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Probability of Wet and Dry Days in Mymensingh

M. M. Ali
Department of Mathematics, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh.

M.G. Hossain
Department of Physics, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh

M.A. Hye
Department of Mathematics, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh.

M.S.U. Talukder
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh.

Abstract/ Executive Summary:

Presence of rainfall or lack of it in a particular day or in a sequence of days in a year may be useful in obtaining economic gains or losses for agricultural and non-agricultural activities. A probabilistic method was used in this paper in order to find out the probability of each week to be dry or wet (days) in Mymensingh. Rainfall data of Mymensingh for 19 years (1975 to 1993) were analysed using Markov Chain Probability Model for predicting the probabilities of wet and dry days of each week in a year. This paper also presents a procedure for calculating probability of a particular day or sequences of wet and dry days for Mymensingh.

Keywords: Rainfall, Wet & Dry days, Transition probability, Markov Chain Model

Location:

Start Date:

End Date:

Program Area: Socio-economic and Policy

Non-commodity: Performance

Objectives:

In this paper a procedure for calculating initial and transition probabilities for a particular day or a sequence of days in Mymensingh is described and presented.

Methodology:

Daily rainfall data for 19 years (1975 to 1993) were collected from the Weather yard of Bangladesh Agricultural University, Mymensingh, Bangladesh and were used in this study. Mymensingh is situated 24°40'N latitude and 90056'E longitude and at an altitude of 14.52 meters above mean sea level (msl). This area represents humid and subtropical climate. Daily rainfall observations were transformed into weekly data (week = 1,2,3, 52; year = 1,2,3, 19). Since 19 years record were used, each week had 19 rainfall events. Analysis of data is based on the assumption that the sequences of wet and dry days in Mymensingh will be the same in the future as they were during the 19 years period from 1975 to 1993. The definition of a wet or a dry day varies from region to region. A wet day in the Mymensingh region was taken as the day having rainfall greater than 2.5 mm and a dry day as having no rainfall or a maximum of 2.5 mm rainfall. Two types of probabilities viz. initial probability and transition probability were used in the analysis. Transition probabilities were computed by considering whether the previous day is dry or wet. They were so named because they indicate the probabilities of changes in the weather from one day to the next. Initial probabilities were computed without reference to previous rainfall history. The ratios for initial and transition probabilities were generated by taking into consideration: i) The number of years the ith day was dry divided by the number of records. ii) The number of the years that the ith and (i-1 )th days were dry divided by the number of years that the (i-1 )th day was dry, where i = 1 ,2,3, .. ... 365.

Source of Information: Bangladesh J. Agril. Sci. 21(2): 207-216, July, 1994

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Rainfall in Mymensingh region varied from 1581.2 mm in 1992 to 3384.5 mm in 1991. The average rainfall was found to be 2492.32 rnrn. Rain occurs most frequently in monsoon season. The average monsoon rainfall in every year was found to be 66.76% .

Analysis shows that there is a high variation of the occurrence of a dry/wet days throughout the year. Result shows that from mid April to mid October there is a maximum probability of wet days. This period approximately coincides with the monsoon in this region.

Knowledge Management: Journal