J. E. Parsons

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 nwater table soils with artificial drainage, is described. Results of sensitivity analyses are presented and model predictions nare compared with results from VS2DNT, a more complex, two-dimensional model. The nitrogen transport component is nbased on an explicit solution to the advective-dispersive-reactive (ADR) equation. Nitrate-nitrogen is the main N pool nconsidered. Functional relationships are used to quantify rainfall deposition, fertilizer dissolution, net mineralization, ndenitrification, plant uptake, and surface runoff and subsurface drainage losses. nA sensitivity analysis showed DRAINMOD-N predictions are most sensitive to the standard rate coefficients for ndenitrification and mineralization and nitrogen content in rainfall. Simulated daily water table depths were within n0.121 m, cumulative subsurface drainage rates were within 0.016 m, and cumulative surface runoff rates were within n0.003 m, of those predicted by VS2DNT for a 250-day period. DRAINMOD-N predictions for NO3-N losses in subsurface ndrainage water only differed from VS2DNT predictions by less than 2.6 kg ha–1. DRAINMOD-N predictions for ndenitrification were within 8%, for plant uptake were within 15%, and for net mineralization were within 26%, of those nsimulated by VS2DNT.

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.

FIELD TESTING OF DRAINMOD-N

This study was conducted to evaluate the performance of DRAINMOD-N, a nitrogen fate and transport model nfor artificially drained soils, based on a comparison between predicted and observed hydrologic and nitrogen variables nfor an experimental site in eastern North Carolina. The site consisted of six plots drained by subsurface drain tubes n1.25 m deep and 23 m apart. Each plot was instrumented to measure water table depth, subsurface drainage, surface nrunoff and subirrigation rates. There were two replications of three water management treatments: conventional ndrainage, controlled drainage and subirrigation. Crops were winter wheat followed by soybean. Results showed the nmodel did a good job in describing the hydrology of the site. On average the predicted daily water table depths were nwithin 0.13 m of observed during the 14-month study period. Differences between predicted and observed cumulative nsubsurface drainage and surface runoff volumes were less than 0.10 and 0.09 m, respectively, for all treatments. nPredictions for the movement and fate of nitrogen were also in good agreement with measured results. Simulated nitratenitrogen n(NO3-N) losses in subsurface drainage water were within 1.5 kg/ha of the observed values for the 14-month nperiod. Differences between simulated and observed total NO3-N losses (subsurface drainage plus surface runoff) were nwithin 3.0 kg/ha. nResults of this study indicated DRAINMOD-N could be used to simulate nitrogen losses in poorly drained soils with nartificial drainage. The model, however, needs to be tested for longer periods of time and under different climatic nconditions and soil types, before it can be recommended for general use.

Simulation of drainage water quality with DRAINMOD

The design and management of drainage systems should consider impacts on drainage water quality and receiving streams, as well as on agricultural productivity. Two simulation models that are being developed to predict these impacts are briefly described. DRAINMOD-N uses hydrologic predictions by DRAINMOD, including daily soil water fluxes, in numerical solutions to the advective-dispersive-reactive (ADR) equation to describe movement and fate of NO3-N in shallow water table soils. DRAINMOD- CREAMS links DRAINMOD hydrology with submodels in CREAMS to predict effects of drainage treatment and controlled drainage losses of sediment and agricultural chemicals via surface runoff. The models were applied to analyze effects of drainage intensity on a Portsmouth sandy loam in eastern North Carolina. Depending on surface depressional storage, agricultural production objectives could be satisfied with drain spacings of 40 m or less. Predicted effects of drainage design and management on NO3-N losses were substantial. Increasing drain spacing from 20 m to 40 m reduced predicted NO3-N losses by over 45% for both good and poor surface drainage. Controlled drainage further decreases NO3-N losses. For example, predicted average annual NO3-N losses for a 30 m spacing were reduced 50% by controlled drainage. Splitting the application of nitrogen fertilizer, so that 100 kg/ha is applied at planting and 50 kg/ha is applied 37 days later, reduced average predicted NO3-N losses but by only 5 to 6%. This practice was more effective in years when heavy rainfall occurred directly after planting. In contrast to effects on NO3-N losses, reducing drainage intensity by increasing drain spacing or use of controlled drainage increased predicted losses of sediment and phosphorus (P). These losses were small for relatively flat conditions (0.2% slope), but may be large for even moderate slopes. For example, predicted sediment losses for a 2% slope exceeded 8000 kg/ha for a poorly drained condition (drain spacing of 100 m), but were reduced to 2100 kg/ha for a 20 m spacing. Agricultural production and water quality goals are sometimes in conflict. Our results indicate that simulation modeling can be used to examine the benefits of alternative designs and management strategies, from both production and environmental points-of-view. The utility of this methodology places additional emphasis on the need for field experiments to test the validity of the models over a range of soil, site and climatological conditions.

