Do Droughts Make Floods Worse?

The answer isn't as simple as you might think!
One statistician famously said, “All models are wrong, but some are useful.” 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 it’s complicated.
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[@PracticalEngineeringChannel]

💡Check out brilliant.org/practicalengineering to get ahead in your career or education. 📖Did you know I wrote a book all about the infrastructure you can spot in your everyday life? store.practical.engineering/

[@blu12gaming44]

Just look up the effect that Ethopia's poor agricultural practices had on erosion. The lack of proper land management caused soil compaction which then led to water just flowing down into tributaries and rivers: leading to dangerous floods that severely eroded the landscape.

[@CodyHvids]

I really enjoy your videos even without any background in engineering or desire to get into engineering there are lots of tidbits that can be applied to personal projects and things to take into consideration before designing or building diy projects that can really help take your project to the next level I'm slowly becoming a fan of over engineering my projects 😂

[@GOAT_GOATERSON]

Could you make a video about the difficulties that permafrost bring with engineering?

[@christopherburgess96]

Okay so I'm only a minute into the video but I noticed something really obvious that IDK if you address later but I just have to comment on it - the moister the grass is, the plumper it is, and the more difficult it is to form a seal with the ground. Can we confirm this is measuring the absorption of the water, and not just fluid leaking out when placed on an uneven surface? I would think it necessary to use bare soil for this experiment. Though it's true that bare soil is not a common natural condition. Otherwise, perhaps this experiment could be repeated with a heavier vessel that will satisfactorily flatten the grass.

[@life42theuniverse]

I’d like to see these experiments again, this time on an incline plane. A low infiltration factor should make more run off, when taken together with the rest of a drainage basin -> flood. Instead of direct from the hose try slow drops/spray from a sprinkler.

[@UncleKennysPlace]

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.

[@DanielleWhite]

Decades ago while growing up on a dairy farm in the northeast US we had a ton of runoff and attendant erosion because after almost a month of nearly no rainfall (what fell was unmeasurably small amounts and very infrequently, not even reaching soil with any cover and rapidly evaporating) we had a rain that started as an absolute downpour. The period where the soil was hydrophobic was brief but it did mean runoff started in a few seconds and that had an outsized effect in that very hilly region.

[@andrewsercer9538]

3"!!! Gahhh! That's more than we've gotten in about 7 weeks! The back yard is finally dead for the first time in 15-20 years. I heard a lot about drought last year and it was fairly bad in my specific location last year .... but this year is way worse (I almost went hyperbolic and said "infinitely worse" ... had to catch myself) and yet I've heard almost nothing.

[@nathanieljames7462]

2 for 1 swimming pools?

[@namantherockstar]

Practical Engineering inspires me.. My parents said if i get 50K followers They'd buy me a professional camera for recording..begging u guys , literally Begging...

[@20chocsaday]

That tells others that they should dig a second swimming pool - outside.

[@danielcarmi305]

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.

[@jbruuuu]

Yes, I noticed the same thing, it may have even been the sole reason for the drastic outcome. That first cup had so much room for water to spread laterally and for the air to rush in and replace the water, it would be nowhere near representative of anything going on with soil infiltration. The other two are probably more reasonable, but still flawed by the pressure variables of the partial vacuum.

[@jasonsmall5602]

Agreed. The difference can be attributed to the different grass states. The live grass allowed more air to get up into the cup. This would have been better if either it was an open tube, as Grady showed or at least to cut the grass so the cup was sitting on the soil.

[@heideknight9122]

So all tests would need a better seal at the base and open top then it should function as intended?

[@sultanofsick]

@@heideknight9122 yes. Grady did it more or less properly, by driving it in and removing "leaking out the side" as an escape.

[@NathanWert]

I was going to say the same thing.

[@WillTellU]

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.

[@johnb8319]

Also to add, it is a reason why plants have hard time growing on dry soil even if moisture is introduced. Highly eroded soil makes it almost impossible for plants to dig with their roots.

[@NikkiTheOtter]

Also desert plants often have shallow roots. A good example actually, on my ranch I was excavating a new septic tank hole, I was about 6' down when the rain hit and filled it to the top. There was still water in the bottom 4 days later. And it took almost a week for it to actually dry out completely

[@jimonthecoast3234]

In the central val!ey water beads on dry soil. There is a 3 foot thick layer called hard pan. Google Forestiere underground gardens, it is where a farmer dug a home under the hard pan, and planted trees that grow. Up though holes in the hardpan. . it gives an underground insight why not even tree roos can establish in soil that saw long periods of drought of many many years

[@NikkiTheOtter]

@@jimonthecoast3234 Never heard it called 'Hard Pan'. It was always 'Caliche'.

[@jimonthecoast3234]

@@NikkiTheOtter I never heard it called that. But I guess they are similar terms for similar soils

[@NickMick9]

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.

[@EdoDave]

I was expecting compaction to be the primary influence for reducing infiltration. This is the first I've heard of soil hydrophobicity and it should be interesting to look into.

[@Fabdanc]

I was waiting for the hydrophobic piece, but you are totally right about compaction!

[@DoubleMonoLR]

Exactly, compaction and the soil types that are more prone to this definitely seems like it should've been mentioned. You can even see this effect in some of the stock footage in the video.

[@austinh1028]

Compaction from the last rain drying out hard- some soil types tend to turn into a hard crust when they dry out, can't imagine much water can penetrate soil that's hard as plywood, quickly pretty much any area that gets flash floods 'regularly'

[@Dragrath1]

@@DoubleMonoLR Compaction is also a problem with wet soils too though it manifests differently there largely forcing water out of the ground however so I can sort of see why it was left out

[@inspirationfollows9692]

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.

[@williamcampbell9859]

Source?

[@Baitrix1]

@@williamcampbell9859 try pouring water on a waterproof jacket, then one thats not waterproof but already soaks. you will see the principle in practice.

[@archstanton_live]

You are kind of right. When you are in a deep arid canyon that drains an area of severe local rainfall (perhaps miles away). The flood that washes you away is a flash flood.

[@Thunterise]

​@@williamcampbell9859Jesus that's obnoxious.

[@franciscoguinledebarros4429]

​@@williamcampbell9859the... Experiments on this video?

[@hiddenloaf2302]

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.

[@yanikkunitsin1466]

Peat based media become more porous when dried out and experience even more dramatic hydrophobicity. There is a lot factors in that, from microporosity, to whicking effect, to loose of ionic components like free silt and clay in dehydration. Moreover, after complete dehydration peats irreversably loose part of its water holding capacity. Best remedy is to use surfactants/wetting agents in water.

