The Terai or Tarai (Hindi: तराई Nepali: तराइ) is a lowland region in northern India and southern Nepal that lies south of the outer foothills of the Himalayas, the Sivalik Hills, and north of the Indo-Gangetic Plain. This lowland belt is characterised by tall grasslands, scrub savannah, sal forests and clay rich swamps. In northern India, the Terai spreads from the Yamuna River eastward across Haryana, Uttarakhand, Uttar Pradesh, Bihar and West Bengal. The Terai is part of the Terai-Duar savanna and grasslands ecoregion. The corresponding lowland region in West Bengal, Bangladesh, Bhutan and Assam in the Brahmaputra River basin is called 'Dooars'. In Nepal, the Terai stretches over 33,998.8 km2 (13,127.0 sq mi), about 23.1% of Nepal's land area, and lies at an altitude of between 67 and 300 m (220 and 984 ft). The region comprises more than 50 wetlands. North of the Terai rises the Bhabar, a narrow but continuous belt of forest about 8–12 km (5.0–7.5 mi) wide.
The Urdu word ترائي तराई tarāʼī means "lands lying at the foot of a watershed" or "on the banks of a river; low ground flooded with water, valley, basin, marshy ground, marsh, swamp; meadow". In Hindi, the region is called तराई 'tarāī' meaning "foot-hill". In Nepali, the region is called तराइ 'tarāi' meaning "the low-lying land, plain" and especially "the low-lying land at the foot of the Himālayas". It has been described as "low, marshy ground".
The Terai is crossed by the large perennial Himalayan rivers Yamuna, Ganges, Sarda, Karnali, Narayani and Kosi that have each built alluvial fans covering thousands of square kilometres below their exits from the hills. Medium rivers such as the Rapti rise in the Mahabharat Range. The geological structure of the region consists of old and new alluvium, both of which constitute alluvial deposits of mainly sand, clay, silt, gravels and coarse fragments. The new alluvium is renewed every year by fresh deposits brought down by active streams, which engage themselves in fluvial action. Old alluvium is found rather away from river courses, especially on uplands of the plain where silting is a rare phenomenon.
A large number of small and usually seasonal rivers flow through the Terai, most of which originate in the Sivalik Hills. The soil in the Terai is alluvial and fine to medium textured. Forest cover in the Terai and hill areas has decreased at an annual rate of 1.3% between 1978 and 1979, and 2.3% between 1990 and 1991. With deforestation and cultivation increasing, a permeable mixture of gravel, boulders and sand evolves, which leads to a sinking water table. But where layers consist of clay and fine sediments, the groundwater rises to the surface and heavy sediment is washed out, thus enabling frequent and massive floods during monsoon, such as the 2008 Bihar flood.
The reduction in slope as rivers exit the hills and then transition from the sloping Bhabar to the nearly level Terai causes current to slow and the heavy sediment load to fall out of suspension. This deposition process creates multiple channels with shallow beds, enabling massive floods as monsoon-swollen rivers overflow their low banks and shift channels. Many areas show erosion such as gullies.
The Terai in Nepal is differentiated into "Inner" and "Outer" Terai and includes 20 districts.
Most of these valleys are 5–10 km (3.1–6.2 mi) wide (north-south) and up to 100 km (62 mi) long (east-west).
The Outer Terai begins south of the Sivalik Hills and extends to the Indo-Gangetic Plain. In the Far-Western Region, Nepal it comprises the Kanchanpur and Kailali districts, and in the Mid-Western Region, Nepal Bardiya and Banke districts. Farther east, the Outer Terai comprises the Kapilvastu, Rupandehi, Nawalparasi, Parsa, Bara, Rautahat, Sarlahi, Mahottari, Dhanusa, Siraha, Saptari, Sunsari, Morang and Jhapa districts.
East of Banke the Nepalese Outer Terai is interrupted where the international border swings north and follows the edge of the Sivaliks adjacent to Deukhuri Valley. Here the Outer Terai is entirely in Uttar Pradesh's Shravasti and Balrampur districts. East of Deukhuri the international border extends south again and Nepal has three more Outer Terai districts.
