Banking winter water for a dry summer

July 2026 has been the driest July in the seventeen-year record: 4.5 mm of rain against a July average of 57 mm. That is the problem in one number.

• 148 — million m³ a year could lawfully be taken at high flows

• 86% — of it arrives between November and March

• 2% — arrives in June, July and August, when it is wanted

• 9 — bursts a year, each lasting about two days

• The water exists — many times more than any scheme here would use.

• It arrives in short bursts. This, not the annual total, is what makes the engineering hard.

• A reservoir alone cannot bridge a drought. It empties every summer and refills every winter, so it carries nothing from one year to the next. Underground storage can — if the water stays put.

• The most promising options are not the obvious ones: a basin that fills itself by gravity, and flood-storage moors that already exist.

• No scheme is being recommended here. Nothing in this study is costed.

The current situation

The Somerset Levels flood in winter and run short in summer. Both happen for the same reason: the ground is flat, the rivers are slow, and the water arrives when nobody wants it.

86% of the water that could lawfully be taken arrives between November and March. Only 2% arrives in June, July and August. Any scheme here is therefore a battery, not a tap: its job is to move water across six months, not to find more of it.

In a drought year there is very little to catch. The surplus is a winter thing — nearly all of it arrives between November and March — so a drought year here means a winter whose floods never came. In the worst year of the record only 21% of the average was available to take.

Water given out in a dry summer was caught the previous winter, or the one before that. It cannot be topped up during the drought, because the drought is the failure of the winter that would have filled it.

What limits a reservoir here

This is flat, low, soft ground with high land value and deep peat in places.

What limits an aquifer

Water can also be put underground into porous rock and pumped back later. This is done elsewhere; the largest UK example supplies London. Underground storage does not evaporate and can hold water between years — but:

• It leaks. Water spreads away from where you put it. A 2 km well field in the local sandstone loses half of what is in it in about four years; a smaller field loses it faster, and the same rock can be either.
• It is fullest when you want to fill it. The water table peaks in February, exactly when the surplus arrives.
• Nothing in seventeen years of local records contains an injection.

The table below is every body, with the same 2 km well field assumed on each so the rows can be compared.

Size of body barely matters; kind of rock matters a lot. Underground storage is not a tank filled to the edges of the rock. It is a mound of water around a ring of wells, so its volume is set by the ring and by what the rock will give up — not by how much rock there is. The 408 km² Oolite body and the 1 km² one offer the same 2.5 Mm³ to the same well field. But the Greensand offers 18.8 and the limestones 1–2, because they differ in how much water drains out of them.

Only the sandstone rests on measured figures. Every other row is estimated from rock type — a fair guess a geologist could argue with, not a measurement, and the last column says which is which. Earlier versions of this report left those cells blank. That was worse: a blank reads as “nothing is known”, and something is.

Read that way the table stops being a shortlist and starts being a question. On these estimates the Greensand and the Exeter Group hold more, and hold it longer, than the sandstone the whole study is built on — and the limestones drain in weeks. If those estimates are anywhere near right, the rock this study picked is not obviously the best one. Nobody has measured them, and that is the point.

The three tests

Every option faces the same three tests.

The one number to hold on to

Every option below is reported the same way, and it is worth fixing what the figure means before meeting it:

What is modelled, and what it is built from

A reader entitled to be suspicious should be able to see what is measurement, what is modelling, and what is assumption.

What turned out not to need modelling

The honest ledger

Real trade-offs — physical, and they will not go away

Model assumptions — choices behind the numbers

Known unknowns

• What the rock will accept when injected. No local precedent — and the model holds that rate fixed across every rock, so a more transmissive body gets no credit for accepting water faster, which is the main thing that would shrink the reservoir in front of it.
• Whether injection wells would clog — the usual failure mode of such schemes, and not modelled at all.
• Costs. Nothing here is costed. Every comparison is “X delivers more water than Y”, never “X is better value”.

