Annual rainfall decreasing or increases depends on location to location, for instance, the northern part of rainfall is falling, but the southern and eastern parts are growing. But dry period rainfall is decreasing especially in the southern part. I suggested you collect BMD data (https://live.bmd.gov.bd/#https://live.bmd.gov.bd/bn/ ) for the last 20-30 yrs for different locations in Bangladesh. It will give you an idea of what was going on.

Salam, Brother.

Many thanks for your answer.

If you allow me to ask a question, if we can calculate the annual rainfall of a location, can we not calculate the rainfall of Bangladesh?

Salam.

Analysis of long-term rainfall trends in Bangladesh

by Abu Raihan Muhammed Khalid

A section in Review of Shamsudduha, M. etal.(2022) The Bengal Water Machine: Quantified freshwater capture in Bangladesh - Science 377, 1315–1319 (2022) 16 September 2022

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Revelle, R.; Lakshminarayana, V. stated: “(T)he average rainfall in Bangladesh is higher, and the potential for increasing groundwater recharge from rain is greater, than in the Indian part of the Ganges Plain”[1].

That potential needs to be revisited because recent studies found a significant and continuous decrease in the annual rainfall in Bangladesh. In a most recent study Getirana, A. etal. stated “(S)atellite data show that annual rainfall rates in Bangladesh have fallen by about 10% over the past two decades, or by 10 millimetres per year”[2]. Another study found the decrease to be “-3.97 mm /year during the time period 1981-2019”[3].

The authors Revelle, R.; Lakshminarayana, V. were correct in one aspect when they stated “Unfortunately, there is a wide variation among different districts”. It still is so according to the Meteorological Report prepared by the Bangladesh Meteorological Department (BMD):

“The amounts of annual rainfall increased during 1981-2010 over extreme southeastern part, Hatiya, Sandwip, Rangpur and Jessore regions. But the amounts of rainfall decreased over Rajshahi division and the regions of Faridpur, Dhaka, Sylhet, Kushtia and Barisal. Substantial increments are found at Hatiya (+9%), Teknaf (+8%) and Jessore (+7%) but the considerable declines are found at Ishurdi (-9%), Barisal (-6%), Rajshahi and Faridpur (-5% each)”[4].

Unfortunately, the BMD Report does not state the trend in the annual rainfall in entire Bangladesh, but a recent study by Rahman, M.M. etal. Found that “all divisional (eight) rainfalls have shown a decreasing trend in annual rainfall with insignificant exceptions in Dhaka and Rajshahi divisions, where the decreasing trend is statistically significant”[5]; another study conducted by Rahman, M.M.; Abdullah, S.A. published in 2023 found a “falling trend in annual rainfall” in most stations out of 34 during the period of 40 years between 1981 and 2020[6]; Noorunnahar, M.; Hossain, M. A.(2023) found a downward trend of annual total rainfall with 5% level of significance[7].

The World Bank Climate Change Knowledge Portal page for Bangladesh published the following ‘Observed Seasonal Precipitation’ data: in the period of 1901-1930 the total annual rainfall in Bangladesh was 2,420.08 mm; in 1931-1960 it was 2,413.93 mm; in 1961-1990 = 2,317.04 mm; and in 1991-2020 = 2,160.14 mm.[8][9] It shows that the annual rainfall of the year 2020 AD has decreased by 259.94 mm from the rainfall of the year 1901 AD; it is a 10.74% decrease[10].

It must be stated here that we have also come across research (conducted in 2016) that concluded that the annual rainfall in Bangladesh has increased “at a rate of +5.53 mm/year during the time period 1958-2007”[11], and the same author in a 2009 study found that “(T)he magnitude of change of annual rainfall computed by Sen’s slope method shows an increase of annual rainfall with a rate of +4.94 mm year-1. However, the Mann-Kendall test shows that the trend is not statistically significant” during the period of 1969 to 2003[12]. However, we must also add that we could not find corroborations of this conclusion in recent studies.

Based on the studies stated above, it is our understanding that the annual rainfall in Bangladesh is decreasing; and is significantly decreasing in the central and northern areas, the main areas for production of rice[13].

References:

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[1] Revelle, R.; Lakshminarayana, V. (1975), ibid.

[2] Getirana, A. etal.(2022), ibid.

[3] Bosu, H. etal. (2020), Trends of Rainfall and Temperature in Bangladesh- A Comparative Analysis of CMIP5 Results and Meteorological Station Data, The Dhaka University Journal of Earth and Environmental Sciences, Vol. 9 (2): 2020, available online at: https://www.banglajol.info/index.php/DUJEES/article/view/55085, accessed last on 17.04.2023.

