What is Groundwater Recharge?
What is Groundwater Recharge?
And why is it important?
A previous article on The Water Droplet discussed groundwater and its importance. Kitchener-Waterloo, Ontario, where I live, is highly dependent on groundwater for drinking water and with summer just around the corner, it’s important to recognize that our demand for water increases in summer months1. Understanding where groundwater comes from is fundamental in learning about the importance of groundwater. This leads to the important concept of groundwater recharge.
What is Groundwater Recharge?
I’ll start by introducing groundwater recharge. In a previous article, I discussed the main components of the water cycle. When rain falls onto the ground a few things can happen to the water:
It can flow down the slope of the ground to a stream or puddle or wetland or lake (this is called ‘runoff’)
It can evaporate into the atmosphere (evaporation)
It can be consumed by plants (transpiration)
It can infiltrate into the ground.
Once water gets into the ground, some of it will continue flowing through the ground until it hits the water table2. The water that reaches the water table is referred to as groundwater recharge.
Recharge Areas
Any area where water gets into the ground is referred to as a recharge area. In other areas, groundwater can seep out of the ground. This is know as a discharge area (sometimes found around stream banks). Recharge areas are mapped using all available data (water levels, geology) and with numerical models that are used to simulate groundwater conditions. In the map below (reference here), the interpreted recharge area for the Waterloo Moraine area is shown in orange.
This is a big area and it’s critical for providing water to the groundwater that supplies water to nearly 100 pumping wells in and around the cities of Kitchener and Waterloo.
Estimating Groundwater Recharge
Precipitation is typically measured in millimetres of water per year3. The amount of precipitation that gets into the ground as groundwater recharge is highly dependent on the nature of the ground surface (think about pouring water on a bucket of dry sand - that water will disappear quickly into the sand. If that same bucket is full of hard-packed clay, the water you pour will just sit on the top surface of the clay).
In the Region of Waterloo, annual precipitation is approximately 900 millimetres per year. Groundwater recharge can be as low as zero, or as high as about half of precipitation (about 450 mm/year, reference here)4
Why is Groundwater Recharge Important?
In a previous article, I discussed where my tap water comes from and I tried to emphasize the importance of groundwater recharge areas. If we look at an aerial photograph of Kitchener-Waterloo and the surrounding area, we can see the difference between the urban areas and the rural farmland that surrounds the cities. Developed areas in the cities of Kitchener and Waterloo show a lot of grey colour that indicate roads, parking logs, building structures. Outside the city, land is mostly green or brown, which indicates farmland or forested areas.
As cities grow, changes in how we use our land are inevitable. However, we need to understand how those land-use changes will affect our environment. The environment includes water resources and we must not forget about the importance of groundwater. If we pave over too much of our recharge areas, our drinking water supply wells will dry up!
Thanks for reading The Water Droplet! Stay tuned for more articles on water!
The City of Waterloo, like many places, has introduced by-laws that restrict outdoor-use of water during summer months. https://www.therecord.com/news/waterloo-region/2021/05/30/waterloo-regions-outdoor-water-use-bylaw-comes-into-effect-monday.html
You can think about the water table this way: If you’re digging a hole in the sand at a beach, you’ll eventually reach water. The elevation that you dig to is the Water table. Depending on how far you are from the shoreline, the depth you will need to dig will vary. If you were to dig numerous holes in different locations on the beach, you could map out the elevation of the water table and come up with an interpretation of the water table surface for that area.
Precipitation includes rainfall plus the water-equivalent of snow/sleet/hail. 30 centimetres of snow might equate to 10%, or 3 centimetres of water, but this can depend on many different factors, such as snow density, air temperature. In Kitchener-Waterloo, precipitation is approximately 900 millimetres per year.
Recharge in small, localized areas can be much higher. During snowmelt events or high-rainfall events, the amount of water available can be very high. This can lead to an increase in local runoff, which can accumulate in small ponds. Researchers at the University of Waterloo have estimated that recharge at these ponds can be very high (as much as 300 millimetres of water!) over a very short period of time (a few days). This is referred to as depression focussed recharge.