United Kingdom Reservoir Transparency

For the COVERED subset (the ~40 strategic supply reservoirs in the DEFRA Water Resources Monitoring Summary plus equivalent SEPA, Welsh NRW and DfI bulletins), data is published primarily monthly as PDF bulletins with accompanying CSV/XLS summaries. The typical covered reservoir therefore sits in the monthly band (40-59). The EA Hydrology API itself explicitly excludes reservoir storage, so the daily-cadence federal API does not improve this dimension for the covered subset. Some water companies (United Utilities, Severn Trent, Thames Water) publish weekly dashboards for a subset of supply reservoirs, but this is not the dominant publication mode for the COVERED subset.

For the COVERED subset, the dominant access mechanism is structured PDFs and CSV downloads attached to the monthly DEFRA bulletin — machine-readable tables but not queryable. There is no documented REST API specifically returning per-reservoir storage time-series. The EA Hydrology API is well-documented (OGC and Linked Data endpoints) but covers river flow and groundwater only. Individual water company portals (e.g., United Utilities, Severn Trent) publish HTML dashboards without APIs. Scotland (SEPA) and Northern Ireland (DfI) publish their own structured bulletins. This combination of structured PDFs/CSVs plus some scrapeable HTML places the dominant channel in the 40-59 band.

v1.3.0 capacity-weighted methodology applied 2026-05-29. United Kingdom total national reservoir storage capacity ~1,800 hm³ across ~570 reservoirs in the EA Reservoirs Act register, dominated by Kielder (~200 hm³) plus the Pennine and Welsh supply reservoirs serving the major water companies. COVERED capacity ≈ 1,200 hm³ via DEFRA Water Resources Monitoring Summary (~30 strategic supply reservoirs in England including Kielder) + SEPA aggregates for Scottish Water (~5 strategic reservoirs) + Welsh NRW (~5 Welsh reservoirs) + DfI Rivers (~2 NI supply reservoirs) + United Utilities and Severn Trent weekly dashboards. coverage = round(100 × 1,200 / 1,800) = 67. The hydropower reservoirs operated by SSE, Scottish Power and Drax in Scotland and Wales remain the principal capacity gap.

Across the ~40 covered reservoirs the median accessible historical record exceeds 20 years. DEFRA's National Hydrological Monitoring Programme (CEH-operated) has published monthly reservoir storage reports since the early 1990s, with archived PDFs and aggregated CSV series available on GOV.UK back to approximately 1993 for the major English supply reservoirs. Welsh NRW and SEPA records similarly extend several decades for strategic supply reservoirs. Median sits firmly in the 20+ years band, placing this dimension at 100.

Monthly cadence is the dominant publication frequency for the COVERED subset. The DEFRA Water Resources Summary is released approximately 4 weeks after the reference date; equivalent SEPA, Welsh NRW and DfI bulletins also publish monthly. A handful of water company dashboards (United Utilities, Severn Trent) update weekly for a subset of reservoirs, but for the typical covered reservoir, monthly is the normal cadence. The EA Hydrology API updates daily for river flow but has no storage equivalent.

DEFRA and CEH publish detailed methodology notes for the National Hydrological Monitoring Programme, including reservoir groupings, capacity figures, aggregation methods and the long-term reference series for percentage-full calculations. Monthly reports include QA notes and explicit capacity references. The Reservoirs Act register, maintained by the Environment Agency, provides standardized capacity metadata. Water company-specific measurement methods vary and are not always standardised nationally, but the dominant federal channel (CEH/DEFRA) provides documented methodology placing this comfortably in the 60-99 'partially published' band.

All UK government data portals, DEFRA publications, EA APIs, water company dashboards, SEPA, Welsh NRW and DfI bulletins are in English. Documentation is clear and well-structured. The EA Hydrology API has thorough English documentation. GOV.UK presentation standards are high and the UI is modern.