WEEKLY RAINFALL ANALYSIS FOR CROP PLANNING IN JUNAGADH DISTRICT OF GUJARAT 3005
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Date
2019-10
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JAU, JUNAGADH
Abstract
The historical rainfall data for the period of 37 years (1981-2017) of Junagadh district
in Gujarat were analyzed for selection of most appropriate probability distribution of rainfall.
From the analysis, it was found that one single probability distribution has not been found
appropriate to represent all the data sets though Gamma distributions, Gumbel max.
distribution and Generalized extreme value distribution were found promising for most of the
data sets. The best-fit distribution has been employed for obtaining the assured quantum of
rainfall pertaining to 23-42 Standard Meteorological Weeks (SMW) at various probability
levels. The minimum assured rainfall of 20 mm and more are expected from SMW 27
onwards at 70% probability. This indicated that the sowing of kharif crops has to be done
during the 27 SMW for maximum utilization of rain water.
Weekly rainfall was analyzed using Markov chain model and initial and conditional
probabilities were estimated for 10 mm and 20 mm rainfall amount. The initial probability of
getting 10 mm rainfall during 24th to 38th SMW is more than 50% except 25th and 26th SMW.
Conditional probabilities of wet week preceded by another wet week of getting 10 mm
rainfall during 23th to 40th SMW were 40% and more. Initial probabilities of getting 20 mm
rainfall during 24th to 34th SMW are more than 40% except 26th SMW. Conditional
probability of wet preceded by another wet week of getting 20 mm rainfall during 24th to 39th
SMW are more than 50% except 25th, 34th, and 35th SMW. Consecutive dry and wet week
revealed that chances of occurrence of 10 mm and 20 mm, 2 consecutive dry and wet weeks
are 10.26-38.46% ,12.82-48.72% and 17.95% to 66.67%, 12.82-53.85% from 23 to 38 SMW.
3 consecutive dry and wet weeks are 1.28-17.09%, 5.92-36.01% and 7.69-49.46% and 4.72-
39.89% from 23 to 37 SMW. Onset and withdrawal of monsoon in most of the year is
observed in 23 SMW and 47 SMW respectively.
Weekly reference evapotranspiration values were estimated by the Penmen Monteith
method.Water balance study by Thornthwaite and Mather. Revealed that water deficit was
found to be 51.40 mm in driest year and maximum water surplus was 42.80 mm. Crop water
requirement of groundnut (bunch and spreading), cotton and wheat are 338.63 mm, 414.08
mm, 818.42 mm and 581.28 mm respectively. Based on the analysis, crop planning in
Junagadh district of Gujarat is suggested.