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A study was undertaken to determine weekly index of moisture adequacy
(I MA) and water availabi I ity periods for various locations in the central zone of Kerala
and to examine the viability of important crops.
Daily rainfall data for the period 1978-97 were collected from 26 stations in the
central zone. Daily data on maximum and minimum temperature, wind speed,
maximum and minimum relative humidity and bright sunshine hours for Kochi,
Palakkad, Pattambi and Vellanikkara were collected. Soil data and crop data were also
collected for the zone. Mean weekly, monthly, seasonal & annual rainfall and
coefficient of variation for all stations were worked out. Spatial variation of mean
rainfall and coefficient of variation during the different seasons were studied. Monthly
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dependable rainfall at 75 per cent and 90 per cent ' •• vere calculated using the ranking
method suggested by Frere and Papov, and its spatial variation over the zone was
studied. Initial and conditional probabilities of receiving 30 mm or more rainfall per
week following Markov chain model for all the rain gauge stations were worked out and
discussed. Water balance elements were computed for the 26 stations following book-
keeping method of Thornthwaite. PET was computed following the modified Penman
method as suggested by Doorenbos and Pruitt.
Annually, the actual evapotranspiration (AET) varied from <400 mm in the
eastern region of the zone to >700 mm in the southern part of the zone. The lowest AET
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of 311 mm was observed at Parambikulam and a highest of 779 mm at Piravom. A
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gradual increase of AET can be observed from northern part to southern part. The
midland and of the central part of the zone had higher values of water deficit (WD)


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compared to the northern and southern parts i.e., more than 1300 mm. Parambikulam
had highest value for WD i.e., 1606 mm. Almost all the coastal region and midland of
the northern and southern parts were found to have lesser WD value « 1300 mm.)
Water surplus (WS) was high in the midland and coastal areas of the zone. The
highest value of WS, 913 mm was observed at Wadakkancherry followed by
Kunnamkulam (614 mm) both in the coastal region. Almost all stations in the coastal
region were found to have higher values ofWS, i.e., more than 400 mm except a small
patch around Kochi.
Water availability periods were identified comparing the AET and PET. It was
found that water availability periods followed the same pattern for most of the stations
in the central zone. It was characterised by a 1 st sub humid period followed by a humid
period and a 2nd subhumid period. The highest number of moist days was recorded at
Piravom (203) followed by Aluva and Perumbavoor(188). At this station the humid
period extended for 170 days which is also the highest value compared to all other
stations. The least number of moist days was observed at Parambikulam (82). At this
station there is a break in the humid period which gave way to a sub humid period.
Most of the stations have total number of moist days more than 150. Generally, the
water availability period starts in the 2nd week of June and extends up to 3rd week of
November.
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The cropping system of the central zone of Kerala was analysed and it was
found that the cropping patterns fall under three major categories (1) mainly paddy (2)
mainly coconut and (3) plantations other than coconut. The viability of these cropping
patterns based on water availability periods and soil information are discussed.