Not rated — no significant reservoirs
Tonga is set aside from the ranking rather than graded. Having no significant reservoir storage is a geographic fact, not a transparency failure — so assigning an “F” would be misleading.
H1 2026 Evaluation
Tonga Reservoir Transparency
N/AWhy it's not rated
No surface reservoirs: relies on a groundwater lens, with no rivers.
Where its water comes from
Groundwater wellfields beneath Tongatapu managed by the Tonga Water Board.
Reference source
Tonga Water Board (TWB)
https://twb.to/Evaluator notes
Tonga is a structural near-zero for surface water reservoirs. The archipelago's predominantly flat coral limestone geology — with the exception of 'Eua Island's forested highlands — does not support river impoundment. Water supply is entirely groundwater-based: a freshwater lens beneath Tongatapu that is increasingly vulnerable to saltwater intrusion and over-extraction. TWB has undergone significant modernisation with ADB support, rehabilitating 17 wellfields, constructing 12 new ones, and deploying smart sensor infrastructure through Direct Control NZ that enables remote monitoring of pump flows and groundwater levels. A 4,000 m³ distribution reservoir was also built for the Nuku'alofa system. A TWB investigation found that approximately 50% of extracted water was unaccounted for, highlighting a major non-revenue water problem now being addressed through metering. Despite this internal data infrastructure, none of Tonga's operational water monitoring data is publicly accessible: no portal, no API, no regular bulletin. The RTI score reflects the compounded absence of both surface reservoir infrastructure and public data disclosure. Tonga's smart monitoring investment positions it well to improve its RTI score in future years if TWB opens its data to public access.
Evaluated by Jaime Delgado · 2026-09-15 · Methodology v1.3.0