Stories From Gnangara Mound
Recharge Story
Recharge is the water that makes it underground, and is a very important contribution to our water supply
Every year Banksia woodlands North of Perth send about 123mm of water to groundwater. Groundwater is water stored underground, fed by water that seeps through the soil. The water that makes it to the aquifer is called recharge. It is important to understand recharge if we want to manage our groundwater supply sustainably.
Not all rainfall makes it to the aquifer. Some evaporates or is used by plants. When plants use a lot of water, recharge can be negative. So, we can summarise recharge in a year, as total rainfall minus evapotranspiration.
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Recharge changes every year, mostly due to rainfall
Variation in recharge from year to year is almost as big as average recharge volume. This variation is 86% of the average recharge. Most of this variation is due to rainfall, where the variation in rainfall is 112% of the variation in recharge. Differences in evapotranspiration from year to year are about half as big as that of recharge (52%).
This means that the different rainfall from year to year is the biggest control on recharge. So, we can try and understand recharge at Gingin better, by understanding rainfall better.
WA experiences wet winters and dry summers
The plot below shows how much rain fell at Gingin each month on average for all the years that recordings were taken. In summer, rainfall amounts are low and more variable. In winter, there is more rainfall and it is more reliable. This pattern of rainfall is typical of the climate in Western Australia.
We can predict recharge in the future based on what we know about rainfall
The total recharge in any year is most closely related to how much rain falls in total over Summer, Autumn and Winter. Having more or less Spring rainfall at Gingin does not change the recharge for the year. Knowing this, predictions of summer, autumn and winter rainfall in the future, can help us predict recharge volumes at Gingin in the future. We estimate that by 2090, average yearly recharge will reduce to 87mm!
Evaporation changes throughout the year and affects the amount of recharge
Evaporation speeds up when conditions are sunny, warm and dry, and slows down when it is cold, dim and humid. Most of the evaporation in the year happens in spring when the plants are active and the Gingin soils are wet following winter rainfall. Helping maintain the ecological balance at the site, both rainfall and evapotranspiration play an important role in the story of recharge at Gingin!
Evaporation is complicated. Many different environmental issues affect how much evaporation happens. For example, wind, temperature, humidity, how wet the soil is (soil moisture), and the number, type and health of plants that use water can all change how much water is evaporating on any given day.
To summarise all these different influences, water scientists separate the problem of how much water could be evaporated by the environment from how much water is available. They describe how much water could be evaporated as a potential evaporation. In a sandy flat area like Gingin, how much water is available to evaporate can be measured using the average soil moisture
When the land is dry and the plants are water stressed (e.g. Summer), the actual evaporation will be less than potential evaporation. When the land is wet (e.g. Spring), the actual evaporation will become closer to the potential evaporation.
Our first plot shows how the maximum actual evaporation we see increases as the potential evaporation increases. But once potential evaporation is large enough, actual evaporation can't keep up and the maximum value doesn't change any more.
We can see that the spring conditions have high potential evaporation. When more rain falls in spring, it is evaporated by the actively growing plants.
Our second plot shows how soil moisture controls how close the actual evaporation is to the maximum rate. The maximum rate would be occurring if the AET/PET value on the y axis was equal to 1. As soil moisture increases, the AET/PET increases, showing that lack of water becomes less important in controlling how much water evaporates.
Summarising Recharge at Gingin
Rainfall, evapotranspiration and the moisture in the soil are all connected and can help us understand what recharge at Gingin will be like in the future. Below we see how each variable influences each other and most importantly, how they all influence recharge!
Click here to see how the last 10 years of rainfall at Gingin compares to historical rainfall dating back to 1889!
Click here to explore the relationship between recharge, evaporation and rainfall yourself!