An underground river

Parts of the River Frome in Gloucestershire have dried out completely in the last couple of weeks. We’ve had a few reports from people concerned about what’s caused it and whether it will return to its former glory. Here are some answers.

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Not all of the river is affected, but a fair stretch to the east of Stroud, in the Frampton Mansell area is now completely dry.

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The drying out is a completely natural phenomenon. It happens most years, and is the result of the local geology.

The River Frome flows over highly permeable (porous) rock formations such as limestone and sandstones which also make up the principal aquifers in the Stroud area. Aquifers are underground water reserves, bound up within rock formations. Also, in its upper reaches, the River is mainly groundwater fed. As the groundwater table declines in dry periods below the bed of the river it experiences the drying out episodes which we are seeing now. Any flowing water is now beneath the surface! Extensive faulting (cracks and gaps in the rocks) in the area also locally contributes to losses of river water into the underlying bedrock via these features.

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When we get some rain the groundwater level will rise back to the surface and the River will reappear.

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Because the process happens quite slowly fish and insects tend to naturally move away from the drying area and then return once flows return.

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A tale of two Marches

My monthly weather updates usually focus on water, but this March has been truly exceptional for another meteorological reason – cold. In the Midlands it’s been the coldest March since 1892 with an overall average temperature over 3c lower than we would expect.

The contrast with last March, which was one of the warmest on record, could hardly be more stark.

This blog compares the two remarkable months and tries to explain the reasons why the weather happened.

March 2012 – Scorchio

March 2012 was exceptionally warm. The month was also dominated by high pressure meaning it was also dry and pretty sunny. All in all a very nice month!

The records show that March 2012 was the third warmest March since records began in 1910 and the warmest since 1957. Overall temperatures were 2.3c higher than average. It also broke the record for the biggest positive anomaly for maximum temperatures (below), being 3.2c above what we would expect.

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Towards the end of the month temperatures soared into the low 20’s which is more than 10c higher than the average maximum temperature for March. The weekend of 24/25th saw glorious sunny, warm weather across the country. Beaches (like Brighton below) and resorts did very brisk business.

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Although it wasn’t wall to wall sunshine, March 2012 was much sunnier than average. The map below shows many parts of the UK, including most of the Midlands, enjoyed 50% more sun than we’d normally expect.

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A look at the daily maximum temperatures for Pershore in Worcestershire shows that 10c or higher was recorded on all but five days and 15c or more on twelve days.

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Coming on the back of 18 very dry months, the warm dry conditions began to lead to environmental problems. Many rivers (like the Teme below) had very low flows and the very warm temperatures meant oxygen levels in some lakes and canals began to drop dramatically. We carried out a number of fish rescues in the Midlands during the month.

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March 2013 – Siberian Spring

March 2013 started cold and, with a very small blip, stayed cold. Compare the maximum temperature chart for Pershore (below) with the one for 2012 (above) and you’ll see that 10c was reached on just one day (26 in 2012!) and 15c wasn’t reached at all (12 days in 2012).

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With an average Central England temperature (a temperature record going back to 1659) of 2.7c it was the coldest March since 1892. For England as a whole it was the second coldest March on record (1962 being the coldest). Here data only goes back to 1910. Average maximum temperatures were more than 3c lower than expected (map below). Also unusual was the fact that March was colder than any of the winter months (December, January and February). The last time that happened was in 1975.

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To make matters worse the month was also very dull. Many areas only saw around 80% of average sunshine, compared with the 150% of 2012 (map above). Prevailing easterly winds brought lots of low cloud off the near continent and North Sea. The satellite picture below (from 23 March) shows a fairly typical setup. Conditions on the ground would have felt particularly cold and raw with temperatures hovering around freezing both day and night.

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Snow fell on many days as the easterly flow encouraged the development of snow showers, particularly across eastern parts of the country. However, the main event happened during the weekend of 23/24th when warmer air off the Atlantic tried to push into the UK but met much colder air resident over us (see 22 March post below). The outcome was heavy snow and blizzards for many parts of the country. North Wales, North West England and Northern Ireland were the worst affected areas.

