Free Online Water Balance Calculator: Catchment Recharge and Yield

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Recharge is what remains of rainfall after evapotranspiration, runoff, and storage change. Multiplied by area it becomes a volume, and the allocatable fraction turns that volume into a defensible pumping budget that leaves water for baseflow and ecosystems.

One equation for the whole catchment

Every drop of rain on a catchment has exactly four fates: it evaporates or transpires, runs off in streams, recharges the aquifer, or changes what is in storage. Write that as an equation and solve for the term you cannot measure, almost always recharge, and you have the water balance, the accounting exercise underneath every wellfield permit and water allocation dispute. The tool above does the arithmetic and the unit gymnastics: recharge in millimeters becomes cubic meters over your catchment, and an allocatable fraction converts that volume into a pumping budget in units a regulator recognizes.

Catchment cross section showing precipitation arrows in, evapotranspiration up, surface runoff along the surface, and recharge down to the aquifer, balancing in one equation
Everything that falls either evaporates, runs off, recharges, or changes storage; solve for the term you cannot measure.

This completes the Hydrogeology and Water Suite in the bar above: the pumping test measures the aquifer, the capture zone and Darcy tools describe the flow, and the water balance says how much of it anyone may sustainably take.

Using the balance honestly

  • Evapotranspiration is the biggest and least certain term; in most climates it consumes well over half of rainfall, and small percentage errors in ET are large percentage errors in recharge.
  • A negative recharge result means your terms do not close, usually because ET is overestimated or runoff includes baseflow that is itself discharged recharge.
  • The allocatable fraction is a policy choice, not physics: pumping the entire recharge dries streams and wetlands, and 30 to 50 percent is a common starting posture.
  • Recharge as a percentage of rainfall is the sanity check: single digits in arid climates, 10 to 30 percent in temperate ones.

What this tool is for

First-pass sustainable yield for a catchment, framing a water license application or objection, and teaching the accounting that hydrology is built on. Monthly balances, soil moisture bookkeeping, and climate scenarios refine the answer; the annual equation here is where every one of them starts.

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