The eight options

A · Reservoir only

**1.16 million m³ per drought summer** — enough to cover **18%** of drought-summer days with a survival ration. A basin caught from the river in winter and drawn down through the summer, with no underground storage at all. It gains nothing whatever from being mature, because it has no memory between years.

B · Aquifer only, no reservoir

**1.26 million m³ per drought summer** — enough to cover **21%** of drought-summer days with a survival ration. River water pumped straight down wells with no basin at all. The cheapest thing to build, but it can only take water while the river is high, and it sends barely-settled flood water into the wells — the arrangement most likely to clog them.

C · Aquifer + reservoir

**1.96 million m³ per drought summer** — enough to cover **32%** of drought-summer days with a survival ration. The conventional managed-recharge scheme: catch the flood in a basin, inject it underground over the following weeks, pump it back in summer. Better than either part alone, and unlike a reservoir it carries water between years. **This is the baseline everything else should be judged against.**

D · Three sites on three rivers

**5.07 million m³ per drought summer** — enough to cover **78%** of drought-summer days with a survival ration. Three separate schemes of the same kind as C, one on each of three river systems, sharing the same total well capacity. Here the landscape does the design rather than a dial. It works for one simple reason — *the rivers do not flood on the same days*, so three modest intakes are busy more often than one large one.

E · Fast rock and slow rock, linked

**0.96 million m³ per drought summer** — enough to cover **16%** of drought-summer days with a survival ration. Two genuinely different rocks working together: fractured limestone that drains over a summer to feed the rivers naturally, passing the rest into sand that holds water for years. **Still the worst performer here, and that is the finding.**

On the limestone, a store that drains over a summer holds 7.4 million m³ — eight times what the sandstone gives at the same speed. The pairing has the storage it always needed. It still comes last.

Would a slower store do better?

The obvious objection, and worth a sweep rather than an argument — because a slower store is not a smaller one. Volume rises in step with the half-life, so asking for 180 days instead of 90 doubles it rather than trading it away. There is no penalty to pay on that axis.

Two further refinements were tried and are kept in the model, off by default: crediting the leak against the day’s river-support release, and holding the fast-to-slow transfer back until the river is too low to abstract from. The first changes almost nothing, because deliberate releases are only 0.02 million m³/yr to begin with. The second moves real water — river baseflow over the record rises from 8.2 to 13.6 million m³, at about 2% of delivery — but not into a drought summer.

F · A basin filled by gravity

**5.67 million m³ per drought summer** — enough to cover **89%** of drought-summer days with a survival ration. The same scheme as C, but the basin sits low enough that the river fills it through a gate when in flood — no intake pump at all. **The best performer of the eight.**

G · An existing flood storage moor

**4.67 million m³ per drought summer** — enough to cover **74%** of drought-summer days with a survival ration. The same idea as F, on ground that is already flooded every winter on purpose. Curry Moor and its neighbours are filled from the river through existing inlets and emptied by an existing pumping station; this option adds injection plant and keeps some of that water instead of pumping all of it away.

H · The best rock nobody has tested

**2.40 million m³ per drought summer** — enough to cover **39%** of drought-summer days with a survival ration. Option C moved onto different ground, and nothing else changed. The Greensand is estimated to hold half again as much water as the sandstone and to keep it half again as long — so this is what better rock is worth, with every other dial held still.

What the three tests show

The cost of being new is real but not fatal. Maturity is worth between nothing (option A) and about 15% (option F). A scheme commissioned in 2010 would have met little of the 2010–11 drought, simply because it had not yet banked anything.

Nothing collapses. Even in six consecutive drought years the driest years still carry enough water at high flow that a scheme keeps delivering something. These schemes do not run dry; they run small.

All options side by side

One chart, one question: on a day in a drought summer, is there enough water to keep a crop alive?

The chart splits into two clear groups. The five options built on one river cover 16–39% of drought days; the three that reach across the landscape cover 74–89%. Every one is sized at its own best configuration, so the gap is not a sizing artefact.

And the gap survives changing the ground underneath. H is option C on the best rock in the study area, and it reaches 39% — still less than half of what the weakest landscape-scale option manages on ordinary sandstone. The difference is not how much water these schemes store. It is how much they can catch.

