The southernmost strip of land is flat plain known as the Terai region covering 17% of the _ total land area of Nepal. Its altitude ranges up to 600 m from the sea level. The climate, here, is hot. The fertile and arable soil of the Terai Region has given its identity as ‘granary (breadbasket) of Nepal’ yielding high amount of food crops. The region mainly supports the sub-tropical forest with hard stem, broad leaves, and evergreen tall trees.
It constitutes a broad stretch of level land extending from beyond the Mahakali Rivers on the west to the Mechi River on the east of Nepal. Because of the occurrence of hills and mountains on the north, the general gradient of this level region runs towards the south. On the basis of structure and surface relief, the terai region is relatively far more homogeneous than any other topographic units of the country. This terai region including inner terai basins encompasses approximately 25% of the total land surface of Nepal. It has been formed out of the age-long deposition of sediments carried by various rivers and streams debouching from the northern hilly and mountainous parts of the country.
The Terai Region is more developed than others in terms of infrastructural development. It supports to more population since it is fertile, plain and gets abundant rainfall. For instance, The Terai accommodates 50.3 % of 2,64,94,504 total people of Nepal while population density is 392 persons per sqr km.
Like the northern hills and mountains, even the Terai region on the basis of location is popularly divided into three sub-units as (a) Eastern Terai (b) Central Terai and (c) Western Terai. But more significant from the geographic point of view are the three north-south sub-units introduced below:
(a) Terai (Outer or Open Terai)
It is the vital part of the Terai region and represents some 13 % of the total area of Nepal. Its elevation from sea level rises from below 100 m. in the eastern section to the maximum of 300m. In the western one and attains the maximum width of 56 km. It is not quite continuous; for it disappears at places where the Churia Ranges extend up to the border. Its eastern section is relatively much broader than the western one. Though limited in size and extent, this Terai section, being composed of fertile alluvium, is of substantial significance from the viewpoint of agricultural production.
(b) Inner Terai
There exist several synclinal valleys bounded by the Mahabharat Range on the north and the Churia Range on the south. These are of tectonic origin and are known as Inner Terai or Doons. In point of dimension, their east-west length varies from 32 km. to 64 km. Though their average width is reckoned to be 16 km, they are much broader in central and western parts where the Churia Ranges extend up to the border. Their elevation from sea level runs from about 200 m. on the east to 400 m. on the west.
These inner Terai basins altogether represent some 8 % of the total area of Nepal. These are basically composed of alluvial soil which grows coarser towards the marginal parts close to the Mahabharat and Churia Ranges. The most widely known inner terai basins from west to east are Dang, Deokhuri, Chitwan, Makawanpur, Sindhuli, and Udaypur. In several physical aspects, they nearly resemble the outer terai plain and prove, therefore, to be potential agricultural areas of Nepal.
(c) Bhabar Tract
It constitutes a narrow stretch of level land which gradually merges with the outer Terai plain on the south and happens to be on the southern foot of the Churia Ranges. It is roughly 200m. High from sea level and represents about 4 % of the total area of the country. This tract is formed of coarser materials like sand, pebbles, conglomerates and rock detritus carried and deposited by the rivers and streams debouching from the Churia Ranges. Since its surface is in this way capped with such loose and coarse materials, water rapidly disappears from the surface; and hence this part of the Terai region is of little significance from the standpoint of agricultural production. Almost all parts of this tract are covered since long with sub-tropical evergreen forest.