Improving surface irrigation

Improving surface irrigation

Surface irrigation is the term used to describe the range of application techniques by which water is delivered to one or more points or edges of a field, and distributes over the field area under the influence of gravity. Surface irrigation is often referred to as flood irrigation, implying that the water distribution is uncontrolled, and, therefore, inherently inefficient. The term encompasses a large range of systems with varying degrees of complexity and performance. It is true that a significant proportion of surface irrigation systems do perform poorly, but, for the most part, much of this is due to design issues or incorrect management of the system rather than an inherent problem of the system itself.

Surface irrigation is the oldest form of irrigation and still remains the most common application technique across the globe. Irrigation was a necessary part of many of the world’s ancient civilizations and evidence of various forms of surface irrigation has been found across Africa, the Middle East, Asia, and South America. Approximately 6000 years ago flood and furrow irrigation was practiced in the area surrounding the Tigris and Euphrates rivers in modern day Iraq (Postel, 1999). The Egyptians and Indians pioneered a system of 'inundation canals' that are dug parallel to the river and rely on regular seasonal flooding (Cantor, 1970). In Egypt, these canals were used to supply water to basin irrigation layouts which resemble those still used in the region to this day. Evidence for similar primitive irrigation systems has also been found in other regions, such as China and Mexico.

Other forms of irrigation have only become a reality in the last 150 years with the development of pressurized sprinklers in the late nineteenth and early twentieth centuries, and the invention of drip irrigation in the mid-twentieth century. Pressurized irrigation, particularly large mobile machines and drip irrigation, is seen as a superior application technique. For millennia, farmers were reliant on a system which offered limited flexibility in terms of timeliness and rate of application. Now they have an engineered solution where perfect application is at least theoretically possible. Despite the perceived advantages of pressurized irrigation, much of the world’s food and fiber is still grown using surface irrigation, and for good reason, as it has a number of advantages. Possibly the most significant of these is capital cost. Pressurized irrigation systems involve a higher capital investment. The highest cost option, drip irrigation, is typically restricted to higher-value horticultural and orchard crops, or in circumstances where great precision of irrigation applications is required. The major capital costs associated with surface irrigation is the grading of the land. This should be performed regardless of the choice of system to ensure adequate drainage of the field during rainfall events. In water-limited circumstances, farmers will only irrigate a portion of the total cropping area and then rotate to other fields in subsequent seasons. This practice, common in parts of Australia, is not compatible with high-cost infrastructure, as it will take much longer to recoup these costs if the field is only utilized every second or third season.

The energy requirements of irrigation are often neglected during farm planning, but as energy prices increase it is becoming an important concern. In many circumstances, surface irrigation requires no on-farm pumping, water flows from the scheme channels or on-farm reservoirs to the fields under the influence of gravity alone. Where pumping is required, pressurized sprinkler and drip systems will always involve considerably higher energy costs than the equivalent surface irrigation layouts. The requirement for reliable and consistent electrical power or petroleum supplies has proven to be a challenge in developing nations. Sprinkler and drip systems are designed to operate at a specific supply pressure and are reliant on access to this electrical power supply 24 hours per day. While not an issue in most developed nations, unreliable power supplies have forced designers to adopt high-system capacities with higher flowrates requiring higher capital costs. Many irrigation distribution schemes have been designed to supply high flow rates for short periods of time instead of lowering the continuing flow rates required for pressurized systems. Adoption of new systems either require on-farm storage or changes at the scheme level.

Surface irrigation can be managed with a low-skilled workforce, while other forms of irrigation require higher skill levels or specialist knowledge. Other forms of irrigation may also require more frequent maintenance and access to technical support. Neglect of these facts has resulted in many pressurized systems failing to achieve anticipated levels of performance (Burney and Naylor, 2012, Kulecho and Weatherhead, 2005, Maisiri et al., 2005).

As previously stated, most of the irrigated area worldwide is under some form of surface irrigation, which, in most cases, is performing at a substandard level of efficiency. It is true that this performance can be increased through adoption of appropriate pressurized systems, but the truth is somewhat more complex. For example, promoters of drip irrigation would suggest that this single technology can save water and help save the water crisis, but as recent research has shown, there is little conclusive evidence that this is true (van der Kooij et al., 2013). Regardless of whether alternate systems offer any benefit, considering the cost and difficulty of the conversion, it is important to first ensure that the existing system is functioning optimally.

The proportion of irrigated land under surface irrigation has declined significantly over the past few decades in countries such as the United States (Koech et al., 2015), but this is likely due to labor savings rather than water efficiency savings. Traditional surface irrigation is more labor-intensive than most other types of irrigation system; therefore, as wages rise in developing nations, it is reasonable to expect that the adoption of pressurized irrigation will increase. Rising labor costs are also the main driver behind the adoption of automation systems, as is described in Section 10.7.

This chapter draws from material written by the author which has been previously presented in a range of articles and theses (e.g. Gillies, 2008, Gillies and Smith, 2015, Koech et al., 2014, Koech, 2012, Smith et al., 2016).

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