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Water Potential: Definition, Equation & Measurement

Instructor: Nichole Miller

Nichole is a research scientist with a PhD in Materials Science & Engineering.

This lesson defines water potential, gives everyday examples of water potential in action, and explains how gravity, pressure, and dissolved materials affect water potential.

Water Towers

Water towers come in a variety of shapes and sizes. Sometimes they display a city name or the mascot of a local sports team. Other times the decorations are more creative like the Earffel Tower, a water tower shaped like Mickey Mouse at Disneyland Paris. Whatever their decoration, all water towers have one thing in common: they tower above the city around them. In fact, their height is what makes water towers work.

Take a look at this schematic of a water tower. Because gravity pulls the water down, you can get flowing water even if you are on the fourth story of the building, because the water tower is even higher.

Schematic showing a water tower delivering water to a four-story building.
Water tower

Potential Energy

We can also talk about the water in a water tower in terms of potential energy. Potential energy is energy that is 'hidden' or stored in some way. Water at the top of the water tower has some stored energy due to its height above the ground, which is why water always flows down if we let it. We therefore say that water in the water tower has a higher potential energy than water closer to the ground.

Water Potential

We could also say that water in the water tower has a higher water potential than water closer to the ground, because water potential is a measure of the potential energy of water. Water potential is measured in units of pressure. Just as air will flow from regions of high pressure (like inside a bike tire) to regions of low pressure, water will flow from regions of high water potential to regions of low water potential.

We've already discussed water potential as a result of gravity, but water potential can also be due to other factors like applied pressure and molecules dissolved in the water. As shown in the equation, the total water potential is the sum of the water potentials due to each of these factors. We'll talk more about water potential due to pressure and dissolved molecules below.

water potential

The Effect of Pressure

Applying pressure to water increases its water potential, whereas decreasing the pressure on water decreases its water potential. Think about what happens when you drink water through a straw. When you suck on the straw, water goes up the straw and into your mouth. By sucking on the straw, you decrease the pressure on the water and therefore lower its water potential. Because water flows from regions of high water potential to regions of low water potential, the water flows out of the cup and into your mouth.

The Effect of Dissolved Molecules

It's easy to understand how gravity and pressure affect water potential, but it is a little more difficult to understand the effect of dissolving something in water. To understand this better, let's take a look at the drawing showing a glass of liquid. In the middle, there is a semi-permeable membrane. Water can travel through this membrane, but dissolved materials cannot.

A drawing showing the effect of water potential due to dissolved molecules
osmosis

We start with the same amount of water on both sides of the membrane, as illustrated by the cup on the left. The concentration, or the amount of the dissolved molecule in a given amount of water, is lower on the left side of the membrane than on the right side. What will happen if we let the glass of water sit for a while? It turns out that some of the water on the left side of the membrane will pass to the right side of the membrane to help even out the concentration of the molecules on the two sides of the membrane.

In other words, water with a high concentration of dissolved molecules has a lower water potential than water with a lower concentration of dissolved molecules. Because water flows from regions of high water potential to regions of low water potential, water will flow toward regions with higher concentrations of dissolved molecules.

One interesting application of this is in plants. Plants control how much water the roots absorb from the ground and where in the plant this water goes by changing the amount of dissolved molecules in different parts of the plant.

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