What is a Water Table?

Today we discuss water tables and why they are important. We also learn what factors can affect the water table.

You may have heard the term “water table” in geology before. It plays an important role with regards to the water we use in agriculture and in the home.

Various factors affect the world’s water tables. Geologists must keep track of them to ensure we have adequate water supplies. Before we examine these factors, let’s look at the definition of a water table.

Water Table Definition

Water tables are the boundaries that exist between unsaturated ground and land that has become saturated with water. Much of the water we use exists below the water table. But one region’s water table will differ from another.

You will find rocks and soil underneath the water table, just as you will above it. But the ground below a water table contains groundwater, often contained in small pockets. This means you can dig down through the water table to access the water below. Geologists call these water pockets “aquifers.”

Water tables do not stay stable throughout the year. For example, the land’s topography may change, which results in the water table changing with it. Other factors, such as the amount of rain or snow in an area, will also affect the height of the water table.

Water table geology experts call the area below a water table the “phreatic zone.” They call the area above the water table the “vadose zone.”

Why are Water Tables Important?

This water table definition does not show you why water tables are important. For centuries, we have broken through these tables to gain access to saturated ground. Take wells as an example. Digging deep enough into the ground results in saturated rocks and aquifers releasing the water they contain. This provides a constant source of water to the well. On a wider scale, groundwater provides up to 98% of the water we drink, making it vital to human survival.

We also break through water tables so we can use the water beneath for agriculture. Irrigational systems often rely on saturated ground to provide the water needed for crops to grow. Groundwater also plays a role in how well plants grow in the wild

In irrigation systems, there are several factors that will affect the water table geology and how well the water beneath the table flows. The depth of the hole determines how much water accumulates in it. Furthermore, some regions are less prone to saturation than others; digging in barren land will not produce much water. Finally, diggers must consider the openings they create in the land. Connecting these openings properly allows water below the water table to flow freely, thus providing easier access.

You also need to consider the height of the water table. As a general rule, the higher the water table, the easier it is to access groundwater. Conversely, on land with a low water table, it’s harder to access the benefits of groundwater. This can lead to less agriculture due to a lack of water. This also has an effect on how much water we have available to drink. It’s also essential that we protect this water from harmful by-products of farming like pesticides.

The Problem

Water table geology plays a crucial role in whether people can live off the land. As mentioned, low water table geology makes it difficult to access groundwater supplies. That is why you see barren areas, such as deserts, where people cannot grow crops.

This is a problem, and there are several factors that affect water table geology. Each of these issues can make it harder to access fresh groundwater. Over time, these problems may affect agricultural production and may directly affect how comfortably we live our lives.

Factors That Affect Water Tables

There are three major issues that affect water table geology. Each contributes to changes in water table height.

1. Climate

The current climate in a region affects the height of the water table. High levels of rainfall will raise the table, as will humidity in the summer. This makes it easier to access groundwater, but also makes it harder for the ground to absorb additional water. Areas with high water tables become prone to flooding due to the oversaturation of the land.

The opposite is true for regions with low rainfall and dry weather. In these areas, there is no water for the ground to absorb, which lowers the water table. While this decreases the risk of flooding, it also makes the water difficult to access. As such, these regions can become barren.

2. The soil

The soil’s composition will also help to determine the level of the water table in a region. For example, soil that contains a lot of clay will not absorb much water. This results in a low water table, as water must travel further down to find suitable soil to accumulate. Similarly, rocky areas and regions with compacted soil usually have low water tables.

By contrast, regions with loose soil absorb water more readily. You will see such soil in rainforests and similar regions. Sand and sandy soil also have high water tables. You can see this for yourself if you dig a hole in the beach. You won’t have to dig too deeply before water starts accumulating in the hole.

3. Human activities

How we treat the land can have an effect on water table geology. Usually, human activity leads to the table lowering. Ground clearing, which is often followed by construction work, changes the topology of the land. This often leads to soil erosion. Of course, concrete over land also prevents soil from absorbing water, which causes it to flow to other areas.

Irrigation for agriculture also lowers the water table. Particularly heavy use of such techniques may make the irrigation systems themselves useless until the groundwater replenishes. Combine this with a dry climate, and you can see the negative effects this would have on crop growth.

Frequently Asked Questions

What is the effect of the water table on bearing capacity?

If the water table is present at a depth less than the width of the foundation from the foundation’s bottom, it will reduce the capacity of the soil. The lesser the depth is, the harder it will be to build a solid foundation because the soil will have a low bearing capacity. And, furthermore, the change in moisture content also affects the soil’s bearing capacity.

What is the unit of bearing capacity?

The allowable bearing capacity is noted with Qa = Qu/F.S., where Qa [measured in (kN/m2) or (lb/ft2)] is the allowable bearing capacity, Qu is the ultimate bearing capacity [again measured in (kN/m2) or (lb/ft2)], and F.S. is the factor of safety.

Let’s look at the Terzaghi Ultimate Bearing Capacity Theory so you can better understand what you’ll need to measure to get the right bearing capacity for your foundation:

Qu = c Nc + g D Nq + 0.5 g B Ng (Ultimate bearing capacity equation for shallow strip footings)

where:

c = Cohesion of soil,
g = effective unit weight of soil (kN/m3) (lb/ft3),
D = depth of footing (m) (ft),
B = width of footing (m) (ft),
Nc=cotf(Nq – 1),
Nq=e2(3p/4-f/2)tanf / [2 cos2(45+f/2)],
N g=(1/2) tanf(kp /cos2 f – 1),
e = Napier’s constant = 2.718…,
kp = passive pressure coefficient, and
f = angle of internal friction (degrees)

Can I pour concrete into a hole full of water?

Yes, you can pour concrete in water.

Professionals do this all the time. You just have a small standing puddle in your basement and that won’t be a problem. The biggest issue with pouring concrete under water is movement. If the water is moving, it can wash away the cement paste that’s holding the sand and gravel together. But if the water is calm, then it’s not a problem at all.

Conclusion

Water tables are a key concept in geology. Knowing where these tables are will allow us to determine how we can access the groundwater below them. We depend on this groundwater to live, as it contains approximately 30% of the world’s entire fresh water supply.

As such, we must take the issues that affect water tables seriously. For example, careful irrigation allows water tables to rise, making the ground more useful for agriculture.

On a larger scale, climate change may affect the world’s water tables for many years to come. It is crucial that we adapt to these changes if we are to continue making use of the groundwater that lies below the world’s water tables.

Knowing about your local water table is especially important if you are thinking about buying a well pump. Check out our guide to learn more.

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Image Sources

By Jojko.sivekOriginal: Fiveless – File:WaterTable.gif and File:WTFluctuations.gif, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=22391558

By T.C. Winter, J.W. Harvey, O.L. Franke, and W.M. Alley [Public domain], via Wikimedia Commons