MAILBAG
‘How do I calculate evaporation, simply?’
S
IR ~ I attempt to
conserve my household's grey water for
eventual use on my garden.
To this end it flows through a
simple gravel and sand filter
which removes any fats and
solids (and also deodourises it)
before entering a lined “dam”.
This is a circular dished affair
about 15m in diameter, lined
with a single sheet of thick
greenhouse plastic (and
therefore, hopefully impervious), over which is laid brick
embedded in sand and
grouted into position. The
hole in which this is dug is of
solid ouklip, of such
impermeability that when it
rains a hole dug in ouklip will
take days, literally, to drain.
The sides of the dam are
gently sloped downwards
towards the centre where the
dam is about 1,2m deep, the
idea being firstly to maximise
the surface area of the water
to enable as much of the
sterilising interaction of the
U-V rays of the sun as
possible, and secondly to
facilitate the exit from the
water of any animal or human
who should stray into its
murky depths.
I have had this ifor a few
years now and in the early
days it would fill to the brim,
and even overflow after rain.
This year, however, it is far
from full, even after the
recent rains, so much so that
there's not enough in it to use
for watering the garden.
I should add that in the past
year I have added a small
solar pump to the set up
which, during sunlight hours,
pumps a steady stream up
into the filter, discharging
back into the dam and
creating a pleasant tinkling
sound, aerating the water a
little and keeping the medium
of the filter, hopefully, in an
anerobic state.
I should also add that our
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household's water usage has
greatly decreased over the
past year as our now-adult
children have one by one
moved out of our home. At
most, therefore, we use 200
litres a day of recyclable
water.
My question, therefore is this:
how much moisture does one
lose through evaporation here
on the Highveld?
I ask, because I really can't
believe (at least I fervently
hope) that the dam and its
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