Barind Tract
Barind Tract largest Pleistocene physiographic unit of the bengal basin, covering an area of about 7,770 sq km. It has long been recognised as a unit of old alluvium, which differs from the surrounding floodplains. In Bangla, it is spelled and pronounced as Varendra Bhumi. Geographically this unit lies roughly between latitudes 24°20´N and 25°35´N and longitudes 88°20´E and 89°30´E. This physiographic unit is bounded by the karatoya to the east, the mahananda to the west, and the northern bank of the ganges to the South. A lower fault scarp marks the eastern edge of the Barind Tract, and the little jamuna, atrai and Lower Punarbhaba rivers occupy fault troughs. The western part of this unit has been tilted up; parts of the western edge are more than 15m higher than the rest of the tract and the adjoining Mahananda floodplain. The southern part of the main eastern block of the Barind Tract is tilted down towards the southwest and passes under lower Atrai basin sediments in the south. The Barind Tract covers most parts of the greater dinajpur, rangpur, pabna, rajshahi, bogra, joypurhat and naogaon districts of Rajshahi division.
The Barind Tract lies in the monsoon region of the summer dominant hemisphere. The tropic of cancer lies south of this region. The climate of the area is generally warm and humid. Based on rainfall, humidity, temperature and wind pressure the weather condition is classified into four types, such as, (a) pre-monsoon, (b) monsoon, (c) post-monsoon and (d) winter. Rainfall is comparatively little in this region, the average being about 1,971 mm. It mainly occurs during the monsoon. Rainfall varies from place to place as well as year to year. For instance, the rainfall recorded in 1981 was about 1,738 mm, but in 1992 it was about 798 mm only. This region has already been designated as drought prone. Its average temperature ranges from 35°C to 25°C in the hottest season and 9°C to 15°C in the coolest season. Generally this region is rather hot and is considered semi-arid. In summer, some of the hottest days experience a temperature of about 45°C or even more in Rajshahi area, particularly in lalpur upazila. In winter it falls to about 5°C in some places of Dinajpur and Rangpur districts. So this older alluvium region experiences extremes that are clearly in contrast to the climatic condition of the rest of the country.
Physiographically this region is divided into three units. These are Recent Alluvial Fan, Barind Pleistocene, and Recent Floodplain. These morphologic units are separated by long, narrow bands of recent alluvium. The floodplain of the Mahananda flanks the west side while the Karatoya delineates the eastern margin. The punarbhaba, Atrai and Old Jamuna with headwaters in the foothills of the himalayas have cut across the Pleistocene and their floodplains separate the units. These and numerous other streams are responsible for the development of a broad Piedmont alluvial plain, which delineates the northern flank of the Tract. The Tista alluvial fan is located to the north of the area. This fan surface of the Himalayan foothills has a slope of approximately 0.43m/km and it overlaps the Barind, which has essentially a flat or somewhat domed surface. South of the Barind Tract are the Recent Floodplains, with a southerly slope of about 0.06m/km.
In the Barind region, three distinct channel patterns are observed. In the north there is a great number of small braided streams, which have built a broad piedmont alluvial plain along the foothills of the Himalayas. The major rivers of these alluvial plains are the Atrai and the Punarbhaba, with entrenched valleys. On the Pleistocene unit, there are numerous small entrenched, tightly meandering streams, which have developed an overall dendritic pattern and flow into the major north-south rivers of the Barind unit. There are some major valleys that separate the Pleistocene unit into some north-south elongated units. These valleys are followed by some major rivers, such as the Mahananda in the west, the Karatoya in the east, the Atrai and the Punarbhaba in between. The largest unit is bounded by the river Punarbhaba and the Atrai. Another large unit is bounded by the Karatoya and the Little Jamuna. Many small channels, mostly of dendritic pattern, flow through the individual units.
However, it is very interesting that the course of the Karatoya is controlled by a southeast trending fault. In the late 17th century, the major stream of this region was tista which, near Jalpaiguri, branched and followed the course of the Depa-Punarbhaba to the west, the Atrai due south and the Little Jamuna to the southeast. All of these rivers discharged into the Ganges. Of these rivers, the Atrai seems to have been the major channel in the late 17th century with the Punarbhaba as the second most important. But now the Tista, Karatoya and Atrai flow into the jamuna (brahmaputra). The river courses, particularly of the Tista, changed suddenly in 1787 during a major flood. This change could have been accompanied by renewed uplift and tilting of the Barind surface. At the present time, the Punarbhaba, Atrai and Little Jamuna still carry some flood flow but they generally can be considered as antecedent streams carrying mainly local runoff water of the uplifted Barind area.
The Barind Tract, which is the largest pleistocene terrace of the country, is made up of the Pleistocene alluvium, also known as older alluvium. Tectonically, this region is situated in the precambrian indian platform, mostly in the saddle and shelf area of the shield. This platform region is covered mostly by Tertiary and Quaternary sediments and Recent Alluvium.
The Barind unit is comparatively at a higher elevation than the adjoining floodplains. The contours of the Tract suggest that there are two terrace levels - one at 40m and the other between 19.8 and 22.9m. Therefore, when the floodplains go under water during the monsoon the Barind Tract remains free from flooding and is drained by a few small streams. About 47% of the Barind region is classified as highland, about 41% as medium highland and the rest are lowland. Agricultural land commonly occupies about 80% of the hill slopes of Barind unit most of the year. As this region is generally free from floodwater, rainwater is the only major source of groundwater recharge. Once there were many isolated small depressions but those have since been converted into agricultural land. This landscape modification has affected the groundwater recharge and has increased dependence on rainwater. Again, the channel migration, mainly the shifting of the Tista and the Atrai and their distributaries over the last couple of centuries, has greatly influenced the climatic conditions of the area. Geomorphic modifications gradually turned this area into a hot region.
