More efficient irrigation is not a silver bullet for the world’s water woes.
Meeting the additional food and material requirements of a growing global population, with an improving standard of living, will likely mean that the demand for water is going to increase substantially. The prevailing view is that many places will run out of water if we don’t do something. Indeed, the recent WEF Global Risks Report 2019 has, yet again, highlighted water crises as something that is of prime concern to business, government, and civil society leaders.
As water for irrigation is one of the biggest users of water globally (at around 70% of total withdrawals of water), solutions to the water crisis are often looked for in agriculture. Drip irrigation and other types of high-tech irrigation are widely held up as a marvel when it comes to saving water — often traditional forms of flood irrigation are said to have typical efficiencies of 50% while drip irrigation is said to have an efficiency of 90% or more.
That’s a saving of 40%, right? So, if we somehow managed to shift all irrigation to drip we’d nearly solve the world’s water problems? Well, no — like this blog and the recent paper in Frontiers in Environmental Science it is based on, make clear.
With an ‘inefficient’ and leaky irrigation system all that ‘lost’ water has to go somewhere — it either percolates into groundwater or runs into surface drains — and in most (but not all) cases it finds its way back to rivers or other wetlands. So the water is not lost to the river basin and is most likely available for other uses or the environment, but it does have to take a roundabout route back to rivers (either quite quickly in surface drains or really slowly through groundwater), which means that the water is not returned in the same place or at the same time as it was taken out. That returned water probably has more pollutants (nutrients and pesticides) from farming in it too.
So, improving irrigation efficiency isn’t generally going to ‘save’ water as it wasn’t lost in the first place. However, when compared with an efficient irrigation system consuming the same amount of water as evapotranspiration, inefficient irrigation can have a bigger impact on river flows and aquatic ecosystems. The time delay between when the water is taken out of the river and when the ‘wasted’ portion of it gets back to a river or wetland means that the water might not be available when species in the river need it.
The fact that it doesn’t go back to the same place it was taken from means that important habitats can be ‘bypassed’. And, of course, the additional pollutants leached out can affect aquatic species. So there is a potential for improved irrigation efficiency to deliver environmentally beneficial outcomes.
Even if there isn’t much or any scope for saving water at a catchment scale, for an individual farmer changing to a more efficient irrigation system is often a sensible choice. Becoming more efficient means they lose less of the water they put on their fields back to the environment. But if that farmer still has the same amount of water available to them (in terms of the amount they are allowed to or able to pump, for instance) then they can, and do, use the efficiency gains to grow more profitable crops that use more water, or grow an additional crop within the season, or expand their irrigated area. Often, more high tech irrigation gives farmers a better yield as they have more control over the timing and distribution of water. Higher yields mean more water is consumed as transpiration by the crop.
All of these factors mean that more, not less, water is lost from the catchment as evaporation and transpiration: water that was previously going back to freshwater habitats.
When large scale programmes to increase irrigation efficiency are implemented by well-intentioned public sector organisations, private businesses, or NGOs, all those individual farmer responses add up to significantly more, not less, water being consumed. There is very little evidence of real savings at the catchment scale from more efficient irrigation and plenty of examples of greater intensification of water consumption. This works out great for the farmers who can take advantage of more efficiency, less so for those downstream (and ecosystems) who find they are getting less water than before.
So, from the perspective of establishing or restoring environmental flows, promoting irrigation efficiency can be something of a double-edged sword. For a given level of crop production, increasing irrigation efficiency can potentially improve river flows from a quality and timing perspective, even if there is no additional flow in the river over the course of a year. But if withdrawals are not somehow constrained the chances are that the collective response of farmers will mean that water consumption goes up, there is less water returned to the environment, and rivers and wetlands will be worse off.
High-tech irrigation is not the silver bullet solution that it is often proposed to be for existing and impending water crises. It certainly has a role to play but, first and foremost, the solution comes back to having good water management and governance, grounded in a good understanding of the hydrology: which is much more difficult than putting in fancy bits of kit.
Water allocations for environmental flows have to be explicitly defined and protected in legislation, not expected to be delivered as the residual of savings that can be made in agriculture. Within the context of consensus-based water allocations between the environment, agriculture, domestic and industrial users (that accounts for all of the trade-offs at a basin scale, and consumed fractions), water managers must also have an effective means of controlling withdrawals by agriculture to forestall increased consumption in response to improved efficiencies.
Only once all that is in place should high-efficiency and high-tech irrigation be deployed.