In many lowland areas, land drainage (subsurface, open channels, or combination of the two) enables land to be used productively by lowering the shallow groundwater table and preventing waterlogging of soils. Drainage networks have been a feature of lowland Canterbury for well over a century, and considerable thought is now being focussed on re-imagining these drainage channels as waterways in their own right, with associated biodiversity values, rather than solely as a network utility.

The combination of a relatively high groundwater table and deep-rooted crops can significantly reduce the demand for irrigation due to the plant’s ability to access water from below. If this concept is factored into future land use decisions, it could help to reduce our reliance on rainfall and rainfall-dependent water sources as the climate changes. Of course, groundwater levels still depend on long-term rainfall, but the groundwater system’s slower response time can buffer out a lot of the variability.

There are other facets of groundwater systems that are beginning to receive more attention, including the role of groundwater in flooding, its contribution to hazards for the built environment, and its potential as an energy source. Groundwater can contribute indirectly to the effects of surface water flooding. If groundwater levels are already high, there is less capacity for the land to absorb rainfall and any floodwaters that spill out of surface channels. When groundwater levels rise sufficiently to intersect the land surface and cause ponding, this is called groundwater flooding. While groundwater flooding typically doesn’t create the same level of damage as river flooding, the nuisance can persist for a long time as low-lying areas prone to groundwater flooding often don’t have good drainage pathways for the water to flow away. We need to be aware of the potential for sea level rise to exacerbate the risk of high groundwater levels and associated flooding. There is a link between this and liquefaction risk: saturated soils are more prone to liquefying during an earthquake.

Ground source heat pumps

Ground source heat pumps (GSHPs) have been a feature of the post-quake rebuild for Christchurch, with several large building projects incorporating them as a heating and cooling solution. These systems are more efficient than air-source heat pumps, taking advantage of the relatively stable temperature of groundwater to extract heat from groundwater in winter, and using groundwater as a heat sink in summer to cool buildings. There is no net take of water: the systems either circulate water through a closed loop situated below the water table or use paired abstraction and injection bores. While GSHPs aren’t commonplace in rural settings, they have potential for milk cooling and dairy shed water heating, as well as heating farmhouses.

Groundwater will continue to be an important part of life in Canterbury. Managing water levels and quality is important for our region’s continued wellbeing. As the climate changes we will need to become even more tuned into the benefits and hazards that groundwater can contribute to.