[@IstasPumaNevada]

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.

[@TheGreatAtario]

Using this same test procedure, I have proven that the most absorbent material of all is… _[checks notes…]_ barbecue grates.

[@anteshell]

That is exactly how it went. The demo wasn't testing the soil's ability to absorb water. It was testing how good of a seal you can get around a cup by pressing it against grass. Of course dry and flat compacted grass gives better seal than fresh and fluffy, untouched grass.

[@falsemcnuggethope]

Even more important than water leaking out is where the air gets in since tha cup had its bottom intact.

[@anteshell]

@@falsemcnuggethope No, it's not any more important. It is quite literally the same thing. Water won't leak out if air wouldn't leak in, and the same in other way; air will not come in if the water cannot get out. Water and air moving into or out of the cup are not two different phenomenon. No, they are parts of the exact same phenomenon.

[@anteshell]

@@renedekker9806 You're just repeating the same thing I said except with more detail, but you're phrasing it as if you'd be arguing against me. Make me wonder what's your goal here? Also, I'm not talking about a generalized concept. I'm also not talking about it in scientific accuracy, so I'm not counting molecules of water that can come out without any air molecules replacing it, etc. I'm talking about this very specific case, and on the level of accuracy that actually matters in practice, so the statement you quoted from me is absolutely correct.

[@TheRandomAustralian]

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.

[@sidikbertualang4099]

besides eucalyptus, what other trees can cause hydrophobic phenomena?

[@AstartesP]

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.

[@hazza2247]

legit idk how that isn’t just obvious to some people

[@57thorns]

And it is pretty obvious with those air bubbles.

[@chuckstestscreen5707]

Yup! (You beat me to it by one minute. Congrats!) The giveaway is the speed at which the air bubbles enter the glass. If he would've driven a clear cylinder into the ground like Grady did even if it's only an inch downward his results would've been drastically different.

[@hazza2247]

@@chuckstestscreen5707 or even just sat something heavy on the cup to squish the grass flat against the soil

[@mesiyathefelinegod9608]

Still if you clear the grass, well watered and healthy grass can soak more water beacause it has it more porosity than dry compact soil.

[@Chriss120]

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.

[@bertjesklotepino]

yes. Well. Have you ever seen the video of John Christy? Not the killer but the scientist? "John Christy Climate Change Denial Testimony Highlights May 13" That is the title of the video. Do not judge the video by its title. It is true what it says, but the word denial is often used to discredit something. Check the video, have a look at those models and the real data, and then dare say again what you just said. Because here in this video you can see yourself that the models do not hold up to reality and do not show the larger picture. But, i am no scientist. John Christy is

[@jovialcupid9687]

and this rule works everywhere In electronics there is rule that u shouldn't use 100% of capacity of devices never, always use 20% stronger/faster solution then u think u should

[@tanker_3301]

I think the thing that helps reduce floods over dry ground is the cracks in the dry dirt and the fact that when fully damp dirt can't absorb any water and so all the water goes to rivers and streams which makes them really dangerous

[@Jobobn1998]

I like this too. Even something extremely precise, like working your way through thousands of Feynman Diagrams and averaging each contribution, is going to ignore an infinite number more of (decreasingly impactful) Feynman Diagrams. So, in the end it's "wrong" in that you can't calculate the infinite number of contributions, but it is useful!

[@monad_tcp]

@@jovialcupid9687 I like the rule on networking of never using more than 60% of the actual total capacity, or bandwidth of a given link.

[@jubadiju]

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.

[@johncarlaw8633]

Baked soils have decreased microbial activity, often become hydrophobic due to waxes and other organic material. Soils often become compacted with decreased porosity affecting mobility of water within the soil. Altered acidity can chemically alter soil grains and bonding. This affects first growth after a severe drought. It isn't just a matter of adding water and everything resets in a year. Soil profile, nutrient mobility is altered. They said hydrophobic effects do not last. The time depends on the soil type and length of drought, how deeply the soil alterations have penetrated. It could be hours of rain for some, many days for others. Hydrophobic soil increases runoff but also alters the way water infiltrates. Infiltrations can form in less hydrophobic channels which can drain to the subsoil but leave large areas of the surface and subsoil still hydrophobic and can remains so for a long time. The channels become saturated and curve down in capacity only slowly wetting the upper soil from below. They will slow runoff for light rainfall but far short of the model capacity for heavy. This depends on the rate and duration of rainfall. Downpours run off and run through to deep subsoil beyond the root zone. Light rains over days don't form much runoff and slowly soak in. Light short rainfall soaks in but very shallow and then mostly evaporates.

[@mrhappyface4181]

@Alexandre-sz2jb There's bacon in your water.

[@charcoaljohnson]

Your ability to explain complex things in a clear way is top tier.

[@SecondFoundation]

"All models are wrong but some are useful". That's an intriguing quote.

[@1boobtube]

That and "A conclusion is the point at which you stop thinking" are a few of my favorites.

[@tomschmidt381]

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.

[@gianni_schicchi]

Cut a hole in the cup. The flat dirt seals the edges, creating a vacuum. That’s not how natural rain works so it’s a terrible test

[@gbprime2353]

That was my assumption as well. The large bubbles coming up from the grass in the middle shot kind of indicate that the seal from cup to ground wasn't the same in all three runs of the experiment. Clearing a level patch of soil in each location and then setting the cups would be a better controlled test.

[@freedomfighter22222]

The air bubbles making it in clearly shows there is a lot of water seeping out under the edges of the middle and left cup, more on the far left than middle one. On the right side there are 0 air bubbles, showing that that cup is well sealed against the edges. That test does not at all showcase how the ground soaks in water, only how fast water runs out of a cup when you leave a large gap, small gap and no gap between the cup and the ground. Because the right cup is so well sealed it is also holding a vacuum making the water in it have less pressure towards the ground, it will take far longer to empty than if it was an equally well sealed tube with a hole at the top. The demonstration gets the point across but it isn't close to actually doing what it is pretending to show. There is also as this video showed no reason the middle cup should empty less fast than the left one, the middle cup would empty faster as there is less water in the ground but no seal due to existing moisture while the cup on the left is resting on soaked ground that should have little room for new water.

[@colatf2]

@@gianni_schicchihe says this in the video but just more politely

[@TheZombieButler]

You folks beat me to it. The hydrophobic soil part does explain something I would see in monsoon season in Arizona (US).

[@moabfool]

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.

[@bramvanduijn8086]

Excellent summary. Using a surfactant seems risky since those tend to have nasty side effects, but I haven't studied surfactants in depth, so maybe I'm just thinking of a specific subset.

[@zlm001]

​@@bramvanduijn8086Generally when I hear the term surfactant I think of harsh detergents and cleaning agents, but the term can describe anything that reduces surface tension in a liquid, usually water. So you could use organic, biodegradable surfactants already found in other natural soils. So I think you're right, the OP has other surfactants in mind other than the subset that we usually think about.

[@andrewharrison8436]

I am on chalk so alkaline soil and there is still a hydrophobic effect. If I am watering a plant with a hose playing on one spot then the water will initially flow downhill past the plant for some distance but over a few minutes that distance will gradually reduce as the water wets the soil around the plant. It would have to be a very intense rainstorm after a drought for this effect to create excess runoff in practice.

[@XIIchiron78]

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.

[@aldproductions2301]

It is true that when I lived in the desert, there were several flash floods that immediately dissapeared after the rain stopped, but we didn't experience floods that stuck around.

[@useazebra]

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.

[@EeDog2]

He mentioned the water pressure variable and from the video you can see it doesn't have a large effect since the rate of adsorption at the very end of the experiment is very similar to halfway when the soil was saturated. Your thunderstorm example makes sense too since that was a brief storm. During a longer storm the hydrophobic/capillary effect is less impactful since the top layer is quickly wetted and the effect is lost

[@michealdrake3421]

This is all really good to know, thanks for posting mate!

[@VoxyGon]

Not sure if you watched the whole thing but he goes over this later in the video...

[@nothrabin]

@@EeDog2 He mentions that the water pressure must be kept stable for an experiment, he doesn't really address the fact that for a good comparison the level of water should be significantly less deep. There are two major concerns regarding flash flooding. One is that it will cause significant runoff for a comparatively very small amount of precipitation. This leads to arid areas experiencing significantly more erosion than you would expect for that level of precipitation. The second concern is that while the level of the flood is likely limited to broadly being very shallow, in certain places where it converges like dry riverbeds it will very suddenly increase in volume and depth. This is why it's so dangerous to hike along dry riverbeds even if you can't see any rainclouds, because if it is raining upstream the water will not soak into the ground and you can quite suddenly find yourself in the middle of a river.

[@aquaticaaficionado2004]

I know that in real life your not going to see water volumes "stacked" like this and it looks like it would add "water pressure" . . but you've got to think of it in the context of the soil. It isn't pressure moving water through the soil it is the water potential gradient. Gradients in soil science (percolation formula, saturated hydraulic conductivity, etc..) deal with area, volume, and rates instead of the pressure. I know it seems pressure would be an unaccounted for variable, but it isn't because of the use of rate any affect pressure has is within the rate measurement. This is more accurate because of the variation in soil types and contents in water itelf (both of which are nearly infinite).

[@TheFoxfiend]

packed clay heavy soil becomes somewhat hydrophobic while dry. In areas where the rain only lasts for short amounts of time the rate of runoff outpaces the initial absorption rate. That's why flash floods are a problem in places like southern Utah, our soil is rich in clay that has baked in the sun. The occasion where we get a long running slow rainfall is practically celebrated because it means the rain has a chance to start saturating soil before it turns to runoff.

[@danielrouw2593]

This, the family farm is on top of 300ft of glatial till. Heavy, high refractory clay, mixed with boulders. Significant errosion whenever we get "normal" amounts of rain after a dry spell. Very annoying getting stuck in mud while there is bone dry powderized clay an inch over requiring a mask to work. Organic matter is worth it's weight in gold.

[@Dragrath1]

Thanks to one of Nick Zentner's uploaded Central Washington University Colloquium series last academic year I learn the geology is even more complex in terms of soil dynamics since areas which experience infrequent episodic flooding with high mineral load erosion like out west will actually experience mineral sedimentation/precipitation of primarily calcium carbonate which further binds the soil together until it effectively becomes a rock termed calcrete. That stuff can't be dug through with soil machinery you need the kid of tools that break through rock since calcrete isn't all that different from concrete or limestone Oh and you also get biological crusts formed from bacteria fungi and algae which will form along surfaces out west now they are fairly fragile unlike calcrete but they will effect the hydrology too.

[@BaronVonScrub]

I would think that the gaps between particles both become smaller due to long-term exposure to gravity/settling, and bake together so they're harder to get apart again. That would also be a pretty big factor, as like the video stated, those gaps are where the water's gotta go.

[@bramvanduijn8086]

@@BaronVonScrub Yeah, and clay particles are flat disks. So when they stick together, they are really stuck. Ever had two plates vacuum together? That effect.

[@danielrouw2593]

@@BaronVonScrub and then when the surface absorbs moisture it swells, sealing and adding more pressure. Water traveling through some clays is measured in inches per hundred years, that's why bentonite/montmorillonite are used as a liner for sealing pits and ponds.

[@dh8203]

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.

[@petawatson5120]

The best sort of that demonstration use a rainfall situlator and takes into account more than just volume of run off (plenty of examples floating around y-tube)

[@ssu7653]

Think this is why he showed that the ground were still "dry" after the first cup was empty. No puddle of water, it soaked into the earth. It show what is happening when it start raining on dry ground, think of the video as the first 5-10minutes of heavy rain after a drought. Given time to overcome the barriers, the dry ground will end up wet and just be behind the curve for water soaking

[@J3scribe]

The footage of a microburst being overtook by an haboob @ 3:54 is amazing. Both are common in southern Arizona. I lived in the Mojave Desert for a couple decades and can attest to the hydrophobia of caliche soils. Clark County, Nevada, where Las Vegas is located, has invested billions of dollars on hundreds of miles of flood mitigation channels, not always with success, as recent events have greatly illustrated. Thank you for another illuminating video, Grady!

[@miroslavhoudek7085]

There's a lot going on in that video. Helicopter spitting sparks or flames @12:40 ? A microburst? Some noticeable large insect in the transparent water recepticle @14:40 ? Such wow.

[@J3scribe]

'there is no god" Yeah, you got that right. @@Alexandre-sz2jb

[@jeffer1101]

I thought the reason the really dry soil was slower to absorb water was more to do with compaction than being hydrophobic. I figured when the soil dried out, that exposed many "pockets of air" (which may have previously been occupied by water), which then allows the soil to settle (light compaction). Then when a large amount of water is applied to the surface, it is harder for the water to penetrate the more tightly compacted soil.

[@WyvernYT]

I suspect that soil compaction may be a factor too - but I don't have any numbers to put onto it.

[@jamesverhoff1899]

It's going to depend heavily on how well sorted (or poorly graded, if you're an engineer) the soil is. If you have, say, a bunch of 1/2" chunks of stone, there's not much that drying is going to accomplish. The stones may shift, but there's going to be a lot of open space no matter what you do. Crusher run, on the other hand, basically turns into something not unlike concrete--all the fines move around when it gets wet and fill those voids. Biotic activity is going to be a factor as well. In most well-watered soils you have a lot of burrowing critters, which make a lot of air space in the soil regardless of soil type (if it's too unstable they'll support the walls with feces). After a drought or fire a lot of those burrows are going to be unused, and the first rain will mobilize sediment to fill them. This will result in an overall lower porosity and transmissivity (the factors which govern infiltration).

[@kitchentroll5868]

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.

[@toddhoward1892]

Write a book.

[@Br3ttM]

Our soil is 6-8 feet of sand on top of clay, and we have a basement which did not have draining material put under and around it when it was built. The water soaks down, then goes sideways above the clay, and into the basement. I wonder how much less water would get in our basement if it just had a foot of gravel under the slab and around the lower part of the walls.

[@ke6gwf]

@@Br3ttM about the same amount, just delayed as it filled the gravel first lol You have to have actual drains to remove the water, or very careful waterproofing to keep it out in those conditions.

[@RobinTheBot]

​@@toddhoward1892short book....

[@Luckmorne]

We had similar hard clay soil and a high water table at our hold house. When they built they dug a big hole and brought in 25x 10-yd dump trucks worth of sand to fill the hole, and put the leach field in that. It still had... issues, during the spring melt. But it worked well enough the rest of the time.

[@omnarasi9637]

I always thought that dry clay (especially in the dust form) is hydrophobic. I felt that a number of times when I water the garden. Adding some good amount of mulch on top of the soil made the run off significantly less, though.

[@NhiTran-nf2nt]

zzz

[@izzycurer1260]

I don't know why, but Grady is one of those youtube personalities that I just find so calming after a hard day. Maybe it's the quiet voice, or the smile, or the chipper tone, or the enthusiasm for the subject matter. And of course, I always love watching the water flow through the clear plastic devices. Thanks Grady.

[@Glenn_AE6YT]

I live in Southern California, where it usually doesn't rain for six to eight months out of the year. Our soil dries out to the point that it is difficult to re-wet. At the first rain, we are all hoping that the first storm will bring a slow, gentle rain that re-wets the soil. Our clay soil acts more like a sponge, where a moist sponge will absorb water more quickly than a dry sponge. That's why you wet a dry sponge out and then wring it dry before using it. Fires or not, the first rain of the season is cause for worry. Take care.

[@dingo4530]

In arboriculture, we use surfactants to help water get into (and out of) clay soils

[@broad_cat]

iirc same as fire fighters using 'wet' water.

[@markedis5902]

What surfactants do you use?

[@dingo4530]

@@markedis5902 my preferred product is Nutriroot, which has a combination artificial and natural surfactants in it. Alternatively you could use an artificial one like Aircover.

[@20chocsaday]

I wonder what happened to the pine trees in Holland with the cracks in the dry soil showing at the base of their trunks.

[@moabfool]

​@@markedis5902 I don't know what Dingo is using, but a major brand is Hydretain.

[@Scuba11Steve]

I'd love to see a part 2 here that includes the effects of clays. Seeing loose soils and sands doesn't seem like what is present after a drought, as lack of vegetation can mean that soils have eroded away. Like the shot you showed in the desert, I'd be expecting hard packed earth, not loose sediments easily infiltrated by water. I'd be curious to see if the topsoil is even of the same content after a sufficient drought.

[@doctorgoose7]

Hi Grady! I've been watching you since your very first few videos - you really caught my eye with the automation systems in your garden. This channel has grown into a level of educational value and professionalism beyond many college classes. You're at outstanding example of engineering and a credit to us all.

[@smivan.]

"Even if they could, it would be an ethical gray area" - gold writing

[@andrewharrison8436]

Was looking for this comment - sublime bit of writing

[@anthonyduffy6953]

​@@Alexandre-sz2jbdidn't ham head marry an 8yr old?

[@hazza2247]

the video with the upside down cups is so silly, the ‘very wet soil’ has way longer grass which just lets the water run out the bottom of the cup whereas the dry soil is basically a flat surface so with the surface tension of the water it is ‘sealed’

[@CookieTube]

True, nevertheless the conclusion is very much correct: dried out hard top soil without vegetation makes for very poor water absorbing.

[@ExploringAI42]

This was an interesting video. The way it was explained to me is that dry soil condenses making it more difficult for water to get into the soil but I guess that would still absorb more water than when the ground his waterlogged and the water has nowhere to go. Maybe it's because I group in an area with soil that contained a lot of clay so, during the drier months, trying to dig was like trying to break through rock.

[@chriscassidy2554]

I think another factor for hydrophobic top layers is clay content. Having worked drainage construction in the Pacific Northwest, I saw this commonly.

[@Beryllahawk]

Great video! I grew up out in west Texas, dry as a bone most of the time, and the worst flash floods were almost always the first few in the autumn after a hot dry summer. I remember older folks around me pointing out how certain areas in the city were getting very dry (reasonably enough, all of them places with no automatic watering for lawns and so forth), and predicting that Wadley Avenue would flood (again). That street was infamous for it, but it was SO consistent that folks could just about judge the severity of the weather by asking if Wadley had flooded. I couldn't tell you, all these decades later, whether those extra-dry areas were connected to Wadley Avenue, but it wouldn't shock me to find that they were. But thunderstorms out west are frequently very intense at the start - gentle rains that go on for a day just aren't all that common - and MOST of Midland's infrastructure was built to handle such flash flooding, because it's just that likely that a big cloudburst will dump half the year's allotment of rain on the city, all in the space of thirty minutes! Also I'd like to say - I appreciated that bit of humor. Bringing down a rainstorm at will would INDEED be a "ethically gray area," haha!!

[@WindsorMason]

Out in AZ there's the same picture, plus mountains for extra sources of flooding. Always funny noticing how short of a timeframe the rain comes down. Do the summer thunderstorms in Texas also typically dump their bucket of water just after sunset?

