Predicting effects of water table management on loss of nitrogen from poorly drained soils

Agricultural drainage and related water management practices affect the quality and quantity of water leaving the field and entering ground and surface waters. The design and management of drainage systems should consider these impacts, as well as effects on agricultural production. In many instances, water quality and environmental requirements have a priority greater than that of maximizing agricultural productivity and profits. This paper describes the application of a simulation model, DRAINMOD-N, to predict the effects of design and management of subsurface drainage systems on nitrogen losses and crop yields. 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. Simulations were conducted for maize production on a Portsmouth sandy loam soil (Thermic, Typic Umbraquult) in the North Carolina Coastal Plain. Agricultural production objectives could be satisfied with Im deep parallel drains spaced 40 m apart or less. Predicted losses of NO3-N were significantly affected by drainage design and management. Increasing drain spacing from 20 to 40 m decreased NO3-N losses by 47 per cent. Nitrate losses can be further reduced by placing a weir in the drainage outlet so as to raise the water level in the outlet and reduce subsurface drainge rates. This practice, called controlled drainage, can be applied in both the growing season and the nongrowing season, and can be varied in intensity (with season) by placing the weir closer to or further below the soil surface. Controlled drainage during both the growing season and winter months reduced NO3-N losses from an annual average of 21.8 kg ha(-1) to 10.5 kg ha(-1) (52 per cent) for a 30 m drain spacing, without reducing crop yields. Analysis of simulated results on a year-by-year basis showed that large losses of NO3-N via drainage water occur in years following droughts when crops remove little of the fertilizer N because of reduced yields. Losses to the environment under these circumstances can be reduced by increasing the intensity of drainage control to minimize subsurface drainage during the following year. In one year, for example, raising the weir in the drainage outlet to a 25 cm depth directly after harvest and holding it there until time for seedbed preparation (one month prior to planting) in the spring reduced predicted NO3-N losses by 77 per cent compared to conventional drainage and by 60 per cent compared to currently recommended controlled drainage practices. The effectiveness of intensive drainage control was not as great in other years, but reduced NO3-N losses by at least 20 per cent in the years analysed. Results of this study indicate simulation modelling can be used to design and guide the management of drainage systems to address both agricultural productivity and environmental objectives.