Effective Rainfall

  • Written by Andrew Davis
  • Posted on Jul 15, 2019
  • Articles

June is turning into an exceptionally wet month although, in this immediate area, we have been spared the torrential rain and floods further east.  The rainfall is welcome as previous months have been very dry and concerns were rising about the prospect of drought this summer, not only for crops but also reserves in reservoirs and aquifers.

It is unusual for recharge in the summer months.  The concept of effective rainfall is that proportion that percolates through the soil to end up in rivers, reservoirs and aquifers.  During the summer, with trees in full leaf, so much water is taken up by growing plants that very little, if any, percolates through the soil.  A mature oak, for example, may transpire up to 300 gallons of water during a hot summer day.  It is only when the leaves fall and plant growth slows that we can expect any recharge of water resources.

Winterbournes are the upper reaches of chalk streams fed by springs when the chalk aquifers are full and usually run from January to May.  An exception was 2012 when the weather had been so dry that the winterbournes did not run at all during the spring.  Then torrential rain in June and July rapidly recharged the aquifer and the winterbournes started running in July!  That year there was certainly effective rainfall in summer and it may well be the case this year too.

Nevertheless, we have been exhorted to cut back on the water we use for drinking, washing, cooking, for industry, to manage this country’s water resources.  But that direct use is only part of the picture, there is also embedded water to consider, the water that we use indirectly or import from abroad.  Every product that we buy has the consumption of water associated with it.  For example, it takes 170 litres of water to produce one pint of beer.  There is the water needed to grow the barley and the hops, to soak the barley to produce the malt and all the water used in the brewing process.

Similarly, according to figures published by the Institute of Grocery Distribution, it takes a staggering 16,000 litres of water to produce every kg of beef.  For some time it has been said that beef is expensive in terms of feed efficiency in that it takes around 7kg of grain to produce 1kg of meat, at least in the American feedlot system, but the cost in terms of water is even greater as it takes 1,000 litres of water to grow 1kg of wheat.

Much of our food is imported with the result that water is being used in countries overseas to provide the produce that we eat.  Every kg of green beans shipped to the UK from Kenya, for example, deprives the indigenous people of water.  A cup of coffee in this country means that 140 litres of water has been lost to the country of origin.  Across the world, 80% of water consumption is used to grow crops, much of which is then exported.

Thirty years ago I went to Israel and stayed by the Dead Sea, which is slowly drying up.  The river Jordan that feeds it is little more than a ditch because of the abstraction to irrigate fruit and vegetable crops in the Galilee region of Israel.  For every glass of orange juice that you drink, 170 litres of water has been used up.

But it is not just food that has embedded water.  A pair of bovine leather shoes has 8,000 litres of embedded water whilst cotton takes so much water to grow and process that a T-shirt has over 4,000 litres of embedded water and a pair of jeans over 10,000 litres!  Perhaps that is something we should bear in mind when we travel abroad and buy a T-shirt as a souvenir.

It is estimated that the average Briton drinks up to 5 litres of water each day and uses another 145 litres for cooking, washing, cleaning and flushing.  But that, accounting for only 5%, pales into insignificance compared with the amount of embedded water we consume.  65% of our consumption is embedded in food, the remaining 30% in other products such as clothes, even electrical goods.  Taking all this into account, one estimate of total consumption in this country is over 3,000 litres per head per day!

In the light of an increasing world population and the impact of climate change, this process is simply unsustainable.  Western demand for food and other goods is putting severe pressure on water resources in developing countries many of which are already suffering from drought.

It is difficult to get accurate figures as they can vary from country to country depending upon how the measurements are made.  For example, it has been estimated that a kg of oranges from Australia has up to three times the embedded water of a kg of oranges from the USA.  Most importantly, similar amounts of water used in drought-prone countries have a much greater impact than in high rainfall countries.

Of course these issues are never simple.  Developing countries are keen to export their goods to earn foreign exchange and to promote their economies.  Nevertheless, it can be argued that importing goods with high levels of embedded water from drought-prone countries, such as cotton from Egypt, is irresponsible and ‘exports drought’.  This is an issue that will assume greater prominence in the years to come.