Watering a villa garden from a well: irrigation demand, pumps and storage

A green villa garden through a Samui dry season is a lot of water. Lawns, palms and flowering beds in sandy coastal soil can drink more each day than everyone living in the house, and that demand peaks exactly when rain stops and delivery trucks get scarce. A private well is the obvious source for garden irrigation, and often a good one. The problems start when the garden is simply connected to the house plumbing and left to fight the showers for water.

How much water a garden actually uses

Irrigation demand is easier to estimate than most owners think, because of one useful fact: a millimetre of water spread over a square metre is one litre. The amount a garden needs per watering depends on the plants, the soil, the weather and the method. A gardener or irrigation designer can give a sensible figure for each zone. Multiply it by the area and you have litres.

As an illustration, 500 m² of lawn given 5 mm a day needs 2,500 litres a day, every day, before the beds and pots. Drip lines on mulched beds use far less per square metre than sprinklers on open lawn, because they put water at the roots instead of in the air and on the paths. Sandy soil near the beach drains fast and needs little and often; heavier soil inland holds water longer. The point isn't the exact figure. It is to know the garden's daily volume before deciding what the well, the pump and the tanks must supply.

Why the garden shouldn't share the house pump

The simplest installation tees the irrigation off the house pressure line. It works in a small garden. On a villa it causes four problems:

• Competition. When the sprinklers and the morning showers run together, the house pressure drops and the garden zones lose their throw. Neither gets what it was designed for.
• The wrong pump. A booster sized to keep house pressure steady is rarely sized for an irrigation zone's flow. It cycles, overheats or never reaches the pressure the sprinklers need.
• Expensive water on the lawn. On a villa with whole-house treatment, every litre of irrigation has been filtered and disinfected first, costing media, cartridges and electricity for water that soaks into the ground.
• One empty tank. With one storage tank for both, a long night of irrigation can leave the house short at breakfast.

A dedicated irrigation line: tank, pump, filter, zones

In layout B the well fills a separate irrigation tank at a steady, sustainable rate, just as it fills the domestic tank. Often a level control gives the house tank priority and sends water to the garden tank once the house is full. The irrigation pump then draws from its own tank, sized to the flow of the largest zone at the pressure its sprinklers or drippers need. Zones are split so that each one matches the pump, rather than asking one pump to water the whole garden at once.

Because the garden doesn't need drinking-quality water, irrigation can usually run on raw well water and skip the house treatment entirely. Raw doesn't mean unfiltered, though. A screen or disc filter at the valve manifold keeps sand and scale out of the emitters, and it needs cleaning on a schedule like any other filter.

Drip lines and iron

Drip irrigation is the most efficient way to water beds, and the most sensitive to well-water chemistry. Dissolved iron and manganese come out of solution when the water meets air at the emitter outlet, and the deposit builds up until the dripper slows and stops. One bed turns yellow while the next is fine, and the cause is a few millimetres of orange crust. See brown or orange well water for the chemistry behind it.

On iron-rich water there are two practical answers. One is to let the iron drop out before the irrigation pump: an aerated or well-vented irrigation tank gives the iron time to oxidise and settle, and the pump draws from above the sediment. The other is to filter it out with the right media. Either way, plan regular flushing of the drip line ends, and test the water before choosing drip over sprinklers.

Dry season: when the well and the garden compete

Every well has a sustainable daily yield, and late in the dry season it is at its lowest. The garden's demand has to fit inside what is left after the house. If a test shows the well sustains a certain volume per day, the house takes its share first and irrigation gets the remainder, not the other way round. Our guide to how much water a well produces explains how that figure is measured. Why that figure drops in the dry months is explained in seasonal groundwater and deep wells.

• Water at night or before sunrise, when evaporation and wind losses are lowest.
• Move lawns to the lowest priority in April. Palms and established shrubs tolerate a dry spell far better than turf.
• Mulch beds. It cuts evaporation sharply and protects drip lines from the sun.
• On coastal plots, watch salinity. Long irrigation runs are the classic trigger for saltwater intrusion, and salty water then damages the plants it is meant to save.
• Collect rainwater for the garden in the wet months where the roof and space allow; every litre stored is a litre the well doesn't have to give in April.

Before you build: an irrigation checklist

• Daily water need per zone, in litres, at the height of the dry season
• The well's tested sustainable yield, and how much is left after the house
• Separate storage and pump for irrigation on anything bigger than a small garden
• Zones sized to the irrigation pump, with a controller and night-time schedule
• A filter at the manifold, plus a plan for iron if the water test shows it
• Priority control so the house tank always fills first
• A storage plan for the whole villa, including irrigation volume

Need to size the complete water system rather than individual components? Send us the property details: garden area and planting, number of bedrooms, pool, and how the villa is used. We'll design the well, storage, pumps and irrigation as one villa water system.