The Midlands marked the boundary of warm and cold air and therefore saw huge variations. Gloucestershire and southern parts of Worcestershire and Herefordshire had an inch or more of rain with widespread minor flooding while upland parts of Shropshire were buried in six foot snow drifts (below).

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The weather remained exceptionally cold through to the end of the month meaning there was little thaw of the lying snow. At the start of April many places still had significant snow cover and enormous icicles!

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The economic impacts of the arctic conditions have yet to be calculated. They will be significant! The tourism industry, hoping for an early boost in difficult economic times, suffered from booking cancellations and greatly reduced attendances, especially in the locations most affected by snow. Below is another picture of Brighton Beach, the same weekend as the one above, but in 2013!

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The farming industry, beleagured by three years of drought and flooding (see bonkers weather post below), suffered dreadfully in some parts of the country. The costs of feeding and housing stock indoors increased and getting crops planted and established was very difficult. Worst affected though were farmers in upland areas, many of who spent the month trying to keep animals alive and rescue them from snow drifts. Some, like the sheep below were lucky, many thousands more were not. Early estimates suggest well over 25,000 sheep and cattle have died as a result of the cold and snow.

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But why?

Remarkably the weather setup for March 2012 and 2013 was quite similar. Both months were dominated by large high pressure systems. High pressure, near the UK holds back rain bearing weather systems in the Atlantic. Consequently both months were drier than average, with 2013 seeing around 65% of average rainfall while 2012 saw a tiny 40%.

But the position that the high pressure systems sat in made a huge difference to the temperatures and type of weather we experienced. In 2012 the highs tended to be located either over the UK or a bit to the south. A Met Office chart, typical of conditions in 2012, showing the positions of weather systems is below. Winds move in a clockwise direction around high pressure systems. Crucially the winds which blow over us pick up their characteristics from the areas they pass over. In the case of 2012 the winds (red arrow) had come over a lot of very warm land and air in southern Europe and north Africa. Clear, warm and sunny conditions were the result.

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In 2013 the high pressure systems set up shop to the north and north east of Britain. Indeed high pressure dominated much of the northern hemisphere for the month. This is quite an unusual phenomenon called northern blocking and has resulted in very cold conditions over northern and eastern Europe but unusually wet and windy conditions for much of the Mediterranean as weather systems get shunted below the blocking highs. The chart below from the end of March 2013 shows winds again going clockwise round the high but this time picking up bitterly cold air from Scandinavia and eastern Europe.

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As we move into the second week of April there are strong signals for a change in the weather. It’s expected that Atlantic weather systems will finally break through as high pressure to the north and east of the UK finally declines. Temperatures should eventually recover to somewhere near average. What comes next is anyone’s guess!

For more on weird weather and some possible explanations for why it’s happening watch the excellent BBC Horizon film Global Weirding http://bbc.in/XrfmiT

Malvern Hills

Chasing the clouds

The last few weeks have been memorable for the cold and snow. Across the Midlands temperatures have not got much above freezing for a couple of weeks. And yet we’ve not really seen any really exceptionally low temperatures at night. Here’s why.

Radiation –┬ábut no nukes!

During the day we get warmth in the form of incoming solar radiation. Admittedly not that much when the sun is low in the sky in the winter but it’s still there. All objects then absorb some of that radiation and then slowly release it back into the air as heat (pics courtesy Hong Kong Observatory).

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While the sun’s up there, even if it’s behind the clouds, incoming solar radiation is usually more than that being lost so it’s generally warmer in the day.

At night, when there is no solar radiation, more heat is lost to the air than is gained so it gets colder. The main controlling factor over how cold it will get at night is cloud. Cloud cover acts like a duvet. Some of the heat radiated up from the surface bounces back of the clouds and warms the air below keeping temperatures up.

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If there are no clouds there is nothing stopping the heat escaping all the way up to space. This is when temperatures can fall to very low levels.

