Effect of Controlled Drainage on Water and Nitrogen Balances in Drained Lands

Abstract

Field studies have shown that subsurface drainage systems can be managed to conserve water and reduce losses of nitrogen (N) to surface waters. The practice, called controlled drainage (CD) or drainage water management (DWM), is a viable alternative for reducing N loads from drained cropland, including millions of acres in the Midwest. This article reviews past studies on the effect of CD on drainage volumes and N losses for a wide range of soils and climatological conditions and uses simulations to examine mechanisms affecting the practice. Results published in the literature show that CD has reduced drainage volumes and N losses in drainage waters by 17% to over 80%, depending on soil properties, crops, drainage intensities, control strategies, and location. This study resulted in the following conclusions. CD reduces subsurface drainage and raises water tables, while increasing ET, seepage, and surface runoff. Seepage, which depends on soil properties and site conditions, is an important factor that often governs the effectiveness of CD. Experiments to determine the effect of CD on drainage volumes and N losses should be conducted on the field or watershed scale so that impacts of seepage are properly represented. Increases in ET in response to CD are important but are rarely greater than 10%. The effect of this increase in water use on drainage water loss is also less than 10% for most locations. CD reduces N losses in drainage water by about the same percentage as its effect on subsurface drainage volume in most cases. The effect of CD on N loss to surface waters depends on denitrification, both in the profile and in reduced zones along seepage paths. For soils that do not develop reduced zones, the effect of CD on N loss may be substantially less than its effect on drainage volume.