[@CptJistuce]

​@@WindsorMasonHere in East Texas, the thunderstorms dump whenever they dang well please. I feel like mid-afternoon is most common, though. West Texas is probably more similar to your weather.

[@hamentaschen]

"If I'm curt with you, it's because time is a factor."

[@karlroebling3316]

If I'm not curt with you, it might be because your understanding doesn't matter?

[@barefootdep]

I love your videos in general, but this one especially. The answers to so many things in this world are often not as simple as people want them to be. Exploring the complicated answers is so satisfying.

[@acawap]

I live on a property that floods - sometimes seriously, but every year I see this effect. My hill has loamy soil that absorbs well, the lower area is very clay heavy. During a drought, the clay becomes like concrete, and if it rains on top of that, the water goes nowhere (except the mole holes). Then in the rainy season it hits a saturation point where the water just doesn't absorb any more, and I get ponds all around my field that won't absorb into the ground for days. It's like a shammy cloth - dry doesn't absorb anything, saturated doesn't absorb anything, but damp absorbs a lot.

[@Benson_aka_devils_advocate_88]

Here in Arizona some of the worst flooding seems to be at the beginning of our monsoon when the ground is the driest. I've always understood it was from the ground having a hard crust on top that prevents the ground from readily soaking up the rain. It'll be interesting to see if what we've been told holds true in the video! Edit: So the answer is no, bit maybe yes! LoL I love that you did the experiment with the hydrophobic sand as that is a really good analogy of the Arizona desert topsoil. As a kid playing with the hose out back I always marveled at how much slower the ground absorbed the water when it was really, really dry during the summer. This seems like it would be a good graduate student project. Collect soil types from around the country and try to figure out a reliable test to figure out which regions will be most affected by drought vs super drought. Especially with the fact that rain patterns are changing over many areas we humans have populated.

[@johnfreeman652]

Already excists websoilsurvey.sc.egov.usda.gov/app/HomePage.htm

[@enigmalfidelity]

Don't forget the 6-12" of clay about a foot below the dirt crust. Another big reason there is no absorption

[@mirjam3553]

Not in the video, but seems to be a thing in my experience: the trodden footpath in a soil like you described will absorb the water slower. There's just less voids for the water to go into. One that's "alive" will be puffier and have more space for water. Roots of plants also take some up, but that's probably not a relevant timescale. And heavy clay soils get compacted more easily than dune sands or compost-y forest soils.

[@Benson_aka_devils_advocate_88]

@@enigmalfidelity I worked for the City of Phoenix water department so I've got more experience digging up our dirt then I care to remember. Most of our soil is pretty consistent down several feet, with the top six inches or so becoming incredibly dense when dry. Once you get down two feet it becomes pretty consistent, with a puffy, powdery feel to it. I don't know if clay is the right word for our soil type. We always refered to it as farm dirt because of how soft and easy it is to dig. On some jobs we could dig a grave sized hole in under an hour, by hand. Not the kind of thing you could do in most clay soils. But I'm no geologist so I might be wrong on the soil type. But I do stand by it being far, far harder when dry. To the point that just a few feet from where the pipe was leaking the ground would often still be rock solid until you get down a couple of feet.

[@Benson_aka_devils_advocate_88]

@@Alexandre-sz2jb Dude, I'm a Baha'i. I'm well aware of God, and the power he has over all of us who believe. Regardless of preference, we are all wanting the same thing. Knock off this crude. No one wants to hear it.

[@Yora21]

In horticulture, where we mostly use nearly pure peat, dry soil can be a serious nuisance. You can water pots with dry peat for minutes and it will only penetrate a few millimeters. It looks soaking wet, but water just runs off at the surface almost completely. The typical solution I've encountered in most places I worked at was to use tables that are build like big tubs, so you can flood the whole thing and leave the pots standing in the water for hours. The pots will eventually soak up water through holes in the bottom. Though there are specialized chemical agents that can be mixed into the water to improve its ability to penetrate dry peat. As I understand it, the effect is caused by air in the small empty spaces between the various soil particles. In many soils, these gaps are so small that the surface tension of water keeps it from squeezing through the narrow openings. If there is already moisture in the soil, then the surface water can link up with it and the water surface is broken, allowing it to get soaked up.

[@user-dg9pu4pe9d]

Getting potting mix to absorb water is a nuisance. Even mixing it together does not always work well. Last year when filling new 16" planters, I stuck them in garbage bags and filled them with water and let them soak overnight before planting. Same idea but practical for home. For just one pot, leaving a hose on mist works nicely.

[@LtKernelPanic]

Interesting video. I’d also suspect that extremely dry soil that has become compacted would also soak up water slower. We’ve been in and out of drought conditions all summer and thankfully have avoided serious flooding after a few big storms moved through. The minor low land flash flooding that occurred isn’t uncommon and pretty much expected after that much rain moved through.

[@WimsMill]

I think you missed 1 type of soil conditions that often occurs with droughts and clay-like soils that are bare. That is the kind of baking or tile forming on the top soil. The really fine clay closes off the underlying clay from absorbing the moisture and thus essentially creating a barrier for infiltration. If there is a sudden downpour, the clay will not absorb any moisture and thus causing a flood.

[@Lunavii_Cellest]

Here in the Netherlands this is a very known thing. Flashfloods happen nearly always after a draught, especialy in the area I live in where we have sandy soil. After a draught the ground is rock sollid and doesn't absorb any rain for quite some time. Enough time for large flashfloods to happen. For this it is important to keep the soil water level as high as possible and to create as much shade as possible to limit evaporation during a draught.

[@RussSchoonmaker]

Anyone who has made pour over coffee knows you have to add a small amount of water to allow the grounds to bloom (saturate) for a short period of time, before beginning the pour over process. Consider this as a model everyone can relate too.

[@joshcahill6880]

I love how civil engineering through this channel explains truly fascinating mechanisms and structures. Answering simple seeming questions with the complex truth of the real world. My granddad was a civil engineer, and while I didn't know him very well, I love learning about the field he dedicated his life to. Thanks for your content Brady, you make civil engineers and educators proud!

[@chrisalex001]

I really like that melody right after the intro; it always sets the stage for an interesting episode.

[@blackmagemasher4031]

I remember my 5th grade teacher telling me Arizona has this problem... He says the ground gets too dry so when it does rain, it doesn't get absorbed quick, it just pools and creates a flash flood...

