Brahamaputra Basin

Brahamaputra Basin
Pynursla belt. This is generally of the order of 11,000 mm per annum, the highest in the world. The
clouds that pass over these 1,830 m high mountain ridges of Garo and Khasi hills of Meghalaya enter
into the Brahmaputra basin and widespread rainfall takes place in the valleyand the mountain
reaches of Bhutan and Arunachal Pradesh. The rainfall in the Brahmaputra valley ranges from 2,125
mm in Kamrup to about 4,142 mm in Tirap Division of the Arunachal. The Lanka area in Nagaon
district on the north of the Khasi and Jaintia hillis lies in the rain shadow region and in consequence,
the annual rainfall there is only about 1,100 mm. The rainfall in the Teesta valley varies from 1,635
mm in West Dinajpur district to 3,945 mm in Jalpaiguri district. Intense rainfall activities triggered by
cloud burst occasionally cause devastating flash floods and landslides in the Brahmaputra valley
region.
A gradual increase of rainfall from the valley bottom towards the lower ranges followed by a
decrease towards the higher ranges is evident from the observed annual rainfall at Dibrugarh (285
cm) in the far eastern part of Assam valley, Pasighat (507 cm) in the foothills, and Tuting (274 cm)
further up in the Himalayas.
Map.4. is showing the annual average rainfall of the basin. It depicts that most of the hilly regions of
Brahmaputra basin experience more than 4000 mm rainfall. Whereas very few region received
rainfall less than 1200 mm. The rainfall distribution map is generated based on the gridded daily
Rainfall data of (0.5 x 0.5°) for 35 years (1971-2005). The average annual rainfall is 2371.21 mm. The
basin is not affected by drought (Source: Drought Prone Areas Program, DPAP, MoRD, 2002).
1.3.2 Temperature
Monthly average Temperature (36 Years, 1969-2004) is shown in the Figure 3. The minimum and
maximum temperature of the basin is 16.16°C and 38.79°C respectively during that period. During
the winter season in December to January; the minimum temperature over the Basin varies from
16.16°C to 17.56°C. In the rainy season, in the month of July, the mean temperature varies from
32.47°C to 36.17°C. Towards the end of the monsoon season, in the month of October, the
temperature over the basin varies from 29.23.0°C to 34.91°C. During the summer season in April and
May, the minimum temperature in the Basin varies from 25.67°C to 27.21°C.
The higher elevation zones of the Himalayan ranges (Arunachal and Sikkim) experience lower
temperatures than the other regions of the basin. These parts also experiences snowfall. The
western part of the Brahmaputra valley experience hot summer in April and May. During summer,
the average maximum temperature in this part of the basin remains above 35°C, whereas the
maximum temperature is 40°C. In the south of the Brahmaputra valley the Southern (Assam)
mountainous region, temperature drops down below the freezing point in some days of December
and January in high altitude places. Most of these places above 1,500 m experience ground frost in
peak winter month but doesnot experience any snowfall.
Rainfall (1971-2004) and Temperature (1969-2004) profile in the basin is tabulated in Annexure II:A
& Annexure II:B .
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