Sri Lanka Reservoir Transparency

For the COVERED subset (~25 strategic reservoirs published by MASL WMS + CEB/PUCSL), publication is consistent and complete: the Mahaweli Authority WMS releases a daily PDF covering the major Mahaweli cascade reservoirs (Victoria ~723 MCM, Randenigala ~860 MCM gross, Kotmale ~171 MCM, Rantambe ~7 MCM) in MCM, acre-feet and fill-%. PUCSL publishes daily 'Generation and Reservoirs Statistics' PDFs for the same hydropower reservoirs (Samanalawewa, Castlereagh, Maussakelle, Polpitiya cascade). The typical covered reservoir has volumetric storage, fill % and elevation reported daily. The principal limitation is PDF-only delivery; structured fields are not exposed.

For the COVERED subset, the dominant access mechanism is PDF only. No REST API, no JSON/CSV feed, and no machine-readable data format exists at MASL WMS, PUCSL or CEB. The Irrigation Department's reservoir status page blocks external requests (HTTP 403). The CRIP data portal returned HTTP 500. PUCSL's Electricity Dispatch Dashboard (gendata.pucsl.gov.lk) offers a web visualisation but no documented API or bulk download. All programmatic access requires PDF scraping.

Conservative capacity-weighted estimate applied 2026-05-29. Numerator/denominator: n_covered ≈ 3,500 / n_total ≈ 5,800 hm³ across ~50 large reservoirs >10 hm³ combining the Mahaweli hydropower cascade (Victoria, Randenigala, Kotmale, Rantambe), CEB Laxapana/Samanalawewa cascade, and Irrigation Department tanks including Senanayake Samudra (~950 hm³, the country's largest), Parakrama Samudra, and the dry-zone historical tanks. MASL + CEB cover the strategic hydropower reservoirs daily. Applying conservative downward adjustment (~-9 points) to recognise that the Irrigation Department's reservoir status page returns HTTP 403 to external requests — making Senanayake Samudra, Parakrama Samudra and most dry-zone irrigation tanks structurally invisible to public users — and that the long tail of smaller irrigation tanks managed by the Irrigation Department is the dominant numerical gap. coverage = round(100 × 3,500 / 5,800) = 60.

MEDIAN history for the COVERED subset is approximately 7–10 years of online-accessible daily data. PUCSL daily PDF archives are searchable back to roughly 2023 on the live site; the MASL WMS server retains a rolling window of monthly archive pages going back ~5–7 years. The Irrigation Department's Hydrological Annuals series (continuous since 1960, 63 volumes) is a strong paper record but is the MAX, not the median — most volumes are not online and the irrigation.gov.lk server blocks external access. MASL Statistical Handbooks (2020, 2022) provide retrospective multi-year tables but not continuous daily series. Scoring the typical covered reservoir: ~7–10 years of accessible daily series.

For the COVERED subset, cadence is DAILY. The MASL WMS publishes daily reservoir status PDFs (confirmed with May 2026 date stamps). PUCSL publishes daily 'Generation and Reservoirs Statistics' reports. Weekly meeting minutes are also available from MASL. The Irrigation Department hydromet network samples every 10 minutes internally, but that real-time feed is restricted. Publicly accessible daily PDF cadence for the covered hydropower cascade is genuinely daily, only constrained by PDF format.

For the COVERED subset, capacity figures are consistently documented in MASL Statistical Handbooks (Victoria 722–723 MCM, Randenigala ~585 MCM active / 860 MCM gross, Kotmale ~171 MCM, Samanalawewa 278 MCM). CEB/PUCSL engineering references provide stage-storage references for the major hydropower dams. Hydrological Annuals (since 1960) document gauging methodology. However, no dedicated open methodology document explains how live storage is calculated from observed water levels for each covered reservoir, and QC procedures are not publicly described. The WMS Seasonal Operating Plan (SOP PDF) on the MASL server is in binary-compressed format without human-readable methodology accessible through web fetch.

English is one of Sri Lanka's official languages and the primary language of government technical communication. All MASL WMS reports, PUCSL daily reports, Irrigation Department pages, and Hydrological Annuals are published in English. The MASL daily PDF uses English column headers (Storage, Spill, % Full). No Sinhala-only barrier exists for the published datasets. The PUCSL Electricity Dispatch Dashboard is also fully English-language.