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The last couple of weeks have been mainly cloudy across the Midlands as the cold easterly airflow has brought cloud with it off the continent and North Sea. So although it has been bitterly cold during the day with snow and icy winds, the cloud cover has prevented temperatures tumbling far at night. A look at the minimum temperatures recorded at Pershore in Worcestershire (below) over that last week or so show values hovering around freezing – so not especially cold.

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Getting the breeze up!

Ironically the other element that has prevented temperatures hitting the floor has been the east wind which has had us all shivering over the last month. The wind mixes up the air, mingling cold pockets with warmer bits. The overall result is to prevent areas of still very cold air forming.

Below – east winds off the continent bringing plenty of cloud over the UK. A very common setup during March.

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Starry starry night

It looks like we might start seeing some more breaks in the cloud over Easter as winds fall a bit lighter. If we do the overnight temperatures could tumble – you have been warned.

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Floody Confusing!

We’re dealing with some very difficult and unusual weather conditions at the moment. A large winter storm in the Atlantic is bumping into cold air over the UK, turning heavy rain to snow in some places.

Our dilemma is that the ground is very wet and rivers very sensitive to rainfall. Our ability to predict flood risk is heavily dependent on getting accurate forecasts of the amount of rain entering the system. At the moment it’s really difficult to say how much of the precipitation will be rain and how much snow (which will not get into rivers immediately).

What’s happening?

The chart below shows the weather fronts (which bring the rain and snow) moving into the UK from the south west. Normally they travel straight across us bringing a spell of rain which then clears away. At the moment there is a big blob of cold air sitting over much of Europe, including the UK. It doesn’t want to shift out of the way and the weather fronts are being slowed and held back over the top of us.

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If we have a look at the forecast temperatures (below) for the same time (Friday night/Saturday morning) the contrast between the cold continental air and warm air pushing from the Altlantic is obvious. As the rain bearing fronts collide with this cold air the rain turns quickly to snow. The challenge is predicting exactly where!

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The difficulty is also illustrated by looking at the Met Office weather warnings that are in place on Friday morning.

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In fact the forecasts and warnings have been very accurate, with very heavy rain and significant flooding in Devon and Cornwall and disruption due to snow in the north Midlands.

However, the problem for us is elevation also makes a big difference to whether rain or snow falls. Overnight much of the high land of Shropshire and Herefordshire experienced significant amounts of snow (below), while lowland areas had over half an inch of rain. Trying to factor these differences into our flood prediction models is very difficult.

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A look at our rain gauges (mm in last 24 hours) on Friday morning (below) shows this well. Some locations recording over an inch of rain, while others, nearby showing very little. The radar for the previous 24 hours was showing fairly continuous precipitation across the entire area.

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The upshot of this uncertainty is that we’ve opened our incident room in Tewkesbury so that we can monitor the situation very closely and launch a quick response should the rivers begin responding quickly.

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The immediate outlook remains uncertain, but we do expect the rain to turn increasingly to snow through Friday night and Saturday. It should then turn very cold and dry. Our thoughts then will inevitably turn to the thaw of all the snow and what that will do to river levels. But that’s another story!

Water scene February 2013 – National update

Just a brief update on the national watery picture.

As in the Midlands, February was a pretty average month in terms of rainfall nationally. As you can see from the maps below this made a refreshing change for the areas that were deluged (blues) from April to December last year.

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With a fairly average January too overall rainfall amounts in the last three months are returning to levels we’d expect. However, if you look at the cumulative amounts for the last six months to a year they are still way more than normal.

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Even though we’ve not had that much rain, the soil remains saturated. This is because temperatures are low so there isn’t much evaporation and there are fewer growing plants to suck water up. As a result when we do get rain it runs off very quickly, causing surface water flooding problems.

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River flows during February were very similar to January with many being average for the time of year. Some of the larger lowland rivers, such as the Trent, Thames and Avon, however, continued to see above average flows.

