Meaning and mechanism
Water budget is a type of balance, similar to an energy balance, that accounts for the water that remains within a defined hydrological boundary.
If the surface is covered, the opportunity for it to seep into the soil decreases, and the rate of leakage can increase. Even after the rain stops, water in the soil and groundwater continues to move.
Relationships and conditions
The experiment aligns the boundary of the domain with P=R+I+E+ΔS. Movement between storage areas is an internal exchange, so it is not counted as leakage outside the boundary or duplication.
The model must clearly define the flood area and time frame. The concentrated storage model does not fully reproduce the actual drainage and rainfall distribution, and therefore is not used for flood forecasting.
What to observe in the simulation
As the impervious area increases, the space for water to seep into the soil decreases and the amount of water flowing into rivers increases. Even when the rain stops, water moves from the reservoir.
It is a concentrated storage model of surface, soil, groundwater, and rivers calculated in mm of depth per uniform basin area. The soil volume is 70 mm and a virtual displacement coefficient is used. R is river discharge outside the basin, I is deep infiltration outside the model, E is evapotranspiration, and ΔS is the sum of the four reservoirs. Surface → soil infiltration is recorded separately as internal movement. We do not predict actual floods or flooding.
Key relationships and symbols
P = R + I + E + ΔS
| Symbol | Meaning | Unit |
|---|---|---|
| P, R, I, E | Precipitation, outflow, deep infiltration, and evapotranspiration | mm |
| ΔS | Change in the amount stored across all reservoirs | mm |
Worked example
What is the difference between I in water balance and surface → soil infiltration?
Since soil and groundwater are also included in the storage, if internal infiltration is added to external outflow, it is counted twice. R is the stream outlet discharge, and I is the water that passed the boundary below the model.
Common misconceptions
- The water that penetrates into the soil does not immediately disappear entirely from the drainage area.