Why a deep well changes between Koh Samui's wet and dry seasons

A common call in March or April goes like this: the well has worked for years, nothing has changed, and suddenly the tank takes all day to fill and the pump keeps stopping. Usually nothing is broken. The well is doing what wells do in the dry season. Understanding why is the difference between fixing the real problem and spending money on a bigger pump or a deeper hole that makes things worse.

A well is not an underground tank

It's natural to imagine a well as a fixed volume of water waiting to be pumped out. In Samui's granite, groundwater actually sits in fractures and in the weathered rock above them, and moves slowly through them. The borehole is a window into that system. What you can take from it at any moment depends on how high the water stands around it, how fast water flows back in as you pump, how hard you pump, and how long the well gets to recover between runs. All four change with the seasons.

Rain, recharge and the delay between them

Samui's main rains come late in the year: roughly late October to mid-December, peaking around November. February to April is the driest stretch. Only part of that rain becomes groundwater. Some runs off, some evaporates, and the rest soaks slowly through soil and weathered rock before it reaches the fractures a well draws from. That journey takes time, so water levels don't follow the weather day by day. In many wells the level keeps falling for weeks after the rains end and reaches its lowest late in the dry stretch, then recovers once the next wet season has had time to soak in.

How strongly and how quickly a particular well responds depends on its ground. Shallow water in weathered material reacts relatively fast to rain; water deeper in the rock reacts more slowly. Fractured granite often holds less water than the sound of "deep well" suggests, so wells in it can fall noticeably when recharge stops. These are general patterns, not a forecast for any one bore. The only way to know how yours behaves is to measure it.

Static level, pumping level and recovery

Three numbers explain most seasonal behaviour. The static level is where water stands when the pump has been off for a while. The pumping level is where it settles while the pump runs; the gap between them is the drawdown. Recovery is how quickly the level climbs back once the pump stops. A yield test records these when the well is new (our guide to how much water a well produces explains the test), but that test describes one point in the year.

In the dry season the static level starts lower, the same pump pulls the level down further, and recovery is slower. A pump set comfortably below the wet-season pumping level can find the water near its intake by April. Nothing about the pump has changed. The water around it has.

The dry season is also the busy season

On Samui, low groundwater coincides with peak demand. High season fills villas with guests, the sun evaporates pools faster, and gardens need water every day. A system that was comfortable in the wet months can be marginal in the dry ones, and the margin disappears all at once. Irrigation is often the biggest single draw, which is why our guide to watering a garden from a well puts the garden on its own supply. A pool topped up from the well adds to the same peak.

Why the neighbour's well behaves differently

Two wells a short walk apart can respond to the same season very differently. They may cross different fractures, sit at different depths, draw at different rates, or have pumps set at different heights. A neighbour reporting no problems tells you little about your own well. Measurements from your own well tell you a lot.

Will a deeper pump or a deeper well fix it?

Lowering the pump buys margin only if there is usable water column below its current position and the pump still sits safely above the screen and the sump. It doesn't make more water flow into the well; it lets you draw the level down further, which can pull in sediment or, near the coast, salt water. Drilling deeper is not automatically better either. A deeper bore may cross another water-bearing fracture, or it may not. On coastal plots it can bring the well closer to the salt water beneath the fresh.

The cheaper fixes usually come first: match the pumping rate to what the well recovers, add storage so the well can pump gently for longer, and take discretionary demand off the well in the driest months.

Pump control that suits the season

• Tank level controls that run the well pump in steady cycles rather than chasing every tap.
• Dry-run protection that stops the pump before the level reaches the intake. See electrical protection for well pumps.
• A lower pumping rate over longer hours where the well recovers slowly, instead of short, hard runs.
• Rest periods built in so the level can recover between runs.
• Enough storage that the house rides through the slow refill, as covered in our guide to tank size.

Measure it: simple monitoring through the year

A dip meter reading once a month, taken after the pump has been off long enough for the level to settle, shows how the well moves through the year. Note how long the tank takes to fill and how long the pump runs each day. Compare December with April. The pattern tells you whether the well simply follows the seasons or is declining from one year to the next, which is a different problem covered in our guide to borehole maintenance.

Warning signs usually arrive weeks before the taps run dry: the tank takes longer to fill, the pump cycles more often, taps spit air, fine sediment appears after long runs, dry-run trips happen late in the day, and near the coast the water starts to taste flat or salty. Planning for the season itself, such as storage and delivery backup, is covered in preparing a villa for the dry season.

If your well gives less water in the dry season, we can check the well, the pump and the storage before anything is replaced. A well inspection measures the levels, the pumping rate and the recovery, and shows whether the answer is a different setting, more storage, or a change at the well itself.