Title. I saw a video showing it happening, but I don’t understand why.
In: 51
Ultra basic answer is if you have an old , dry dish sponge and put it under running water ( not a tub or sink filled with water. ):
Initially the water’s going to roll over the dry sponge because it’s going to take some time for the sponge (or dirt in this case ) to actually absorb the water. All the holes that the sponge has that act as passages of water, all shrank and it’s more difficult for the water to pass INTO and absorb into the sponge. After some time of surface werness, the holes in the sponge become larger and Water is allowed to go further into this sponge.
Basic terms, that’s exactly what happens to dirt when it’s too dry. All the small passageways have closed , from becoming so dry and shrinking, and it’s harder for water to actually get into the dirt. instead it’s easier for the water to roll over the top.
Source: I also have a basement that will flood if there’s too long of a time without rain. If I know it’s been dry out for a couple days, and know a storm is going to hit in the next day, I will actually water the ground in front of my house a little bit to prevent the water from rolling into my basement. By watering lightly over an hour, the soil will absorb more, and move water out into my yard, instead of into the dry cinder blocks of my basement wall
Do you have a “Hard sponge” that need to be soaked for couple minutes to be fully usable? Same thing applies to the soil
Because water has already created a path in the wet soil for more water to follow. Dry soil is hard and compacted, making the water work harder to get through because it has to create a path from scratch.
Density. The ground is less dense and has more room to let the water in. Wheras dry hard baked earth like from a drought is more dense and doesn’t let itself be open to the concept of wet.
Its like its lover has been away for a long time. And the earth, while parched, is feeling scorned. It wants the cool touch of its lover the water. But is unwilling to be hurt again. It doesn’t want to experience that loss. So it hold onto its dryness. But the water lets the earth know that its here now. It needs to restore the earth. And sure it comes and goes, but earth knows that. And it knows that water loves the earth. So after a while the earth lets down its guard and accepts the water back into its heart. And they once again achieve an equilibrium of acceptance.
That video was misleading/exaggerated. There was pretty clearly some leakage from the bottom of the cups that were over the live grass, but a better ‘seal’ over the dry grass.
Not that the phenomena isn’t real, and others here have given good explanations, but it isn’t as profound a difference as that video portrayed.
You see this same behavior with a chamoix to dry your car. The instructions are to wet it first so that it better absorbs water. When you wet it the towel expands a bit so more water can get in quicker.
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
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.
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.
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.
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