The Thar Desert, also known as the Great Indian Desert, is a desert located in western India and southeastern Pakistan. It lies mostly in the Indian state of Rajasthan, and extends into the southern portion of Haryana and Punjab states and into northern Gujarat state. In Pakistan, the desert covers eastern Sind province and the southeastern portion of Pakistan's Punjab province. In Pakistan's Punjab province it is known as the Cholistan Desert. The Thar Desert is bounded on the northwest by the Sutlej River, on the east by the Aravalli Range, on the south by the salt marsh known as the Rann of Kutch (parts of which are sometimes included in the Thar), and on the west by the Indus River. Its boundary to the large thorny steppe to the north is ill-defined. Depending on what areas are included or excluded, the nominal size of the Thar can vary significantly.

According to the WWF definition, the desert proper has 92,200 square miles (238,700 km²). Another source gives the area of the Thar Desert as 446,000 km² extending 805 km (about 500 mi) long and about 485 km (about 300 mi) wide, with 208,110 square kilometres in India. Of the Indian portion, 61% falls in Rajasthan, 20% in Gujarat and 9% in Punjab and Haryana combined. The greater portion lies in Rajasthan, covering about three-fifths of the total geographical area.

There are three principal landforms in the desert region i.e.

* The predominantly sand covered Thar
* Plains with hills including the central dune free country
* Hills

It is a desolate country where sand is piled up into huge wind blown dunes (technically this is known as an erg). The sand dunes are of three types viz., longitudinal parabolic, transverse and barchans. The first type, running NNE-SSW, i.e. parallel to the prevailing winds, occurs to the south and west of the Thar. The transverse dunes, aligned across the wind direction, to the east and north of Thar and barchans, with the conclave sides facing the wind in the interior, predominant in Central Thar. On the whole the Thar Desert slopes imperceptibly towards the Indus Plain and surface unevenness is mainly due to sand dunes. The dunes in the south are higher, rising sometimes to 152 m whereas in the north they are lower and rise to 16 m above the ground level.

The Aravalli ranges form the main landmark to the south-east of Thar Desert. The more humid conditions that prevail near the Aravallis prevent the extension of Thar Desert towards the east and the Ganges Valley. In the heart of the sand covered area, the bare, dune free country of Barmer, Jaisalmer and Bikaner present an anomaly.

The soils of the Arid Zone are generally sandy to sandy-loam in texture. The consistency and depth vary according to the topographical features. The low-lying loams are heavier and may have a hard pan of clay, calcium carbonate (CaCO3) or gypsum. The pH varies between 7 and 9.5. The soils improve in fertility from west and northwest to east and northeast. Desert soils are Regosols of wind blown sand and sandy fluiratile deposits, derived from the disintegration of rock in the subjacent areas and blown in from the coastal region and the Indus Valley. The desert soils occupy the districts of Jodhpur, Bikaner, Churu, Ganganagar, Barmer, Jaisalmer, and Jalore. The Thar consists mainly of the wind-blown sand. The area is covered not only by sheet of sand but also of rocky projections of low elevations which constitute the older rocks of the country. Water is scarce and occurs at great depths, from 30 to 120 m below the ground level.

Some of these soils contain a high percentage of soluble salts in the lower horizons, turning water in the wells poisonous. Being poor in organic matter they show a low loss on ignition. They contain varying amount of calcium carbonate.

Stretches of sand in the desert are interspersed by hillocks and sandy and gravel plains. Due to the diversified habitat, the vegetation and animal life in this arid region is very rich. About 23 species of lizard and 25 species of snakes are found here and several of them are endemic to the region.

Some wildlife species, which are fast vanishing in other parts of India, are found in the desert in large numbers such as the Great Indian Bustard (Ardeotis nigriceps), the Blackbuck (Antilope cervicapra), the Indian Gazelle (Gazella bennettii) and the Indian Wild Ass (Equus hemionus khur) in the Rann of Kutch. How these animals and insects survive in these harsh conditions, under such high temperature and without drinking water and green vegetation are amazing. They have evolved excellent survival strategies, their size is smaller than other similar animals living in different conditions, and they are mainly nocturnal. There are certain other factors responsible for the survival of these animals in the desert. Due to the lack of water in this region, transformation of the grasslands into cropland has been very slow. The protection provided to them by a local community, the Bishnois, is also a factor.

The Desert National Park, Jaisalmer, spread over an area of 3162 km², is an excellent example of the ecosystem of the Thar Desert, and its diverse fauna. Great Indian Bustard, Blackbuck, chinkara, desert fox, Bengal fox, wolf, desert cat etc. can be easily seen here. Seashells and massive fossilized tree trunks in this park record the geological history of the desert. The region is a haven for migratory and resident birds of the desert. One can see many eagles, harriers, falcons, buzzards, kestrel and vultures. Short-toed Eagles (Circaetus gallicus), Tawny Eagles (Aquila rapax), Spotted Eagles (Aquila clanga), Laggar Falcons (Falco jugger) and kestrels are the commonest of these.

Tal Chhapar Sanctuary is a very small sanctuary in Churu District, 210 km from Jaipur, in the Shekhawati region. This sanctuary is home to a large population of graceful Blackbuck. Desert Fox and desert cat can also be spotted along with typical avifauna such as partridge and sand grouse.

The soil of the Thar Desert remains dry for a longer period in the year and is therefore most prone to wind erosion. Due to the high velocity of winds in these areas the dust is blown off with it and deposited on fertile lands causing a problem of shifting sand dunes. The movement of the shifting sand buries fences and blocks roads and railway tracks. Permanent solution to this problem of shifting sand dunes can be provided by fixation of the shifting sand dunes with suitable plant species and planting windbreaks and shelterbelts. They also provide protection from hot or cold and desiccating winds and the invasion of sand.

The local tree species for planting in the desert region are not many and they are also very slow growing. The introduction of exotic tree species in the desert for plantation has become necessary. Many species of Eucalyptus, Acacia, Cassia and other genera have been tried from Israel, Australia, USA, Russia, Southern Rhodesia, Chile, Peru, Sudan etc. in Thar Desert Acacia tortilis has proved to be the most promising species for desert afforestation and has helped in checking the spread of desert soils to other areas and fixing of sand dunes. The jojoba is one other promising species of economic value found suitable for planting in these areas.

The Rajasthan Canal system is the major irrigation scheme of the Thar Desert and is conceived to reclaim it and also to check spreading of the desert to fertile areas.