MSc

The study entitled “Rainfall Probability Analysis for Kerala”, was conducted to
estimate the probabilities and amounts of normal, excess and deficit rainfall during
southwest and northeast monsoon periods, to determine the conditional probabilities for
monsoon to be normal, excess and deficit when the monthly rainfall is normal, excess and
deficit and to determine the assured monthly and weekly rainfall in the five agroclimatic
zones of Kerala. Daily rainfall data from 1983 to 2019 collected from the five stations viz.
Vellayani, Kumarakom, Vellanikkara, Pilicode and Ambalavayal representing southern
zone, problem area zone, central zone, northern zone and high range zone respectively
formed the database for the study.
The daily rainfall data were converted to annual, seasonal, monthly and weekly
rainfall data series for further analysis. Significant increasing trend is observed in annual
rainfall at Vellayani. Non-significant increasing trend in annual rainfall is also observed
at Vellanikkara, Pilicode and Ambalavayal. But, Kumarakom exhibited a non-significant
negative trend for annual rainfall. Except the stations of Kumarakom and Vellanikkara,
other stations have non-significant increasing trend in southwest monsoon rainfall. Only
Vellayani has a non-significant increasing trend in northeast monsoon rainfall. All other
stations exhibit declining trend for northeast monsoon rainfall over the period 1983-2019.
The exploratory analysis showed that wide variation exists in rainfall over
different agroclimatic zones. The rainfall variability was visually made possible by the
use of box-and-whisker plots constructed for annual, seasonal, monthly and weekly
rainfall over five stations. The boxplots explained the average and dispersion of rainfall
data in terms of quartiles. In all the stations, annual, southwest monsoon and northeast
monsoon rainfall did not follow normal distribution which was evident from the uneven
boxes and dissimilar whisker lengths. A station-wise comparison showed that annual
rainfall as well as southwest monsoon rainfall increased from south (Vellayani) to north
(Pilicode) of Kerala and northeast monsoon rainfall declined from south to north. Outliers
representing extreme rainfall events were also detected using boxplots which made the
rainfall distribution skewed. At Ambalavayal, extreme annual rainfall (3093 mm) and
extreme southwest monsoon rainfall (2380 mm) happened in 2018 and at Pilicode,
extreme annual rainfall (4803 mm) occurred in 1994.The northeast monsoon witnessed
extreme rainfall in 1992 (764 mm) and 2010 (950 mm), at Vellanikkara. The analysis of
monthly rainfall using boxplots clearly depicted bimodal distribution at Vellayani and
unimodal distribution at Pilicode. Thus, it was appropriate to use median and Inter
Quartile Range (IQR) as the measures of central tendency and dispersion, rather than
mean and standard deviation, as former measures were not affected by extreme
observations.
Conditional probabilities were estimated to determine whether the seasonal
rainfall was excess, normal or deficient when monthly rainfall was excess, normal and
deficient. Excess, normal and deficient rainfalls were defined in terms of quartiles for the
purpose. At Vellayani, there was more than 50 per cent probability for the southwest
monsoon to be in excess if the rainfall in the month of June was excess. At Kumarakom,
there was a 60 per cent chance for receiving excess rainfall in southwest monsoon when
July and September had excess rainfall. Rainfall in June and July determined excess
rainfall in southwest monsoon at Vellanikkara with 70 per cent probability. There was
70 per cent chance to receive excess rainfall during southwest monsoon at Pilicode when
August or September received excess rainfall. Ambalavayal had experienced excess
rainfall with 80 per cent possibility when July had excess rainfall. Similarly, each station
had varying effects of rainfall in October and November towards northeast monsoon
season. This analysis can provide information about when to take precautionary measures
to anticipate the threat of floods and droughts in near future.
The assured amounts of rainfall varied during different months and weeks in each
station. Selection of crops (short duration varieties for low rainfall regions), cropping
pattern and other decision making strategies for harvesting rainfall or drainage
management can be made on the basis of assured rainfall in different months. At
Vellayani, 23rd (4th June to 10th June) and 24th (11th June to 17th June) standard
meteorological weeks (SMWs) are the rainiest weeks. The rainiest week is 28th SMW (9th
July to 15th July) at Kumarakom and Ambalavayal, whereas, it is 24th SMW (11th June to
17th June) at Vellanikkara and Pilicode. On farm operations and crop management plans
like decision of times of sowing, irrigation, fertilizer application, harvesting, etc., could
be executed on the basis of assured weekly rainfall during the different crop periods.