[@WyvernYT]

That effect is famously being demonstrated at the Black Rock Desert in Nevada this week, to the inconvenience of a lot of people who were hoping to be able to drive home from Burning Man 2023.

[@JackLeMetis]

Great video, I work with peat moss, the stuff absorbs LOTS of water, but when dry it's almost impossible to make it absorb water; water runs off it and/or peat floats on top. My real life experience with water absorbtion

[@JackLeMetis]

From Eastern Canada

[@kenbrown2808]

I've got a history of rehydrating baled peat moss, and yeah. it's a slow process.

[@nicholas1460]

You replied "Great video" to a 16 minute video 4 minutes after it was posted? 2x replay?

[@blancothevanchannel]

Grady, I love your videos. In the West, where clay soils go dry they take time to re hydrate. An agricultural engineer I once knew always talked about losing hydraulic conductivity, and how important it was to keep soil watered.. When the clay soils are dry, there is no capillary action to help pull water down..

[@frogjmon]

I just installed a drywell in my backyard to disperse water from my sump discharge, all this discussion of soil water absorption is especially relevant to me right now. Thanks for the clear explanations!

[@gambit-6181]

I think one of the things potentially overlooked here is the way in which your models were carried out. Rain spits on the ground, it doesn't create a volume above the ground like in your example. Like you said in your video, some plant owners choose to submerge their plants rather than sprinkle them in order to water them properly. You can even see this phenomenon on sponges. Sprinkling water on a bone dry sponge will not initially absorb water very well compared to a slightly damp one, but if you submerge it I doubt you would see much difference. In your models you've submerged the ground with water rather than trying to simulate rainfall. Floods also don't usually occur because the dry ground got oversaturated and became submerged, usually they occur downstream because upstream they weren't absorbed properly. This make sense if the upstream ground is bone dry and was not able to absorb the water, so it simply flowed downstream causing the flood there instead. If you want to correctly determine whether or not it increases flood risk would be to add that variable of allowing runoff, water that wasn't able to be absorbed before it flowed downstream.

[@michealdrake3421]

I noticed this too. I put a comment somewhere suggesting a modified experiment to account for some of that

[@ashreen4330]

I wonder if soil compaction is taken into account. If soil is compacted I am pretty sure it will absord slowly. Example with potting soil is pretty bad since if its not compacted it gets water through pretty easy. Also another thing that is wrong in this test is that water is going out at the end, that wont happen in real life conditions.

[@mathewritchie]

I have had experience with this I emptied a drum of wash water on cracking clay and 120l of water disappeared in seconds but have often seen water run straight off sand.Water repellent sands are common in Perth .

[@dielaughing73]

Yes I live in God's Sandpit too. Builders speak of 'gutless sand' because it contains so little clay or organic matter any hole you dig just fills itself in.

[@OutbackCatgirl]

Bassendean soils here in Australia often get hydrophobic. My backyard soil, which is characteristic of the Jandakot region, remains bone dry besides a thin surface layer, but despite that dryness it also drains quickly. I know this due to recently trying to pre-soak soil to put in a couple of endemic shrubby plants from a lot that was being renovated (the owner let us dig them out) and after ten minutes of thoroughly wetting the whole area a shovel turned up bone dry soil less than an inch beneath the surface. At that point we just went and got some native potting mix and dug a great big hole to fill it in with.

[@numbthumbhero8529]

The shot of the clouds dumping rain at 3:54 was friggin awesome.

[@sdtfoxon]

Hi Grady! Love the content as ever! I have a video suggestion 😊 the UK government has shut all school buildings that use Reinforced Autoclaved Aerated concrete, after a collapse of concrete previously deemed safe. I'd love to have your take on the engineering side- what is RAAC, why was it thought safe, what changed and why is it now an issue? Thanks!

[@noterix1164]

I love this channel so much. your videos are so informative and engaging. thank you for all the work you put into Practical Engineering, Grady !!

[@Pyromaniac2450]

This is really cool, thank you for taking the time to make a covey explaining how you did that and what everything was. Keep it up, you def deserve more subscribers

[@JoshDoingLinux]

Before watching this I’ll put a hypothesis in. I think dry ground would absorb water more slowly. I think this because as soil dries out it settles more under its own weight not having to make room for the water that usually inhabits it’s myriad of gaps. Then when water is reintroduced it has to force its way through the gaps, and the path of least resistance is to just pile up or run downhill

[@JoshDoingLinux]

I was incorrect. Fair enough

[@nosleep7026]

anyone who has had a plant at home would also attest to this

[@JoshDoingLinux]

@@nosleep7026I have never had a house plant. I was more thinking about the individual particles of earth

[@vepr5596]

Based hypothesizer.

[@CookieTube]

There are very much different types of "dry ground" and many use that term willy nilly. Baked dry hard top soil DOES make it MUCH harder for water to get absorbed! The engineer in this video did NOT do a good job explaining things IMO. The proof is in the comments; many people interpret what he said wrong and/or take it in the opposite way. And/or are jumping to very much wrong conclusions. He was very right in one thing though: It is complicated. But if you need to take one thing, and only one thing, from it: then, YES, dried out hard baked ground (a generalization) makes it much much harder for water to be absorbed. Although that viral video in the beginning, which he showed, might not have been the best and correct experiment (but neither were this engineer's experiments), the conclusion of it IS correct.

[@heronimousbrapson863]

This happened in British Columbia in 2021. After a summer of extreme heat and fire, those same areas so affected were stricken with massive flooding the following November due to warm, wet air (an atmospheric river), bringing in large amounts of heavy rain.

[@brentdaloney6292]

Thanks for the video. I always assumed that when it rained after a drought, that the rain would just saturate the top inch or two of soil and before that moisture could work it's way to deeper, the rest of the rain would just run off. Glad i learned that's not necessarily the case.

[@brendakrieger7000]

Thanks so much for covering this topic! I've actually wondered about exactly this.

[@peetiegonzalez1845]

A dry Shamwow is terrible at cleaning up spills. It has to be damp for the capillary effect to work. I haven't watched the video yet but it's very well timed given the absolute carnage at Burning Man.

[@CookieTube]

Which is exactly why he made this video. He isn't dumb, this wasn't coincidence. This is profiting from trending topics, as a youtuber.

[@peetiegonzalez1845]

@@CookieTube Well, yes. Thank you for agreeing with me.

[@nightpups5835]

This reminds me of some work going on in montana building artificial beaver dams to better retain water up near the snow bank and has reduced a lot of dry summer issues.

