How much rain is 'enough'? Inside the soil moisture deficit model
"It hasn't rained in a while" is doing a lot of work in most lawn-watering advice. A while compared to what? A dry week in a wet month isn't the same as a dry week in a genuine drought, and no fixed rainfall number tells the two apart.
Rainfall in, evaporation out
Turf Alert tracks a running soil moisture deficit (SMD): rainfall in, evapotranspiration out — the same water-balance accounting agronomists use, not a rain-gauge reading on its own. The watering trigger fires once that accumulated deficit crosses 60mm.
That number wasn't picked to sound reasonable — it was calibrated against the 2016-2025 backtest specifically to separate a genuinely wet year from a genuinely dry one. At 60mm, 2021 (one of the wettest UK summers on record) correctly shows zero drought stress the entire year. At the same threshold, 2022 and 2025 — both notably dry — show sustained stress for 150+ days. A single number, doing the job a "hasn't rained in X days" rule can't: telling a real drought from a normal dry spell.
Why one alert wasn't enough
The original version of this trigger fired once, on the day the deficit first crossed 60mm, and stayed quiet after that. A real case in 2026 showed why that's not enough on its own: the deficit climbed from 61mm to 275mm over three and a half months — a genuine, worsening drought, with a real hosepipe ban and visibly dying grass along the way — while the alert stream showed exactly one message, sent on day one.
Two fixes came out of that:
- Escalation: a fresh alert fires every time the deficit deepens by another full 60mm since the last one — "this got meaningfully worse," not just "still going."
- A plateau backstop: some dry spells stay painfully dry without the deficit climbing much further — a long plateau rather than a worsening trend, which the escalation rule alone wouldn't catch. Checked against the full 86-year backtest, that's a real, common pattern: 15 of 86 years (17%) have 60+ days of continuous stress that never cross a single escalation step, with two years (1942, 2023) running 129 continuous days. A reminder alert now fires every 42 days into a plateau episode — landing on 5-10 total alerts for a genuinely severe multi-month episode, clearly more than the original single alert, nowhere near daily-nag territory either.
What this means for your lawn
A deficit that's just crossed 60mm calls for deeper, less frequent watering — or, if you're comfortable with it, letting the lawn go dormant rather than fighting a losing battle against evaporation. A deficit that keeps escalating past that is a genuinely different situation from one that's merely crossed the line once and stayed there — worth knowing which one you're actually in, not just that "it's been dry."
Check your own region's current deficit on the live forecast, or sign up to get the entry alert and every real escalation after it, automatically.
The same soil-moisture accounting also drives how far the mow interval stretches under stress — see the science behind Turf Alert for how it's been checked against 86 years of UK drought history.