Several protected areas were established in the Terai since the late 1950s:
Based on the Köppen–Geiger climate classification system, the Nepal Terai experiences a tropical savanna climate type with dry winters and hot summers, a mean annual temperature of 20–28 °C (68–82 °F), a mean annual rainfall of 1,600–1,800 mm (63–71 in) in the west and 2,500–3,000 mm (98–118 in) in the east.
Tharu and Dhimal people are the indigenous inhabitants of the Terai forests. Several Tharu subgroups are scattered over most of the Nepal and Indian Terai. They used to be semi-nomadic, practised shifting cultivation and collected wild fruits, vegetables and medicinal herbs. They have been living in the Terai for many centuries and reputedly had an innate resistance to malaria. Dhimal reside in the eastern Nepal Terai, viz Sunsari, Morang and Jhapa districts. In the past, they lived in the fringes of the forest and conducted a semi-nomadic life to evade outbreaks of diseases. Today, they are subsistence farmers.
Maithils inhabit the Indian Terai in Bihar and the eastern Terai in Nepal. Bhojpuris reside in the central and eastern Terai, and Awadhis live in the central and western Terai. Bantawa people reside foremost in two districts of the eastern Terai in Nepal.
Following the malaria eradication program using DDT in the 1960s, a large and heterogeneous non-Tharu population settled in the Nepal Terai. Pahari people from the mid-hills including Bahun, Chhetri and Newar moved to the plains in search of arable land. In the rural parts of the Nepal Terai, distribution and value of land determine economic hierarchy to a large extent. High caste migrants from the hills and traditional Tharu landlords who own agriculturally productive land constitute the upper level of the economic hierarchy. The poor are the landless or near landless Terai Dalits, including the Musahar, Chamar and Mallah. Several Chepang people also live in Nepal's central and eastern Terai districts.
As of June 2011, the human population in the Nepal Terai totalled 13,318,705 people in 2,527,558 households comprising more than 120 different ethnic groups and castes such as Badi, Chamling, Ghale, Kumal, Limbu, Magar, Muslim, Rajbanshi, Teli, Thakuri, Yadav and Majhi speaking people.
The Muslim invasion of northern India during the 14th century caused Hindu and Buddhist people to seek refuge from religious persecution. Rajput nobles and their entourage migrated to the Himalayan foothills and gained control over the region from Kashmir to the eastern Terai during the next three centuries.
Until the mid 18th century, the Nepal Terai was divided into several smaller kingdoms, and the forests were little disturbed. By the 16th century, the rulers of Palpa and Makwanpur controlled the mid-western Terai and extended this control to the eastern Terai by the 17th century. They controlled the area of today's districts of Saptari, Siraha, Dhanusa, Mahottari and Sarlahi. The rulers of Makwanpur controlled the central Terai region of present-day Nepal, and the rulers of Vijayapur controlled today's Sunsari, Morang and Jhapa Districts. The Shah rulers conquered the eastern Nepal Terai in the 1770s. They also conquered land in the eastern Terai that belonged to the Kingdom of Sikkim. The Tulsipur State in the Dang Valley of Nepal's western Terai was also an independent kingdom, until it was conquered in 1785 by Bahadur Shah of Nepal during the unification of Nepal. Since the late 18th century, the Shah rulers encouraged Indian people to settle in the Terai and supported famine-stricken Bihari farmers to convert and cultivate land in the eastern Nepal Terai. From at least 1786 onward, they appointed government officers in the eastern Terai districts of Parsa, Bara, Rautahat, Mahottari, Saptari and Morang to levy taxes, collect revenues, and capture Indian elephants and Indian rhinoceros. At the end of the 18th century, between 200 and 300 elephants were caught annually using snares or nooses.
The far-western and mid-western regions of the Nepal Terai called 'Naya Muluk' (new country) lay on the northern periphery of the Awadh dynasty. After Nepal lost the Anglo–Nepalese War in 1816, the British annexed these regions in the Terai when the Sugauli Treaty was ratified. But as reward for Nepal's military aid in the Indian Rebellion of 1857, they returned some of this region in 1860, namely today's districts Kanchanpur, Kailali, Banke and Bardiya. To promote economic development of the Nepal Terai, people from the hills were invited to settle in the region. Since only a few moved to the Terai, Indian people were encouraged to settle. Immigration of Indian people increased between 1846 and 1950. They settled in the eastern Nepal Terai together with native Terai peoples.
The Indian Terai remained largely uninhabited until the end of the 19th century, as it was arduous and dangerous to penetrate the dense and marshy malarial jungle. Dacoit gangs retreated to the Terai jungles, and the area was considered lawless and primitive by the British, who sought control of the region's valuable timber reserves. The region was densely forested with stands of foremost Sal.
Heavy logging began in the 1920s. Extracted timber was exported to India to collect revenues. Cleared areas were subsequently used for agriculture. But still, the Terai jungles were teaming with wildlife.
Inner Terai valleys historically were agriculturally productive but extremely malarial. Some parts were left forested by official decree during the Rana dynasty as a defensive perimeter called Char Kose Jhadi, meaning 'four kos forest'; one kos equals about 3 km (1.9 mi). A British observer noted, "Plainsmen and paharis generally die if they sleep in the Terai before November 1 or after June 1." British travelers to Kathmandu went as fast as possible from the border at Raxaul to reach the hills before nightfall.
Malaria was eradicated using DDT in the mid-1950s. Subsequently, people from the hills migrated to the Terai. About 16,000 Tibetan refugees settled in the Nepal Terai in 1959–1960, followed by refugees of Nepali origin from Burma in 1964, from Nagaland and Mizoram in the late 1960s, and about 10,000 Bihari Muslims from Bangladesh in the 1970s. Timber export continued until 1969. In 1970, the king granted land to loyal ex-army personnel in the districts of Jhapa, Sunsari, Rupandehi and Banke, where seven colonies were developed for resettling about 7,000 people. They acquired property rights over uncultivated forest and 'waste' land, thus accelerating the deforestation process in the Terai. Between 1961 and 1991, the annual population growth in the Terai was higher than the national average, which indicates that migration from abroad occurred at a large scale. Deforestation continued, and forest products from state-owned forest were partly smuggled to India. Community forestry was introduced in 1995. Since the 1990s, migration from the Terai to urban centres is increasing and causing sociocultural changes in the region.
Since the early 1950s, several political parties advocated for autonomy and independence of the Nepal Terai, such as the Nepal Terai Congress and Janatantrik Terai Mukti Morcha. Several armed groups were formed, which pursued this aim using violent means. In 2013, more than 24 Madheshi political parties were registered for the Constituent Assembly of Nepal election.
The most significant border dispute of the Indo-Nepal boundary in the Terai region is the Susta area. In the Susta region, 14,500 hectares of land is generally dominated by Indian side with support of Seema Shashatra Bal (SSB) forces.
After the 2008 Nepalese Constituent Assembly election, Indian politicians kept on trying to secure strategic interests in the Nepal Terai, such as over hydropower energy, development projects, business and trade. The government of Nepal has accused India of imposing an undeclared blockade in 2015. The Indian government denied allegations of any involvement in the blockade.
The Terai is the most productive region in Nepal with the majority of the country's industries. Agriculture is the basis of the economy. Major crops include rice, wheat, maize, potato, peas, lentil, mustard, sugar cane, ginger, turmeric, cardamom, garlic and chili. Fruits comprise mango, lychee, guava, papaya, banana and jackfruit. The Terai is also known for beekeeping and honey production, with about 120,000 colonies of Apis cerana.
The Mahendra Highway crosses the Nepal Terai from Kankarbhitta on the eastern border in Jhapa District, Mechi Zone to Mahendranagar near the western border in Kanchanpur District, Mahakali Zone. It is the only motor road spanning the country from east to west.