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1,319,635 Views ⢠Sep 5, 2023 ⢠Click to toggle off description
One statistician famously said, âAll models are wrong, but some are useful.â And even something as simple as the flow of water into the soil has so many complexities to keep track of. Like most answers to simple questions in engineering and in life: the answer is that itâs complicated.
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YouTube Comments - 1,778 Comments
Top Comments of this video!! :3
We found out, during our midwest drought this summer (which is still continuing), that 3" of rain in 44 minutes will "outrun" the soil's ability to absorb it, regardless of how thirsty it is! It did top up the swimming pool ... and flooded the basement.
3.2K |
Not to forget, dry areas with fewer plants will make the soil less porous, because there are fewer plant roots. So while the dry top layer can absorb more water, it can't transfer the water down deeper as well as soil with regular moisture that has more plants with roots all over the place.
It's a well known fact plants are great for preventing soil erosion. And that's related to permeability and other similar things.
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I think one important thing to note in the viral cup demonstration is the fact that no bubbles were rising into the cup on the dry soil. The cups on the wet soil were letting in air from around the base which bubbled up into the cup and equalized the pressure above the water, whereas the cup on dry soil seemed to be forming a partial vacuum above the waterline inside the cup. This partial vacuum may have inhibited the water from seeping into the dry soil, slowing its absorption into the ground below.
1K |
A factor missing from the demos that may account for some of the differences is soil compaction. Soils in long term drought conditions get packed more tightly which I would imagine slows infiltration. It is intuitive to think about when you consider how much softer the same patch of ground gets when it is wet, there is more open space for the particles to move around in. Recently dried soils will retain some of those gaps until they settle down packing more tightly leaving less room for sudden rainfall.
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I think it's also important to highlight the difference between an areal flood and a flash flood - hydrophobic ground conditions in drought-impacted areas can lead to more flash floods as water slides across the ground surface instead of absorbing, while moist ground conditions can contribute to more areal floods.
1K |
As a gardener my theory on the hydrophobic nature of dry soil what I've noticed is when soils dry out they tend to compact and become less poros. In addition there tends to be less roots breaking up the soil further increasing its density. As water soaks in the soil tends to open back up.
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I think the biggest effect in the original demo is the fact that the edges of the cups are resting on the grass blades, leaving lots of spaces for water to leak out between the cup and the ground. Well-watered grass would be a lot thicker and springier than flat, dry, dead grass. It was the first thought I had when I first saw the demo and immediately dismissed it as useless.
930 |
One of the things I learned about in my 2nd year Environmental Hydrology class was the massive effect fire has on the interaction between water and the soil. Especially in Australia, wildfires and burnoff will combine with the high oil content in Eucalyptus trees to form a barrier to water infiltration on the surface of the soil, significantly exacerbating runoff and worsening flood events.
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That original video had "air tight" dry grass vs long grass blades making the containers not air tight air rushing in, that's the biggest difference of the original video, he did not consider the air intake from the bottom of the glass vs no air intake on the dry grass.
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I really like that 'all models are wrong, but some are useful'
Any model will neglect some small effects, but they manage to show the larger picture.
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Writing from Australia here. Our land definitely gets super hydrophobic during the droughts. We had just as intense flooding during the dry years as well as the last few La Nina years.
Really seems to matter how much vegetation and organic material there is on top. Exposed soil gets baked in the sun.
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I think the type of flood we are talking about matters. Very dry conditions can cause unexpected flash floods even with relatively little rainfall, especially if the terrain favors accumulation. And, loss of plant/root mass and soil cohesion due to a long drought can make mudslides more likely or severe, too.
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"All models are wrong but some are useful". That's an intriguing quote.
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That was a fun video. Watching the three cup infiltration demo at the beginning I wondered if some of the water in the glasses was not even making it into the soil because the cups were slightly suspended by the grass. I appreciate your garage demos to help visualize the effects.
I agree about the tremendous impact of soil types. I grew up in the Chicago area with a lot of clay soil. During droughts our yard turned into "concrete." Now we live in southern NH with very sandy soils that water percolates through quickly. We have a small stream on our property and it is fun to watch the flow change during rain storms.
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Your ability to explain complex things in a clear way is top tier.
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This is an important consideration when determining the irrigation interval and volume for a clay soil. Dry clay doesn't want to take water, but once it gets wet it holds water well. Most watering advice says to water deeply and less frequently, but when dealing with clay it's important that the soil remain damp so the first few minutes of watering don't try to run off. It's a tricky balance. That or the soil needs more humus, small aggregate, and possibly a surfactant to aid water retention.
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Drought state gardener here. The tube demo adds water pressure as a variable, which does not reflect IRL semi-arid conditions. A pressure gradient helps overcome the hydrophobic conditions of the surface soil.
My area is Holdaway silt loam soil, high summer temps, and 10% humidity. Our surface soil desecates quickly, making it hydrophobic most of the time. Summer rain is usually intense thunderstorms, so .5to 5 cm of water in under half an hour. In those conditions, you can dig into the soil after a storm and find runoff happening on soil that is barely wet 5mm down. Even puddles drain slowly as the infiltration rate of native soil can be 1mm per hour.
Slightly wet soil absorbs water much better because the wetted soil breaks the surface tension of the water, allowing it to flow in via capillary action.
Easy experiment to show this: pack potting soil in a clear tube with holes and float it on water. The soil may not wet, even after days of floating. Potting soil must be wetted and mixed before using it to start seeds or the hydrophobic nature of the potting soil means some cells of the germination containers will hide pockets of desecates, hydrophobic soil.
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I was first introduced to the idea of infiltration when my father did percolation tests to determine how large a leech bed was needed for a septic tank system in the flood plane of the Ohio River. The humus layer was very thin, less than 2 inches in most places, atop a substrate of layered alluvial clay with almost no aggregate, pretty much dense yellow clay as far as a core could be drilled. Memory says they stopped drilling at 60 feet. The percolation test results were not good. The engineer my father had hired did the math and came back with a dire face, telling my father that he'd need to buy more land to fit in the right sized leech bed for even a modest septic tank system or build a small water treatment system that would need regular service. I can't say I was surprised by this, because all while the house was being built, I had seen even a modest rain shower left the soil around the construction a muddy, sodden mess until solar evaporation dried everything to a flint hard mass. Fast forward a year and my father was suing the land developer that had sold him the lot. So, thus was I educated to the reality of soil infiltration.
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During the 3 inverted cup demo, the thing that I noticed right away was that more watered grass seems to lift the cup a lot more than the dry dead grass, and that appears create a larger gap for the water to escape and crucially for air to flow into the cup. This would have a huge effect on the speed that the water can escape the cup. That demonstration really doesn't say anything about how quickly the soil is absorbing water since it could just be flowing away.
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