How the dry-spill test works.
Everything here is built from public data and republished as public data. This page is the whole method, written plainly: what we read, how we decide a discharge happened with no rainfall to point to, and where the data can mislead.
What we measure
The record is a monitor activation, not a verified discharge. Water companies operate Event Duration Monitoring (EDM) on storm overflows and publish status in near real time. We cross that status against rainfall recorded at the nearest gauges. When an overflow is discharging and no rainfall has been recorded, we describe it as discharging outside the permit's stated basis — rainfall and snowmelt. We never assert why, and we never assert legality.
Sources & licences
The dry-spill definition
A discharge is classified as a dry spill only when rainfall at the assigned gauges stays below every one of these thresholds. Two of them look back well beyond the last day, because saturated ground keeps overflows running long after gauges flatten.
For reference, a discharge is treated as rainfall-associated when the trailing 6-hour total reaches 2 mm or the trailing 24-hour total reaches 6 mm. Historical instants are walked at 15-minute granularity.
The two guards, in plain English
The 7-day antecedent guard. A wet week leaves the ground saturated. Overflows can then run for days on genuine infiltration after the rain has stopped and the gauge reads zero. So a discharge shortly after a wet week is never called dry, even if the last 24 hours were dry.
The sustained-dry (drain-down) guard. Immediately after real rain, catchments drain down and overflows tail off. To avoid catching that tail, the dry condition must hold for the whole discharge, or for the trailing 12 hours, whichever is shorter, before we call a discharge dry.
When the test cannot run
A false dry-spill is the one mistake this product will not make. If the rainfall evidence is thin, the discharge is marked unverified rather than dry. The test does not run when:
- the nearest usable gauge is more than 10 km from the outfall;
- the freshest reading is older than 90 minutes (the gauge has gone quiet);
- no assigned gauge supplied at least 80% of its expected readings over the trailing 24 hours.
Known failure modes
- Monitors misfire. Companies note that even weed growth can trigger an activation. An activation is not by itself a verified discharge; where the feed provides a confidence caveat we surface it.
- Gauges are sparse. West of Exeter the gauge network thins, so some genuine discharges sit in the unverified bucket rather than being called either way.
- Emergency overflows. Permits separately allow emergency operation on pump or power failure, and the EDM feed does not distinguish those from storm activations. We acknowledge that defence before it is made: a dry classification describes the rainfall record, not the cause.
- Two methodologies. Audited annual returns count spills on a 12/24-hour basis; the live feed reports raw durations. We never mix the two in one figure without labelling which is which.
Correction policy
Classifications can change as late gauge data arrives. When that happens the event is reclassified and the change is recorded on its permalink — every per-event page carries its full reclassification history in public. We would rather show a correction than hide one.
Feed health
A source going quiet is a data point in itself. Last successful poll and the share of polls that succeeded over the trailing seven days, per source: