Perth, where I live, is a city built on what is, geologically, a giant sandpit. This is a curse for gardeners, who struggle to grow anything on the infertile and dry sands. But, it is a blessing for the city. That's because if you go down deep enough, the sandpit is saturated with groundwater - and by using that groundwater to water gardens and vegetables through the rainless summers, we enjoy green and pleasant parks AND amazing beach weather. Awesome. In the low parts of the landscape, the groundwater emerges as wetlands - windows into the water table - and creates vibrant habitats.
You have to pay back what you use, and for this shallow aquifer, that payback comes when winter rainfall recharges the aquifers. Ideally we would balance the water use from the groundwater systems against this replenishment. But Perth's groundwater has been out of balance for a long time. We can tell, because the water levels in the aquifers are lower than in the past - much lower in some parts of Perth.
Knowing how much recharge is entering the aquifers is obviously really helpful to getting to a balanced state. Rainfall has been dropping - how has this impacted the recharge?
To answer this question we did a literature review of all the studies that explored recharge over the Gnangara Groundwater Mound north of Perth (https://www.sciencedirect.com/science/article/pii/S0022169424004025). Kudos to Simone Gelsinari who lead the study and did the massive compilation and review of the literature!
We also reviewed the processes that controlled recharge and their susceptibility to climate changes - schematically the measurements and processes are shown in the figure below.

We developed simple, analytical models to link these different factors together so we could look out for surprises that might cause fast and large changes to recharge.
We found that recharge has been dropping as rainfall did - but not just because there was less rainfall overall - also because each drop of rain that fell produced less recharge. As the climate dried, recharge declined faster than rainfall. This was true for all soils and vegetation types, as shown below.

The really big contributor to this was the shrinking length of the winter season - which means there's less opportunity for rain to wet the soil and allow water to reach the groundwater.
This problem is most extreme over vegetation with deep roots - like native woodlands and pine plantations - the plot below shows the big changes in recharge under banksias as a % of rainfall.

This might mean that we need to rely more on recharge from cleared land and urban areas to refill the aquifers in the future. Our ongoing work monitors recharge from different land use types, so we will be able to compare different land uses monitored with the same methodology to understand the recharge behaviour over several years.
This paper was great to write - it has explanatory models, process reviews, data analysis - I feel like it shows off a lot of our group's toolkit. The message is a bit sobering, but the more we know, the better we are positioned to plan and manage the impacts of the changing climate on our precious water resources.
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