J. W. Gilliam

Hydrologic and water quality impacts of agricultural drainage

Abstract While some of the worlds most productive agriculture is on artificially drained soils, drainage is increasingly perceived as a major contributor to detrimental off‐site environmental impacts. However, the environmental impacts of artificial or improved agricultural drainage cannot be simply and clearly stated. The mechanisms governing the hydrology and loss of pollutants from artificially drained soils are complex and vary with conditions prior to drainage improvements and other factors: land use, management practices, soils, site conditions, and climate. The purpose of this paper is to present a review of research on the hydrologic and water quality effects of agricultural drainage and to discuss design and management strategies that reduce negative environmental impacts. Although research results are not totally consistent, a great majority of studies indicate that, compared to natural conditions, drainage improvements in combination with a change in land use to agriculture increase peak runof...

DRAINMOD-N, A NITROGEN MODEL FOR ARTIFICIALLY DRAINED SOIL

DRAINMOD-N, a quasi two-dimensional model that simulates the movement and fate of nitrogen in shallow water table soils with artificial drainage, is described. Results of sensitivity analyses are presented and model predictions are compared with results from VS2DNT, a more complex, two-dimensional model. The nitrogen transport component is based on an explicit solution to the advective-dispersive-reactive (ADR) equation. Nitrate-nitrogen is the main N pool considered. Functional relationships are used to quantify rainfall deposition, fertilizer dissolution, net mineralization, denitrification, plant uptake, and surface runoff and subsurface drainage losses. A sensitivity analysis showed DRAINMOD-N predictions are most sensitive to the standard rate coefficients for denitrification and mineralization and nitrogen content in rainfall. Simulated daily water table depths were within 0.121 m, cumulative subsurface drainage rates were within 0.016 m, and cumulative surface runoff rates were within 0.003 m, of those predicted by VS2DNT for a 250-day period. DRAINMOD-N predictions for NO3-N losses in subsurface drainage water only differed from VS2DNT predictions by less than 2.6 kg ha–1. DRAINMOD-N predictions for denitrification were within 8%, for plant uptake were within 15%, and for net mineralization were within 26%, of those simulated by VS2DNT.

Sorption of organic phosphorus compounds in Atlantic Coastal Plain soils

Organic phosphorus (P) can comprise a significant amount of the total P in animal wastes, yet there is little information on the potential for organic P to be transferred from soils to watercourses. We examined the adsorption of organic P compounds to soils typical of the southeastern United States, i.e., Blanton Sand (loamy, siliceous, thermic, Grossarenic Paleudult), Cecil sandy clay loam (fine, kaolinitic, thermic, Typic Kanhapludult), and a Belhaven sandy loam (loamy, mixed, dysic, thermic, Terric Medisaprist). The behavior of four organic P compounds was studied: adenosine 5′-triphosphate (ATP), adenosine 5′-diphosphate (ADP), adenosine 5′-monophosphate (AMP), and inositol hexaphosphate (IHP); while KH2PO4 (ortho-P) was used as an inorganic reference. Laboratory studies were conducted to determine the effects of concentration (0–130 μg P mL−1), pH (4.6–7.6), and soil properties on P adsorption. All the organic P compounds had greater adsorption than KH2PO4 on the Blanton and Cecil soils at all concentrations and ranges of pH. In the Belhaven soil, IHP had the greatest sorption followed by KH2PO4 and the nucleotides (ATP, ADP, and AMP, respectively). Adsorption of organic P was positively correlated with soil organic matter and Fe and Al contents. The greater sorption of some organic P compounds over that of ortho-P suggests that these compounds may pose less of a threat to water quality, although this preferential sorption may increase soluble P in situations where there is displacement of ortho-P by organic P added in manures.

