Rainwater harvesting in Israel Palestine - how much would it provide?

News that the UK is rightly considering water metering prompts me to write again about drier, less peaceful countries.

I have blogged many times about the excellent work of Friends of the Earth Middle East, who bring Israeli and Palestinian communities together to work on a common problem - water management. 

I have long argued - into a vacuum - that the European Union should provide guttering, piping and storage facilities so that the Israeli-Palestinian people can work to save every drop of water that falls on their roofs.

Today I make a very rough calculation on the amount that this project could provide.

Here goes:

The population of Israel/Palestine is 11 million.

Let us assume that they live 11 to a house.
I have no idea how many there are really, but 11 makes the calculation easier.

Take the average rainfall in Israel Palestine is 50 centimetres.
If there are 1 million houses with a roof area of 10 sq metres,  every year each house can harvest 5 cubic metres of rainwater, or 5000 litres.

A human needs 1.86 litres of drinking water a day. Say 2 litres, to keep us away from decimal land.

5000 litres gives 2500 days (7 years) of drinking water, so each house will keep its 11 inhabitants in drinking water for about 8 months of the year.

If compost toilets are used, a good proportion of the water is excreted as urine, to feed the compost, and therefore to return to the land as fertiliser.

Now, because I know that people delight in trying to destroy any concept that challenges their conviction that there is NOTHING we can do to help the situation in Israel Palestine (apart that is from the overthrow of capitalism worldwide and its replacement by the dictatorship of the proletariat/anarcho-syndicalism/libertarianism [delete as appropriate]), let's avoid misunderstanding. I'm not saying they will all have to drink only the harvested water. Sure, in some situations it could be filtered and processed for drinking. In other situations it could water the vegetable garden. In each case, rainwater saved is mains water spared. The point is, water is water, and, along with sun, soil and toil, it is the stuff of life.

The other point is that in setting up the water harvesting equipment, the work project, especially if it is collaborative and inter-communal, will divert energy away from political hatred and all that flows from it.

I may have over estimated the amount, or got my sums wrong. I may also have under-estimated the amount, since there will be large roof spaces belonging to offices, factories, stores &c, and these may be enough to bring the amount harvested to a whole year's worth of drinking water.

I am also aware that water harvested is water that does not find its way down to replenish aquifers. This is true. But water harvested is a tiny proportion of the total rainfall on the land, so it is an insignificant loss to the aquifers, and balanced by the fact that less will need to be abstracted from the aquifers.

There are other things that can be done to improve water management. I've mentioned dry toilets. I will also mention that forests can be seen as aerial aquifers, so reafforestation can assist with rainfall.

The point is that a huge water harvesting project, along the lines already pioneered by Foe Middle East, would give the European Parliament something useful to do in making a Middle East war somewhat less likely.