cd-rainfall method a quick method to make a groundwater balance

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