Controlled drainage

What

Controlled drainage is where water table depth is regulated by an adjustable raised structure at the outlet of subsurface drainage, such as a tile. Maintaining desirable water depths through controlled drainage has agronomic and environmental advantages that vary with regional climate, soils, and management. Internationally, controlled drainage is considered a best management practice.

Simplified diagram of conventional and controlled subsurface drainage. In controlled drainage, adjustable gates maintain the water table at its desired height, while allowing excess water to flow over the gate and out the tile outlet.
Image source: Carsten et al., 2020

Why

Controlled drainage can minimise contaminant loss from paddocks by regulating the outflow of water at the end of a drainage system using a control structure. Control structures on drains allow farmers to hold water and nutrients in the soil when needed, and steadily release water from drains in wetter months to prevent paddocks from becoming waterlogged. Therefore, controlled drainage has multiple benefits for both crop growth and water quality by giving the farmer the ability to keep nutrients in the soil, reducing subsequent loss to waterways.

A recent international review of mitigation measures targeting nutrient losses from agricultural drainage systems included controlled drainage, constructed wetlands, denitrification beds, saturated buffer zones, and integrated buffer zones. The study found the load of nitrate was substantially reduced by all five drainage mitigation measures. The mitigation measures mainly acted as sinks of total phosphorus, but occasionally, also as sources. The various factors influencing performance included design, runoff characteristics, and hydrology. This resulted in large variations in the reported removal efficiencies.

Read more about the results of the study here.

References

Carstensen, M. V., Hashemi, F., Hoffmann, C. C., Zak, D., Audet, J., & Kronvang, B. (2020). Efficiency of mitigation measures targeting nutrient losses from agricultural drainage systems: A review. Ambio, 49, 1820-1837.

Drury, C. F., Tan, C. S., Gaynor, J. D., Oloya, T. O., & Welacky, T. W. (1996). Influence of controlled drainage‐subirrigation on surface and tile drainage nitrate loss (Vol. 25, No. 2, pp. 317-324). American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.

Houlbrooke, D. J., Horne, D. J., Hedley, M. J., Hanly, J. A., Scotter, D. R., & Snow, V. O. (2004). Minimising surface water pollution resulting from farm‐dairy effluent application to mole‐pipe drained soils. I. An evaluation of the deferred irrigation system for sustainable land treatment in the Manawatu. New Zealand Journal of Agricultural Research, 47(4), 405-415.

Houlbrooke, D. J., & Monaghan, R. M. (2009). The influence of soil drainage characteristics on contaminant leakage risk associated with the land application of farm dairy effluent. AgReserach report prepared for Environment Southland.

Monaghan, R. M., Paton, R. J., & Drewry, J. J. (2002). Nitrogen and phosphorus losses in mole and tile drainage from a cattle‐grazed pasture in eastern Southland. New Zealand Journal of Agricultural Research, 45(3), 197-205.

Nguyen, L., & Sukias, J. (2002). Phosphorus fractions and retention in drainage ditch sediments receiving surface runoff and subsurface drainage from agricultural catchments in the North Island, New Zealand. Agriculture, Ecosystems & Environment, 92(1), 49-69.