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Groundwater levels continue to show rapid rises in response to last years heavy rain as the water continues to percolate slowly down. Some sites, such as Skirwith (below) on the Eden Valley sandstone aquifer in Cumbria, are now recording record high levels for the end of February (solid black line).

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You can read our full national water report for February here: http://bit.ly/XoBB9x

The Midlands water scene – February 2013

After the extremes of 2012, February has been a pretty average month.

Rainfall in most parts of the Midlands was a little below normal, only the lowland parts of Shropshire recording significantly more than average. Indeed the far east and west of the area only received three quarters of February’s normal rain.

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This means that for two months running total rainfall in the Midlands has been less than the long term average. Is it the start of another trend?

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Despite the smaller amount of rain, river flows were generally above average for the month. This is partly because they were still recovering from flood levels at the end of January and due to continued run-off from saturated soils.

We’ve added a new monitoring site to our monthly water reports on the River Lugg at Butts Bridge in Herefordshire.

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Groundwater continues to respond to the heavy rain of the last year in most places. It can take many months for rainwater to find its way through the soil and into the deeper aquifers (water-storing rock).

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The delayed response can be clearly seen in the sandstone aquifers in Shropshire (below). They are, however, responding dramatically now!

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In contrast, the Nottinghamshire sandstone aquifers (below) are proving more stubborn and levels remain low despite all the rain.

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The first week of March has continued the drier trend and the immediate outlook is for much colder (but still fairly dry) conditions. We’ll see how it all pans out, but with recent experience I wouldn’t bet on anything!

You can read our full Midlands water report here http://bit.ly/Y0zfKk

2012 – A year of bonkers weather

It’s climate week. We’re encouraging people to start taking action to prepare for more extreme weather in the future. Whatever your views on climate change there is increasing evidence that extreme weather events are already getting more common in the UK and the trend is likely to continue in a warming world.

So let’s have a look at some of the conditions we experienced in the Midlands during 2012

A parched landscape

The year started with all parts seeing exceptionally dry conditions. Two successive dry winters and a dry (but dull and cool) summer in 2011 meant that by March the Midlands had recorded the driest 18 months on record. In the chart below red bars represent months that were drier than average. The bigger the bar the drier it was.

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The dry conditions were having a dramatic impact on the amount of water in our rivers. The flows in many such as the Severn, Avon and Wye were less than a quarter of what we would expect.

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The headwaters of some rivers such as the Teme in Shropshire (pictured below) and Lathkill in Derbyshire actually began to dry out completely and we had to begin rescuing fish that were getting stranded in isolated pools.

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Farmers and growers were getting increasingly worried about their ability to water crops, particularly if the weather stayed dry through the summer. We worked with them, relaxing restrictions on abstraction licences (which farmers need to take water out of rivers) and offering advice on creating new on farm storage reservoirs.

We were also getting very concerned about the amount of water stored in the rock beneath us (aquifers). Aquifers are vital both for use by water companies to supply mains water and also to individual homes and businesses that have their own private supply. By March levels right across the Midlands were at or lower than those recorded in the drought of 1976.

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Our models and predictions were telling us that even if we had above average rainfall over the summer of 2012, water resources would continue to be very low through the period. As a result the Midlands formally went into drought on 16th April.

Little did we know what lay in store!

The heavens open

On Wednesday 4th April it started to rain across the Midlands. Everyone was relieved and grateful for the respite in the drought conditions. The problem was that it didn’t stop! Every month except May until the end of the year was wetter than normal and in many cases much, much wetter.

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Indeed, by December the preceding nine months were the wettest in the Midlands since records began in 1910 (chart below). Even more remarkably, when taken as a whole across England and Wales, 2012 was the wettest year on record. This really is amazing when you consider that the first quarter of the year saw virtually no rain!

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Some of the rain we experienced was very intense, particularly in June and July, resulting in many incidents of flash flooding. One example of this happened on 14th July when very heavy thunderstorms developed over Shropshire. Up to 2 inches of rain fell in a few hours leading to some serious flooding problems around Ludlow and Tenbury (pics below).

