eli5: Why does wet earth (like right after rain) absorb more water then dry earth (like right after a drought)?


Title. I saw a video showing it happening, but I don’t understand why.

In: 51

You know when you have two big droplets of water on a flat surface, and you nudge one towards the other, when they make contact they appear to fuse together fast and aggressively? This is related to what’s known as “surface tension”.

A different phenomenon you may have noticed is when you take say, a dry paper towel, and make a corner touch some water, the paper towel kinda “sucks up” water into itself. This phenomenon is known as “wicking”.

I believe the reason water penetrates wet soil faster is related to a combination of these two. I believe the dry topsoil behaves like the flat surface and water kinda “beads up” on it, whereas on moist soil the rain water would “connect” to the water in the soil and then wicking would encourage it to enter the soil.

If a soil expert shows up they might be able to explain a bit better.

Take out a dry kitchen towel, preferably the sponge kind. Pour water on it. You see that the water jusr flows off.

Now soak it and wring it dry. Then pour water on it again and it soaks in nicely.

The mechanism is same in both earth and the towel. When they get really dry the material collapses, becomes compact and dense. Add moisture and they are nice and loose.

Why is this? Because water doesn’t actually go in to the earth or the towel, but between the fibres or particulates that make it up. If the material is really dry, the gaps are not big enough for water to fit in to it.

As you might have noticed, the wet towel, wet sponge, or just wet dirt weighs more and takes up more space. This is because there is actually more stuff in it, water, and this expands the material like little wedges pushing the parts of the material open, so much that water can soak into it.

In addition to compaction, dry earth still contains the less volatile polar/hydrophobic molecules from broken down organic matter, preventing water from penetrating. The longer the drought, the higher concentration of these molecules.

I’m a geotechnical engineer, so I’d say I’m pretty well qualified to answer this.

In ELI5 terms – clay soils when they dry up, shrink in size (several reasons for this, which is part of a theory called total stress, but that’s beyond the scope of ELI5). The water content of the soil gives it some volume, less water, less volume. This causes the tiny little holes in the soil body to close up. This is where the water lives in clay soils, in these tiny little holes. As you add more water it becomes softer, take away the water, it becomes harder.

Rehydrating the soil, it needs time to “soak” to absorb the water back to a happy moisture content. If you pour water over the top it tends to flow over because it cannot absorb that quickly, due to the little holes being less prevalent.

Because of this principle, you tend to get a phenomena called heave, or shrink swell which is when clay soils can move (in some degrees measurable in 10s of millimetres) over the course of a year, and can lead to cracking or other structural issues with buildings and structures.

There’s more to it, but that’s as basic as I can make it without getting into total and effective stress, cohesive and granular soils etc.

The real reason is that water has a very high surface tension. It is hard for gravity to pull a drop of water into dry pores between soil particles. Once water DOES manage to infiltrate the pore space, it becomes easier for more water to move through. If the pore spaces are already wet, it is easier for the soil to absorb more moisture until it gets 100% saturated.

Fluids with LOW surace tension will infiltrate soil much more easily.

Viscosity is a factor here, too, but viscocity and surface tension vary together