WED 2006

Rainfall

The country belongs to sub-tropical regions where monsoon weather prevails throughout the year. The average temperature ranges from 7.20C to 12.80C during winter and 23.9 to 31.10C during summer. The average annual rainfall varies from 1229 to 4338 mm (WARPO, 2000). The total precipitation in the dry regions is low but the rainfall often occurs in sudden heavy storms, which sometimes lead to flooding and soil erosion.

Rainfall is observed at 327 stations in the country, 31 by BMD and 296 by BWDB. The observation have been collected from both organizations and entered on the NWRD. Rainfall (R) is based on the monthly averages for a standard 30-year period. Dependable rainfall (DR) is the rainfall that is exceeded four years out of five for any month. Evapotranspiration (ETo) is the evapotranspiration from grass. Data for estimating ETo has been collected at 30 stations since about 1960 and entered on the NWRD.

The pattern of rainfall excess and deficit, the difference between rainfall or dependable rainfall (DR) and reference crop ETo (in mm) is tabulated by season and by the hydrological region. The annual dependable rainfall exceeds the reference ETo except in the NW and SW hydrological region. Rainfall (in mm) in Bangladesh by hydrological region is given in Table 1.

Hydrological Region	Rainfall	Dependable Rainfall (DR)	Evapotranspiration (ETo)
South East SE	2271	1746	1275
North West NW	1739	1332	1332
North Central NC	1956	1529	1316
Rivers & Estuary RE	2318	1810	1325
South West SW	1665	1259	1381
North East NE	3194	2595	1233
South Central SC	2307	1793	1287
Eastern Hills EH	2445	1733	1360
Bangladesh	2360	1725	1320
MPO 1991			1553

P = Rainfall; DR= Dependable Rainfall; ETo= Evavotranspiration
Source: WARPO, 2000

Table 2: Contribution Seasonal Balance of Rainfall and Potential Evapotranspiration (in mm)

Hydrological Region	Monsoon June-October					Dry Season (November -May)
Hydrological Region	R	DR	ETo	R- ETo	DR-		R	DR	ETo	R-ETo	DR-
					ETo						ETo
SE	1909	1161	551	1358	610		588	200	724	136	524
NW	1539	852	581	958	271		346	131	751	405	620
NC	1550	901	568	982	333		511	206	748	237	542
RE	1972	1169	564	1408	605		516	174	761	245	587
SW	1469	872	572	897	300		365	101	809	444	708
NE	2511	1535	534	1977	1001		896	417	699	197	282
SC	2016	1222	537	1479	685		485	144	750	265	606
EH	2241	1280	566	1675	714		512	148	794	282	646
Bangladesh	1901	1124	559	1342	865		527	140	759	227	564

P = Rainfall; DR= Dependable Rainfall; ETo= Evavotranspiration
Source: WARPO, 2000

During the dry season (of 7 months) in some regions the evapotranspiration exceeded the amount of rainfall by an amount of more than 0.5 times the rainfall. They are Northwest, Southwest, North Central and South Central Hydrological regions. So, for that duration within a year these regions may be considered as dry region. Seasonal variation of rainfall and Evapotranspiration contributes to the Rainfall/ ETo as is shown in Table 3.

Table 3: Contribution of Seasonal Variation of Rainfall and Evapotranspiration

Attempts were made to identify the aridity in Bangladesh using meteorological data. It was observed that Bangladesh does not fall within the annual aridity limit of 20 according to Martonne Indices. however, if the monthly data were used seasonal aridity could be observed. The seasonal aridity increases from 4 months (November-February) in the northeast/ southeast regions to 6 months (November-April) in the northwest region. WARPO advocates dry month periods of 7- month duration (November - May).

On the basis of annual rainfall frequency of rainfall (25mm, 50 mm), maximum, minimum and frequency of temperature (250C, 300C and 350C), clear days, cloudy sky and first available rain of the season, Bangladesh has been divided into 7 seven agro climatic zones. Out of these, the zones E, F and G possibly be considered as relatively dry zones. These would be part of the agro ecological zones of the Barind and Madhupur Tract and the Ganges Floodplains.

