CD-Rainfall method
A quick method to make a
groundwater balance
CD-Rainfall method
The Cumulative Departure-Rainfall (CD-Rainfall)
method is a method to rapidly understand
groundwater conditions.
It only requires:
Periodic rainfall data
Ground water level data
CD-Rainfall method
Above average rainfall results in an increase
of water tables.
Below average rainfall results in a decline
It uses the assumption that in normal conditions
groundwater levels quickly react to rainfall:
Recharge - Rainfall
1. Water level – Mean rainfall2. Water level – Above mean rainfall3. Water level – Below mean rainfall
In the CD rainfall you calculate:
Departure (difference) from normal rainfall for the
period
Then calculate the cumulative departure (=CD)
And compare this with groundwater levels
Take a close look at the next table >>
CALCULATION OF CUMULATIVE DEPARTURE RAINFALL (CD-RAINFALL)
MONTHS RAINFALL DEPARTURE CD RAINFALL WATER LEVEL
J 50 -76,5 923,5 2
F 90 -36,5 887 1,9
M 175 48,5 935,5 2,2
A 230 103,5 1039 2,4
M 310 183,5 1222,5 2,8
J 300 173,5 1396 3,3
J 229 98,5 1494,5 3,5
A 105 -21,5 1473 3,4
S 30 -96,5 1376,5 3,2
O 5 -121,5 1255 2,8
N 0 -126,5 1128,5 2,6
D 0 -126,5 1002 2,2
TOTAL RAINFALL 1524
AVERAGE MONTHLY RAINFALL 127 Differencewith monthly
average
Cumulativedeparture
Comparing the ‘cumulative rainfall
departure’ with groundwater levels
allows us to understand recharge and
overuse
‘No overuse’:
Water level follows CD-rainfall
0
100
200
300
400
J F M A M J J A S O N D
Ra
infa
ll
0
100
200
300
400
Av
era
ge
Ra
infa
ll
Rainfall
Average rainfall
CD rainfall - Cumulative departure from average rainfall
0500
100015002000
J F M A M J J A S O N D
cd
rain
fall
0
2
4
wat
er
leve
l
CD RAINFALL
WATER LEVEL
Rainfall – water level relation
‘No over-use’: Water level follows CD-rainfall
Another ‘normal’ situation
Overuse:
water level drops even though
CD rainfall goes up
Recharge and discharge to a river:
The relation between CD-rainfall and
water levels is not clear
Presentation based on Presentation based on
work of Wim Boehmerwork of Wim Boehmer
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