THE NITROGEN SIMULATION MODEL, DRAINMOD-N II

DRAINMOD-N II is a field-scale, process-based model that was developed to simulate nitrogen dynamics and turnover in the soil-water-plant system under different management practices and soil and environmental conditions. It is an enhanced version of the nitrogen (N) simulation model, DRAINMOD-N, that simulates a more complete N cycle, adds a carbon (C) cycle, and operates at different levels of complexity. Processes considered in the model include atmospheric deposition, application of mineral N fertilizers including urea and anhydrous ammonia (NH3), soil amendment with organic N (ON) sources including plant residues and animal waste, plant uptake, organic C (OC) decomposition and associated N mineralization/immobilization, nitrification, denitrification, NH3 volatilization, and N losses via subsurface drainage and surface runoff. Nitrogen pools considered in the model are nitrate-nitrogen (NO3-N), ammoniacal nitrogen (NHx-N) and ON. DRAINMOD-N II includes a submodel that simulates C dynamics in the soil-plant system using a C cycle similar to that of the CENTURY model. A simplified approach is used to simulate temporal changes in soil pH; consequently, the model determines the composition of the NHx-N pool and, if necessary, changes its operation mode. DRAINMOD-N II simulates N reactive transport using a finite difference solution to a multiphase form of the one-dimensional advection- dispersion-reaction equation. Model output includes daily concentrations of NO3-N and NHx-N in soil solution and drain flow, daily OC content of the top 20 cm soil layer, and cumulative rates of simulated N processes.

Effect of Drainage System Design and Operation on Nitrate Transport

ABSTRACT THE computer simulation model, DRAINMOD, was modified to predict nitrate movement from artificial-ly drained soils with high water tables. Nitrate concentra-tions in surface runoff, subsurface drainage and seepage waters were assumed to be constant and independent of the drainage system design for a corn-soybean rotation on these high water table soils. Total annual nitrate outflow was determined for alternative drainage system designs and operational procedures. The results showed that trafficability and crop protection requirements can be satisfied by several different drainage system designs. For the poorly drained soil considered in this study, there was a three-fold difference in NO3-N outflow among systems that satisfied drainage objectives. The amount of nitrate that leaves the field through drainage waters can be reduced by using controlled drainage during the winter months and during the growing season. However if the controlled drainage systems are not used properly an overall increase in nitrate outflow may result.

A computer simulation study of pocosin hydrology

The hydrology of pocosins is dependent on plant, soil, site, and climatological factors. A simulation study was conducted to determine the effects of natural factors, such as depressional storage, and changes in drainage and land use on pocosin hydrology. The water management model DRAINMOD was used to simulate the hydrology of drained and undrained pocosins. Hourly rainfall data from a 33-year period of climatological record were used in DRAINMOD to predict evapotranspiration (ET), subsurface drainage, runoff, and water table depth on a day-by-day basis. Results were summarized to determine annual and long-term average effects. Pocosins with a large amount of surface depressional storage have water ponded on the surface during most of the year, high ET, and low surface runoff. As the depth of depressional storage decreases, average annual ET decreases, and runoff increases. Average annual runoff predicted for a natural pocosin on a Portsmouth soil near Wilmington, North Carolina increase from 277 mm to 384 mm as the depth of depressional storage was decreased from 300 mm to 5 mm. However, year-to-year variation in annual runoff was much greater than the effects of all other factors considered. Drainage ditches at spacings greater than 400 m had no effect on average annual runoff for constant surface depressional storage and conditions analyzed in this study. Decreasing the ditch spacing from 400 to 100 m increased average total annual outflow by only 7%. However, more than half of the predicted outflow for the 100 m spacing occurred as subsurface flow compared to less than 5% of the total for the 400 m spacing. Conversion from natural pocosin vegetation to a managed pine forest with a deeper rooting zone decreased predicted annual outflow by about 9%. Conversion to agricultural uses increased predicted average outflow by 7% compared to natural conditions.

Prediction of nitrogen and phosphorus losses as related to agricultural drainage system design

Abstract The purpose of this study was to determine the effects of differing degrees of improved agricultural drainage on the long-term efflux of N and P from poorly drained soils common to the North Carolina Coastal Plain. The effect of using controlled drainage on efflux was also investigated. Computer simulations were used to predict the losses from 6 soils for a 20-year period. The simulations indicate that both drainage system design and management can have significant effects upon N and P efflux in drainage water. Drainage systems designed to give good subsurface drainage lost 17–35 kg ha−1 year−1 more NO3-N than systems with poor subsurface drainage. Good subsurface drainage decreased total P lost by 0.2-0.4 kg ha−1 year−1 on the mineral soils studied. The increase in N lost because of installing a good subsurface drainage system can partially be offset by utilizing controlled drainage. Under the conditions simulated in this study, controlled drainage reduced the nitrate efflux by as much as 34%, but the reduction varies with soil and management conditions. Controlled drainage does, however, result in a small increase in P efflux under the conditions simulated.

