Water Budget / Water Balance
A water budget (also called hydrologic budget or water balance) is a method used to measure the amount of water entering, stored within, and leaving a drainage basin.
Inputs: Precipitation (rain, snow, hail, etc.)
Outputs: Runoff (river discharge) and evapotranspiration
Storage: Water held in soil or bedrock
It helps in water resource management, irrigation planning, and flood prediction. Water budgets vary spatially (between different geographic areas) and temporally (across seasons and time). It can be shown as an equation or a compound line graph.
General Water Balance Equation
P = R + E + ΔS
Where:
- P = Precipitation
- R = Runoff
- E = Evapotranspiration (potential)
- ΔS = Change in soil or groundwater storage
Components of Water Balance
-
Precipitation (P):
This is the main water supply, coming from rain, snow, hail, sleet, and sometimes dew or fog. -
Evapotranspiration (E):
Combined process of evaporation (water turning to vapor) and transpiration (water loss from plants).
- Actual Evapotranspiration (AE):
Water actually used by evaporation and plants under current conditions. It depends on water availability, temperature, and humidity. - Potential Evapotranspiration (PE):
The amount of water that would evaporate if unlimited water was available. It shows water need rather than actual use. Solar radiation strongly influences PE because it drives energy for evaporation.
- Soil Moisture Storage (ST):
Water stored in the soil at any given time.
- Field Capacity: Maximum water soil can hold.
- Fine-grained soils (clay) hold more water than coarse-grained (sandy) soils.
- Change in Soil Moisture Storage (ΔST):
The difference between current storage and the soil’s field capacity. It can range from 0 (completely dry) to maximum storage.
Positive and Negative Water Balance
-
Positive Water Balance (Surplus):
Occurs when precipitation exceeds potential evapotranspiration and the soil is at field capacity.
Extra water runs off the surface and increases stream discharge. This can lead to flooding. -
Negative Water Balance (Deficit):
Occurs when water demand (PE) is greater than available water and the soil has no moisture left (ST = 0).
Deficits usually require irrigation to meet water needs.
Key Points
- Surplus (S): P > PE and soil saturated → runoff and flooding risk.
- Deficit (D): PE > AE and soil dry → irrigation needed.
- Temporal Variation: Water balance changes by season (e.g., surplus in wet season, deficit in dry season).
- Spatial Variation: Different regions have different budgets due to climate and soil.
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