4.1.6—Water body steady state

For a defined water body and time period, storage changes according to its inputs, natural outputs and harvesting.

$ΔS = I − O − H; at steady state, ΔS = 0, so H = I − O$

ΔS is change in storage, I is total input, O is natural output and H is harvested water. All quantities must use the same volume-per-time unit and the same system boundary.

Example: a lake receives 180 million m³ yr⁻¹ and loses 150 million m³ yr⁻¹ naturally. At steady state, H = 180 − 150 = 30 million m³ yr⁻¹. This is an arithmetic maximum; a sustainable quota may be lower to protect ecological flows and allow for drought and uncertain recharge.

A balanced annual budget does not guarantee sustainability in every season: state the boundary, time period, units and ecological allowance.