Water is essential to all life on Earth.

It's a vital building block of all living organisms - humans, animals and vegetation.

Water, in particular freshwater - is an important resource to understand and protect.

In this video series, I'll be talking about the hidden water - the water in the subsurface that we refer to as groundwater.

Groundwater is part of the larger water cycle.

Rain seeps deep into the subsurface and becomes groundwater.

It travels through the ground and it reemerges at the surface - in rivers, lakes or the ocean.

From these water bodies it'll eventually evaporate, form clouds - and return to the surface as precipitation.

To understand groundwater, let's look at what happens when rain hits the surface.

Parts of it will evaporate directly from the surface as water in gas form.

Another part of the water seeps into the ground.

From here, it's very likely it'll be absorbed by plant roots - and go back to the atmosphere as transpiration from the leaves.

The part of the water that escapes the plant roots - continues to travel into the ground and becomes part of the groundwater.

The water will travel in the pore space between sand grains in the subsurface.

Initially, this pathway will be mostly vertical, driven only by gravity.

At some point, so much water has accumulated - that the pore space between the grains is completely filled with water.

This is what we see with the red color I added to the water here.

At this level, our water droplet has reached the groundwater table.

The part of the precipitation that forms new groundwater - is referred to as the recharge.

The recharge ranges from almost nothing in dry regions - to about a meter in very wet regions.

In Denmark, where we are now, the recharge is about ten centimeters per year - or the equivalent of about 100 liters per square meter.

From this point onwards, the water will travel not driven by gravity - but by pressure gradients from high pressure to low pressure.

The particle tracks will be curved pathways - bringing the water deep into the subsurface, and eventually - back to the surface at a river, the ocean or a lake.

At the point where the groundwater reenters the surface - the direction is upwards. Pushed by overpressure - the water is entering into the bottom of lakes or streams.

It may also appear directly on land forming a natural spring.

This particular spring in the middle of Denmark slowly grows into a small river - and reenters the ocean after about 160 kilometers through the Danish landscape.

In the general case, springs may be located very far - from the actual point on the surface where the water entered into the ground.

This is the case in oases and deserts.

Let's focus on the groundwater table.

It's not always below ground.

It's visible in the landscape in many places.

This is where we have rivers and lakes.

When you look at a lake like this one, you're looking at the groundwater table - intersecting the surface topography.

So, how old is the groundwater?

It varies quite a bit.

In areas where the pathway of the water in the ground is short and shallow - it may be days or maybe a few years old.

In other areas where the water pathways are deep and long - the water may be very old, easily thousands of years.

In Denmark, where we're standing now, the groundwater that we use for drinking - is between five and fifty years old.

We have a few deep wells where we extract water that's a few thousand years old.