Jonathan M. Duncan

Mechanisms driving the seasonality of catchment scale nitrate export: Evidence for riparian ecohydrologic controls

Considerable variability in the seasonal patterns of stream water nitrate ( NO3−) has been observed in forested watersheds throughout the world. While many forested headwater catchments exhibit winter and early spring peaks in NO3− 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 NO3− concentrations and watershed export. Higher NO3− export from June to September is particularly surprising, given that these summer months typically have the years lowest discharge. A key challenge is identifying the source(s) of NO3− 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 NO3− 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 NO3− production and NO3− transport mechanisms is required to identify critical points in the landscape and over time that disproportionately drive patterns of watershed NO3− export.

Climate Variation Overwhelms Efforts to Reduce Nitrogen Delivery to Coastal Waters

We calculated watershed nitrogen (N) retention (inputs–outputs)/inputs) each year from 1999–2013 for nine sub-watersheds along an urban–rural gradient near Baltimore MD to determine how land use and climate influence watershed N flux. Retention is critical to efforts to control coastal eutrophication through regulatory efforts that mandate reductions in the total maximum daily load (TMDL) of N that specific water bodies can receive. Retention decreased with urbanization as well as with increases in precipitation with retention decreasing from an average of 91% in the forested sub-watershed to 16% in the most urban sub-watershed. Export was 23% higher, and retention was 7% lower in winter (November–April) than during the growing season. Total N delivery to Baltimore Harbor varied almost threefold between wet and dry years, which is significant relative to the total annual export allowed for all non-point sources to the harbor under the TMDL. These results suggest that expectations for TMDLs should consider watershed land use and climate variability, and their potential for change if they are to result in improvements in receiving water quality.

Tapping Environmental History to Recreate America’s Colonial Hydrology

To properly remediate, improve, or predict how hydrological systems behave, it is vital to establish their histories. However, modern-style records, assembled from instrumental data and remote sensing platforms, hardly exist back more than a few decades. As centuries of data is preferable given multidecadal fluxes of both meteorology/climatology and demographics, building such a history requires resources traditionally considered only useful in the social sciences and humanities. In this Feature, Pastore et al. discuss how they have undertaken the synthesis of historical records and modern techniques to understand the hydrology of the Northeastern U.S. from Colonial times to modern day. Such approaches could aid studies in other regions that may require heavier reliance on qualitative narratives. Further, a better insight as to how historical changes unfolded could provide a “past is prologue” methodology to increase the accuracy of predictive environmental models.

Variable Nitrate Concentration‐Discharge Relationships in a Forested Watershed

&NA; 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−.

Dynamics of nitrate concentration-discharge patterns in an urban watershed

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 three-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 USA 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.