**6. Irrigation Water Productivity**

This paper has largely been written from an engineering perspective, which predominantly defines WUE as the ratio of the irrigation water beneficially used by the plant or pasture to the water supplied through irrigation. This section will briefly discuss irrigation water productivity, with a focus on plant genetics and agronomic practices used to achieve higher yields using less water.

Through plant breeding, scientists have managed to develop high-yielding crop varieties. This implies that, all other factors kept constant, with the same amount available water, farmers can achieve a higher irrigation water productivity. Detailed information on how the plant WUE can be improved through molecular genetics is described in Ruggiero et al. [38]. The research provides an overview of the manipulation of genes that strongly impacts on WUE such as these that control root traits and stomatal development. Some genetically modified varieties are also resistant to pests and diseases, leading to higher yields. A study conducted within the cotton industry in Australia found that the water use productivity had increased by 40% over a period of ten years as a result of yield increases achieved by developments in plant breeding, the use of genetically modified varieties, and improved crop and water managemen<sup>t</sup> systems [5].

Deficit irrigation, which is the application of less water than that is required by the plant or pasture, is a strategy that is often used when water is limiting. In a trial conducted in a dairy region of Victoria, Australia, where pasture is often irrigated, Rogers et al. [39] demonstrated that lucerne under deficit irrigation can fully recover once full irrigation is restored, and thus ideal forage can be grown under water limiting conditions. Tejero et al. [40] in a trial undertaken in a citrus orchard in Spain concluded that deficit irrigation strategies have the potential to improve WUE. Du [41] proposed the adoption of deficit irrigation strategies in areas of China where conventional irrigation is no longer sustainable because of water shortages.

### **7. Water Consumption at the Basin Scale and Trends in WUE**

### *7.1. WUE and Water Consumption at the Basin Scale*

The need for water users to achieve greater WUE is often seen as a prerequisite for saving water for the benefit of other users as well as the environment. However, literature reviewed suggests that a higher WUE does not necessarily equal to net water saving, particularly at the basin scale.

When seen from the dimension of a water basin, what may be assessed as a loss in one perspective (e.g., deep drainage losses that may occur in surface systems), may be a gain in another way (e.g., recharge of groundwater resources). Some research has shown that significant improvement in delivery and on-farm WUE may in fact lead to a decline in groundwater resources [21] or reduce water for environment and downstream users [3]. Therefore, although improvement of on-farm irrigation WUE may lead to water savings on the farm, it will not necessarily be beneficial on a catchment or basin scale [15].

An overall increase of water consumption at the basin scale may occur if water savings ultimately leads to an expansion of the irrigated area [42]. This was demonstrated by research conducted in Morocco which saw the overall water consumption rise as a result of subsidised drip irrigation kits promoted as a means of increasing productivity and saving water [43]. In this example, although the drip system is generally regarded as water-efficient, farmers were found to shift to more water-intensive crops and generally used the "saved" water to expand the acreage under irrigation. This view has been corroborated by other studies, for instance a FAO-funded research project undertaken in North Africa and the Middle East region [44]. The study found that at the field scale, water saving may appear to be substantial, but at the basin scale, the total water consumption may actually increase while the crop water productivity gains for the most important crops may be modest at best. In a study undertaken in India, the widespread adoption of water efficient methods such as the sprinkler and drip systems were found to have the capacity to substantially reduce overextraction of groundwater resources, but half of the water saved was reused to expand the area under irrigation [45].

Figure 4 is used as an example to demonstrate the simultaneous increasing uptake of water-efficient technologies (sprinkler and drip) and the increasing total area under irrigation (especially between 1994 and 2013) in the Unites States. This suggests the potential reuse of any water savings to expand the area under irrigation. The graph in Figure 4 also shows the corresponding decrease in surface irrigation systems that are generally regarded as inefficient.

**Figure 4.** Area of land in the Unites States irrigated using different methods. (Plotted from data obtained from: USDA 1990, Table 4; USDA 1999, Table 4; USDA 2006, Table 4.6.1; USDA 2009, Table 4; USDA 2014, Table 28) [46].

An analysis of the MDB in Australia showed that the environment may become the unintended casualty (receive less water on average) of the increases in WUE driven by the adoption of water-efficient technologies [47] with most of the saved water being reused. The reuse of the saved water seems to be corroborated by the trend of the total irrigation water use in Australia between 2002 and 2017 (Figure 5). The graph shows that the total irrigation water use between 2002 and 2006 was above 10,000 gigalitres (GL) but reduced to a low of just above 6000 GL in the four-year period: 2007–2008 and 2010–2011. The reduced irrigation water use in the period 2006–2011 was as a result of a severe drought that drastically reduced the availability of water for irrigation. In the period 2012–2014, the water use increased back to a similar level to the early part of the available data (approximately 11,000 GL), effectively signalling no net water saving. There was a decrease of irrigation water use in 2014–2016, but increased slightly in 2016–2017 to just over 9000 GL. The trends appear to be largely dependent on weather patterns.

**Figure 5.** Total irrigation water use in Australia. (Plotted from data obtained from: Australian Bureau of Statistics (ABS), 2004–2018.)

While Figure 5 shows the trend of irrigation water use in the whole of Australia, Figure 6 is specific to the MDB, which consumes the bulk of the water used for irrigation in the country as previously discussed. In addition, Figure 5 shows the trend of the irrigated land in the basin and correlation of the irrigation water use with the area irrigated, meaning when farmers have access to more water, they irrigate more land (and vice versa). Therefore, it is likely that water saved as a result of the water-efficient technologies and practices adopted is reused as suggested by the studies quoted above.

**Figure 6.** Irrigation water use and area irrigated ('1000 ha) in the Murray Darling Basin, Australia. (Plotted from data obtained from: ABS 2004–2018.)

However, there are strategies that can be used to attain a good balance between improved irrigation efficiency and environmental conservation, including groundwater recharge. A typical example is the water saving initiatives funded by the Australian Government, with the understanding that the water saved is released for environmental use [6]. The regulatory return of the saved water to the environment therefore mitigates the "rebound effect" phenomenon, which suggests that the increase in efficiency of use of a resource may lead to an increase in the rate of consumption of that resource [48].

Many other studies undertaken in different parts of the world have also linked widespread adoption of water-efficient technologies to overall increase in water consumption mostly due to expansion of land under irrigation, and not a decrease as intended. Nonetheless, as shown by a study undertaken in Spain, the water-efficient technologies have come with other side benefits such as reduced use of fertilisers and better accounting of water use [49].

In the literature reviewed for this study, an almost unanimous view that emerges is that overall reduction of water at the basin scale cannot simply be attained through the promotion or subsidies provided for water-efficient technologies. These technologies will thus need to be used in tandem with other measures such as incentives for conservation [45] and regulations to limit water allocation [44], among others.

Another interesting dimension is the nexus between the irrigation methods deemed to be generally more water-efficient, energy consumption and greenhouse gas emissions. For instance, modelling has demonstrated that although pressurised irrigation systems such as sprinkler and drip methods are generally more efficient and productive, they are more energy-consuming (compared to conventional systems such as furrow irrigation), resulting in the production of additional greenhouse gas emissions [50]. The energy costs in many countries (in the case of irrigation electricity for pumping water) has been rising steadily. This is thus likely to impact on the adoption of the water-efficient but high-energy consuming irrigation methods.
