J. Wendell Gilliam

Modeling hydrology and sediment transport in vegetative filter strips

The performance of vegetative filter strips is governed by complex mechanisms. Models can help simulate the field conditions and predict the buffer effectiveness. A single event model for simulating the hydrology and sediment filtration in buffer strips is developed and field tested. Input parameters, sensitivity analysis, calibration and field testing of the model are presented. The model was developed by linking three submodels to describe the principal mechanisms found in natural buffers: a Petrov‐Galerkin finite element kinematic wave overland flow submodel, a modified Green‐Ampt infiltration submodel and the University of Kentucky sediment filtration model for grass areas. The new formulation effectively handles complex sets of inputs similar to those found in natural events. Major outputs of the model are water outflow and sediment trapping on the strip. The strength of the model is a good description of the hydrology within the filter area, which is essential for achieving good sediment outflow predictions or trapping efficiency. The sensitivity analysis indicates that the most sensitive parameters for the hydrology component are initial soil water content and vertical saturated hydraulic conductivity, and sediment characteristics (particle size, fall velocity and sediment density) and grass spacing for the sediment component. A set of 27 natural runoff events (rainfall amounts from 0.003 to 0.03 m) from a North Carolina Piedmont site was used to test the hydrology component, and a subset of nine events for the sediment component. Good predictions are obtained with the model if shallow uniform sheet flow (no channelization) occurs within the filter. q 1999 Elsevier Science B.V. All rights reserved.

Nitrogen Removal in Streams of Agricultural Catchments—A Literature Review

Excess nutrient loads have been recognized to be the major cause of serious water quality problems recently encountered in many estuaries and coastal waters of the world. Agriculture has been recognized in many regions of the world to be the largest single source of nitrogen emissions to the aquatic environments, and best management practices have been proposed to reduce nutrient losses at the field edge. As a result, there is growing awareness that nutrient management must be handled at the watershed scale. However, the key to nutrient management at the watershed scale is the understanding and quantification of the fate of nutrients both at the field scale and after they enter the aquatic environment. There has been widespread evidence since the late 1970s that nitrogen can be removed from water during its downstream transport in watersheds or basins. Although this information is becoming crucial, no overview has been proposed, so far, to qualitatively as well as quantitatively summarize available information in the literature. For this reason, we propose a review on the biogeochemical processes involved in nitrogen removal in streams, the rates of removal reported, and the factors influencing those rates. Nitrogen removal rates in agricultural streams should be expected to vary between 350 and 1250 mg N m−2 day−1. Mass transfer coefficients (coefficient evaluating intrinsic ability of a stream to remove nitrogen) values in agricultural streams may vary between 0.07 and 0.25 m day−1, although these values correspond to values obtained from reach scale studies. Reviewing values obtained from different measurement scales has revealed that results from incubations or experiments performed in the laboratory clearly underestimate mass transfer coefficients compared to those reported at the reach scale, from severalfold to more than one order of magnitude. Nitrogen removal rates and efficiency in streams are the highest in the summer, and this is critical for receiving ecosystems, which are most sensitive to external inputs at this period of the year. Removal efficiency is the lowest in winter in temperate climates due to high flow and loading combined with lowest removal rates. In-stream processes, on an annual basis, may remove at the watershed scale as much as 10 to 70% of the total N load to the drainage network.

Effects of Regeneration on Hydrology and Water Quality of a Managed Pine Forest

Intensive forest management practices such as drainage, harvesting, site preparation, regeneration, and fertilization have been frequently blamed for problems related to excessive nitrogen, phosphorus, and sediment in receiving waters. Two 25 ha experimental watersheds (D1 – control; D2 – treatment) on a pine plantation in eastern North Carolina have been monitored since 1988 to study the hydrologic and water quality effects of various silvicultural and water management treatments using a paired watershed approach. Data from a two-year calibration period (1988-90) and a four-year regeneration period (2000-03) were used for the analysis. This study period recorded both the highest (2330 mm in 2003) and lowest (850 mm in 2001) rainfall of the 16-years (1988-2003) of record at this site. Nearly seven years after planting, water table elevations returned back to pre-treatment conditions. However, peak flow rates and consequently annual outflows were generally higher on the treatment watershed D2 compared to the control watershed (D1), indicating that the outflows on the treatment watershed may not have completely returned back to base line conditions. Average outflow nutrient (NO3-N, TKN, and Total-P) concentrations for the treatment (D2) watershed for the period from 2000 to 2003 were, however, similar or somewhat lower than their expected values. Although sediment concentration seems to have slightly increased compared to the calibration period, regeneration did not seem to have any effect by the third year after planting, The water quality concentrations were also much lower than the data reported for agricultural lands in the same region. These results will be evaluated and reported soon in the context of prior data after harvesting in 1995 and planting in 1997 to detect the actual effects of regeneration.