[4] Met Report, no. 08/2016, ISSN 2387-4201, Climate, prepared by Bangladesh Meteorological Department, 2016, Paragraph 6.2.1, Rainfall, at page 53; available online at: http://live4.bmd.gov.bd/file/2016/08/17/pdf/21827.pdf , accessed last on 16.04.2023. Similarly, Rahman, M.N.; Azim, S.A.(2021), Spatiotemporal evaluation of rainfall trend during 1979-2019 in seven climatic zones of Bangladesh, Geology, Ecology, and Landscapes, https://doi.org/10.1080/24749508.2021.2022425, concluded a “descending annual trend” of 6 out of the 7 climatic zones of Bangladesh.

[5] Rahman, M.M. etal. (2022), Analysis of Long-term Rainfall Trends in Bangladesh, Preprint Version 2 posted 15 Nov, 2022 at ResearchSquare, https://doi.org/10.21203/rs.3.rs-564948/v2, accessed last on 17.04.2023.

[6] Rahman, M.M.; Abdullah, S.A. (2023), Long-Term Trend and Variability of Rainfall in Bangladesh, J Math Techniques Comput Math, 2(1), 18-29, ISSN: 2834-7706, available online at https://www.opastpublishers.com/open-access-articles/longterm-trend-and-variability-of-rainfall-in-bangladesh.pdf, accessed last on 25.04.2023.

[7]Noorunnahar, M.; Hossain, M. A.(2023), Trend Analysis of Rainfall Data in Divisional Meteorological Stations of Bangladesh, Ann. Bangladesh Agric. (2019) 23 (1): 49-61, ISSN 1025-482X (Print) 2521-5477 (Online), available at https://bsmrau.edu.bd/aba/wp-content/uploads/sites/320/2019/09/article-5.pdf, accessed 25.04.2023.

[8] The World Bank Group Climate Change Knowledge Portal page for Bangladesh, https://climateknowledgeportal.worldbank.org/country/bangladesh/climate-data-historical#:~:text=Bangladesh%20is%20a%20very%20wet,mm%20of%20rainfall%20per%20year., accessed last on 16.04.2023. World bank presented the amount of annual rainfall divided in four seasons, DJF - December, January and February, MAM - March, April and May, JJA - June, July and August, SON - September, October and November. We have added the four figures and calculated the annual total. We have also calculated the variation in annual rainfall amount between 1901 AD and 2001 AD and found a decrease of the annual rainfall.

[9]The unit of precipitation is linear depth, usually in millimetres (volume/area), or kg m–2 (mass/area) for liquid precipitation. For detailed information regarding methods of measuring precipitation please see World Meteorological Organization (WMO), Guide to Instruments and Methods of Observation (WMO-No. 8 ), Volume I –Measurement of Meteorological Variables, 2021 edition, Chapter 6: Measurement of Precipitation, available online at https://library.wmo.int/doc_num.php?explnum_id=11386#page=245, accessed last on 30.04.2023. 1 mm rainfall means every one square meter area is filled with the water of height 1 mm. 1 square meter = 1 m (1000 mm) length × 1 m (1000 mm) breath. Therefore, 1 mm rain means 1000 mm length × 1000 mm breath × 1 mm height, please see FAO (1985), Irrigation Water Management: Training Manual No. 1 - Introduction to Irrigation, Chapter 4- Rainfall and Evapotranspiration, available online at: https://www.fao.org/3/r4082e/r4082e05.htm, accessed last on 30.04.2023.

[10] Calculation of the trend is conducted by this author.

[11] Shahid, S., (2012), Rainfall Variability and Changes in Bangladesh during the Last Fifty Years, in Martin, O.E.; Roberts, T.M. (Eds.), Rainfall: Behavior, Forecasting and Distribution, Nova Science Pub Inc., 2013, ISBN-10 ‏ : ‎ 1620815516. The Chapter is available online at: https://www.researchgate.net/publication/286565576_Rainfall_variability_and_changes_in_Bangladesh_during_the_last_fifty_years#:~:text=The%20magnitude%20of%20change%20of,1%20in%20last%20fifty%20years., accessed last on 12.04.2023

[12] Shahid, S; Khairulmaini, O.S. (2009), Spatio-Temporal variability of Rainfall over Bangladesh during the time period of 1969-2003, Asia-Pacific Journal of Atmospheric Sciences 45 (3): 375-389, Pre-print version, collected from vhttps://www.researchgate.net/publication/228278691_Spatio-Temporal_Variability_of_Rainfall_Over_Bangladesh_During_the_Time_Period_1969-2003, accessed last on 25.04.2023.

[13]Mainuddin, M. etal.(2022), Long-term spatio-temporal variability and trends in rainfall and temperature extremes and their potential risk to rice production in Bangladesh, PLOS Clim 1(3): e0000009. https://doi.org/10.1371/journal.pclm.0000009, accessed 25.04.2023.