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Water Budget Equation

The document discusses water budgets and hydrology. It defines key terms like catchment area and presents the water budget equation. It also provides two example problems applying the water budget equation to calculate changes in water storage in a lake and the amount of water not available for runoff from a catchment area.

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Lapiz, Nathalie Suzette
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171 views9 pages

Water Budget Equation

The document discusses water budgets and hydrology. It defines key terms like catchment area and presents the water budget equation. It also provides two example problems applying the water budget equation to calculate changes in water storage in a lake and the amount of water not available for runoff from a catchment area.

Uploaded by

Lapiz, Nathalie Suzette
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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HYDROLOGY

The Science of Water


WATER BUDGET
EQUATION
CATCHMENT AREA

Catchment Area (a.k.a. watershed, drainage area, or drainage basin)


-is the area of land draining into a stream or a water course at a given
location.

A catchment area is separated from its neighboring areas by a ridge


called divide.
Schematic Sketch of a Catchment of River A at Station M
WATER BUDGET EQUATION
Mass inflow – mass outflow = change in mass storage

Vi - Vo = ΔS

Where: Vi = inflow volume of water into the problem area during


a time period
Vo = outflow volume of water from the problem area
during a time period
ΔS = change in the storage of the water volume over and
under the given area during the given period
WATER BUDGET EQUATION
ΔS = Ss + Ssm + Sg

Where: Ss = surface water storage


Ssm = water in storage as soil moisture
Sg = water in storage as groundwater

P – R – G – E – T = ΔS

Where: P = Precipitation
R = surface runoff
G = net groundwater flow out of the catchment
E = evaporation
T = transpiration
ΔS =change in storage
Rainfall-Runoff Relationship
R = P– L

Where: R = runoff volume


P = precipitation
L = losses (water not available to runoff due to infiltration,
evaporation, transpiration, and surface storage)
Problem No. 1
A lake had a water surface elevation of 103.2m above datum at the
beginning of a certain month. In that month, the lake received an average
inflow of 6.0 m3/s from surface runoff sources. In the same period, the
outflow from the lake had an average value of 6.5 m3/s. Further, in that
month, the lake received a rainfall of 145mm and the evaporation from
the lake surface was estimated at 6.10cm. Write the water budget
equation for the lake and calculate the water surface elevation of the lake
at the end of the month. The average lake surface area can be taken as
5000 ha. Assume that there is no contribution to or from the groundwater
storage.
Problem No. 2
A small catchment of area 150 ha received a rainfall of 10.5cm in 90
minutes due to a storm. At the outlet of the catchment, the stream
draining the catchment was dry before the storm and experienced a
runoff lasting for 10 hours with an average discharge of 1.5m3/s. The
stream was again dry after the runoff event. (a) What is the amount of
water which was not available to runoff due to combined effect of
infiltration, evaporation, and transpiration? What is the ratio of runoff to
precipitation?

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