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250
200
Rainfall/ETp (mm)
150
100
50
0
Figure 6. Mean monthly rainfall and potential
evaporation (ETp) (FAO)
Probability (%)
Rainfall (days/month)
Figure 7. Probability of different monthly rainfall
amounts in Anantapur (CSWCRTI)
8
6
4
2
0
100
80
60
40
20
0
-20
Jan Feb Mar Apr May Jun Jul Aug Se p Oct Nov Dec
Ana ntapur Kurnool ETp
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
20 mm 40 mm 60 mm 100 mm
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 8. Mean number of rain days for Anantapur
Figure 6 compares the mean
monthly rainfall at Kurnool and
Anantapur with potential
evaporation 3 . It can be seen that
Anantapur’s mean monthly rainfall
is below 100 mm for every month
except September. This figure also
gives an indication of the relatively
higher rainfall at Kurnool during
the early south-west monsoon and
relatively higher rainfall in
Anantapur during the north-east
monsoon. Mean monthly rainfall is
less than potential evaporation for
both locations for every month
except September.
Figure 7 shows the probability
of monthly rainfall exceeding 20,
40, 60 or 100 mm at Anantapur. It
can be seen that the highest
probability of monthly rain
exceeding 60 mm is during
September-October. Hence, this is
the period during which large
volumes of runoff are most likely to
be generated.
Figure 8 presents the mean
number of rain days per month for
Anantapur. As the number of rain
days is not well correlated with the
rainfall, this also indicates that
rainfall events tend to be smaller
during the early and late parts of
the south-west and north-east
monsoons respectively.
3 Potential evaporation can be defined
as the rate of evaporation from an
extensive surface of 8 – 15 cm tall, green
grass cover of uniform height, actively
growing, completely shading the ground
and not short of water. The ratio
between rainfall and potential
evaporation provides a good indication
of the aridity of an area.
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