Nutrient and sediment removal in forested wetlands receiving pumped agricultural drainage water

The effectiveness of two forested wetland buffer areas at removing sediment and nutrients from pumped agricultural drainage water was evaluated in a two-year field study. The movement of these potential pollutants during pumping events was determined by sampling water quality at 36 stations distributed over each wetland. Automatic water sampling continued after pumping events to determine the nutrient and sediment removal rates in the water left standing on the wetlands. The total volume of water pumped during small events was effectively stored on the wetland until displaced by subsequent pumping or removed by evapotranspiration. Nutrient and sediment concentrations of this stored water, which had several days of residence time on the wetland, were near background levels before leaving the wetlands. Nutrient concentrations leaving the wetlands during these small events ranged from 0.03 to 0.04 mg/L for total phosphorus (TP) and from 0.0 to 0.1 mg/L for nitrate nitrogen (NO3-N). Pumped water completely traversed the wetland during the less-frequent, larger pumping events. Nutrient and sediment concentrations at the wetland outlet were often higher than background concentrations, ranging from 0 to 70 percent of the inflow concentrations. Nutrient concentrations leaving the wetlands during these larger events ranged from 0.06 to 0.12 mg/L for TP and from 0.0 to 4.7 mg/L for NO3-N. Sediment, TP, and NO3-N were removed from drainage water standing on the wetland. This nutrient and sediment removal was described using a first order decay model. Deposition of sediment was observed only within 800 m of the pumps, and was not resuspended during subsequent large pumping events.

Hydrology and Water Quality of Forested Lands in Eastern North Carolina

More than 100 site years of hydrology and water quality data spanning 25 years (1976-2000) were compiled from research and monitoring studies on forest stands with natural vegetation and tracts managed for timber production. A total of 41 watersheds located on poorly drained to very poorly drained soils on flat divides between coastal streams were included ranging in area from 7.3 to 6070 ha. Hydrology and nutrient concentration data from the study sites are used to examine how variation among sites may be related to soil type, drainage intensity, vegetation, and physiographic setting. The median annual hydrologic response (outflow as a percentage of precipitation) among the sites was 31%, with an interquartile range of 26-35%. Nutrient concentrations in forest outflow were generally low for most study sites compared with typical values for other land uses. Mean seasonal concentrations of nutrient fractions in drainage from 75% of the study sites were <1.8 mg/L for total N (TN), <0.08 mg/L for total P (TP). Concentrations of Org-N, TN, and TP were all consistently higher in drainage from organic soils compared with mineral soils for both paired comparisons and the overall data base. TN exports from 75% of the study sites were less than 6.5 kg/ha/yr, predominantly as Org-N and TP exports from all forest sites was <0.36 kg/ha/yr.

Using the DRAINMOD-N model to study effects of drainage system design and management on crop productivity, profitability and NO3–N losses in drainage water

Abstract The environmental impacts of agricultural drainage have become a critical issue. There is a need to design and manage drainage and related water table control systems to satisfy both crop production and water quality objectives. The model DRAINMOD-N was used to study long-term effects of drainage system design and management on crop production, profitability, and nitrogen losses in two poorly drained soils typical of eastern North Carolina (NC), USA. Simulations were conducted for a 20-yr period (1971–1990) of continuous corn production at Plymouth, NC. The design scenarios evaluated consisted of three drain depths (0.75, 1.0, and 1.25 m), ten drain spacings (10, 15, 20, 25, 30, 40, 50, 60, 80, and 100 m), and two surface conditions (0.5 and 2.5 cm depressional storage). The management treatments included conventional drainage, controlled drainage during the summer season and controlled drainage during both the summer and winter seasons. Maximum profits for both soils were predicted for a 1.25 m drain depth and poor surface drainage (2.5 cm depressional storage). The optimum spacings were 40 and 20 m for the Portsmouth and Tomotley soils, respectively. These systems however would not be optimum from the water quality perspective. If the water quality objective is of equal importance to the productivity objective, the drainage systems need to be designed and managed to reduce NO3–N losses while still providing an acceptable profit from the crop. Simulated results showed NO3–N losses can be substantially reduced by decreasing drain depth, improving surface drainage, and using controlled drainage. Within this context, NO3–N losses can be reduced by providing only the minimum subsurface drainage intensity required for production, by designing drainage systems to fit soil properties, and by using controlled drainage during periods when maximum drainage is not needed for production. The simulation results have demonstrated the applicability of DRAINMOD-N for quantifying effects of drainage design and management combinations on profits from agricultural crops and on losses of NO3–N to the environment for specific crop, soil and climatic conditions. Thus, the model can be used to guide design and management decisions for satisfying both productivity and environmental objectives and assessing the costs and benefits of alternative choices to each set of objectives.