Saving water: Flood irrigation explained – African Farming

Flood irrigation is one of the oldest and most-used methods to irrigate crops.

Flood irrigation is a way of controlling, distributing and delivering water by means of gravity. It’s suitable for use on any crop.

The irrigation cycle for flood irrigation is usually longer than for other systems because it penetrates deeply. It is generally used on smaller areas and in the corners of a field where centre pivots cannot reach.

Also read: Flexipump – custom made for the smallholder farmer

ADVANTAGES

• One of the advantages of flood irrigation is low energy costs.
• Because water flows at ground level over the field, less electricity is needed to pump water than for a pressurised system.
• Drip and micro-irrigation require filtered water, but this isn’t necessary for flood irrigation, so there’s no added energy cost for filtration.
• Another advantage of flood irrigation is that it can work with lower-quality water because the water doesn’t come into contact with the leaves of the crop.

DISADVANTAGES

• Flood irrigation is very labour-intensive, as water flow has to be monitored and water has to be directed to prevent wastage.
• Generally, one labourer is needed to oversee 20 ha.
• Also, land preparation for flood irrigation involves the levelling of lands to a predefined gradient.
• The soil type and flow rate of the supply stream determine the gradient.
• Sandy soil lets water infiltrate the soil quicker than clay soil does, as clay can hold more water.
• Fields containing sandy soil will need a steeper gradient to get the same infiltration depth as the flatter gradient for clay soils.
• Farmers should compare the cost of levelling land with the developmental cost of other systems.
• Usually, the operating cost of flood irrigation is lower than for other systems.
• According to the South African Agricultural Research Council Institute for Agricultural Engineering, one of the commonest criticisms of flood irrigation is the wastage of water.
• A properly designed, constructed and operated system, however, can help to overcome this. The fact that several thousands of hectares of land are under controlled flood irrigation in the United States bears witness to this.

“If designed correctly, flood irrigation will give crops uniform, deep-penetrating irrigation which is beneficial for crops having deep root systems. Without proper operating practices, any irrigation system can waste water.”

PREVENTING WASTAGE

• The gradient and the flow rate are important design considerations.
• A steep gradient with a rapid flow rate causes erosion, can damage crops and the upper end of the field won’t receive the required infiltration of water.
• If the water flows too slowly (the gradient is too flat, or the flow rate too low) the infiltration of water at the upper end of the field that receives water first, will be too deep.
• Water wastage will occur and infiltration won’t be adequate. It all has to do with the contact time of water on the soil.
• You have to get the correct quantity of water to infiltrate to the correct depth for the plant to use.

The following techniques can help reduce water wastage:

Levelling fields is important, because water is transported using gravity. Since water flows downhill, it won’t reach high spots in the field. De Wet says farmers can use laser levelling equipment to construct the correct gradient according to the design. This technique uses a rotating laser beam to control land-levelling equipment to move and construct a land surface having a uniform gradient.

Surge flooding is another important technique. Instead of releasing the water all at once, it’s released at intervals to allow for better infiltration. Predetermined intervals also reduce unwanted runoff.

Also read: Irrigation – which system should you use? Watering vegetables: Use water wisely Watering the veggie field or patch

• This article was written by Wilma den Hartigh and first appeared in Farming SA.