Effect of Riparian Buffers and Controlled Drainage on Shallow Groundwater Quality

As a result of recent surface water quality problems in North Carolina, riparian buffers and controlled drainage are being used to reduce the loss of nonpoint source nitrogen. The effect of controlled drainage and riparian buffers as best management practices (BMPs) to reduce the loss of agricultural nonpoint source nitrogen from the Middle Coastal Plain has not been well documented. A two-year study was conducted to determine the effectiveness of controlled drainage, riparian buffers, and a combination of both in the Middle Coastal Plain of North Carolina. It was thought that raising the water table near the ditch might enhance NO3-N removal. Controlled drainage did not raise the water table near the ditch to a greater degree than observed on the free drainage treatment. Over seventeen storm events, the riparian buffer (free drainage) treatment had an average groundwater table depth of 0.92 m, compared to 0.96 and 1.45 m for the combination (riparian buffer and controlled drainage) and controlled drainage treatments, respectively. Treatment locations were assigned randomly. Strictly by chance, the free drainage treatment was installed along a ditch with a shallower impermeable layer compared to the impermeable layer on the controlled drainage treatments (i.e. 2 m versus 3-4 m deep). Percent NO3-N concentration decrease between the field/buffer wells and ditch edge wells for those treatments averaged 22 and 35%, 75 and 51%, and 77 and 69%, for the deep and mid depth wells, in each respective treatment. The distance from the ground surface to the top of the well screen ranged 2.1-3.5 m for the deep wells and 1.5-2.1 m for the mid depth wells, with most at 3.0 m and 1.8 m, respectively. Although apparently more nitrate was removed from the groundwater on the controlled drainage treatments, the controlled drainage treatment water table was not raised closer to the ground surface compared to the free drainage treatment. Nitrate removal effectiveness was attributed to local soil and landscape properties. Differences in local soil properties between the combination riparian buffer and controlled drainage treatment and controlled drainage only were not obvious; however, the impermeable layer on the free drainage treatment was closer to the surface compared to the other treatments.

Effect of Riparian Buffer Width and Vegetation Type on Shallow Groundwater Quality

Agricultural nonpoint sources are a large contributor of nitrogen to many rivers in the Eastern United States, including the Neuse River in North Carolina. Best Management Practices (BMPs) such as nutrient management, riparian buffers, and controlled drainage have been shown effective at reducing nutrient transport from agricultural fields under certain landscape conditions. As a result, recent regulations in North Carolina have mandated that agricultural operations must use a combination of BMPs to reduce the loss of nitrogen. The Middle Coastal Plain is characterized by intensive agriculture on sandy soils with deeply incised or channelized streams. The effectiveness of riparian buffers has not been well evaluated for this landscape. This study was conducted to compare the effect of riparian buffer vegetation type and width on shallow groundwater quality in the Middle Coastal Plain of North Carolina. Five riparian buffer vegetation types were established as follows: cool season grass (fescue), deep-rooted grass (switch grass), forest (pine and mixed hardwood), native vegetation, and no buffer (no-till corn and rye rotation or pasture). These vegetation types were established at two buffer widths, 8 m and 15 m, perpendicular to channelized streams for a total of 10 plots on each of six stream replicates (ranging from intermittent to perennial). Each plot was 24 m long parallel to the stream. A groundwater monitoring well nest was installed at the field/buffer edge and the stream edge in the middle of each riparian buffer plot. Wells were installed at three well depths per well nest. Most deep, middle, and shallow wells were 3.0 m, 1.8 m, and 0.6 m deep from the ground surface to the top of the perforated section, respectively. The perforated section was 0.6 m long. Land use adjacent to the riparian buffer plots was agricultural and included beef cattle pasture along one replicate stream, dairy cattle pasture along two replicates, and row crop agriculture along three replicates. Wells were sampled for 23 months beginning July 1997. Nitrate nitrogen concentration was significantly lower (alpha = 0.1) on approximately half the sampling dates at the middle well depth on the 15 m wide riparian buffer plots compared to the 8 m wide plots. Buffer width was not a significant variable at the deep well depth. Effect of vegetation was not significant at any time. Nitrate removal from the groundwater was greater and less temporally variable in the deep wells than the mid depth wells. For plots where flow was toward the stream and dilutional effects taken into account, nitrate concentration decreased 69 and 28% as groundwater flowed beneath the 8 m wide riparian buffer plots toward the ditch; 84 and 43% beneath the 15 m plots, in the deep and mid depth wells, respectively. Overall, the 15 m wide buffers were approximately 15% more effective than the 8 m buffers at groundwater nitrate removal, while vegetation did not seem to play a key role. Reasons may be attributed to soil and hydrologic variability on the site, immaturity of vegetation, and differences in localized groundwater flow paths. Results of this study indicate that riparian buffer effectiveness was closely linked to the site hydrology. Establishment of buffers along streams where groundwater flowed away from the stream did not result in lower groundwater nitrate levels. Implementation of riparian buffers without knowledge of the site hydrology may lead to minimal water quality benefits.