10. Reducing Demand: Improving Irrigation Efficiency

Water scarcity occurs when demand exceeds supply, as shown in the diagram below.

Fig. 1 This is the diagram I made and introduced in my first post to depict water scarcity and a function of supply and demand

Falkenmark’s (1989) Water Stress Index assumed that demand is fixed (see my second post) and that the only way to reduce water scarcity is to increase supply. So far, I have investigated increasing supply via increased groundwater access and improved rainwater harvesting. However, in reality demand is not fixed and water supply cannot expand forever. Therefore, increased water use efficiency is also needed, a process broadly referred to as 'sustainable intensification' (Tilman et al. 2011). My next two posts will investigate reducing water demand via more water efficient irrigation techniques and more water efficient crops. In line with Howell (2003)’s categorization I am going to discuss three types of irrigation efficiency: (1) conveyance, (2) application and (3) storage efficiency.

Fig. 2 I am investigating reducing water demand in terms of water use efficiency in the field. In other words, getting “more crop per drop”. 

(1) Conveyance efficiency:

This is the ratio between the water that reaches the field and that diverted from the irrigation source/storage reservoir.

This can be improved by reducing the distance between the water source and the field (e.g. via local rainwater harvesting or small-scale groundwater pumps) and by reducing seepage or leaks from the canals/pipelines that convey the water. There seems to be limited literature on lining canals for SSA but I found a successful example from Sri Lanka (Meijer et al., 2006), where concrete lining reduced seepage by 50%. I also found an experiment from Pakistan (1994-1999), which used 200km of old irrigation canals to test 9 basic lining types for 'ease of construction, watertightness, durability and cost' (Snell, 2001). Additionally channels can be covered to reduce evaporation losses. A project was launched in 2012 in Gujarat (India) aiming to cover 19,000km of canals with solar panels: simultaneously generating energy and reducing evaporation. Although, the scale and cost of this particular project may not be feasible for small-holder farmers in rural SSA, other more viable methods for covering canals exist. 

Fig. 3 (a) construction of an unlined irrigation canal, outside Doolow in southern Somalia. (Source: FAO, photo by Frank Nyakairu) (b) workers in Hidalgo County, Mexico spread concrete over a synthetic canal liner made of polyester (Source: Texas AgriLife Extension, photo by AskarKarimov)

(2) Application efficiency:

This decribes how much water reaches the crop root zone compared to how much water is applied to the field.

Conventional sprinkler systems have low application efficiencies because they spray water rapidly and widely across the entire field. Water is lost as evaporation from the spray (before it reaches the soil), as surface runoff (if water application exceeds infiltration rate) or as water that lands on soil outside of the root zone (and subsequently evaporates/percolates). Drip irrigation systems largely overcome these problems by dripping water very slowly in small volumes directly onto the plant root zone. Consequently, they can reduce irrigation requirements by up to 60% (FAO, 2016; Narayanamoorthy et al., 2016). Although drip irrigation tends to require more expensive equipment, organizations such as ‘Rooted In Hope’ (a sustainable development NGO) have developed low cost ‘drip kits’ that are ‘durable and affordable to poor farmers’. They are currently raising funds to supply drip kits to kitchen gardens in Kenyan schools in Kenya (click here to donate).  Application efficiency can also be improved by increasing soil infiltration capacity using conservation agriculture techniques, such as no-till, cover cropping and crop residue retention (Hobbs, 2007). This reduces losses from runoff and evaporation.

Fig. 4 (a) convention sprinkler system with low application efficiency (b) drip irrigation system with much higher application efficiency

    

(3) Storage efficiency:

This relates to how much of the water that reaches the crop root zone is retained vs how much evaporates/percolates before the plant has had the chance to absorb it.

Sandy soils characterize much of SSA. These have particularly low storage efficiencies (i.e. water rapidly drains through them) because sand is inert and lacks microporosity. The water retention capacity of such soils can be improved by adding 'soil conditioners' (products that improve the physical characteristics of soil). Organic matter is a good soil conditioner because it provides a hydrophilic surface for water to 'stick' to (Weil & Brady, 2016). Soil organic matter content can be increased via conservation agriculture, as discussed above (Hobbs, 2007). Biochar (charcoal used for soil amendment rather than for fuel) is another potential soil conditioner because it has a sponge-like structure (see photo below) that increases soil porosity (Ulyett et al., 2014). However, its effectiveness is variable (Lehmann & Joseph, 2015). In 2004 a Japanese company launched a new foamed glass soil conditioner called “Porous Alpha” made from waste glass bottles and reinforced with calcium (see video below), which is intended to be low cost and sustainable. They have carried out successful crop trials in Morocco, where it reduced irrigation requirements by 50% and are starting trials in Kenya and Senegal.

Fig. 5 (a) No till agriculture in a Kansas corn field (Source: Hunter College, City University of New York) (b) scanning electron microscope image showing the open pore structure of biochar (Source: Schmidt & Taylor, 2014)

N.B. in this post I discussed soil management techniques (for increasing infiltration and water retention capacity) in terms of increasing irrigation efficiency. However, the same techniques apply to increasing rainwater harvesting (RWH) capacity, as explained in a previous post. Whether soil RWH increases supply or decreases demand depends on how you define terms. It decreases demand for irrigation but increases supply of water in the root zone. 

The three main barriers to widespread adoption of more efficient irrigation techniques in SSA are (1) high cost, (2) lack of accessibility and (3) lack of education about their importance/effectiveness.