[@jcarey568]

Ironically, Pennsylvania has removed many dams to restore fish runs. They were originally built to power mills or electric generation.

[@nightpups5835]

@@jcarey568 ahh, so the difference comes from permeable dams and nonpermeable dams, so water behind a beaver dam isn't stopped, just slowed a lot, so life can still move through the dams, while the dams in penn don't allow that life flow. Also beaver dams make much smaller bodies of water. Basically not all things called dams are equal

[@tanushkalazaretto8465]

completely agree. It is necessary to preserve not only water, but also all natural resources. Protect and multiply.

[@michaelrafferty4414]

I'd be very interested in seeing those same experiments with a fixed volume of water and performed on both flat areas and run off channels or artificial rivers (with and without levees) to understand the effect of terrain and engineering on run off volume

[@kuromad]

I always thought it was this: Soil contains more than just sand. And water does more than just seep through. Some material is like a sponge and swells when moist, some material clumps together when moist. Bigger particulates leave more room for water to flow, so if a draught causes the soil to compact, then it would be a logical conclusion that it slows the absorption of water. I also notice this with my plants. The soil level goes down and when properly watered it will sometimes rise above the normal level (and fall on the ground) due to the roots taking up more space than when I potted the plant.

[@R0b1et]

Nice video Grady. My experiment is far from rigorous, and you did a great job of doing it far better and yet still visual. The effect of hydrophobic surface seems to occur in most natural soils, as you say once its overcome, infiltration starts. It's why the drought increases flash food risk, but has little impact on fluvial floods.

[@rv6amark]

Really good discussion of how all of the variables (some of which are almost impossible to define) make outcomes appear to be "not logical". I spent over 40 years wrestling with this in my career as a mechanical engineer...and I didn't have to deal with rain absorption in soil! Here in California we deal with the layer of a fire-scorched earth crust every fall when the rains start. These processes are simply a part of our natural environment.

[@mjustjeanette7026]

I love learning about civil engineering because of your channel. Yay for more.

[@AT-zr9tv]

Great video, thank you. I am careful to let my grass grow during summer, so as to avoid dehydration of the soil.

[@tomgosy]

I've always been told that the worst times to get heavy rain are when the ground is bone dry or water logged. I think the biggest factor is a large volume of rain in a short time

[@stevenblue1979]

I was sad you didn't mention clay. I happened to start a new garden plot this year in a spot with very high clay soil and a stubborn quantity of rocks (~1" diameter). I started tilling it by hand and decided at the half way point to simply take half of the plot down about 6" to replace with top soil & mulch. So I've got half a plot with clay/rock and half a plot with nice top soil. Watering daily, each half is very different. The clay side sets almost to concrete when dry (90F + 20% humidity doesn't take long to dry; especially when water absorption is so slow to start)--puddles form after watering for just a few seconds. Watering continuously creates streams that simply flow to the lowest part of the garden. The other side of the garden, rich with purchased topsoil, can be watered for 30 seconds continuously with puddles only just barely forming by the end. I haven't the patience to wait to see if it will eventually form a river the way the other side will. A side effect of this is that the clay/rock soil doesn't support as many plants--only specific ones survive and tend to grow very slowly. Ironically, leaving the lawn in place would have prevented this drainage problem--the grass roots kept the top clay layers loosened up and shaded.

[@RealVercas]

A very important variable that's missing here is how compressed the soil is. For soil that sees a lot of traffic (by foot, automobiles, farming equipment, etc.), long droughts allow the soil and any dead vegetation on top to compress far more than it would if it were wet.

[@CookieTube]

This!!! And that is what is wrong with this video IMO. The experiment he did with the sand is completely unrealistic, that is not what a generalized dried out hard top soil looks like.

[@henrik.norberg]

The missing component is clay. Dirt and sand soak up water quite fine when dry, clay do not as it get hard as rock and only softens after being in water for some time.

[@Dracomancer273]

Soil samples from the site are an important headache for any large enough building project

[@henrik.norberg]

I have about 10-20 cm (4-8 in) top soil and then only, what we call in Sweden, blue clay, on my property. When it has been very dry for a long time my soil can take a few mm of rain and then it is like it hit rock and only flow on top. So my absorption graph would be very different compared to the video. I am VERY happy that my property slopes down to my 200 m2 pond.

[@Slide61]

Another great video!! Good job! A couple of additions to consider. Capillarity is one. Clayey soils can be hydrophobic initially after a severe drought for extended periods. This is due to their capillarity. It pulls the soil tightly together. As you mentioned hydrophobic soils are relatively common post wildfire. Duff builds up in soil - leaves, needles, organics in general. When it burns a waxy substance is released which penetrates a few inches into the soil. It's like putting a sheet of plastic over the soil.

[@YZBot]

I have "soil" in my yard that does the hydrophobic thing when it's really dry. It's a very powdery clayish dirt with very little organic material in it. It turns muddy for the first few minutes of having water on top. Water will pool on top before soaking past the top inch or so. After that though, you can pour buckets of water on it and it just slurps it up. There is also no vegetation where this dirt is, and it's gets very hard packed.

[@AlanTheBeast100]

A problem with the first demo (cups) is the airtightness. In nature, there is nothing like that - it's "open" to the ambient air pressure. So the one on the "dry" sand would permit the least amount of air to enter the cub at the rim whereas the grass leaves plenty of gaps. Solution: put a small hole at the top to let air in (a few mm across will do) - or do the experiment as Grady did it (tube driven partway into the ground). As to impermeability on the surface, you can see this in some soils after a hard rain - they form a sort of hard shield or shell that is much denser than the soil below. It eventually soaks up some water (possibly from underneath) and reverts to its former self. As mentioned in the film, all sorts of "biofilms" will repel water. Good video - as always!

[@christiananthonysalazar6296]

Fantastic video! This topic is so important, and you've explained it in such a clear and engaging way. Understanding the connection between droughts and floods is crucial for addressing climate-related challenges. Keep up the great work in educating us about these environmental issues!

[@jessechristensen1074]

What about soil compactness? Dry soil seems to be much harder which would require more pressure needed to both penetrate the hard top layer and expand the soil when the soil absorbs the water. If there's more clay in the soil, that would seem to allow the soil to become more compact and limit the rate of which water can flow to the deeper layers too.