The rhythm of each option

A single number per drought summer cannot show when a scheme hands water over, and that turns out to be most of the difference between them. The next two charts put all eight on the same clock.

The same thing shows in how steady each option is from year to year. A’s best year delivers 5 times its worst; F’s ratio is 1.5. That is what “smoothing” means in practice — not a gentler daily curve, but a smaller gap between a good year and a bad one.

What a scheme looks like from the inside

One option in detail, to show the mechanism the table hides. This is C, the conventional basin-plus-wells scheme, over eight years.

The same view of the two options that reach across the landscape shows why each of them works, and what each has to respect.

Option G has a constraint none of the others do: the moor it would use is not empty ground. It is held to water levels agreed between the Drainage Board, the Environment Agency and Natural England, for farming and for birds.

What is worth exploring, and why it works

⭐ Let the river fill the basin itself

Most promising

What it is. A basin sitting low enough that when the river runs high it simply flows in through a gate. No pump. When the flood passes the water drains back to the river, or is pumped underground at leisure.

Why it works. The water arrives in nine bursts a year, two days each. To catch a whole burst by pumping you would need a machine moving 560,000 m³ a day — used nine days a year. A gate costs almost nothing to run and has no such limit.

What would stop it. The ground has to be in the right place: below the river when it floods, above it when it does not. The screen found candidates, but the estimate of flood level behind them is the weakest number in the study.

⭐ Use the moors that already flood

Most practical

What it is. Several moors are already flooded deliberately in winter to protect towns, with inlets and pumping stations built and operating. Also use that water — put some underground before the moor is drained.

Why it works. Most of the cost is already spent. The structures exist, the operating practice exists, and the moors already fill and empty on roughly the right rhythm.

What would stop it. Those moors are managed to agreed levels for farming and for birds, and those agreements would have to be reopened. It needs the Drainage Board and Natural England, not just an engineer.

⭐ Spread across several rivers

Most robust

What it is. Three modest schemes on three different rivers instead of one large one.

Why it works. The rivers do not flood on the same days. Three modest intakes are busy more often than one large one, so they catch about 10% more water for the same total pumping.

What would stop it. Three sites means three lots of land, consent, access and maintenance. The water case is good; the cost case is untested.

Worth knowing about, but not the answer

A reservoir on its own

Water straight down wells, no basin

A fast-draining store feeding the rivers

What this study does not say

• It does not recommend a scheme. Costs would decide between these options and none has been estimated.
• It does not claim a return period. The six-year drought is a stress test built from real years in an invented order.
• It does not predict what the rock will accept. No injection has ever been made here.
• Land ownership, designations and consents are unchecked for every candidate site.
• Clogging — the most common way such schemes fail in practice — is not modelled at all.
• The survival threshold is this study’s own, not an agronomist’s.

What the next steps would be

1. Survey river levels at the candidate basins. Cheap, quick, and it decides whether the best-performing option is real.
2. Talk to the Drainage Board about the flood-storage moors. If they can double as recharge basins, most of the capital is spent.
3. Cost the options. Wells against earthworks is the trade that decides this.
4. Do one injection test. It would remove the largest single uncertainty in the study.

Somerset Levels & Vale of Taunton feasibility study, version 2 · 13 August 2026. Sources: Environment Agency hydrology and abstraction data; British Geological Survey mapping and report WD/97/34; Defra Crop Map of England 2024; Somerset Drainage Boards Consortium Water Level Management Plans; OpenStreetMap contributors. Every figure is reproducible from raw data with a single command.

Where every number comes from

Every figure above is produced by a background chapter, and every chapter regenerates from raw Environment Agency, BGS and Defra data with one command. They are working documents rather than prose, and they sit behind this report rather than beside it.

• The background chapters — all forty-one, grouped by layer: the water, the rock, the demand and the scheme.
• Earlier versions — the drafts this report grew out of, and the corrections between them.

← All notes · More from Aquifer Storage