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Analysis of Monthly Rainfall for Water Management Planning

M.N. Islam
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh-2202 Bangladesh

Abstract/ Executive Summary:

Monthly rainfall data of Dhaka for 35 years, Chittagong for 39 years and Sylhet for 34 years were used and analysed by different probability equations. The Weibull formula was found to give the best results. The square root plots provided better linear fit. Monthly rainfall at different probabilities were calculated. It was observed that probable maximum rainfall was obtained during July for Dhaka and Chittagong station and June for Sylhet station. Monthly maximum rainfall at 70. 75, 80, 85 and 90% probabilities were 275,259,2<10,213 and 182 mm for Dhaka station; 552, 484, 441, 380 and 324 mm for Chittagong station, and 686, 635, 576, 484 and 436 mm for Sylhet station, respectively. Excess and deficit rainfall periods at 70 and 80% probabilities were evaluated. For Dhaka, Chittagong and Sylhet station, the durations for storage and disposal of excess water were found from mid April to mid September, May to October and mid March to October, respectively. The remaining months were classified as deficit period. This information is extremely useful tor developing irrigation and drainage plans for an area.

Keywords: Monthly rainfall, Probability, Water management

Location: Dhaka, Chittagong and Sylhet Districts

Start Date:

End Date:

Program Area: Resource Development and Management

Commodity: Water management

Objectives:

To find out the best formula for plotting position
To predict monthly rainfall at different probabilities and
To find out excess and deficit moisture and its variation with time.

Methodology:

To carry out any probability analysis, past data for several years are necessary. For the present study, monthly rainfall data of the three greater districts of Dhaka, Chittagong and Sylhet were collected from the meteorological department, government of Bangladesh, Dhaka. Although data for several years were collected but 3-5 years data were missing for these stations. For Dhaka station, out of 40 years (1953-1992) 39 years data were used and for the Sylhet station, out of 37 years (1956-1992) 34 years data were used. Initially, an attempt was made to find out the best formula for plotting position. Analysis was then carried out using Weibull's method. Different steps involved in the analysis are: (a) tabulating the data according to the months of the year. (b) arranging the data in a descending order. (c) giving the data a serial number, m (i.e., m = 1 for the largest magnitude). (d) calculating plotting position, Fa' as: Fa =(m/N+1)x100. (e) calculating the plotting position for small or no rainfall, Ga, as: G, - lOOP + (l-P)Fa,(f) plotting the values of Fa or G, on the x-axis of an arithmetic probability paper and values of rainfall for different transformations (i.e., square root, cube root and logarithmic) on the y-axis. (g) determining the magnitude of rainfall for 70, 75, 80, 85 and 90 percent probability. (h) finally, plotting the probability for different months on a normal graph paper.

Source of Information: Bangladesh J. Agril. Sci. 24(2): 127-135, July, 1997 ,ISSN 0379-4296

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Probable maximum rainfall was obtained during July for Dhaka and Chittagong station and June for Sylhet station. Monthly maximum rainfall at 70. 75, 80, 85 and 90% probabilities were 275,259,2<10,213 and 182 mm for Dhaka station; 552, 484, 441, 380 and 324 mm for Chittagong station, and 686, 635, 576, 484 and 436 mm for Sylhet station, respectively. Excess and deficit rainfall periods at 70 and 80% probabilities were evaluated. For Dhaka, Chittagong and Sylhet station, the durations for storage and disposal of excess water were found from mid April to mid September, May to October and mid March to October, respectively. The remaining months were classified as deficit period. This information is extremely useful tor developing irrigation and drainage plans for an area.

Knowledge Management: Journal