Water Budget Calculator

Calculate the water balance for a watershed or region. Determine storage change from precipitation, evapotranspiration, runoff, and groundwater outflow.

Inputs

Precipitation

Evapotranspiration

Surface Runoff

Groundwater Outflow

Tab to move between fields

Results

Storage Change (ΔS)

150 mm

≈ 2 credit cards

Budget Status

Surplus

ET (% of Precip)55.6%

Runoff (% of Precip)22.2%

GW Outflow (% of Precip)5.6%

Storage (% of Precip)16.7%

Volume Change per km²150,000 m³

Aridity Index (P/ET)1.8

Runoff Ratio0.222

Precip Volume900,000

How to Use This Calculator

Enter Precipitation, Evapotranspiration, and Surface Runoff.
Set Groundwater Outflow.
Review Storage Change (ΔS) (mm) and Budget Status.
Use ET (% of Precip) and Runoff (% of Precip) to inform your decision.
Use the chart to visualize the results and explore different scenarios by adjusting inputs.

How the result changes with Precipitation

Precipitation	Storage Change (ΔS)	Budget Status
1,000	250 mm	Surplus
3,500	2,750 mm	Surplus
6,500	5,750 mm	Surplus
9,000	8,250 mm	Surplus

What each input means

Precipitation: Total precipitation input for the budget period in millimeters
Evapotranspiration: Total water lost to evaporation and plant transpiration in millimeters
Surface Runoff: Total surface water leaving the watershed boundary in millimeters
Groundwater Outflow: Net groundwater leaving the basin through subsurface flow in millimeters

How this is calculated

Formula

ΔS = P - ET - R - GW_out

Worked example, using the default values

Identify Input Parameters
4 parameters
Precipitation = 900, Evapotranspiration = 500, Surface Runoff = 200, Groundwater Outflow = 50 = 4 input(s) provided
Calculate Storage Change
Storage Change
150 = 150
Calculate Budget Status
Budget Status
Surplus = Surplus
Calculate ET
ET
55.6 = 55.6
Calculate Runoff
Runoff
22.2 = 22.2

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