydroclimate variability. This work highlights the value of both long‐term and high‐frequency c‐Q data collection for calculating and analyzing solute fluxes.

Widespread deployment of sensors that measure river nitrate (NO 3 −

The relationship between solute concentrations and discharge can inform an integrated understanding of hydrological and biogeochemical processes at watershed scales. Recent work from multiple catchments has shown that there is typically little variation in concentration relative to large variations in discharge. This pattern has been described as chemostatic behavior. Pond Branch, a forested headwater catchment in Maryland, has been monitored for stream nitrate (NO3−) concentrations at weekly intervals for 14 years. In the growing season and autumn of 2011 a high‐frequency optical NO3− sensor was used to supplement the long‐term weekly data. In this watershed, long‐term weekly data show that NO3− concentrations decrease with increasing discharge whereas 6 months of 15‐minute sensor observed concentrations reveal a more chemostatic behavior. High‐frequency NO3− concentrations from the sensor collected during different storm events reveal variable concentration–discharge patterns highlighting the importance of high resolution data and ecohydrological drivers in controlling solute export for biologically reactive solutes such as NO3−.

Considerable variability in the seasonal patterns of stream water nitrate (NO 2 3 ) has been observed in forested watersheds throughout the world. While many forested headwater catchments exhibit winter and early spring peaks in NO 2 3 concentrations, several watersheds have peak concentrations during the summer months. Pond Branch, a headwater catchment in Maryland monitored for over 10 years, exhibits recurrent and broad summer peaks in both NO export from June to September is particularly surprising, given that these summer months typically have the year's lowest discharge. A key challenge is identifying the source(s) of NO 2 3 and the mechanism(s) by which it is transported to the watershed outlet during the summer. In this study, we assessed multiple hypotheses (not mutually exclusive) that could account for the seasonal trend including proximal controls of groundwater-surface water interactions, instream processes, and riparian groundwater-N cycling interactions, as well as two distal controls: geochemical weathering and senescence of riparian vegetation. A combination of long-term weekly and limited duration high-frequency sensor data reveals the importance of riparian ecohydrologic processes during base flow. In this watershed, patterns of seasonal stream water NO 2 3 concentrations and fluxes depend fundamentally on interactions between groundwater dynamics and nitrogen (N) cycling in the riparian zone. Groundwater tables control nitrification-denitrification dynamics as well as hydrologic transport. Our results suggest that in many watersheds, a more sophisticated exploration of NO 2 3 production and NO 2 3 transport mechanisms is required to identify critical points in the landscape and over time that disproportionately drive patterns of watershed NO 2 3 export.

[1] Enhanced consideration of the hydrogeomorphic template of watersheds is critical to understanding watershed nitrogen budgets. We developed a framework to estimate the spatial distribution and temporal dynamics of soil moisture and soil oxygen in surficial soils to scale nitrogen transformations for a forested watershed (Pond Branch) in Maryland, USA. We sampled soil cores in upland, hillslope hollow, riparian hollow, and riparian hummock landscape positions in different seasons for biogeochemical fluxes including measurement of N 2 gas produced via denitrification. We extrapolated these rates in space and time with information derived from in situ soil oxygen and soil moisture probes to scale fluxes from plots to the catchment level. We addressed three questions: (1) How important are seasonal, daily, and storm event variations in soil oxygen for denitrification? (2) How is denitrification spatially distributed through the watershed? (3) How important is denitrification to the watershed nitrogen budget? We found that microtopography within the riparian zone is a significant influence on soil oxygen dynamics and therefore redox-sensitive biogeochemical processes such as denitrification. Riparian zone hollows (lower topographic positions) represented 0.5%-1.0% of the catchment area, but accounted for >99% of total denitrification. Interestingly, topography was a much stronger controller of oxygen than rainfall, which had little influence on temporal variation in soil oxygen levels. Spatial and temporal extrapolations of measured rates suggest that a minimum of 16%-27% of atmospheric nitrogen deposition is lost to denitrification. These results suggest that the importance of denitrification in the nitrogen budget of forested watersheds depends fundamentally on the presence of landscape elements, such as riparian hollows that function as "hot spots" of activity.Citation: Duncan, J. M., P. M. Groffman, and L. E. Band (2013), Towards closing the watershed nitrogen budget: Spatial and temporal scaling of denitrification,

Extensive time and financial resources have been dedicated to address nonpoint sources of nitrogen and phosphorus in watersheds. Despite these efforts, many watersheds have not seen substantial improvement in water quality. The objective of this study is to review the literature and investigate key factors affecting the lack of improvement in nutrient levels in waterways in urban and agricultural regions. From 94 studies identified in the academic literature, we found that, although 60% of studies found improvements in water quality after implementation of Best Management Practices (BMPs) within the watershed, these studies were mostly modeling studies rather than field monitoring studies. For studies that were unable to find improvements in water quality after the implementation of BMPs, the lack of improvement was attributed to lack of knowledge about BMP functioning, lag times, nonoptimal placement and distribution of BMPs in the watershed, postimplementation BMP failure, and socio-political and economic challenges. We refer to these limiting factors as known unknowns. We also acknowledge the existence of unknown unknowns that hinder further improvement in BMP effectiveness and suggest that machine learning, approaches from the field of business and operations management, and long-term convergent studies could be used to resolve these unknown unknowns.

Ultrasound and magnetic resonance imaging (MRI) are both capable of diagnosing full-thickness rotator cuff tears. However, it is unknown which imaging modality is more accurate and precise in evaluating the characteristics of full-thickness rotator cuff tears in a surgical population. This study reviewed 114 patients who underwent arthroscopic repair of a full-thickness rotator cuff tear over a 1-year period. Of these patients, 61 had both preoperative MRI and ultrasound for review. Three musculoskeletal radiologists evaluated each ultrasound and MRI in a randomized and blinded fashion on 2 separate occasions. Tear size, retraction status, muscle atrophy, and fatty infiltration were analyzed and compared between the 2 modalities. Ultrasound measurements were statistically smaller in both tear size (P=.001) and retraction status (P=.001) compared with MRI. The 2 image modalities showed comparable intraobserver reliability in assessment of tear size and retraction status. However, MRI showed greater interobserver reliability in assessment of tear size, retraction status, and atrophy. Independent observers are more likely to agree on measurements of the characteristics of rotator cuff tears when using MRI compared with ultrasound. As tear size increases, the 2 image modalities show greater differences in measurement of tear size and retraction status. Additionally, compared with MRI, ultrasound shows consistently low reliability in detecting subtle, but clinically important, degeneration of the soft tissue envelope. Although it is inexpensive and convenient, ultrasound may be best used to identify a tear, and MRI is superior for use in surgical planning for larger tears. [Orthopedics. 2017; 40(1):e124-e130.].