Climate simulation and rainwater collection

While I do not deny climate change, I am sceptical of climate modelling.  One only has to look at the UK handling of the Covid crisis where their responses were based on simulation models produced by SAGE (Scientific Advisory Group for Emergencies).  They caused terrible harm to the UK economy.  Only in the final wave which still showed scenarios of virtually everyone being infected and overwhelming the NHS did the UK government show them the middle finger and do the opposite to what they recommended.

Simulations rely on lumped parameter models where they aggregate the properties of the components.  Scientists and engineers like to draw a little box around, say, a litre of air and say that it’s at a certain temperature and pressure.  What does that actually mean?  The problem is that 1lt of air contains 25 billion trillion molecules, each whizzing around like demonic bumping cars doing their own thing at the fair.  Temperature is given by the average speed of the molecules and pressure given by the average momentum with which they hit the sides of the real or imaginary container.  The averages work out well in steady state or laminar flow conditions.  The problem is that the world is seldom like that – it’s generally chaotic.  The world’s largest supercomputer would not be able to solve exactly for the turbulent wake coming off the rear wing of a F1 car or even a single one of its eddies.  How then can we expect to model the world’s weather systems?

If that is too much to think about, consider the last two decades of rainfall figures for Tygerberg, Cape Town.  The annual rainfall varies from 363 to 880mm or by a factor of 2.4.  Retrospective averages are just about meaningless and hence future predictions are useless.

Notwithstanding that, climate change is real and it’s here to stay like the unwanted mother-in-law.  In the last decade, Cape Town found itself staring down the barrel of Day Zero when the water would run out.  This prompted every man and his dog to make some contrivance to collect the rain water that didn’t fall.  Except for a fortunate few, it was a futile effort but luckily the weather turned just in time.

The graphs for annual rainfall in Tygerberg over the last 20 years below show the futility of trying to catch rainwater in the Western Cape during the critical summer months.  The exception is around Table Mountain which has its own micro-climate.  Newlands, for example, has a prodigious rainfall ranging from a maximum of 2303mm in 2001 to a minimum of 1019 in 2011 in the last 20 years.

When it comes to rainwater collection, I base my calculations on the Tygerberg numbers which are broadly representative of where the bulk of Cape Town’s population resides, namely away from Table Mountain.  I also assume a 150sqm roof collection area which represents a large house of 200-250sqm as not all the roofs are amenable for collection to a common tank.  

During the worst year (2009/10), only 7.7mm rain fell over 5.6 months of summer!  That would be a total collection of 1155lt or 6.8lt/day.  That’s only enough drinking water for a family of four and the pet gerbil.

The next two worst years weren’t much better with ~30mm falling over a similar period for a total of 4500lt or 27lt/day.

The best year over that period saw 157mm fall over 6.2 months for an average collection of 125lt/day or 3800lt/mth – only enough to sustain a careful family at a basic level and that does not include watering the garden or filling a pool during the height of summer which alone consumes about that.

Even rain-soaked Newlands experienced a minimum of 17mm over a 4.5 month period in 2010/11 giving a collection of only 18.5lt/day.

It must also be borne in mind that the first mm of light/misty rain never makes it to the gutters as it is evaporated on the hot summer roof or absorbed into the tiles and galvanising.  This means that the worst-case year of 7.7mm over 5.6 months for Tygerberg would probably have resulted in 0lt of rain collected!

I suspect that a similar situation pertains in all the inland areas over their dry winter months.  At least their pools aren’t evaporating like crazy then.  To the best of my knowledge, only the east coast down to Port Elizabeth gets a better distribution of rain through the year and tanks are probably viable.

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