Scratching the surface of water use efficiency
A number of irrigation schemes have converted from open race to pressurised pipe distribution, eliminating race leakage and by-wash losses. Overall, much less water is now being used to irrigate each hectare, with increased efficiency of water use.
On the face of it, increasing water use efficiency seems like it must be good thing: less stress on our precious water resources, and more "crop per drop". A win-win surely? Unfortunately, the answer isn't that simple and requires a bigger-picture view of the catchments that irrigation is occurring in. International research on this issue concluded that increasing the efficiency of on-farm water use rarely results in less water being used at a catchment scale. There is also potential for unintended adverse consequences.
The Land and Water Regional Plan and the Canterbury Water Management Strategy both aim to increase efficient water use in Canterbury. In part, this has driven the changes from border dyke to spray systems. Other drivers are the opportunity to use water over a greater area (if the amount of water originally allocated is retained by the consent-holder or irrigation scheme shareholder), and reduction in labour costs. These factors generate benefits for individual water users, but not necessarily for the hydrological system.
There are many ways of defining “efficiency” in relation to irrigation, depending on the viewpoint (agricultural production, water allocation, hydrology) and the scale (paddock, farm, catchment). By any definition, inefficient use of water does not remove it from the hydrological system – it shifts it from one place to another: “losses” at the paddock, farm or irrigation scheme scale may be re-captured elsewhere, and may have other benefits. Inefficient use of water from a river or a deep aquifer source may result in more water recharging the shallow groundwater system and increasing flows in spring-fed streams. If inefficient water use has been occurring for many decades, the “losses” may have become an integral part of the local water resource system: for example, water supply bores may depend on groundwater being at a certain level, or the ecosystems in lowland streams may have adapted to the current flow-regime.
Water use and water quality are closely and inextricably linked. If water use efficiency increases, but land-use practices remain the same, there is less water flowing through the system to dilute the load of contaminants such as nitrates. This can result in higher contaminant concentrations in shallow groundwater and the surface waterways that are fed from it.
Let’s be clear, I’m not endorsing wasteful use of water. However, we need to ensure irrigation efficiency is not considered in isolation, and that the benefits for individuals are balanced with the broader system benefits.
So, what is the solution?
The same researchers that I’ve referred to earlier suggested five steps:
Robust water accounts, increasing transparency of how much water is being used, and where, and where the losses go.
Limits on water use (we already have this through seasonal volumes and other consent conditions).
Evaluating the effects, including risks, of increasing water use efficiency.
Understanding whether the benefits of increased water use efficiency outweigh the costs.
Water allocation policies that drive the desired outcomes at all scales.
A common thread through all of these steps is the need for robust and comprehensive data, enabling better understanding of water use and the catchments that it occurs in.