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Annual and Seasonal Trends of Precipitation in Moulvibazar District If Bangladesh from 1950 to 2014 Using Mann-kendall Test

M S A Talucder*
Climate-Smart Agriculture Lab, Department of Agroforestry & Environmental Science, Sylhet Agricultural University, Sylhet-3100, Bangladesh2Interdisciplinary Program in Agricultural & Forest Meteorology, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea

Abstract/ Executive Summary:

Investigating long term trend of precipitation is important for understanding the effect of climate change. The objective of this study was to document and examining the seasonal and annual trends of precipitation in Moulvibazar district of Bangladesh from daily precipitation data of Sreemangal Meteorological Station of Moulvibazar for a period of 65 years (1950-2014) using the non-parametric Mann-Kendall test. Annually, 65years average precipitation was 2420±390 mm whereas dry season (December-March), pre-monsoon (April-May), monsoon (June-September) and post-monsoon (October-November) seasons average were 131±89 mm, 639±223 mm, 1447±276 mm and 203±111 mm, respectively. Dry season showed the most inconsistency (coefficient of variation, CV 68%) while monsoon season showed the most consistency (CV 19%) among the seasons. The findings of the non-parametric Mann-Kendall test reveal that significant decreasing trend of rainfall was only found in monsoon season time series. No significant trend was found in either annually (decreasing), dry season (decreasing), pre-monsoon season (increasing) and post-monsoon season (decreasing).

Keywords: Precipitation, Trend analysis, Climate change.

Location: Sreemangal of Moulvibazer district

Start Date: 00-00-1950

End Date: 00-00-2014

Program Area: Crop-Soil-Water Management

Commodity: Precipitation

Objectives:

The present study focuses on the temporal (inter-seasonal and inter-annual) variability and trends of precipitation in more details in Moulvibazar district using long time series data sets covering 65 years (1950-2014). The analysis will improve the general understanding of the seasonal and annual precipitation variability and trends over Moulvibazar district of Bangladesh.

Methodology:

Study site: Sreemangal (24.30ºN,91.73ºE, 21.95 m above msl) of Moulvibazar district of Bangladesh under the North-East Hydrological Region of Bangladesh (WARPO, 2001) was selected for this study. This region is an interesting study area because of its diversified terrestrial and wetland ecosystems. Meteorological data collection: Daily precipitation (mm) of Sreemangal of Moulvibazer district for a period of 65 years (1950–2014) was collected from the Bangladesh Meteorological Department (BMD). BMD follows the World Meteorological Organization (WMO) regulations for quality control of data. From the meteorological point of view, there are four climatic seasons in Bangladesh. They are: Dry (December-March), Pre-monsoon (April-May), Monsoon (June-September) and Post-monsoon (October-November). It may be noted here that for the computation of seasonal values for the dry season, the data of December for one year has been used with the data of January, February and March of the following year to represent the dry season values of the following year.Statistical analysis: Seasonal and annual means, standard deviation and coefficient of variation were calculated from 1955 to 2014. The normal was defined as the average for the period from 1955to 2014in this article. Trends of annual and seasonal rainfall data were analyzed by Non-parametric Mann-Kendall (M-K) test (Mann, 1945; Kendall, 1975; Helsel and Hirsch, 2002) using the R statistical package. Before applying the Mann-Kendall test to the time series of annual and seasonal precipitation time series, presence of serial correlation was investigated by examining the autocorrelation (ACF) and partial autocorrelation (PACF) plots produced by R whether the annual precipitation levels are serially correlated. If most of the vertical spikes in the ACF and Partial ACF plots produced by R fall within the horizontal band defined by the blue dotted lines beyond which auto correlations and partial auto correlations would be deemed to be significant. In this study, ACF and PACF were found insignificant. Therefore, the Mann-Kendall test was applied as it is without corrects the P-value with block bootstrapping protocol. The presence of serial correlation among the time series can be investigated visually with the help of the ACF and PACF functions in R, which compute the autocorrelation and partial autocorrelation corresponding to the time series of annual and seasonal precipitation. The output of this test produced by R was reported the Kandall’s tau statistic and the 2-sided P-value for testing the hypotheses “Ho: no trend” versus “Ha: monotonic trend (upward or downward)”.

Source of Information: J. Sylhet Agril. Univ. 2(2):183-188, 2015 ISSN: 2308-1597

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Achievement/Findings:

The findings of the non-parametric Mann-Kendall test reveal that significant decreasing trend of rainfall was only found in monsoon season time series. No significant trend was found in either annually (decreasing), dry season (decreasing), pre-monsoon season (increasing) and post-monsoon season (decreasing).

Knowledge Management: Journal