Every number in a water-supply plan is either measured or modelled. In 87.5 % of the state's watersheds it can only be modelled — and no portal on the market tells you which one you are reading.
Four people ask one question in four vocabularies — and none of their tools answers it.
Writing an Urban Water Management Plan or a §10910 Water Supply Assessment. Can I commit to this yield through a dry period?
Is there unappropriated water here — or is this reach closed under Order 98-08?
Where does the next monitoring dollar go?
Which watersheds am I making decisions about blind?
All four are asking the same thing: what is the evidentiary standing of the number I am about to rely on?
3,910 of 4,469 HUC12 subwatersheds
239,521 km of 262,259 km
1,196 of 2,597
2,078 USGS sites — California's knowledge of its own rivers is four-fifths somebody else's budget
Two independent routes, one answer. We joined the 2,597-site gage inventory to the 4,469 watersheds and got 87.5 % ungaged. The Water Board's own per-watershed column — huc12_prop_needgage, the share of each watershed's stream length that needs a gage — averages 0.868. Neither figure was derived from the other.
Somebody is taking water out of these watersheds. The licences are public. The measurement used to be.
≈ 15,000 diversions that were once inside a gaged watershed and are not now
across 557 watersheds
classed inactive gage — measured once, no longer
Two thirds of the gap is not absence of ambition. It is withdrawal.
SB 19 directed DWR and the State Water Board to prioritise the gaging gap. The 2022 plan tiered every watershed in the state and recommended reactivating 161 of roughly 900 dormant gages.
the highest-priority gaging needs the state identified for itself
78.6 % of the state's own priority list, four years on
and only 25–30 % of those are fully USGS-funded — the same story nationally
And a blank tier does not mean “fine”. 3,557 watersheds — 79.6 % of the state — carry no tier at all. That means not prioritised, not adequately gaged. Rendering a null as green inverts the finding.
Of the 4,170 segments California did assess, 2,235 — 53.6 % — are listed as impaired; of 905 lakes and reservoirs, 581 (64.2 %). Against a network of 1.6 million reaches, the assessed set is a rounding error. Any portal that paints “Not Listed” green is asserting something the record does not support.
The state publishes its own ignorance as a data layer. That is unusual, and it is the most useful thing in the whole catalogue: it is the denominator that makes every other water quality number readable.
Three books — promised, measured, assessed — published at three different grains, by two agencies, on partitions that do not nest.
Click any block and the whole app adopts that watershed — click it again and it lets go. In this basin, 25 of 43 subwatersheds — holding 51.6 % of the diversions — have no qualifying gage. Three are on the state's own Tier-1 list. Every reading in the rail is a filter you can switch on and off, and the layer drawer opens onto 264 catalogued California services, so the basin is a starting point, not a limit.
The literature says California's water rights total roughly five times its mean annual runoff. We cite that. We do not reproduce it — because the published record carries 39 fields on the diversion register and not one of them is a quantity of water. The volume is missing from the record, not just from us.
AQUARIUS and KISTERS sell the platform that manages gage data. Aquaveo, DHI and WEAP sell the model that fills the gap. Esri sells the platform and Arc Hydro the delineation.
We have no gage network, no model licence and no monitoring contract to defend. That is the only reason we can put the ungaged 87.5 % in the middle of the screen.
Runs on Strata or ArcGIS, on-prem, MIT. It reads the state's record; it does not become one.