Browsing by Author "R. Wayne Skaggs, Committee Chair"

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Effects of Drain Depth on Nitrogen Losses in Drainage in Shallow Water Table Soils
(2004-04-09) Arnold, Laura Ann; Michael J. Vepraskas, Committee Member; R. Wayne Skaggs, Committee Chair; George M. Chescheir, Committee Member
A two-part study (a field study and a modeling study) was undertaken to investigate the effect of shallow subsurface drains on nitrate-nitrogen (NO3-N) losses in drainage effluent relative to deep subsurface drains. The field study was conducted in the Lower Coastal Plain of North Carolina over a two year period between May 12, 2001 and April 30, 2003. Crops grown on the site were corn in 2001 followed by wheat and soybean in 2001/2002. The field was nearly flat and the soils were Portsmouth sandy loam and Cape Fear loam, which, under natural conditions are very poorly drained. The site was subdivided into eight 1.8 ha plots, each drained by three parallel drains spaced 23 m apart. Drains in plots one through three were shallow with an average drain depth of 0.86 m. Drains in plots four through six were deeper, with an average depth of 1.20 m. Precipitation, water table depth and subsurface drainage rates were measured continuously during the study. Subsurface drainage was sampled and analyzed to determine NO3-N concentration in the effluent. Measurements of water table depth and subsurface drainage quantity and quality were made within each plot. Observed results indicate the average flow quantity from the shallow drains was reduced by 37% in Year 1 and by 26% in Year 2 relative to the average flow from the deep drains. However the difference in flow was not statistically significant. Average water table depth in the shallow plots was significantly more shallow than in the deep plots at the 10% level. Drain depth had no statistically significant effect on drainage water quality. For Year 1, the average NO3-N losses from the shallow drains and deep drains were 21.7 and 28.0 kg ha-1, respectively. For Year 2, the average NO3-N losses from the shallow and deep drains were 28.9 and 23.5 kg ha-1 respectively. For the entire study period, total average NO3-N loss from the shallow drains was 50.6 kg ha-1 and the total average loss from the deep drains was 51.5 kg ha-1. Observed behavior in shallow plot 2 was an anomaly. More subsurface drainage occurred and more NO3-N was lost from plot 2 than any other plot. Based on analysis performed in conjunction with predicted results from the modeling study, it was determined that plot 2 was not hydraulically isolated and was receiving surface runoff from an adjacent plot. The second part of the study was a modeling study to further investigate the effect of drain depth on nitrate loss in a way that eliminated the plot to plot variation that was inherent in the field study. Hydrologic models of the research site were developed using DRAINMOD 5.1. Nitrogen dynamic models were developed using DRAINMOD-N II. Initial simulations were performed to calibrate the models using data collected during the field study. Then long-term simulations were performed for the period November 1991 to March 2003 using the calibrated models. A wheat-soybean-corn crop rotation was used. Simulations were performed for each plot with subsurface drains at 0.85 m and 1.2 m. Based on DRAINMOD predictions for the plot 2 through 6, reducing drain depth from 1.2 to 0.85 m resulted in 17% reduction in subsurface drainage, 30% increase in surface runoff, and 4% increase in evapotranspiration over an 11 year period. Based on DRAINMOD-N II predicted results, averaged for plots 2 through 6, the 0.85 m drain depth resulted in 32% less NO3-N loss in subsurface drainage, compared to drains at a depth of 1.2 m.
Modeling Nitrogen Transport and Transformations in High Water Table Soils
(2004-02-03) Youssef, Mohamed A.; John E. Parsons, Committee Member; George M. Chescheir, Committee Member; R. Wayne Skaggs, Committee Chair; J. Wendell Gilliam, Committee Member
Development of management practices that reduce nitrogen (N) losses from agricultural lands has been the focus of research over many years. Development and testing of such practices is a complex task since it requires understanding of N dynamics in the soil-water-plant system, which is regulated by a large number of interacting physical, chemical, and biological processes. Nitrogen models are useful tools for developing and evaluating management practices for sustainable agriculture. The model, DRAINMOD-N was originally developed to simulate N dynamics in artificially drained soils. However, the model was based on a simplified N cycle, which restricted its applicability. A new version of DRAINMOD-N, referred to as DRAINMOD-N II, was developed and field-tested in this study. DRAINMOD-N II simulates N dynamics and turnover in the soil-water-plant system under different management practices and soil conditions. It considers a detailed N cycle, adds a simplified carbon cycle, and operates at different levels of complexity according to the conditions of the system being simulated. Processes considered in the model are atmospheric deposition, application of mineral N fertilizers, soil amendment with organic N sources, plant uptake, mineralization, immobilization, nitrification, denitrification, ammonia volatilization, and N losses due to leaching and surface runoff. DRAINMOD-N II driving hydrologic variables are predicted by the water management model DRAINMOD 5.1. DRAINMOD-N II was tested with a six-year data set from the North Carolina Lower Coastal Plain. The experimental site consists of eight 1.7-hectare instrumented, subsurface drained plots. The site was planted to a corn-wheat-soybean rotation. Water table depth (WTD) midway between the drains, subsurface drainage flow rates, and meteorological data were automatically measured and recorded. Flow proportional drainage water quality samples were collected and analyzed to determine N concentrations and loads. Results of the simulations showed good agreement between predicted and observed WTD. On average, predicted WTD was within 11.8 to 13.9 cm of observed values. The average coefficient of determination (R2) for WTD was in the range of 0.71 to 0.77. There was also good agreement between predicted and observed subsurface drainage rates. On average, predicting annual subsurface drainage was within 5.7-12.1% of observed values. The average R2 values for predicted versus observed subsurface drainage ranged from 0.65 to 0.73. The DRAINMOD-N II simulations showed good agreement between predicted annual nitrate-nitrogen (NO3-N) drainage losses and observed values. On average, predicted annual NO3-N leaching losses were in the range of 19.9-46.0%. The high errors in predicting annual NO3-N leaching losses were mostly induced by errors in predicting WTD and drainage rates. The model did an excellent job in predicting cumulative drainage and NO3-N leaching losses over the whole simulated period. Cumulative drainage and NO3-N leaching losses over the six-year period was within 1.3-13.2% and 2.2-10.1% of observed values, respectively. In spite of relatively large discrepancies on an annual basis, in some years, errors in predicting cumulative NO3-N losses over the six-year period were remarkably small. Results of this study indicate that DRAINMOD-N II can be reliably used to simulate N dynamics and turnover in agroecosystems. However, this evaluation of the model should be regarded as an incomplete test because it relied primarily on the literature rather than field and/or lab measurements to parameterize the model. Further research is needed to test DRAINMOD-N II based on independent measurement of the model input parameters.