First, Read the Brook Before You Touch It

Before you dig a single spade into a bank, spend a fortnight watching the water. A brook that looks generous in February can be a trickle by August, and it is the summer low flow that decides what you can realistically irrigate. Measure it roughly with a bucket and a stopwatch: time how long it takes to fill a 10-litre container, then divide ten by the seconds to get litres per second. A brook running at just 2 litres per second is moving roughly 170 cubic metres a day — plenty for a large vegetable garden, but only if you take a small fraction of it.

Next, work out your available head — the fall in height between your proposed intake and the furthest point you want to water. Pace it out and check with a level or a length of clear hose filled with water. Even 30–60 cm of fall over 50 metres will move water along a channel very happily; you simply cannot push it uphill without a pump.

Know the Rules Before You Alter the Flow

In England you generally need an abstraction licence from the Environment Agency to take more than 20 cubic metres (20,000 litres) per day from a surface water source. Wales, Scotland and Northern Ireland have their own regulators and their own thresholds, so check with the relevant body rather than assuming. Even below the threshold, most abstractions still need registering.

Separately, physical works in a watercourse usually require consent. On an ordinary watercourse that means the lead local flood authority at your council; on a main river it is the Environment Agency. As a riparian owner you own the land up to the centre of the brook, but you do not own the water itself, and you must not:

• raise water levels or increase flood risk for neighbours upstream or downstream;
• block the passage of fish, eels or sediment;
• take so much that the brook runs dry below your intake.

Always leave a residual flow — a continuous trickle past your take-off point. It keeps the ecology alive and it keeps you on the right side of everyone downstream.

Building a Controlled Intake, Not a Dam

The classic mistake is to throw a barrier across the brook and hope. Instead, construct a small headwall on one bank with a sluice board or a removable gate that lets you control how much water enters your channel. Set the board so water spills over it and continues down the brook rather than backing up.

Around the intake, protect the bank with stone, gabion baskets or heavy timber piling — the turbulence at a take-off point scours earth banks quickly. Fit a coarse trash screen ahead of the gate and check it after every storm. Immediately behind the intake, dig a silt trap: a sump 60–90 cm deep and about a metre square that you can clean out each autumn. It will save your channel from silting solid within a season or two.

Designing the Channel

Aim for a fall of roughly 1 in 150 to 1 in 300. Steeper and the water scours the bed; flatter and it stagnates and silts. A channel 30–45 cm wide and 20 cm deep is ample for a smallholding, and a shallow trapezoid profile with sloping sides is far more stable than a vertical-sided trench.

Lining depends on your soil. Heavy clay will seal itself after a few weeks of running. Sandy or gravelly ground needs a butyl liner, or a puddled clay base if you have clay to spare. Whichever you choose:

• keep the bed smooth and free of obstructions that collect debris;
• grade the banks at about 45 degrees and sow them with grass or plant marginal species such as water mint, flag iris and marsh marigold;
• fence the channel from livestock, or leave a generous uncultivated margin either side.

Planting the margins is not decoration. It shades the water, filters run-off, and gives amphibians and invertebrates somewhere to live.

Getting Water to the Crops

Gravity is cheapest. Run the channel to a header tank — an IBC on a plinth works well — and let the fall do the rest. Remember that 1 metre of head gives you only about 0.1 bar of pressure, so low-pressure drip tape and soaker hose will work from a tank 1–2 metres up, but sprinklers will not. For those, or for long runs uphill to an orchard or pasture, use a small pump, ideally solar-powered for summer use.

Water early in the morning or late in the evening to cut evaporation losses, and mulch heavily. A gravel or bark mulch over drip tape can halve the volume you need. For pasture, a simple herringbone of shallow cut channels can flood-irrigate a paddock in rotation, but only run it when the brook is high and shut it off the moment flows drop.

Wildlife, Maintenance and Winter

Brown trout, bullhead and eels all use small brooks, and trout spawn between roughly November and March. Do any in-channel work outside that window, and keep gravel beds and riffles undisturbed. Never let the channel take the whole flow, and keep it shaded along at least part of its length.

Each autumn, lift your sluice boards, drain the channel, clear the silt trap and cut back excessive marginal growth. Check the intake after storms and keep a simple logbook of what you abstract and when — it is both a legal requirement in most cases and a genuinely useful record of how your brook behaves year on year. Looked after properly, a small brook will quietly water your fruit, vegetables and grass for decades.