Where You At Q4: What was the total rainfall in your area last year?

8656011418_5cd2d0a179_z
Rubber duck in a rainwater butt

The fourth question in the Where You At? bioregional quiz is:

What was the total rainfall in your area last year?

I think that annual average rainfall in Essex is about 600mm, but I don’t know how recent years have compared to that. The nearest Met Office ‘climate stations’ to me are at Writtle (c.29km away) and Shoeburyness (c.10.5 km away) – checking the Met Office site I see that for the period 1980-2010:

Writtle
average annual rainfall recorded: 591.8mm
with Days of rainfall >= 1 mm (days): 108.1

Shoeburyness
average annual rainfall recorded: 514.9mm
with Days of rainfall >= 1 mm (days): 101.0

rain

Around the country, there are also a number of independently-run weather stations collecting local data and we have one in our town: Burnham-on-Crouch Weather Station. They have a dashboard on their website giving up to date weather information:

weather dashboard

They also have a section with historical data and from there I can ascertain that in Burnham-on-Crouch, the average annual rainfall Since 2006, has been: 518.4 mm (close to the Met Office Shoeburyness average for 1980-2010) and that the rainfall recorded for the current 12 month period is a little higher: 530.0 mm.

This is less than half the UK rainfall total for 2017 of 1133 mm and less than the average annual rainfall than places like Sydney, Australia (1083.4mm); Rome, Italy (740.7mm); Barcelona, Spain (640mm); Singapore (2340 mm); and Miami, Florida, USA (1314.1mm).

Essex & Suffolk Water (my water supplier) are currently consulting on a new draft Water Resources Management Plan which includes water demand and supply forecasts for a 40 year planning period from 1 April 2020 to 31 March 2060. In it they indicate something of the local challenge:

The Essex and Suffolk supply areas are located within some of the driest areas of
the country and as such face particular challenges including growing demand,
uncertainty from climate change and a general lack of new intrinsic water resources. ESW has always fully embraced the concept of the ‘twin-track approach’ to maintaining water supplies through a combination of demand management and
water supply schemes and initiatives.

This is why Essex & Suffolk Water draw on a catchment area beyond their titular counties (as noted in Where You At Q1: Trace the water you drink from precipitation to tap).

One useful thing you can do with the average rainfall data for where you live is calculate how much water you could capture of a roof. You need two other pieces of data – the area of your roof, and what’s known as the ‘run-off co-efficient’ of your roof (which is the average percentage of rainwater that runs off the type of material used on your roof).

Brad Lancaster’s book Rainwater Harvesting for Drylands and Beyond, Volume 1 gives the run-down on how this works (relevant extract here), which I base the following on:

To calculate the runoff in litres from a slate roof which is 9x 10 m (90m²) in an area with my local rainfall – averaging 519.4mm a year:

90m² roof x 519.4mm average annual rainfall = 46.745 litres of rain falling on the roof in an average year.
(90m²  x 519.4mm = 46,745ltr/average year)

Multiply the above figure by a slate roof surface’s runoff coefficient of 0.9 (i.e. 10% of rainfall is absorbed/deflected/etc.):
46,745ltr x  0.9 = 42,071.4ltr running off the roof in an average year – and potentially available to capture and store.

According to the water watchdog, the Consumer Council for Water, the average annual water usage in a 4 person home is 165m³ (165,000ltr) – so nearly 40% of that water demand could be met by harvesting the water off our 90m² slate roof.

 

waterbyuse
Domestic water consumption by use, from At Home
with Water (Energy Saving Trust, c.2013)

 

Without changing water consumption habits that rainwater could potentially fulfill the domestic water demand for toilet flushing, washing machines, washing cars and watering the garden – tasks that don’t need ‘drinking quality’ water.

Some decent government legislation around the codes for new buildings – requiring rainwater capture – plus incentives and advice for existing homeowners could probably mitigate drought risk, increase household resilience and avoid large infrastructure costs. That’s the kind of thinking and action I would like to see in Essex.


Next up: I’ll try and answer – When was the last time a fire burned in your area?

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