Will El Nino impact water supplies this summer?
El Nino and La Nina (which has been in driving seat for the past two summers) are the two extreme ends of the El Nino Southern Oscillation (ENSO), which is a fluctuation of sea-surface temperatures in the Pacific Ocean that occurs on a cycle of three to seven years. While some parts of New Zealand can experience drought under both El Nino and La Nina, the last two summers in Canterbury have been wetter than average, which has taken the pressure off for water users.
The weather patterns generated by El Nino in New Zealand tend to be westerly flows, with more rain in the west. A strong El Nino can lead to drought in eastern parts of New Zealand. While this typically doesn’t create the life-threatening conditions that the WMO is warning about, it can have a significant impact on water demand and supply, particularly for non-alpine water sources: demand is high, and supplies can be on restrictions for long periods.
El Nino’s impact on atmospheric conditions tends to be greatest from December to February, coinciding with the peak of our irrigation season. It is too soon to say for certain that Canterbury will experience drought conditions next summer, however previous El Nino events have caused severe droughts. These include 1972–73, 1982–83, 1997–98 and 2015–2016 — years that will feature in the memory of farmers who experienced them. In simulations of water supply and demand in Canterbury, combining historical climate and river flow data with current levels of irrigation and municipal supply, 1972–73 stands out as the summer in which the greatest pressure was put on water resources.
The Canterbury Strategic Water Study was initiated by local and central government following the 1997–98 drought, due to concerns that Canterbury was “running out of water”. One of the key findings of this study, is that Canterbury has sufficient water, but it is often in the wrong place at the wrong time. Water storage is a critical part of addressing this issue, and while some progress has been made on building storage in Canterbury, there is a long way to go if we want water supplies across the whole region to be resilient to severe drought. Improving resilience to shorter-term climate variability puts us in a much better position to deal with longer-term climate change.
In the 2015–16 drought, irrigators with access to water from the alpine rivers coped well, particularly those who also had access to stored alpine water. In contrast, some foothills-sourced water supplies were on either full or partial restriction for much of the time from Christmas through to the end of the irrigation season. The following summer, groundwater supplies were under pressure due to the succession of several years with high demand in summer and low recharge over winter, causing low water levels.
While we don’t know for sure what the 2023–24 summer will bring, it is best to be prepared by managing water from the beginning of summer as if a drought is a certainty. Unlike floods, droughts can sneak up on us: we often don’t know we’re in one until it is well underway. Water users with annual volume limits on their resource consents will need to keep a close eye on water use, as there is a higher chance of limits being reached.