[const]

The [const] section defines named fixed values. A constant is a single number given a name, so a value that recurs across the model — a threshold, a factor, a water-year month — is written once and referenced by name wherever it is needed.

A constant may be used within any dynamic expression. That is the scope: const.* is an expression term, not a general text substitution, so it does not resolve in places that take a plain literal rather than an expression.

[const]
const.low_flow_threshold = 120
const.demand_factor = 1.15
const.wy_month = 7

The name includes the const. prefix, and that is exactly how you reference it — the key on the left of the = and the reference in an expression are the same string:

[node.pump]
type = unregulated_user
flow_threshold = const.low_flow_threshold
demand = const.demand_factor * data.raw_demand

Constants are resolved at load, wherever they sit in the file, so a [const] section may appear before or after the nodes that use it.

Optimisable

A constant is an optimisable parameter: give its address const.<name> a search range in an optimisation, and the optimiser tunes it like any node parameter. This makes [const] the natural home for free values you want to calibrate — a demand multiplier, a routing coefficient, a rule threshold.

Rules

• Each value must be a plain number — not an expression. Use a [var.*] block for a computed value.
• Names are bare lowercase identifiers (after the const. prefix).
• Constants are fixed for the whole run; they never vary with time.