Swale Keyline Comparison
In part 1 we mentioned the Keyline plow developed by P.A Yeomans and sold to this day by the
Yeoman’s Plow Company. Well now listen to Darren Doherty discussing
the choice to use a Keyline plough for broad scale water absorption versus using an on-contour
swale as the main technique for water absorption. What I did was, it was a pretty lovely conversation
as you can imagine is, and because there’s a lot of people pretty annoyed at the swale
centricity of permaculture these days, you know, and I just find it really, and I don’t
have a problem with swales used in the right context. But what I do have a problem with is people
using carte blanche solutions, and that includes Keyline ploughing. Right, and other techniques. You got to be very specific. Anyway what I tried to rationalize was how
much water do these structures actually collect in a rainfall event? So what I did was I drew a hectare on graphing
paper and I worked out, well if I’m using a Keyline plough and the shanks on the Keyline
plough are a meter apart on my plow. And I’ll rip say to about 12 inches of depth. So in one hectare I have 100 rip lines going
across alright? Because there’s one meter across, a hectare
is 100, so that’s 100 by 100. So what it looks like on a surface, sorry,
on a cross section is that’s basically what Keyline ploughing looks like in a cross section
with a meter in between each rip. So that sort of triangular zone there at the
foot of the plow here, it’s actually lifting, it goes through, and it breaks the compaction
into a “V” out like that at the foot of the ploughs here, it’s releasing the compaction
here, and this stays as it was at that spacing. If you bring the shanks closer, well then
the whole lot is dispersed. Now because it was difficult and I didn’t
go it, I was estimating it on paper, I estimated I got about a 50% change in bog density. Bog density is basically a volume entry measurement
on the density of soil, so weight per volume. So if I release all of this compaction in
the soil now there’s a lot more air space. And air space equals water space. So I basically, I said “look, I got about
50% increase bog density, so that’s all this airspace per volume.” So what I calculated was the volume of that
section per meter of length. So I was able to calculate a square meterage
of increased water catchment, right? Now what I worked out ultimately that across
a hectare, I was able to take within that 12 inch deep or 30 centimeter deep zone, 37.5
millimeters of rain. Which is early an inch and a half in one of
these. Right? Now that’s not accounting for transpiration,
extra seepage that’s going to occur, the fact that I’m using a Keyline pattern which
is ultimately moving water, it was just the pure amount of volume that is there in between
all of the new aggregates that I’ve created. So I did that. Now I looked at that, and I priced that out
per square meter, and I priced that out at about 100 dollars an hour, and it worked that
out to about 5 cents a square meter was the price. I’ve got the full article there that I wrote. Anyway, now the other thing that I did was
I went to swales as an alternative structure. So I put 5 swales, you know swales that are
conventionally sized swales. And I put 5 of them, each of them at 20 meters
of spacing just as a starting point. I worked out that swales, a normal size swale
that a lot of people out there are building, something like 2 meters across and 1 and a
half meters across, and another 1 and a half meters across there about. And about X amount of height. And then I did the same thing, I worked out
that the swale embankment has a low bog density because it’s obviously fluffed up and not
compacted, and then you also got a water storage right here because water is coming behind
here in the cut. And that cut typically most people are not
ripping so it’s, you know, people are using an excavator or a rotor or a bulldozer or
a shovel. And then that is not a porous system here,
that’s actually a compacted system, this is where the uncompaction is which is in the
embankment itself. So I worked that out the total volume of airspace
for water space, and it was about double what the Keyline was if I put 5 of them there. But the cost per hectare to do that was 1500
dollars per hectare to install that, and it worked out at about 75 cents per square meter
to build swales. That’s at 2000 dollars per cubic meter,
2 dollars a meter to move that. I don’t know what it costs here, but if
you’re paying 190 bucks, 150 bucks for an earth mover per hour for this conventional
equipment, that’s what it’s gonna cost you about 2 dollars a cubic meter. So I worked out that. Then I brought it down and I thought to be
fair I need to bring the swale down to the equivalent amount here. Now the swale caught, and I have to look at
the article which I can do quickly. So the Keyline cultivation cost was, oh sorry,
it’s 2 cents per cubic meter, not 5 cents.