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However, it was the persistence of the rain rather than the intensity that resulted in the long and drawn out flood events that plagued the Midlands over the last nine months. The soil became completely saturated meaning that any rain that did fall found its way very quickly into streams and rivers. There have been seven significant periods of flooding in the Midlands since last April and most of them were caused by relatively small amounts of rain. To make matters worse, several of the wet spells have been close together meaning that river levels have not had chance to recover from the previous flood event.

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River levels and flows were generally well above average for the whole period from April to December. The chart below highlights the exceptionally high flows on many Midlands rivers through November and December. Peak levels observed on the big rivers such as the Severn, Avon and Wye were generally between a one in ten and one in twenty year return period and many towns (Worcester, Tenbury, Gloucester and Evesham) saw the highest levels since the catastrophic flooding of 2007.

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The hydrograph from the River Teme at Stanford Bridge in Worcestershire is typical of many Midlands Rivers. Frequent periods of high levels and showing a rapid response to any heavy rainfall. The flood warning theshold at Stanford Bridge is about 3.8 metres. This was breached nine times during 2012.

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The regular flooding kept us (and our emergency response partners) very busy. Our incident room in Tewkesbury was open for fifty days between April and December and our flood barriers at Shrewsbury, Ironbridge, Bewdley, Worcester and Hereford were up and down on a very regular basis. Many of our new defences, built since the 2007 flood received their first real tests. Sadly, despite all our efforts, many homes and businesses did flood during 2012, although many thousands more were protected.

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Ironically farmers and growers who in April were worried about whether they would have enough water for their crops found their ground waterlogged and, in many cases, completely inaccessible. Crops have been ruined and feed costs for livestock have escalated as normal grazing has been restricted or prevented.

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The intense rainfall over the summer also led to many instances of soil erosion. Thousands of tonnes of soil was washed off fields, damaging agricultural land, polluting local streams and in some case blocking roads!

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On balance

Eighteen months of dry weather have now been more than balanced by an exceptional period of wet conditions. When we declared drought in April we said that it would not be resolved by a few weeks of rain (I remember doing the interviews!). We were right! It’s taken the wettest year on record.

Even water levels in the aquifers, which take much longer to respond to rainfall, have now recovered (see chart below). Indeed, in some parts of the country (such as Dorset) groundwater levels are so high that they are causing flooding problems of their own. Who would have guessed that in April?

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The future

So have we had a vision of the future during 2012 or just been subjected to the natural variability of the British weather? I don’t know, and I’d suggest neither does anyone else. It’s not possible to attribute any single event to climate change. However, if we do accept that the world is warming, then the typeof weather we’ve seen during 2012 (and indeed in the years before) are more likely to occur. Warmer air holds more water and changes to ocean temperatures may change the global scale air circulations (such as the jet stream) which have a huge impact on our weather. Extremes (of all sorts) are more likely.

One of the most dramatic (and scary) model predictions on future climate change in the UK that I’ve seen is for river flows (below). Based on medium/high emissions scenario (which is closest to where we are) the model suggests that by 2050 summer flows on rivers like the Severn and Avon could be 50-80% lower than now. That, clearly, would be game changing! Conversely average winter flows could be slightly higher than now

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Whether you believe in climate change or not, we’re encouraging everyone to think about extreme weather and how we can be better prepared. Over the last decade tens of thousands of people across the Midlands have been affected by extreme weather events. Surely a bit of thought about your levels of risk and how it may affect you, your family or your business in future makes sense!

Try these links:

Climate week – all the details http://www.climateweek.com/

Check your flood risk http://www.environment-agency.gov.uk/homeandleisure/floods/31650.aspx

Practical actions for businesses and organisations http://bit.ly/Umoa2r

Check out how vulnerable you are to extreme weather events http://www.environment-agency.gov.uk/research/137639.aspx

Climate proof your business http://bit.ly/Iawxab

Advice to farmers on coping with adverse weather http://bit.ly/Xkre4q