Irrigation efficiency is a term used to describe how much of the water applied through an irrigation system gets stored in the soil within the root zone of the plant and becomes available to the plant.
Efficiency is determined by:
• How well water is distributed across the irrigated area.
• The effectiveness of scheduling operations.
• The effect of wind. Wind can have a significant effect on sprinkler performance. Spray patterns change, fine water droplets are blown away and evaporation losses in the air are increased. Farmers should, where possible, avoid irrigating when strong winds are blowing.
• The impact of losses.
Atmospheric losses can be significant. With heat, water that is exposed to the air evaporates. Spray droplets in the air, especially small droplets that are suspended in the air and easily transported by wind, evaporate before they reach the plant or infiltrate into the soil. On hot, windy days these losses can be significant. It is often technically better to irrigate at night, or to avoid irrigating during the hottest times of the day. This is, however, difficult if the farmer lives some distance from the field. Evaporation from the soil surface is reduced when plants provide a full shade canopy, or when a mulch layer is in place. Good irrigation management includes the management of atmospheric losses.
Conveyance losses occur between the water source and emitter. While some of these losses can’t be avoided, leaks should be repaired as quickly as possible.
Soil losses are inevitable. The nature of irrigation is such that the perfect distribution of water is never achieved. If enough water is provided in the drier areas of the field, some water in wetter areas will pass below the root zone of the crop and not be available to plants.
Systems are designed to keep these losses to a minimum. Distribution patterns will, however, change as emitters wear. The way in which water moves through and is stored in the soil is also affected by tillage and land use practices. When individual types of irrigation systems are discussed in more detail in later sections, typical ranges of irrigation efficiency are given. These graphs represent the normal range of efficiencies measured in field tests. Further variations can occur as a result of local soil or climatic conditions.
Field tests and systems evaluations can help a farmer to improve maintenance and management, increasing efficiency and production. Farmers should aim to achieve the best possible water use efficiency for their systems. Less efficient systems have reduced crop yields and waste water, energy, fertilisers and labour.
Next month we shall look at flood irrigation. This series is published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.