Artificial Drainage Variant

Occurrence and Hydrological Flow Pathway

A significant variation from the natural hydrological drainage pathway occurs in areas where artificial drainage has been installed to make the land more suitable for a variety of land uses. Artificial drainage is typically used where soils are poorly to imperfectly drained or are slowly permeable and can be installed either as parallel open ditch drains or subsurface tiles or pipe networks. Artificial drainage has also been used to maintain a lower water table depth and divert seepage water to prevent saturation of the soil zone.

Knowing where subsurface drains exist can be particularly challenging, especially if historical records are not available for your property. Subsurface drainage is typically installed in mineral soils in swales or depressions in the landscape and follows the natural contour of the land to the drainage outfall. In peat soils, the more traditional vertical placement to a connector drain is typically used. In dry periods, these areas may appear greener as water may remain in these areas for longer.

Landscape Characteristics

Artificial drainage increases drainage vertically through the soil which minimises the occurrence of runoff and lateral flow, and the transport of all contaminants surficially. The natural ability of the soil to filter and adsorb contaminants is increased. Some phosphorus, sediment, and microbes may be lost through the artificial drainage network but often in lower quantities than would have been discharged by surficial runoff.

The downside to artificial drainage is as the soil becomes more oxygenated, it behaves more like an oxidising environment above the drain and the ability to remove nitrate nitrogen naturally through denitrification is reduced. Nitrate nitrogen is transported to surface waterways through the artificial drainage network at a much faster rate than naturally oxidising settings, especially where piped subsurface drainage is present. Partial denitrification occurring in the soil zone also produces both the harmless dinitrogen gas, which makes up the majority of our atmosphere, and nitrous oxide, a harmful greenhouse gas.

The role of landscape in regulating contaminants when artificial drainage is present.
Artificial Drainage Variant Contaminant pathway (dominant hydrological pathway) How the landscape regulates water quality contaminants Risk to receiving environment
Dilution Resistance to erosion Filtration and adsorption Attenuation: N-Reduction Attenuation: P-Reduction
Artificial drainage Artificial drainage includes both open ditch drains and subsurface drainage. Likely where agricultural soils have impeded drainage or a shallow water table. Pathway is most active during the wetter months. Low Moderate – Moderately high Moderate – Moderately high Low - Moderate Moderate – Moderately high Concentration & load to surface water

Contaminant Profile

Susceptibility of the landscape modified with artificial drainage for contaminant loss.

Nitrogen, phosphorus, and microbes require a source or input for losses to occur. Sediment risk is elevated if nutrient status is also elevated.

Artificial Drainage Variant Nitrogen Phosphorus Sediment Microbes
Nitrate & Nitrite Ammonium & Ammonia Organic (Dissolved & Particulate) Particulate Dissolved Reactive Particulate Particulate
Artificial drainage Moderately high Moderately high Moderately high Moderate Moderately low Moderate High

Key Actions

Where artificial drainage is present, limit the amount of surplus nitrogen in the soil especially prior to the wettest months when drains are most active. Maintaining soil structure is also critical to prevent runoff from occurring and transporting all contaminants surficially. Identify where your subsurface drains are and avoid these areas for effluent irrigation and other nutrient applications.