Occurrence of rainfall in the Barind area has been analysed to compare the amount and distribution during 1975-93 with those of the period 1902-74. The mean annual rainfall of the Barind Area has changed from 1374.6 mm in the past (1902-74) to 1491.1 mm in recent areas. There are incidences of no change and even a decrease in rainfall southwestern, southeastern and western parts. The Monsoon season (June- October) received 83.3% of the mean annual rainfall. The pre-monsoon season of March- May received 14.3% (213.1 mm) while the bone-dry season of November- February got only 2.4% (35.2 mm).

Although the mean annual rainfall in this area has gone up in recent years, the increase has almost entirely been occurred during the monsoon (1278.5 mm vs. 1389.4 mm). The rainfall in the dry months, when it is really needed, remained the same (101.2 mm in the recent years vs. 96.1 mm in the past).

According to the data registered by BWDB, the annual evaporation of the area ranges from 370 mm to 1120 mm. The ratio of annual rainfall to evaporation for this area is 1.33 but considering the dry months the ratio of rainfall to evaporation would be 0.18. According to the Length of Growing Period (LGP) concept advocated by FAO, areas with an LGP of less than 1 day are hyperarid (true deserts); less than 75 days arid, 75 to less than 120 days (dry) semiarid, 120 to less than 180 days (moist) semiarid. These areas together correspond closely to the areas denominated as Drylands. The length of the rainfed growing season ranges from 170-180 days in the west- central west to 280- 290 days in the extreme north east.

The humid climatic situation due to monsoon rainfall, regular flooding and 170-290 days of annual growth period are viewed as conditions where desertification is unlikely to occur where as advocated in favour of desertification using the term synonymously with land degradation. Considering the lower rainfall and its distribution areas lying west of the longitudinal line of 89.5 0 E might be described as dry regions of Bangladesh. A region that has annual average rainfall much lower than the national average of 2200 mm may be considered a vulnerable area for occurrence of drought and land degradation.

According to the criteria set by the CCD for defining a dry region the ratio of annual rainfall to potential evapotranspiration may be a maximum of 0.65. Taking into consideration of annual rainfall and corresponding evapotranspiration data no region within Bangladesh can be termed as part of dry region. But Bangladesh experiences long spell of dry months (7 months) and moderate to severe droughts are spread over a region of 5.46 M ha. Since Bangladesh has two distinct periods of Dry season and the Monsoon having duration of seven and five months respectively, seasonal influence may be given emphasis in outlining dry regions. The value of the ratio of annual rainfall to potential evapotranspiration (a maximum of 0.65) may be extrapolated for dry seasons and used to delineate a dry region. For a spell of 7-month dry season, the threshold may be approximated to 0.38. For example, though the R/ETo values in favour of the SW and SE hydrological regions for the whole year is much above the threshold value of 0.65, the same for the dry period (of 7 months) is in the range of 0.17-0.45. These two regions also have the least annual rainfall and falls within the moderate to severely affected area of the Drought Map of Bangladesh.

Considering the distribution of rainfall and evapotranspiration regimes and the drought condition in the country, it is proposed that the regions fulfilling the following conditions may comprise dry regions in Bangladesh. The conditions are:

(ii) dry season (November – May) Excess Evapotranspiration (ETo-R) should be more than 400 mm; and

With this assumption made and applied on the rainfall and evapotranspiration data available for the agro ecological zones of Bangladesh, the Northwest, Southwest and North central zones can be considered as dry region of the country. It may be seen that the drier agro climatic zones E, F, G falls into the dry regions as defined above. It is further proposed that the dry regions may be divided into two sub regions on the basis of severity of dryness as shown on Table 4.

The Rivers and Estuary hydrological region; the coastal region and the Sunderbans are considered as non-dry. The dry map of Bangladesh shows that the dry zones are extended over an area of 6.442 M ha. The extent of the dry zones are given in the Table 5.

Climate Change in Asia: Bangladesh Report, Regional Study on Global Environmental Issues, ADB