PREDICTING EFFECTS OF WATER-TABLE MANAGEMENT ON LOSS OF NITROGEN FROMPOORLY DRAINED SOILS

Agricultural drainage and related water management practices affect th e quality and quantity of water leaving the field and entering ground and surface waters. The design and management of drainage systems shou ld consider these impacts, as well as effects on agricultural producti on. In many instances, water quality and environmental requirements ha ve a priority greater than that of maximizing agricultural productivit y and profits. This paper describes the application of a simulation mo del, DRAINMOD-N, to predict the effects of design and management of su bsurface 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 Port smouth sandy loam soil (Thermic, Typic Umbraquult) in the North Caroli na Coastal Plain. Agricultural production objectives could be satisfie d with Im deep parallel drains spaced 40 m apart or less. Predicted lo sses of NO3-N were significantly affected by drainage design and manag ement. 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 we ir 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 nongrowin g 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 cen t) 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 t o the environment under these circumstances can be reduced by increasi ng the intensity of drainage control to minimize subsurface drainage d uring the following year. In one year, for example, raising the weir i n the drainage outlet to a 25 cm depth directly after harvest and hold ing it there until time for seedbed preparation (one month prior to pl anting) in the spring reduced predicted NO3-N losses by 77 per cent co mpared to conventional drainage and by 60 per cent compared to current ly recommended controlled drainage practices. The effectiveness of int ensive drainage control was not as great in other years, but reduced N O3-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 gui de the management of drainage systems to address both agricultural pro ductivity and environmental objectives.