Inputs minus outputs change storage. A river can continue flowing by drawing down water stored earlier.
A drainage basin is the area feeding a river and its tributaries. Its topographic divide separates surface runoff from neighbouring basins. Water flows downstream from the source towards the mouth. Groundwater boundaries can differ from surface divides, so real water budgets may include underground transfers.
Precipitation supplies water. Snowmelt can supply liquid water to high-altitude headwaters connected to tropical rivers. If snow is already counted as storage within the chosen system, melting is an internal transfer, not new precipitation; do not count the same water twice.
The main outputs are evapotranspiration and river discharge at the outlet. Potential evapotranspiration (PET) is the loss that could occur with sufficient water under the specified atmospheric and surface conditions. Actual evapotranspiration (AET) is what really occurs. During a dry season, AET can fall below PET because soil water is scarce.
For a simple basin with no other transfers: precipitation = AET + outlet runoff + change in storage. Use the same period and compatible units, often mm over the basin. A negative storage change means depletion. Discharge is a rate, commonly cubic metres per second; convert it to a volume or basin depth before placing it in a monthly budget.
Rainfall timing, heat, vegetation and geology make budgets vary. Wet-season rain can refill soil and groundwater before runoff rises. In a dry season, stored groundwater sustains baseflow while evapotranspiration draws down soil moisture. Humid does not mean water supply is uniform across all months or basins.
Step by step
Set the boundary
Specify the basin, time period and units.
Account for transfers
Separate precipitation and outputs from internal movements such as snowmelt.
Calculate storage change
Subtract AET and outlet runoff from precipitation in the simplified budget.
Worked example: An illustrative monthly budget
A basin receives 120 mm of rain, loses 70 mm through AET and exports 80 mm as runoff. Storage change is 120 - 70 - 80 = -30 mm. Earlier stored water supplies the extra output; there is no breach of water conservation.
Watch out for this
Whenever PET is high, actual evapotranspiration must be equally high.
PET describes demand under sufficient water. AET is limited when vegetation and soil cannot supply that water.
Check your understanding
For the stated simple budget, rainfall is 50 mm, AET 40 mm and runoff 35 mm. What happens to storage?
- It rises by 25 mm.
- It cannot be calculated because PET is missing.
- It falls by 25 mm.