[@snubbedpeer]

We experienced this at a campground in Germany where no rain had fallen in a long time. The ground was dry and dusty and the grass wasn't green and healthy. Suddenly a shower of torrential rain hit and the water stayed on the surface and rose to above our ankles in the deepest areas. Luckily our tent was pitched on high ground. After a while, maybe 20-30 minutes the water was all absorbed and we had no further problems. I suppose that this is similar to clothes in a washing machine and the presoak program, it takes a while to get the absorption started and wet the textiles properly.

[@Verlisify]

I'm worried for society if anyone believed the test in the beginning of the video. Water clearly seeped out of the gaps in the thicker grass (The cup rises as the grass decompresses) while the airtight cup didn't allow any drainage for the dry grass.

[@majorgnu]

It's such an obviously flawed experiment. It immediately made me question the honesty of the person running it.

[@hawkeye481]

We use wetting agents on the turf to enhance infiltration and prevent runoff when irrigating in droughts and even have some used almost year round to prevent dry spots.

[@aaronwilliams1249]

A couple of years ago while camping during the California drought I poured water on the detritus under the trees and the water just beaded up and would not soak in at all. This is in the middle of a forest, where the loose soil full of pine needles would usually allow water through like a sieve. To say it was dry was an understatement. To make matters worse, the smoke from fires got so bad face masks were required and you could not see the sun, only an orange glow.

[@Supreme-King]

The soil type and sun exposure are the deciding factor. Some types of soil literally get baked into a brick like texture by the sun and take a bit to soften with water. It still turns into mud in minutes under sustained rainfall.

[@Miracle12348]

Thank you for adding subtitles, I can’t hear that well because of a ear problem I have and it’s really helpful when someone add subtitles to a video 😊. Also the video was amazing❤.

[@teatowel11]

My experience in Australia is that long droughts (multiple years) kill vegitation and also result in compacted soil. With less vegitation' there seemes to be more run off also, it causes more topsoil erosion which is heartbreaking for farners who have been waiting years for rain.

[@mittfh]

Another factor is rain intensity: most infrastructure in the UK is built with the assumption of monthly rainfall being fairly consistent, whereas it's increasingly common to have a largely dry month, then the entire month's allocation of rain arriving over a weekend. This also contributes to soil erosion on sloped terrain as the rain can be intense enough to lift the top layer of soil and include it in the run-off.

[@rain0aldwaib]

you did a nice job explaining this !!! however there 2 factors i really wished you talked about which is the rate of the rain drop and the angle of inclination of the soil (especially if the soil had some hydrophobic materials)

[@ShukenFlash]

I haven't touched HEC-RAS or HEC-HMS in a few years but when I saw the CN table I knew that had to be coming. For as long as that software has been around it still does a very good job of estimating runoff and its nice and simple to use.

[@acfsambo4996]

Im wondering, especially after a long drought, if the soil/dirt has conpacted more and there is less space between the padticles for the water to fill. Would be intersting to have tested the same volume of soil but with one compacted and the other not.

[@hardlyb]

In Southern California, the mountains - which are very frangible (basically giant piles of gravel and rocks) - have chaparral growing on them, and when that burns, the waxy stuff which makes it drought tolerant is vaporized. Apparently it often condenses a few inches below the surface of the ground, and so when it rains, the first few inches become a river of mud sliding down the sides of the mountains, capable of carrying huge boulders (much bigger than a car) down the hills. The results were particularly dramatic in Santa Barbara a few years ago.

[@calipete]

After a long, hot summer in southern California, we had two days of heavy rain. When the rain ended, we wanted to till the soil, but the rental company wouldn't rent us a tiller because they didn't want it trashed with mud. We went back home and pushed a shovel into the ground. The water penetration was less than 1/2 inch. The rental company agreed to rent us the tiller. :) Life hack: Years later we learned that if the soil/lawn is too dry to readily absorb water, spraying the ground/lawn with a mild surfactant soap, using a Miracle Grow lawn sprayer hose attachment, will open the soil right up to drink up all water you give it. Cheers!

[@donaldmcsween4533]

Coweeta Hydrologic Laboratory in North Carolina did pioneering research on runoff and infiltration rates for various treatments of forest and agriculture land in the mountains. They also were able to help design roads and skid trails that were less impactful to water quality in timber harvesting operations.

[@misteral9045]

Ive been loving the disaster analysis and the engineering in plain sight series, but this is a wonderful return to just asking nerdy questions. I feel like a follow up video could be an excuse to return to the stream/river research institute.

[@IceFire2050]

The problem I always had with that cup demo video was that the 3 cups were on grass. The first cup, with the well water grass, the grass was very healthy and growing upward. The 3rd cup it was pretty much dead and laying flat on the ground. The 2nd cup was in-between. The seal he got with the cup on the ground was much tighter on the 3rd cup while the 1st cup it had healthy grass creating much larger gaps for water to run out around the edge of the cup.

[@Tera_GX]

I remember well the 2013 Colorado floods. I live in the basement and awoke to find the window wells were actually filling up. It's easy to feel like the "mile high city" doesn't have to worry about extreme flooding in this semi-arid climate, but it sure does. More basic flooding isn't too rare and it does have bad impacts. It's tough begging for rain to come, then rain comes ALL at once to cause problems.

[@Curtistopsidae]

Nice demos as usual. I'd never heard of hydrophobic soil before, but it makes sense. Others have pointed out that at least some soils compact over time during prolonged drought (which may contribute to hydrophobicity?). I'd also add that the associated plant deaths also changes the soil conditions since roots act like channels for water to follow into soil. Curious how that changes after a plant dies

[@aluminumfalcon552]

This is why I love my smart timer for my sprinklers, it only waters based on the estimated soil moisture and it factors in rainfall, when it does water, it goes in multiple short waterings to maximize infiltration and minimize runoff. MUCH less water wasted to keep my lawn alive.

[@AlecMuller]

In permaculture, we use earthworks to increase infiltration and reduce erosion. Swales slow down surface water to give it time to soak in, and keyline-patterning (gradual-slope channels that move water from wet valleys out to dry ridges) gives water more surface area to support vegetation that holds soil in place and consumes water.

[@BUD420MAN]

Here in Arizona , I see hydrophobic dirt all the time. We even have what we call moon dust is some areas that resembles brown/tan baby powder . You would think the powder would soak the water right up but... when it rains the dirt floats for decent amount of time before it will finally soak up the water . Without having any knowledge before hand, I imagined it was due the sand / dirt being baked and changing the absorption rate of the dirt.

[@ahmadganteng7435]

Mind blowing knowledge.. Thanks Grady