Tilshead Borehole Levels: 26th January 2020

Despite being dry last week, the rain of the week before caused the water level in the aquifer to rise again.  It did so steadily throughout the week, only peaking in the early hours of yesterday morning.  Today, the level has started to fall slightly.  Unsurprisingly, the Environment Agency has not updated the Flood Alert status since Monday 20th January and the groundwater Flood Alert for the Salisbury Plain area remains in force.

Last week, we were suggesting that we might be lucky and get two consecutive rain-free weeks, and this was echoed by the Environment Agency who did not expect rain until a frontal system moved into the UK after 3rd February.   It seems we were wrong about this as we are now expecting rain today and tomorrow with some forecasters suggesting a further period of rain on Thursday and Friday.  In the longer term, moving into February, the weather seems to be initially very unsettled, with the risk of a little more rain, then steadily becoming more settled towards the middle of the month.   That all suggests that we might see water levels fall steadily for the next 4 or 5 days and they may even fall below the 99.00m AOD flood level for the Tilshead borehole, but after this, the level will rise again for a further week.  It seems unlikely that the Environment Agency will lift the Flood Alert just yet, or if they do, it could be re-instated within a matter of 4-5 days.

Tilshead Borehole Levels: 19th January 2020

Water levels in the River Till and in the Tilshead borehole fell steadily over last weekend and throughout most of the week.  However, the moderately heavy rain from the early part of the week through to the middle slowed the rate at which the borehole level fell.  That meant the level didn’t fall below the 99.00m AOD flood level until Thursday instead of Wednesday which I thought might be the case.  The water level bottomed out at mid-day on Friday and since then has been rising slowly.   Similar things have probably been happening at other boreholes on Salisbury Plain which might account for the fact the Environment Agency’s groundwater Flood Alert remains in force as of 10:00 today; although that status has not been updated since Wednesday of last week.

I can’t tell you how much rain actually fell in the village last week as Janet Abbot’s rain gauge, which has done such sterling service over many years providing data for Janet’s weather website, appears to have a terminal fault.  Although it is generating numbers, they seem to be random and Janet thinks the rain gauge has a “faulty see-saw”; who knew rain gauges kept themselves entertained!  Hopefully, it will soon be sorted out, as the data it provides is very useful.   In seeking an alternative, I found a rainfall gauge that was giving out data, of sorts, for Larkhill.  Unfortunately, it didn’t give 24-hour cumulative totals, so it wasn’t particularly useful, but it did express the rainfall in different units to Janet’s, which got me thinking.  Janet’s gauge gives an area independent depth of rainfall in millimetres whereas the Larkhill gauge expresses the rain measurement as litres per square metre.  Of course 1mm of rain equates to 1 litre per square metre and on Wednesday we had a 12 hour period where 12  litres of rain per square metre fell; or 12 mm.   Now 12mm of rain falling on Salisbury Plain doesn’t sound a lot, but what does that mean in volumetric terms?

Well,  the online version of Encyclopedia Brittanica gives the area of Salisbury Plain as 775 sq km and that equates to 775,000,000 sq m of ground and 12 litres of rain fell on each one of those.   That means a total of 9.3 billion litres of rain.  I don’t know about you, but I can’t get my head around big numbers like that.  Is 9.3 million cubic metres of rain any better?  How about it being the rough equivalent of 93,000 Blue Whales being dropped on Salisbury Plain – and none of us heard a thing!  Maybe 37,200 Olympic swimming pools worth of water then?

Enough nonsense, what about the week ahead.  It seems we are going to be in the grip of the highest high-pressure system ever recorded in the UK, which is going to divert all the wet Atlantic weather to our north.  That suggests the next week and possibly the next two weeks will remain largely dry, with any precipitation, including snow, being to the north of us.  There is little rain in prospect until the middle of February, which bodes well for water levels which should start to fall again in the next couple of days.  I would expect the Flood Alert to be lifted by the Environment Agency in the next few days.


Tilshead Borehole Levels: 12th January 2020

Although we have had a little rain over the last week, it clearly hasn’t amounted to much as the water levels in the River Till have continued to fall and the inundation in the meadow to the south of the village has now virtually gone.  The water level in the Tilshead borehole has also dropped throughout the week,  with the biggest drop being over the last 48 hours.  That said, the level is still above the 99m AOD level which is the flood alert trigger level and the environment Agency re-issued their flood alert for groundwater flooding on Salisbury Plain yesterday, writing:

“Groundwater levels remain high across Salisbury Plain. As a result of the more settled weather over the past few days, readings in some boreholes appear to be leveling off or falling slowly, however, there is still the possibility of flooding. Although the weather has been more settled recently there is a risk of more rainfall for overnight Saturday and early next week which has the potential to sustain high groundwater levels in this area into next week. Reports indicate that properties are pumping and sewage systems could be impacted and roads affected. We encourage residents with pumps and other property resilience measures to put them in place and ensure they are working…”

If the current rate of fall is maintained and there is no further heavy rain, then we would guess the Flood Alert will be dropped by mid-week. However, the system is finely balanced and heavy rain could reverse the current downward trend over the same period.

The short-term weather prospects are mixed with showers overnight and rain tomorrow, with further showers and rain during the week.  The following week could see even more rain, but beyond that is a period of greater weather uncertainty.  If the weather pattern becomes more settled, then it could get a lot colder and drier than it is now; but it might not.  There is still the possibility of snow in February, but it may be confined to the north of the country so don’t start hunting for the sledges just yet.

Tilshead Borehole Levels: 5th January 2020

The first report of 2020 can at least offer a little good news.  With no significant rainfall over the last week, the water levels in the River Till have fallen slowly, but steadily.  The floodwaters in the meadow to the south of the village are receding and the footpath from Church Street towards Stonehenge Campsite, which had been inundated, has also seen the floodwaters recede; though the path has become a muddy morass.  The Environment Agency renewed the two Flood Alerts for the River Wylye and its tributaries and the Salisbury Plain area late last week.  We suspect that for the River Wylye may be lifted fairly soon, but suspect the groundwater alert may stay in place for significantly longer, even in the absence of more significant rain,  There is some moderate to heavy rain and drizzle in the short-range forecast for next week.  This may fall on Monday and Tuesday and maybe again on Thursday.  The impact on an aquifer that is already pretty much at capacity is hard to judge. Later in the month, we have the prospect of further heavy rain, much colder weather, and even snow.