H21A-01
Controls on Watershed Biogeochemistry by Climate, Land Cover, and Soils
Water and elemental discharge from catchments is largely controlled by climate, soils, and land cover. Seasonal temperature and rainfall patterns determine the water available for stream discharge, and average annual air temperature is inversely correlated with the proportion of rainfall that is annually discharged as stream flow. At higher temperatures, an increasing fraction of the soil water is evapotranspired to the atmosphere as water vapor rather than discharged as stream flow. Hydrologic soil drainage properties determine whether precipitation is primarily directed horizontally as overland flow or vertically as infiltration to groundwater for baseflow. Concentrations of N in groundwater, primarily nitrate, reflect the surface land uses, particularly agriculture and residential areas with septic systems. However, hydric soils act as a trap for anthropogenic nitrate in groundwater, probably by denitrification in oxygen-poor, C-rich, micro-environments. Likewise, the P content of surface soils controls the P concentration in overland flow due to leaching of soluble P and erosion of particulate P. At the watershed scale, concentrations of N and P in stream discharge are augmented in proportion to the fraction of the basin in anthropogenic land uses such as agriculture and urban areas, which contribute nutrients via application of fertilizers and disposal of human waste. This anthropogenic fraction of a basin's land uses represents the human footprint upon the land which primarily determines the elevated N and P losses from the basin. N is relatively easily sampled because it is primarily transported as highly soluble nitrate in groundwater-supported baseflow; however, P sampling is more difficult because transport largely occurs episodically following storm events when both stream flow and P concentrations are high during brief periods. As a result, P concentrations and fluxes are almost certainly undersampled and underestimated compared to N fluxes, and fewer of the biogeochemical processes influencing P transport have been clearly identified.
<a href='http://www.hpl.umces.edu/gis_group/'>http://www.hpl.umces.edu/gis_group/</a>
H21A-02
The Influence of Ground Water and Watershed Processes on Nutrient Delivery to the Chesapeake Bay.
The interaction between ground water and other watershed and hydrologic processes is having a significant impact on nutrient delivery to, and water-quality conditions in, the Chesapeake Bay and other estuaries. Ground water transports a significant amount (about half) of the water and nitrogen to streams in the watershed and is therefore an important pathway for nitrogen to reach Chesapeake Bay. The amount of ground water contributing to total flow in streams ranged from 16 to 92 percent with a median value of 54 percent, and varied in different hydrogeologic settings. The residence time of ground water in the surficial aquifer ranged from modern to 60 years old, but did not appear directly related to different hydrogeologic settings. Ground-water nitrate loads contributed nearly half (48 percent) of the total nitrogen load to streams, with a range at individual sites of 17 to 80 percent. Factors affecting nitrogen transport in ground water include spatial and temporal variation of nitrogen inputs, ground-water age, and aquifer processes that lead to denitrification. Additional watershed characteristics, including soil permeability and in-stream loss, are contributing to losses of about 80 percent of nitrogen and 95 percent of phosphorus that is applied to the land surface and transported to estuarine waters of the Bay. The highest loss rates were in the smallest streams and lowest loss rates were in the largest streams. Climate variability and its subsequent influence on streamflow also had a large impact on the amount and timing of nutrient and sediment loads to the Bay. Human-induced changes of nutrient sources in the Bay watershed are the primary causes for their increased concentration in rivers; streamflow variability has resulted in differences of up to 500 percent between the annual phosphorus loads, and up to 300 percent for annual nitrogen loads. While the multiple processes affecting nutrient delivery make restoring water-quality conditions in the Bay very difficult, the information is being used to better plan and target the types of management actions that may provide the greatest water-quality improvement to the Bay ecosystem.
