METHODS OF IRRIGATION

Several methods of irrigation are used today depending on water availability, the type of irrigated crop and the financial investment the grower is willing to make. A firm knowledge regarding the irrigation techniques available can help one understand the different types of deficit irrigation.

Flood irrigation:

In flood irrigation, a large amount of water is brought to the field and flows on the ground among the crops²⁵. In regions where water is abundant, flood irrigation is the cheapest method of irrigation²⁴ and this low tech irrigation method is commonly used by societies in developing countries²⁵. It should be applied only to flat lands that do not concave²⁴ or slope downhill²⁵so that the water can evenly flow to all parts of the field, yet even so, about 50% of the water is wasted and does not get used by the crops²⁵. Some of this wasted water accumulates at the edges of a field and is called run-off. In order to conserve some of this water, growers can trap the run-off in ponds and reuse it during the next round of flood irrigation²⁵. However a large part of the wasted water can not be reused due to massive loss via evaporation and transpiration^{24, 25}.

One of the advantages of flood irrigation is its ability to flush salts out of the soil²⁴, which is important for many saline intolerant crops. However, the flooding causes an anaerobic environment around the crop which can increase microbial conversion of nitrogen from the soil to atmospheric nitrogen²⁴, or denitrification, thus creating low nitrogen soil.

Surge flooding is an attempt at a more efficient version of conventional flood irrigation in which water is released onto a field at scheduled times, thus reducing excess run-off²⁵.

Furrow irrigation:

Furrow irrigation is actually a type of flood irrigation in which the water poured on the field is directed to flow through narrow channels dug between the rows of crops²⁵, instead of distributing the water throughout the whole field evenly. The furrows must all have equal dimensions, in order to guarantee that the water is distributed evenly²⁴. Like flood irrigation, furrow irrigation is rather cheap in areas where water is inexpensive²⁴.

Spray irrigation:

The more modern spray irrigation in all its various forms, is a more expensive type of irrigation, requiring more complex machinery than flood irrigation, but it utilizes water more efficiently²⁴, reducing the amount of water needed to irrigated a field. That said, even more water is lost through evaporation in spray irrigation compared to flood irrigation and plant diseases due to excess moisture can occur at over watering²⁴.

In spray irrigation systems, a long hose is set to a water source on one side and on the side reaching the field, water is released through spray guns²⁵.

The center-pivot system is an efficient way to irrigate a large field with minimum machinery. This system is built of many triangular metal frames on wheels that hold the central hose above the field²⁵. The hose transports water from a pump at the center of the system and water is sprayed through sprinklers along the tube²⁵. The whole structure circulates the field spraying water, with the water source as the center of the circle²⁵. The disadvantages of this method, and other types of traditional spray irrigation, are the electric motors needed to help the system roll in a circle and the large amounts of water (about 35%) that evaporate or get blown away by winds before they even reach the ground²⁵.

The Low Energy Percision Application (LEPA) center pivot system is a more efficient irrigation method than the conventional center pivot system, boosting the irrigation efficiency from about 60% to more than 90%²⁵. This rise in effectiveness is also due to the decline in the electricity usage, but mostly because the water is applied directly onto the crops and not sprayed out into the air²⁵. This system also consists of a central hose, but instead of high power sprinklers, pipes hang from the central hose and attached to the bottom of each pipe, very close to the ground, is a nozzle that sprays water directly onto the crops²⁵. This way, less water is lost through evaporation compared to traditional spray irrigation- more than 90% of the water applied is used by the crop and less electricity is required²⁵.

Drip irrigation

While drip irrigation may be the most expensive method of irrigation, it is also the most advanced and efficient method in respect to effective water use²⁴.

Usually used to irrigate fruits and vegetables, this system consists of perforated pipes that are placed by rows of crops or buried along their root lines and emit water directly onto the crops that need it²⁵. As a result, evaporation is drastically reduced and 25% irrigation water is conserved in comparison to flood irrigation²⁵. Drip irrigation also allows the grower to customize an irrigation program most beneficial to each crop.

Water high in salts should be filtered before use since otherwise they may clog the emitters and create a local buildup of high salinity soil around the plants if the irrigation water contains soluble salts²⁴.