The Barind is floored by the characteristic Pleistocene sediments known as the Madhupur (Barind) Clay. The madhupur clay is reddish brown in colour, oxidised, sticky and rather compact. JP Morgan and WG McIntire assumed these deposits to be of fluvial origin. Actually they were deposited in the late Pleistocene time towards the end of the last glacial period. Nearly 18,000 years ago, the last glacial period reached the peak of activities. During that time the sea level dropped to 100m to 130m below the present-day level. Then the ice-melt water flowed into the bay of bengal. Right at that time the northeast monsoon climatic condition commenced in the subcontinent. Rainfall was not then dominating the Bengal plain. Bengal rivers only flowed with ice-melt water through their narrow and incised valleys. Later, 12,000 years ago, the southwest monsoon climate started dominating the region, and brought heavy rainfall over the Bengal Basin.
The monsoon rainfall was even more intense than at present. The narrow Bengal rivers could not discharge this huge quantity of water, supplied by huge rainfall as well as the ice-melt water from the Himalayan Range. The rivers spilled over the banks and deposited sediments over the large Bengal plain washing the sediments to the south where the sediments prevailed in the Barind Tract and madhupur tract and also in the lalmai area. Afterwards, climatic conditions started to change, making temperature rather hot, which readily influenced the Bengal river courses, the sea level started rising again and the depressed regions began getting more sediments but these Holocene (Recent time) sediments could not reach such a height that they would be deposited over the earlier Madhupur sediments. There is another school of thought that suggests a tectonic origin for the Pleistocene high terrace. This school says that the Pleistocene sediments were deposited in large depressions of the Barind and Madhupur regions. After the deposition due to the neotectonic movement, these regions uplifted and formed high terraces.
The Barind Tract is rich in mineral resources as it rests upon the Precambrian Indian Shield of the Bengal Basin. Of the mineral resources, coal, peat, hardrock, limestone, white clay and glass sand are very important. Actually these mineral deposits are found within the platform area below the Pleistocene rock units of the Barind Tract. High-grade bituminous coal deposits have been discovered in Bogra, Rajshahi, Rangpur and Dinajpur districts. These are found in small isolated basins, known as Grabens, located within the Precambrian basement below Pleistocene sediments of the Barind Tract. This coal belongs to the Gondwana formation of the Permian period and is of good quality. Limestone is also found in the shelf area of the platform located in the southern part of Barind Tract beneath the Pleistocene rock units. This limestone belongs to the Eocene period and is an important raw material for the manufacture of cement. Hardrock is another precious resource of the Barind region. Actually the whole platform area is composed of Precambrian igneous and metamorphic rocks. This hardrock is an essential building material commonly used for constructing roads, bridges and other structures. White clay and glass sand are generally found in the upper part of the basement rocks right below the Barind rock units. These minerals are widely used for the manufacture of ceramic wares, electric goods and many other industrial items.
Rapid population growth along with modifications of the landforms of the Barind Tract has been degrading the biophysical environment of this region. The climatic condition in this region has changed. There is very little rainfall and the weather remains hot by the daytime but becomes cooler by late night. Since rainwater is the main source of groundwater recharge in this area, the climatic change that disfavours abundant precipitation has adversely affected the groundwater recharge system. The withdrawal of more groundwater than its recharge causes the successive lowering of the groundwater table of the Barind region. This phenomena have eventually been greatly affecting the environmental parameters and if it persists the environment of the Barind Tract will become rather unfavourable for habitation in the near future. Besides lowering of the water table another noticeable change is the decrease in forest area. According to some reports from the British colonial times about 42% area of this Tract was covered by forests in early 19th century. Statistical reports of the land survey since 1849 showed that forests covered about 55% of the Barind lands. But by 1974, about 70% land of the region had been changed into cultivable land.
The Barind almost became an arid region due to massive deforestation. Also, due to its extreme dry nature and relatively low rainfall the vegetation cover decreased distinctively and the area could be picked up in satellite images as a hot and dryland. As the area was considered a low potential area for groundwater development, agriculture used to depend on monsoon rainwater. As a consequence, there used to be only one crop and the Tract was a food deficit area. With initiatives from local engineers, there have been new investigations for groundwater resources and it was found that there were good aquifers to be developed for large-scale irrigation.
A project named the Barind Integrated Area Development Project (BIADP) was initiated in mid-1980s to develop groundwater irrigation in the area. Under this project thousands of irrigation deep tubewells have been installed, which facilitated dry season irrigation for cultivation. As a result agricultural production has increased and the area has become a food surplus area. Apart from providing irrigation, there have been other programmes such as tree planting and excavation of ponds and khals to arrest the degradation of the environment. At the beginning, the project generated a lot of concern among environmental scientists about sustainability of the groundwater resources. However, 15 years of its operation has proved it to be a success. Apart from providing irrigation, other connected development schemes such as road development have had a positive impact on the socio-economic conditions of the area. Environmental degradation has also been checked and positive results such as higher rainfall and higher vegetation coverage are evident. The project has been renamed as the Barind Multipurpose Development Authority (BMDA) since the early 1990s and now covers a large part of the Barind Tract. [Kazi Matinuddin Ahmed] [Ahmed, Kazi Matin Uddin Professor of Geology, Dhaka University]
