Impact of a First-Order Riparian Zone on Nitrogen Removal and Export from an Agricultural Ecosystem

Riparian zones are reputed to be effective at preventing export of agricultural groundwater nitrogen (N) from local ecosystems. This is one impetus behind riparian zone regulations and initiatives. However, riparian zone function can vary under different conditions, with varying impacts on the regional (and ultimately global) environment. Rates of groundwater delivery to the surface appear to have significant effects on the N-removing capabilities of a riparian zone. Research conducted at a first-order agricultural watershed with a well-defined riparian zone in the Maryland coastal plain indicates that more than 2.5 kg/day of nitrate-N can be exported under moderate-to-high stream baseflow conditions. The total nitrate-N load that exits the system increases with increasing flow not simply because of the greater volume of water export. Stream water nitrate-N concentrations also increase by more than an order of magnitude as flow increases, at least during baseflow. This appears to be largely the result of changes in dominant groundwater delivery mechanisms. Higher rates of groundwater exfiltration lessen the contact time between nitrate-carrying groundwater and potentially reducing riparian soils. Subsurface preferential flow paths, in the wetland and adjacent field, also strongly influence N removal. Simple assumptions regarding riparian zone function may be inadequate because of complexities observed in response to changing hydrologic conditions

Modelling leaf angle distributions with non-vertical symmetry

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Effect of shallow subsurface flow pathway networks on corn yield spatial variation under different weather and nutrient management

Ground water availability can be a major spatially variable factor of crop yields. In soils with the infiltration-restricting layer, ground water can be organized in the network of channels that conduct water laterally in wet periods and become water storage and water subsidy sources for plants in dry periods. The objective of this work was to quantify the relationships between the distances to the subsurface flow pathway network and corn yield for different weather conditions and nutrient management. Corn yield was monitored across the manured and chemically fertilized fields at the USDA-ARS OPE3 experimental site in Maryland. Data were collected during dry, normal, and above normal years in terms of the amount of precipitation from planting to physiological maturity. The subsurface flow pathway network was delineated using ArcGIS from data on topography of the infiltration-restricting layer found mostly at depths between one and three meters. The geographically weighted regression was used. Adjusted determination coefficients of regressions ranged from 0.485 to 0.655. Decrease of the adjusted determination coefficients from a dry to normal year and an increase from the normal to wet year was found. Factoring the subsurface flow pathway network influence into crop management can be an important component of precision farming strategies

Surface and Subsurface Nitrate Flow Pathways on a Watershed Scale

Determining the interaction and impact of surface runoff and subsurface flow processes on the environment has been hindered by our inability to characterize subsurface soil structures on a watershed scale. Ground penetrating radar (GPR) data were collected and evaluated in determining subsurface hydrology at four small watersheds in Beltsville, MD. The watersheds have similar textures, organic matter contents, and yield distributions. Although the surface slope was greater on one of the watersheds, slope alone could not explain why it also had a nitrate runoff flux that was 18 times greater than the other three watersheds. Only with knowledge of the subsurface hydrology could the surface runoff differences be explained. The subsurface hydrology was developed by combining GPR and surface topography in a geographic information system. Discrete subsurface flow pathways were identified and confirmed with color infrared imagery, real-time soil moisture monitoring, and yield monitoring. The discrete subsurface flow patterns were also useful in understanding observed nitrate levels entering the riparian wetland and first order stream. This study demonstrated the impact that subsurface stratigraphy can have on water and nitrate (NO3-N) fluxes exiting agricultural lands, even when soil properties, yield distributions, and climate are similar. Reliable protocols for measuring subsurface fluxes of water and chemicals need to be developed