Application of a Three-Dimensional Water Management Model

ABSTRACT COMPUTER simulations were conducted to evaluate the effects of channel water level control on the conservation and management of shallow groundwater resources in agricultural drainage districts. Drainable water stored in the profile, crop evapotranspiration, and relative corn yields were used to evaluate the conservation of stream flow and rainfall. The effects of management of shallow groundwater were determined and evaluated for the availability of channel water for withdrawal. The simulations were performed for a 0.734 km^ area adjacent to a section of Mitchell Creek near Tarboro, NC for 2 yr, 1983 and 1984. Channel water level control increased the simulated drainable water stored in the profile by 85 mm in 1983 and by 84 mm in 1984. Simulated relative corn yields were increased in land areas upstream from the water control structure. Yield increases were greatest for land within 100 m of the controlled channel. Channel control provided sufficient shallow drainable groundwater for 17 days of drought compared to only 4 days without control for the periods simulated.

stream Water Levels Affect Field Water Tables and Corn Yields

ABSTRACT A section of land about 1800 m wide and 4000 m along Mitchell Creek in Edgecombe and Pitt Counties, North Carolina, was studied for three years, 1980, 1981, and 1982. During 1980 and 1981 the deep channel in these sandy soils caused a water table drawdown of about 3 m near the stream. The water table was affected 884 m away. Corn yields near the creek were one-half those at 800 m from the creek. Stress-day indices varied inversely with yield. In 1982 a fabric dam, filled with water that automatically controls water levels in the creek and allows floods to pass, was installed in the creek. Stream water level control caused a significant rise in water table levels in the fields, and corn yields were 23% more than yields without water table control within 488 m of Mitchell Creek. This increased value of yield in 1982 will pay for the high costing prototype dam in about 15 years if these results continue

Simulation of controlled drainage in open-ditch drainage systems.

Abstract Subsurface water management was analyzed for a typical open-ditch drainage system in eastern North Carolina. The water management model, watrcom , was used to simulate conventional drainage, controlled drainage, and subirrigation for the 1982–1986 growing seasons. The effects of water management on drainage outflow, relative corn yields, and irrigation water requirements were analyzed. The effect of inflow from upstream uncontrolled drainage areas was also considered. During the dry years controlled drainage increased relative yields over conventional drainage; however, predicted yields were less than those obtained with subirrigation because of drought stresses. In 1984, rainfall exceeded pet and yields predicted for controlled drainage were similar to those obtained with subirrigation. Controlled drainage also decreased drainage outflows. Both average yields and yields midway between the drainage ditches were analyzed for all water management treatments. The relationship between yield and distance from the drain was dependent on hydraulic conductivity. Predicted yields for conventional drainage were higher at the midpoint than those near the ditch due primarily to drought. However, yields near the ditches tended to be higher than those at the midpoint for controlled drainage and subirrigation.

Irrigation Water Supplied by Stream Water Level Control

ABSTRACT STREAM water levels in Mitchell Creek near Tarboro, North Carolina, were satisfactorily controlled for drought and flooding conditions by a water-inflatable dam (Fabridam). The reported 4-year period covers normal rainfall years in 1982 and 1985 (405 and 473 mm of rain for the corn-growing season and 80 and 82 mm of irrigation used, respectively); a dry year in 1983 (349 mm of rain and 175 mm of irrigation used); and a high rainfall year in 1984 (659 mm of rain during the corn-growing season and only 2 mm of irrigation used). By controlling the stream water level, runoff and drainage were redistributed, and adequate water was stored in the soil profile and shallow aquifer (2 to 7 m below the surface) to furnish irrigation water for eight center pivot systems, four volume gun hose reel systems, and one controlled-drainage/subirrigation system, irrigating 327 ha. Before the stream water level was controlled, only two center pivot systems and two volume guns, irrigating 79 ha, were able to operate and then only part time. Corn yields (4-year average) in fields with stream water level control were 27% more for nonsprinkler-irrigated fields and 71 % more in sprinkler-irrigated fields than in fields without stream water level control. These increased yields would pay for the Fabridam in 15 years, pay the cost of irrigation, and without tax break consideration give an annual return for management of

Development And Testing Of A Water Management Model (Watrcom): Development

100/ha.

DEVELOPMENT AND TESTING OF A WATER MANAGEMENT MODEL (WATRCOM): FIELD TESTING

ABSTRACT Drainage is required in many agricultural watersheds in the southeastern United States for flood prevention and to sustain agricultural production. These drainage improvements often increase the severity of summer droughts by lowering water tables. A computer simulation model, WATRCOM, has been developed to assist in evaluating drainage improvements and the feasibility of using channel water level control. A finite element solution of the Boussinesq equation coupled with water balances in the unsaturated soil and on the surface is used to simulate water movement in three dimensions. Varying soil types and boundary conditions in land areas with irregular drainage channel networks can be considered. Model results are compared to published solutions for drainage to parallel drains. Solutions are also presented for flow in regions near intersecting drains and compared to solutions in regions with parallel drains.