Boro Boy wrote:
Chadwick wrote:Temperature and rainfall is measured with thermometers and rain gauges, not hosepipe bans.
Don't understand your point; the last few days there has been rain storms, sunshine and drizzle but still the leaves fall and the bark on the trees exfoliates. This is not what happened in previous years at this time (and we never got as far as a hosepipe ban this year) and I'm asking if its happening like this in other areas of the country where members are based....
The point we're all making is that this year has seen an unusually long and hot dry spell. The trees use more water when it's hot; this - and the lack of rain - has caused the level of groundwater at tree root depth to reduce, leaving the trees short of water. They cannot support their leaves without water, so the leaves wither and die - creating an early autumn effect.
Hosepipe bans come into effect when our reservoirs are running low. When we've had hosepipe bans in the past, it's because we've had several generally dry (if not particularly hot) years previously, and we've not filled up the reservoirs fully. The reservoirs are fed by rainfall over the year, particularly winter and spring. But it's not just direct rainfall: most of the country has large underground reserves in the underlying rock. This is where the long term water is naturally stored and we pump it either directly into the network, or to top up reservoirs. If water levels in these aquifers are low, as they are after dry years, then even a relatively short dry spell can cause a water shortage.
The reason why there are hosepipe bans in the northwest of the UK and Northern Ireland, but not yet elsewhere is due to geology and geography. The aquifers across most of the country are currently reasonably well stocked. The past two years have seen rainfall only just below average, and we haven't had a summer like this one to put a strain on the supply. Also, spring this year was quite wet, so the reservoirs were topped up just before the summer. (Incidentally, that same spring rainfall was a mixed blessing to the trees. Those with good drainage were encouraged to grow and produce more leaves, which they now can't support. Those with poor drainage were put under stress and weakened, making them also less able to withstand the current dry spell.) Back to the reservoirs. I said that most of the UK has underground water-retaining rocks. Guess where hasn't? That's right, the northwest and Northern Ireland. These areas are much more reliant on short-term rainfall - which they normally get in abundance, but not this summer.
As well as the natural factors, you have to factor in our man-made water management. Some water companies are better at bringing in water from outside their area and spreading the risk of supply shortage over multiple sources. Also, some companies are better at controlling their water. United Utilities (the company imposing the NW ban) appears to one of the worst offenders.
Finally, although it may rain, much of that water does not get into the ground or reservoirs. In built up areas it is simply directed into the sewers and out so sea. A sudden downpour - like this weekend's thunderstorms - also just runs off. The ground can only absorb so much water at a time. When it is baked hard, the water just bounces off. When it is sodden, any excess water just bounces off until the previous rainfall is able to drain away. This is how you get flash floods. The only way to replenish the underground aquifers and the water table generally is with a prolonged period of steady, gentle rain.
I hope that extended version explains why hosepipe bans and tree health are not directly linked. You can have a long hot summer without hosepipe bans, and the presence or absence of a ban is not an indicator of the climate or weather.
tl;dr. See expressman's post.
PS. Yes, leaves are falling early outside London. I'm in Milton Keynes and it's happening here.