And, converted to units that actually make sense:
- 1 Inch = 25.4 Millimeter of rain
- 1 Acre = 4046.85642 Sq. meter of forest/wetland/parking lot
- 750 Imperial gallon = 3409.569 Liter = 3.409569 Cubic meter of runoff
- 27000 Imperial gallon = 122744.484 Liter = 122.744484 Cubic meter of runoff
I’d ratio it down to figures per 1 mm or 1 cm of rain, but I’m not sure if it actually scales linearly that way.
I assume that they are using 1 inch (2.54cm) of rain per hour. I also assume that they aren’t accounting for more complex factors like evaporation, transpiration, depth to water table, etc. If all of that is true, they are using an infiltration rate (how fast water can get into the ground instead of flowing over top of it) of 0 cm/hour (0 inch/hr) for pavement and 2.47 cm/hr (0.9725 in/hour) for undisturbed soil (forest/wetland). The calculation goes: Rainfall – Infiltration Rate = Runoff. I wouldn’t go as low as 1 mm rain/hour because ignoring the complex factors above will cause issues. I also don’t advise pushing very high in rainfall for the same reason. The real infiltration equation is logarithmic, but between about 2 and 5 cm/hr it is usually functionally linear. For 1 cm/hr rainfall you would have 100,000 L/hectare (100 m^3/ha, 40.5 m^3/acre) of runoff from pavement using these numbers and 0 L/ha runoff from undisturbed soil. That’s because the infiltration rate assumed for undisturbed soil is 2.47 cm/hr which means the soil can absorb nearly 2.5 times the amount of rain that falls in a 1 cm/hr event.
And now for (extra) too much information - there is a red flag here for anyone who works with soils besides not having correct units for rainfall. Forest soils are extremely diverse. Wetland soils are extremely diverse. Forested wetland soils are extremely diverse. To represent them with one infiltration rate is somewhere between inaccurate and arbitrary. Average across the world, country, city, or whatever for infiltration rates means very little when you’re trying to manage storm water. The solution is local knowledge and local planning/action. I will happily forgive the infographic for inaccuracy and over simplification if it gets anyone to make any sort of positive change though.
your description is much more informative. so you could say that “with 1nch/h of rain, average soil will have zero flooding and tarmac will have 1inch/h flooding”.
but how could you get numbers that really inform policy? that really tell you how to change your land use?
what’s the difference for tarmac with grass verges, tarmac with rows of trees in little holes, tarmac with an average drainage system?
what’s the difference between farmland, versus farmland with a certain density of tree lines?
Hard questions to answer without specifics. There is already lots of research on those different areas and even more on stormwater infrastructure (like rain garden design for huge development). I don’t think thinking of anything as ‘average soil’ is very useful when actually making decisions because the range is extreme. Some soils like heavy clay with high water table can’t take more than 0.15 cm/hr. Practically, that’s not much better than pavement unless you live in an area where it never rains, but just drizzles tiny bits at a time. What is more important for local decision making is looking at your local soils and weather patterns. If you’re looking for an overview of soils in your area you can try a search for ‘soil mapping’ if you’re lucky there will be an interactive mapping site. If you’re not lucky, it will be a hard copy. If you’re really, really lucky no one will have done a soil survey in your area and you get to be the one to go out learning and recording what is going on below your feet :) Anyway you get it, your soils maps will be accurate at the city level at best, but if you’re looking at a specific project - like how do I best deal with stormwater on the street in front of my house - you are going to have to get your hands dirty. Here is a link on how to do that.
thanks. i guess to restore the over-grazed land, he could just plough it, then sow something deep routed, and wait one summer?
and in cities these measurements must be meaningless, because each patch had been dug up so many times, and to different depths, and filled in with different kind of soil and different structures.
are there more creative options? for example some architect once advocated putting city buildings on stilts. the land (the ground level) belongs to everyone. so it should all be open (sometimes green) spaces. are there any crazy ideas like that to help with urban flooding?
I’ve been away a while, so sorry for the late response. Yeah, you could improve infiltration on compacted pasture by tilling and planting select crops and then fixing the grazing practices that caused the problem in the first place.
You’re right that soils in cities can be harder to predict because of all the human disturbance. Even so, we don’t usually build our houses in areas with crappy drainage because that would mess with the stability of the foundation. So you can assume that most buildings are surrounded by soils with decent infiltration capacities.
Crazy ideas for how we could make cites function more like they are a part of the real world (nature)? The stilts one is interesting. We could move deep enough underground to leave room above us, too. There are plenty of ideas out there, some are totally reasonable that won’t get done for this or that reason. There is some actual momentum towards building up instead of more sprawl and including green infrastructure in building codes.
Thanks for the answer. I really find this stuff interesting, though superficially it doesn’t sound like it should be.
Yeah, you could improve infiltration
Thanks. So there is an easy way to make ruined soil productive again. That’s the answer I was hoping for.
that would mess with the stability of the foundation
So the water would rest against the walls, or etch a channel down around the foundation. I never thought of that.
It all reminds me of this: https://99percentinvisible.org/episode/depave-paradise/
Don’t get me started on suburban sprawl. It’s so needless and so bad in every way.
the water would rest against the walls
Out of my expertise here, but I think there are ways to waterproof structures so that being constantly submerged isn’t a problem.
etch a channel down
This is on the nose for what I was thinking. It can be even more of a problem if the builders weren’t as careful as they should be when they compacted the ground below your foundation. Any air pockets they left will be filled in by soil movement. The other issue is that soils shrink and swell when they dry or get wet. That can cause real movement over time.
Thanks for the link, I’d never heard of that, but the subsidence (30ft!) caused by aquifer draw down is the result of that shrinkage on a very large scale like they say. I am surprised there are any buildings standing. It makes me wonder what will happen if they succeed in recharging the aquifer.
I love the type of gardening that they preach on there though. It would make me happy to see more places moving that way.
