Groundwater levels — what can we expect?
Groundwater levels in Canterbury’s aquifer systems are driven by a combination of land-surface recharge (water infiltrating through the land surface and soil profile), river recharge and abstraction pressure. Water levels in the future represent a combination of these factors over various timeframes due to time lags in the system: what happens in spring and summer can be determined in part by the abstraction from the previous irrigation season, and winter recharge. The remaining piece of the puzzle is what happens over the rest of winter and early spring.
Low groundwater levels can create issues with well performance, particularly if levels have trended down over time since the well was drilled. Waiting until you run into problems to contact a driller about deepening isn’t ideal, as there’s likely delays. Water takes from bores near spring-fed lowland streams (or takes from the streams themselves) often have consent conditions tied to the stream’s flow, which in turn is dependent largely on the state of the groundwater system that feeds the stream. At present, some groundwater consents in Canterbury are adaptively managed, based on an assessment of groundwater levels before the start of the irrigation season: very low water levels can result in no water being available, which can be difficult to plan for. Adaptive management, in some form, is likely to become more common in the future, meaning that this will become an issue for an increasing numbers of groundwater users.
On the other hand, while high groundwater levels have benefits for lowland streams and their ecosystems, they can cause issues for land that is prone to waterlogging.
This winter, groundwater levels are above median (i.e. levels that have been exceeded more than 50% of the time historically) almost everywhere in Canterbury. Environment Canterbury have published data showing that in May 2022, 60% of wells that they monitor were at or above median levels for the time of year. Irrigation demand was lower than average over the 2021-22 summer, resulting in less pumping, we’ve also had some significant rainfall recharge events. This is in stark contrast to the end of summer 2021, when only 7% of wells were above the median. However, we aren’t currently at historically high levels.
Even without long-term weather outlooks, it’s possible to forecast groundwater levels several months into the future. Because the levels are driven by current and past conditions, an “envelope” of predicted groundwater levels can be generated, based on the results of computer simulations using the historical climate data from every year — wet, dry and average.
The graph shows last year’s monitoring data from a 20m deep well near Leeston. The dots are the monthly level measurements, the solid line is the median measured level for each month, and the light blue shaded band is the range of measured levels.
Following on from the most recent measurement (20th July) the predicted range of water levels is shown in green, with the median forecast indicated by the dotted line. It’s showing that even if conditions from now on were the driest on record, we are still likely to head into spring with water levels in this bore around the median level. For at least the first part of summer, we are unlikely to face any issues related to low groundwater levels. Tools like this can assist groundwater users in understanding whether they are likely to face any issues over coming months.