Water Catchment
1 December 2008
Here on our Kona coffee farm we can get
100 inches of rainfall per year. That's a lot of rain. Our coffee
trees like all that water but their roots can't stay wet or they'll rot.
So how do you give a tree that much water while still letting its roots dry
out? Easy, you live on a volcano.
Being a 14,000 foot tall bump in the middle of the ocean, the volcano does an
excellent job of squeezing rain out of the humid tropical trade winds all
year long. For growing coffee it's important to be at just the right
spot on the mountain. The beaches are too hot, the peaks are too cold,
some places are too wet and others are too dry. We're at 2000' on the
leeward side of the mountain in the middle of a tiny micro climate that is
perfect for growing Kona coffee.
The volcanic "soil" looks more like a pile of sharp rocks than soil.
It may not look like soil but all those young lava rocks are chock full of
rich nutrients that the coffee trees love. Lava rocks are also quite
porous so our ground drains well. Without good drainage the ground
would be constantly wet which would rot away the coffee tree roots.
That well drained soil is great for growing Kona coffee but it's not so
great for digging a well. Digging through all that volcanic rock is
prohibitively expensive and there's no guarantee of success because there's
not much ground water in the area.
Our farm is too high and too rural for city water. The closest city
water is about a half mile away and several hundred feet lower in
elevation. I suppose we could lay some pipe and pump the city water
up to our farm but that certainly wouldn't be easy or cheap.
Instead, we catch our rainwater. Our house has a steel roof and all
the gutters connect to a pipe which leads to a giant water tank. Two
tanks actually. The barn roof connects to them too. We also have
two more tanks in our back field that are fed by yet another catchment
roof.
The barn's roof is 32'x60' (roof pitch doesn't matter, only it's shadow
area), the house roof is 52'x44' and the catchment roof in the back is
36'x30'. That's 5288 square feet of catchment area. An inch
of rain is approximately 0.625 gallons of water per square foot.
That means it takes about 24 inches of rain to completely fill our 80,000
gallons of tank capacity.
Our tanks are currently full and typically stay that way. 80,000
gallons is far more water than we need for the house. The extra water
is in case we ever need it for irrigation. It's not enough for
full-time irrigation.
To keep them strong and healthy a coffee tree needs about a gallon of
water per day. With 600 trees per acre, that adds up fast. Even
if we only watered a couple thousand trees at a time we could still run out
of water within a couple weeks if we weren't careful.
The worst dry spell we've had so far lasted for a couple months. We
got a little rain during that time but not enough. Our mature trees
did fine, it was only the younger ones that needed water. It was
harvest season so our coffee mill was using a lot of water too. By
the end of the dry spell our tanks were down to about 25% of capacity.
Some of the neighbors had already started trucking in water because their
tanks were empty.
A water catchment system is relatively easy to build and maintain. You
might think that once it's installed, there's nothing left to do.
You'd be wrong. I have no idea how many total feet of water pipe we
have but it's a lot, several thousand feet at least. With that much pipe
there's always something broken.
The easiest way to break the irrigation lines is with the mower. The
larger feed lines are mostly buried but all the drip lines are above ground
and the mower likes to eat them, The buried lines break less often but
being buried doesn't mean they can't break, it just means the leak is
harder to find.
One day I heard my neighbor's workers out in their field, spinning the tires
of their large truck. I knew immediately where they were stuck.
I also knew that they were probably way too close to my main irrigation
pipe. I went out to help but sure enough, I ended up with a 10 foot
high geyser of water.
Another time a friend was hunting for some food when he missed the pig
and hit the irrigation pipe instead. It would have been a great shot
if he was aiming at the pipe. The pipe was already broken elsewhere so
there wasn't any geyser this time, only an expensive sounding thunk.
Besides all the broken pipes, the water catchment tanks occasionally require
maintenance too. I'm guessing that our tanks are nearly 20 years
old. The tanks themselves are galvanized steel which lasts a long
time but the fabric tank covers need to be replaced occasionally. A
sagging cover collects dirt and a ripped cover lets in mosquitoes.
The last thing you want is a tank full of breeding mosquitoes.
The cover on our main tank has needed replacing for almost a year now.
I pulled it off recently so I could drive around my remote control
boats. The water was crystal clear when I pulled the cover off
but within days an algae bloom started to take hold. I had to
heavily chlorinate the tank to get rid of the algae before I could
clean it.
Even though the tanks are covered it's amazing how much junk they can collect
on the bottom. We keep our tanks tightly covered so we didn't find any
dead birds or other animals, just lots of dirt and rust. The dirt is
from wind blown stuff that gets on the roof then washes into the tank.
The rust is from the metal roof which may need to be replaced soon.
We filter our water before it comes into the house but it's still important
to keep the tanks clean. Keeping the tanks covered and clear of debris
(so critters can't crawl in) is the most important thing. We get so
much rain that our tanks are always flushing themselves out and stay pretty
clean on their own. Catchment tanks actually require more maintenance
during dry spells.
The idea of harvesting free, clean water from the sky sounds groovy but
for most people it's probably easier to just pay the city. I did not
install our irrigation system and if it wasn't already here I doubt I'd put
one in. We usually get enough rain that irrigation isn't
necessary. If we get so little rain that we have to irrigate then
there won't be enough rain to keep the tanks full. So irrigating
from water catchment only works when we don't need it.
Water catchment for the house is a different story. We can't get city
water and we can't drill a well so catchment is our only choice. I
have heard horror stories about dirty tanks with dangerous pathogens and
old tanks that burst and create a giant wave that destroys the house.
We keep our tanks clean and I inspect them occasionally so we shouldn't
have any problems. As long as we're careful I think our water can be
cleaner than the city's. For us, water catchment has proven to be a
cheap and easy solution.
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