<a href='http://chesapeake.usgs.gov'>http://chesapeake.usgs.gov</a>
H21A-03 INVITED
Spatiotemporal hydrological patterns in the landscape: Water quality implications
Linking spatiotemporally varying hydrologic processes and streamwater quality remains an important research challenge and a critical knowledge gap for improving management strategies. Indeed, the very term "nonpoint source pollution" implies that we cannot generally identify specific flowpaths connecting streams with their surrounding watersheds. Areas where the soil is saturated to the surface play important roles in terrestrial biogeochemistry and watershed hydrology, especially in humid, well-vegetated areas, like the northeastern US. In this talk we present an overview of our work to identify and predict the spatial and temporal distributions of surface saturation and to improve our understanding of how these areas impact water quality. We are finding that areas especially prone to saturation, i.e., hydrologically sensitive areas, account for a disproportionately large fraction of storm runoff and nutrient loss. We suggest targeting these areas when developing water quality protection strategies and will present some recent examples. As our understanding of the relevant hydrological and biogeochemical processes controlling water quality improves, we are making simultaneous improvements in the tools available for environmental-protection professionals, taking advantage of new technologies. At the same time, technological advances in areas like molecular biogeochemistry and nanotechnology are providing exciting new opportunities to investigate landscape hydrological and transport dynamics.
<a href='http://www.bee.cornell.edu/swlab/SoilWaterWeb/index.htm'>http://www.bee.cornell.edu/swlab/SoilWater Web/index.htm</a>
H21A-04 INVITED
Using Stable Isotopes of N and O in Nitrate as Indicators of Denitrification in Agricultural Watersheds
We assessed the feasibility of using the natural abundances of $^{15}$N and $^{18}$O in groundwater and stream water nitrate as indicators of denitrification in agricultural watersheds on the Maryland Coastal Plain. On one farm, we sampled shallow groundwater at the edge of croplands and within adjacent riparian buffers. Groundwater nitrate concentrations were lower inside the buffers than at the cropland edge. Patterns of isotope abundance were consistent with denitrification because both $\delta^{15}$N and $\delta^{18}$O increased as nitrate concentration decreased, reflecting the preferential denitrification of nitrate containing lighter isotopes. We also analyzed the $\delta^{15}$N and $\delta^{18}$O of nitrate in baseflow from 15 third-order streams. Baseflow nitrate concentrations increased with the proportion of agricultural land in the stream's watershed (r$^{2}$= 0.74). The residuals of the regression of nitrate concentration vs. percent agriculture were inversely correlated with the $\delta^{18}$O of the nitrate. That is, watersheds with lower nitrate concentrations than would be expected from the proportion of agricultural land had higher $\delta^{18}$O composition in the nitrate. This is consistent with the hypothesis that denitrification decreases nitrate concentrations to different extents in the different watersheds. The effect of denitrification on $\delta^{15}$N in stream nitrate was less clear, possibly because $\delta^{15}$N is also influenced by nitrate source, being higher in nitrate derived from manure than in nitrate derived from inorganic fertilizer. Our results suggest that the $\delta^{18}$O of nitrate could be used to assess denitrification at the scale of entire watersheds.
H21A-05 INVITED
Propagation and dissipation of watershed biogeochemical signals in stream networks
Examining the loss of elements from watersheds is an important tool for interpreting and understanding biogeochemical cycling. Yet attributing these biogeochemical signals to particular landscape features (e.g. land use) or specific events (e.g. disturbance) requires certain assumptions to be made. Element output patterns have been variously linked directly to vegetation, to soils, to geology, or to instream or riparian processes. As the final portion of the flowpath, stream ecosystems can exert disproportionate control on the timing, magnitude and form of watershed element outputs. In this talk we present a review of classical interpretations of watershed nitrogen export, present a literature synthesis of the relationship between soil solution and stream nitrate concentrations, and conclude with new data documenting how element output signals from an urban upper subcatchment are muted and transformed during stream transport through a protected lower watershed. This research has important implications for improving our understanding of the relationship between streams and their catchments, and for effective planning and design of river restoration efforts.