AGRO-CLIMATIC ZONES OF ASSAM

Based on rainfall, terrain and soil characteristics, Assam State has been broadly delineated into the following six agro-climatic zones:

A. North Bank Plains

B. Upper Brahmaputra Valley

C. Central Brahmaputra Valley

D. Lower Brahmaputra Valley

E. Barak Valley and

F. Hills

This delineation has also been accepted by the Department of Economics and Statistics of the Government of Assam (World Agricultural Census Report, 1970-71) and by the World Bank (Assam Agricultural Development Project Appraisal Report Credit 1535a-IN). Zoning has been shown in Map 1 and described in the following paragraphs.

The area, population, density and percentage of rural population are presented in Table 1.

Zone (A) North Bank Plains:

This zone comprises the districts of Lakhimput, Dhemaji, Darrang and Sonitpur with an area of 14421 km².

Physiography, climate and soils: This zone can be divided in to 3 parallel belts of district physiography: (1) in the foothills of Himalayas, alluvial soils are found with dense forests. On the south of this belt there are small tea plantations extending from Subansiri river to river Barnadi; (2) the central belt comprises old alluviums which are acidic. Near the river banks there are new alluvials which are either neutral or less acidic; and (3) the low lying riverine belt lies by the side of Brahmaputra. The low lands grow deep water rice known as bao and on the eastern side of the belt in the Darrang district jute is grown extensively.

The climate is characterized by an average rainfall of 1000 mm and high humidity of more than 80%. The maximum temperature rises upto 37⁰C in July-August and the minimum falls to 5⁰C in January. Fifty per cent of total rainfall comes during 7 month period of the rainy season.

Zone (B) Upper Brahmaputra Valley:

This zone comprises the districts of Sivasagar, Jorhat, Golaghat, Dibrugarh and Tinsukia with an area of 16,192 km².

Physiography, climate and soils: The topography slopes down gradually from the hills towards the Brahmaputra. It has got half a dozen important tributaries of the Brahmaputra. These tributaries start in the hills of Nagaland and Arunachal Pradesh and traverse the zone rapidly to join in the Brahmaputra.

The climate is characterized by high rainfall, i.e., more than 2000 mm per annum and high humidity (more than 80%). The maximum temperature rises up to 37⁰C in July-August and minimum falls to 5⁰C in January.

The soils are mostly new alluvium near the Brahmaputra and old alluvium in the central belt of the zone.

Zone (C) Central Brahmaputra Valley:

This zone comprises the district of Nagaon and Morigaon with an area of 5561 km².

Physiography, climate and soils: This zone is situated in the center of the State is encircled by hills on all sides, except on the north where it is bounded by the Brahmaputra. Because of its physiography, this zone is like a basin and is inundated during the monsoon. A number of rivers traverses through this zone. These rivers start in the Karbi Anlong and flow into the Brahmaputra. About 30% of the area in this zone comes under rain shadow belt where the rainfall is much lower (600 mm) than other areas of the Assam plains (1600 mm). The maximum temperature rises upto 38⁰C in July-August and minimum falls to 8⁰C in January. Both new alluvial and old alluvial soils are found here. Compared to lower Brahmaputra Valley, soils here are lighter in texture and are not underlain by rocks and aquifers.

Zone (D) Lower Brahmaputra Valley:

This zone comprises the district of Kamrup, Dhubri, Bongaigaon, Nalbari, Barpeta, Kokrajhar and Goalpara with an area of 20148 km².

Physiography, climate and soils: On the north of this zone lie the folded ranges of the Himalayas, and in the south the Shillong plateau. The mighty Brahmmaputra flows through the zone. The northern part of the zone is characterized by small hillocks and some low lying areas here and there. Flood plains of Brahmaputra extending up to the river Jinjiram bordering Meghalaya, constitute the southern part of the zone.

The average rainfall in the zone is about 1700 mm per annum. Rainfall in the south-eastern part of the zone is low and it increases towards the north and the west. The shallow rivers flowing from the Bhutan hills with torrential currents cause enormous loss of animal lives, properties and crops every year. The maximum temperature rises upto 31⁰C in July-August and minimum falls to 10⁰C in January.

Soils of this zone consist of new alluvium on both the banks of the Brahmaputra and old alluvium towards the foot hills. Soils are mostly sandy and sandy loam in texture. Soils of the zone are acidic in reaction, though a large area is also covered by nearly neutral soils.

Zone (E) Barak Vally:

This zone comprises the district of Cachar, Hailakandi, Karimganj with an area of 6922 km².

Physiography, climate and soils: This zone is separated from the Brahmaputra Valley by the two hill districts viz., Karbi Anlong and North Cachar. This zone has a total area of 6962 km². This zone is bounded in the north by North Cachar hills, in the east by Manipur hills, in the south by the hills of Mizoram, and in the west by Bangladesh and Tripura. The zone is characterized by undulating topography. The hills and hillocks, locally known as `tillas' predominate the land surface. The plains have a great deal of marshy lands. There are two important rivers, viz., Barak and Kushiara in this zone.

The climate is characterized by high rainfall (more than 2000 mm), high temperature and high humidity. Maximum temperature rises up to 37⁰C in July-August and minimum falls to 9⁰C in January.

Alluvial soils in the flood plains are fertile. Red loam soils in the submontane tracts are relatively more deficient in plant nutrients. The Barak plains have a great deal of low marshy lands. Organic soils are found in the swampy `bheels'. Most of the soils are acidic in nature.

Zone (F) Hills Zone:

This zone comprises the district of Karbi Anlong and North Cachar Hills with an area of 15322 km².

Physiography, climate and soils: Both the districts of the zone are characterized by undulating topography. The North Cachar hills are high and steep. In Karbi Anlong the hills have gentle slopes.

Rainfall and temperature differ substantially among the different parts of the zone due to varying altitudes and location of hills and valleys. The total rainfall is about 1,144 mm in North Cachar hills and 600 mm in Karbi Anlong. The maximum temperature goes upto 37⁰C and minimum to 9⁰C at Haflong.

The predominant soils in this zone are lateritic on the slopes and red loams in the valleys. The soils developed in the plateau vary greatly in age and composition. In Karbi Anlong district there are considerable plains areas on the north adjoining the districts of Golaghat and Nagaon. Here the soils are mostly old alluvial.

Source: National Agricultural Research Project (NARP)