1
Hydrologic Equation or
WaterBalance Equation
Conservation of Mass
Amount In – Amount Out = Change in Storage
I – Q =
S
Water Balance Equation can be
applied to:
• Conservative properties: mass, energy,
momentum
A defined time period: seconds minutes
A defined time period:
seconds, minutes,
day, year, decade
• A defined control volume:
pore space to an
entire watershed
Assumption of WaterBalance
Equation
Mass density of water is conserved.
In most hydrological situations this is valid.
Situations where this assumption is not valid:
1) Lakes in cold regions that experience freezethaw;
2) Deep regional groundwater flow systems where
geothermal heat flow can have an effect on the
flow; and
3) Waters with high salt content or sediment loads.
I – Q =
S
The parameters of the above equation often have
units of length:
in, cm, ft, m
Or volume:
ft
3
, m
3
, cm
3
, gallons, liters, acreft
To obtain volume units, need to multiply the length
term by the crosssectional area of interest.
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 Spring '08
 STAFF
 Hydrogeology, Hydrology, inches, Miami

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