"Len Wood" wrote in message
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On Thursday, 29 October 2015 02:08:50 UTC, jbm wrote:
I am absolutely and thoroughly stuck on something the local council has
asked me to look into.
Over the last seven years, the rainfall in this area has been well below
normal for 5 of them. Currently, we are over 300mm short of what we
would normally have expected in that time. Doesn't sound a lot, but it
represents 6 months of normal rainfall.
In 2011 we had just under 400mm, 60% of normal, and the following year,
despite numerous thunderstorms, deluges and flood alerts along the River
Nene, several springs in the area dried up, and have not flowed since.
Result = steams with no water in them, local lakes well below level,
with any pollution entering them not being diluted sufficiently not to
cause problems. One lake lost all its waterfowl in July due to
contamination from fuel oil from a local industrial estate. What I have
found is enough evidence to prove that the ground water levels are
severely depleted, with the water table at least 300mm below what it was
5 years ago.
So I would appreciate it if some of you knowledgeable meteorologists out
there would care to hazard a guess at the following. Having experienced
so many dry years recently, what are the chances of getting some
exceptionally wet ones, with steady and moderate rain to start
replenishing the ground water, without the majority of it disappearing
straight into the rivers as surface run-off? What we need is a lot of
water, and I mean a lot, getting down to that water table as quickly as
possible. Any ideas anyone? We have to make a decision shortly as to
what to do with the lakes - leave them as they are, dredge out all the
****e and see what happens, or fill them in and be done with it. And a
reasonably intelligent prediction on future rainfall might help in that
decision.
jim
a very dry and rainless Northampton
Hi Jim,
If you look at the record of annual rainfall for Plymouth 1874 to 2014 you
see how variable it is with no longterm trend, but clearly trends on the
decadal timescale.
You can see an increasing trend after the drought of the mid seventies,
and then a decrease and now slight increase again.
I have also put in dropbox the link to the graph of summer and winter
rainfall.
There is a significant increasing trend in winter rainfall and a less
convincing decrease in summer rain. Hence no trend in annual rainfall over
the longterm.
https://www.dropbox.com/s/u0s7p9kroj...02014.jpg?dl=0
https://www.dropbox.com/s/9lfjie7ko4...0rain.jpg?dl=0
If I was you Jim I would hold fire, but if the water usage has gone up in
recent years and/or the surface morphology has changed then you are stuck.
Hi Len, interesting stuff, but I have some questions about the graphs?
1. How consistent was the raingauge site over the years.
My experience of looking at Dartmoor gauges has revealed that just a small
change in location (even a few hundred metres) can make a statistically sig.
difference to rainfall. Cowsic is a case in question, due to changes in
aspect from one side of a valley to another.
2. In the 19th Century rainguage height was not consistent, in fact a lot of
gauges were elevated. Scientifically this was proven to produce lower
rainfalls than rainguages on the ground due to turbulence effects.
In my Dartmoor study I have been very careful to only use data where the
raingauge location has been consistent over the decades and where the height
of the gauge above ground has been consistent. I think that is important.
3. Have you done a statistical Student's T test to measure significance of
the changes, I suspect that they are very sig (esp. winter). but with the
provisos of my points one and two above.
Jim, you need to look at your local area rainfall over the decades too, if
you can of course?
Will
--
http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm
Will Hand (Haytor, Devon, 1017 feet asl)
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