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Rainfall Features and Effect of Extreme Events on Statistical Parameters of Rainfall at Mymensingh.

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

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

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

M.A. Hye
Department 0f Mathematics, Bangladesh Agricultural University, Mymensingh-2202, Banqladesh.

Abstract/ Executive Summary:

Rainfall is the most important natural hydrologic event. The rainfall data of Mymensingh for a period of 19 years starting from 1975 to 1993 were analysed. The results revealed that 21st week and 42nd week were the beginning and end of monsoon in this area in which the ratio of wet ard dry rainfafl weeks was 11: 1 5. Different statistical parameters such as means. rainfall percentage, standard deviation and coefficient of variations have been worked out for the weekly, monthly and yearly data. The extreme events in the rainfall have been found out and their effects on the statistical parameters have been discussed. The coefficient of variations varied inversely with an increase and directly with decrease in the monthly rainfall for the months having scanty rainfall. The results indicated the uniformity of distribution of rainfall in the region in which Mymensingh is situated.

Keywords: Statistical parameters, Rainfall, Extreme events

Location: Weatheryard of Bangladesh Agricultural University, Mymensingh

Start Date: 00-01-1975

End Date: 00-12-1993

Program Area: Crop-Soil-Water Management

Non-commodity: Rainfall

Objectives:

To interpret the annual, seasonal and monthly rainfall and prediction of amount of rainfall to be occurring over different intervals of time.

Methodology:

Daily rainfall data for 19 years (1975 to 1993) were obtained from the Weatheryard of Bangladesh Agricultural University, Mymensingh (24°46'N Lati- tude and 90056'E Longitude), Bangladesh. Rainfall data were transformed into two sets viz. the monthly data (month = 1,2, ... 12, year = 1,2 .... 19) and weekly data (week = 1,2, ... ,.52, year = 1,2, ... 19) for 19 years. Since 19 year record was used, each month and week had 19 rainfall events. Statistical parameters such as mean, standard deviation and coefficient of variations were calculated. The monthly rainfall event for which the residuals exceeds five times the probable error was declared as an extreme event (Mishra, 1987). This is because, the chance of occurrence of an event as great as five times the probable error of a single measurement is less than one in a thousand. Similar procedure was followed for Mymensingh with the yearly rainfall data.

Source of Information: Bangladesh J. Agril. Sci. 21(1): 157-165. January, 1994

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Maximum rainfall occurred in June and July at Mymensingh ranging from 457.78 to 501.37 mm. and minimum of 8.95 mm in January. Considering the rainy months (monsoon), the highest rainfall of 501.37mm occurred in July. The variability of rainfall as measured by the percent coefficient of variation (Cv) for different months varied from 32.71 to 195.02. However, the highest Cv was observed in November and the lowest in July. The rainfall of the wettest year 1991 was 136.15% of the average of 2485.5 mm white the driest year of 1992 was 63.61 %. The low value of 19.189% of the coefficient of variation produced an indication of the uniformity of the rainfall in the region in which Mymensingh is situated. The 21st week being the critical week for rainfall in the Mymensingh region. All hydraulic structures including irrigation and drainage channels must be repaired before that time so as to carry any excess water safely away from the fields. The design of our structures should not be based on the extreme events but on the consideration of the most probable events i.e. on the mean value of the probable events. Any design on extreme events will involve very high construction costs and the structures will be over designed. These extreme events are likely to occur say only once in 19 years or so.

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