August 10, 2026 · Updated September 12, 2026

A water tank pays off when you water beds that are dry

A rainwater tank earns its cost when you pour the water onto beds that are dry and leave alone the ones the last storm already soaked. Reading that takes two signals. Recent rain tells you whether the garden as a whole needs anything. Your soil tells you which beds are running low.

Why rain and drying pull apart

After a storm the soil holds water, then loses it three ways. Plants drink it, the sun evaporates it, and gravity drains it down and out. Each bed loses at its own rate. A shallow bed in full sun dries faster than a deep bed in part shade. One soaking rain leaves your beds at different moisture levels within a few days.

So "it rained on Tuesday" makes a weak watering rule. The rain reached every bed the same, while the drying ran at a different speed in each one. A tank of free water spent by habit soaks beds that do not need it and empties faster than it should.

What recent rain tells you

Rain guidance works at the garden level, one call for the whole space. A forecast of roughly 5 mm or more in the next day or two lets you hold off. If 5 mm or more already landed over the last three days, the soil stays covered and no watering is due. When neither holds, the garden wants water. That signal reads best off a station's own rain gauge, and it falls back to the forecast without one.

This is the right place to start with a tank. If the last three days brought real rain, the tank stays capped. You spend nothing on a garden the rain already watered. Once you decide to water, the best time to water your garden covers when to do it.

What the soil tells you, bed by bed

Once the garden as a whole needs water, a soil probe picks the beds. A probe assigned to a bed sends a dry warning once its last two readings both drop under that bed's cutoff. The cutoff starts at 25% by default, and you can override it per bed. Two low readings in a row keep a single dry spike from calling for water the bed does not need. Siltwell flags the bed that has gone dry by name. The hose stays in your hands, so you carry the tank water to the bed that asked for it.

Reading our own garden

Between mid-July and mid-September our probed beds spread out under the same weather. Raised bed 3 swung from 32% up to 62%. The ground bed also ran 32% to 62%, but hit its low on a different day. Raised bed 1 held a narrower range of 39% to 54%. Raised bed 2 ranged from 36% up to 57%. The same weather produced four different curves. Watering them all on one schedule would have soaked at least two beds that were already fine.

A rain gauge measures what fell on open ground. It cannot tell you what a leaf canopy caught before the water reached the root zone, or where your tank's overflow already ran. Two beds under the same 0.3 inch of rain can end the day at different depths of wet soil, so the probe reading decides the last step.

How to spend the tank

Run the tank as the reward for a garden that has gone dry. Save it for those days and skip the daily pour. Wait until recent rain clears the hold-and-covered check, then pour into the beds a probe reports low and pass over the rest. To set a sensible cutoff for each bed above the flat 25% default, work through a per-bed threshold from a season of readings. That is how a tank of caught rain lasts through a dry September instead of draining in the first week.