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Water Catchment
1 December 2008


Rainfall 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.

Book 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 at least 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.

Boat 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.

Cleaning 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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