Controlled Drainage

Controlled drainage is a method of water harvesting for crop production, manipulating the underground water level. As it can only be applied to level fields (with a slope of maximally 1%), its applicability is strongly limited (BUSMAN & SANDS 2002).

With controlled drainage, water loss can be reduced by between 15 and 35 per cent. As infiltration of water is reduced, fewer nutrients are washed out. Therefore, one main advantage of this system compared to other measures (such as, for example conventional drainage or surface groundwater recharge, subsurface groundwater recharge and other precipitation harvesting measures) is the reduced loss of nitrate (see also the nutrient cycle) (BUSMAN & SANDS 2002). On the other hand, surface runoff is higher, because the soil has less space to hold back water.

Controlled drainage systems differ from conventional drainage by simple water control structures placed at various locations within the system. The height of the stored water can therefore be manipulated in order to optimise the groundwater level and store infiltrated rain underground for crop production.

Many different designs for controlled drainage systems exist, which can be differentiated between manually or automatically operated structures.

Controlled drainage systems are sensitive to topography and the amount of rainfall, making it important to consider the following limiting factors: for one part, the ground needs to be even with a constant slope of less than 1% (in order to allow accurate control of the water level according to crop root depth). For steeper grounds, this precipitation harvesting method cannot be implemented economically. Instead, other measures such as field trenches, bunds, micro basins and planting pits would be better suited. Furthermore, controlled drainage systems are not useful for very dry areas, as they can only help to reduce short-term related water stress. Longer time periods without rainfall need other rainwater harvesting measures, such as field trenches, bunds, conjunctive use or micro basins.

Adapted from COOK et al. (2006)

In Illinois (USA), installations of a controlled drainage system lead to average costs of $120 USD per hectare. However, these costs may vary according to the complexity of the topography.

Costs may be lower if already existing drainage systems are converted in order to enable controlled drainage. Due to the existing structure available, lower investments are needed, ranging from $50 to $100 USD per hectare.

Adapted from COOK et al. (2006)

Water quality - and therefore human health conditions - can be improved through controlled drainage systems: as contaminated water is stored in the soil profile over an extended amount of time, its chemical concentration (such as nitrates and other fertilisers or pesticides) is lowered, leading to a decreased wash out into the ground water and therefore higher water quality and better premises for human health.

Operating a controlled drainage system is complex compared to other rainwater harvesting measures. The main reason is the need for broad knowledge about the best time to raise or lower the water in order to effectively run a drainage system.