<strong>Brahamaputra</strong> <strong>Basin</strong> 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 <strong>Basin</strong> 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 <strong>Basin</strong> 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 . 12
<strong>Brahamaputra</strong> <strong>Basin</strong> Rainfall in mm 1,200 - 1,400 1,400 - 1,600 1,600 - 2,000 2,000 - 2,500 2,500 - 3,000 3,000 - 4,000 4,000 - 5,000 5,000 - 6,000 > 6,000 13 Map 4. Annual average rainfall
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