Owners ask how deep the well will be. The more useful question is how much water it will give, and how reliably it will give it in April. Depth is mostly a cost input. Yield is the thing the property actually lives on.
Yield is a rate, not a quantity
A well does not hold a fixed amount of water waiting to be used. It intercepts water moving through the rock, and yield describes how fast that water can be drawn without pulling the level down to the pump. It is a flow over time, which is why two wells with the same standing water level can support very different households.
It is also why hitting water at 45 metres means little on its own. Striking water and sustaining a useful flow from it are separate events.
Why the plot next door is not a forecast
In granite country water travels through fractures and weathered zones rather than through the rock itself, and those pathways are irregular. A borehole that intersects a well connected fracture network can outperform one fifty metres away that missed it, at the same depth, on the same hillside.
Nearby wells are still worth knowing about. They tell a surveyor which depths have been productive in the area and how the ground tends to behave. They just cannot promise the same result on your plot.
What test pumping measures
Once a well is drilled, cased and developed, it is pumped at a controlled rate for a sustained period while the water level is monitored. Three things come out of that.
- Yield: the flow the well holds without the level continuing to fall
- Drawdown: how far the level drops from rest while pumping at that rate
- Recovery: how quickly the level returns once the pump stops
Recovery is the one that gets skipped and later regretted. A well that draws down heavily and recovers slowly can look acceptable during a short test, then struggle under a villa's real pattern, where demand arrives in bursts around morning and evening rather than as a smooth line.
Season changes the number
A yield figure carries a date. Water levels on the islands move between the wet and dry parts of the year, so a well tested in November and the same well in April are not necessarily telling the same story. A test report should state when it was done, and a system designed close to a well's limit should be sized with the dry season in mind rather than with the day of the test.
From yield to pump
The pump is chosen to work inside the well's limits rather than at the edge of them. Three inputs decide it: the sustainable yield, the pumping level measured during the test, and the flow and pressure the property needs.
A pump that outmatches its well is the most common fault we get called to. It empties the bore faster than the formation refills it, runs dry, cycles on and off, and burns out well before its time. The remedy afterwards is usually a smaller pump reset at a properly calculated depth, which is a much cheaper conversation before installation than after a failure in high season.
Where storage does the work
A modest yield is not a problem in itself, provided storage sits between the well and the house. Tanks let the well produce steadily through the day while the property draws hard for a few hours. A villa topping up a pool, running showers and irrigating in the same evening is asking for a peak flow that very few wells could supply directly, and does not need to.
Sizing that storage is a demand calculation rather than a rule of thumb. It shifts with occupancy, garden area, and how easily a delivery truck can reach the property if the well is ever out of service.
When yield falls later
Yield is not fixed for life. A gradual decline usually points to the screen or the formation around it silting up, and sometimes to the pump rather than the well. A sudden drop is more often mechanical. Either way the diagnosis starts by comparing how the well behaves now against the figures it produced when it was new.
If a well is being planned, quoted, or has started behaving differently, the yield figures are usually where the answer sits. Deep Well Samui can test a new bore, review an existing one against its original numbers, and specify a pump and storage arrangement that matches what the ground actually gives.