The accounting identity
Change in storage = inputs − outputs
For a catchment, the terms may include precipitation, stream inflow and outflow, evaporation, transpiration, groundwater exchanges and abstraction. For a tank, the relevant terms are different. Choose the system first; do not borrow a catchment equation for a household storage vessel without adapting its terms.
Make the units agree
| Quantity | Conversion |
|---|---|
| 1 litre per second, continuously | 86.4 m³ per day |
| 1 millimetre over 1 m² | 1 litre |
| 1 millimetre over 1 km² | 1,000 m³ |
| 1 million m³ per year | 1 MCM/year |
These are dimensional conversions. A continuous-flow conversion does not imply a pump actually operates for 24 hours. Include its operating schedule, downtime and seasonal variation when estimating delivered volumes.
Keep a source ledger
For every term, record the value, unit, observation period, geographic coverage and source. A yearly abstraction estimate and a single discharge measurement are not equally informative. Keep missing measurements visible instead of turning them into zeros.
Separate gross supply from water delivered after physical losses. Also distinguish physical losses from non-revenue water: billing and metering issues can affect revenue without representing the same physical loss of water.
A worked learning example
Suppose a hypothetical system has 100,000 m³ of gross annual supply and loses 10% before delivery. Delivered supply is 90,000 m³. With demand of 95,000 m³ and no other terms, the remaining gap is 5,000 m³. This arithmetic says nothing about the quality or legal availability of the water, or when a shortage occurs.
A yearly surplus can coexist with a summer deficit. Move to a monthly or daily balance when timing matters. Include storage capacity and an operating rule rather than assuming that all wet-season water can be carried into the dry season.
Check the unexplained remainder
An unclosed balance may reflect missing fluxes, measurement error, inconsistent periods or a wrong boundary. Investigate it. Do not automatically label the remainder as groundwater recharge or leakage.
Use the annual supply calculator to explore a deliberately simplified service-system balance. For research context, see the SMART reports, which document more detailed catchment and groundwater modelling.
Read the evidence
Updated 2026-10-23. Historical findings retain the dates of their original publications. Editorial contact: [email protected].