Rajasthan Quiz: Climate and Soils of Rajasthan

It lies in the extreme west of the state, farthest from both branches of the south-west monsoon, and parts of the district record under 200 millimetres a year, which is why the sand dunes there are the largest and the most mobile in Rajasthan.

These are the showers of December to February, brought by western disturbances that travel from the Mediterranean across Iran and Pakistan into north-western India, and they are valued because they water the standing rabi crops of wheat, gram, barley and mustard without any cost to the farmer.

The Tropic of Cancer crosses the southern part of Banswara, the southernmost district of Rajasthan, so the whole of the state except that small corner lies north of the tropic, in the subtropical belt.

The soil there is formed from the weathering of the basalt of the Deccan trap, so it is clayey, holds moisture well and cracks in summer, and it carries the state's soybean, cotton and wheat.

The Thar desert and the arid plains west of the Aravalli take up the greater part of Rajasthan, and their soil is overwhelmingly sand, low in nitrogen and organic matter, porous so that water sinks away quickly, but often well supplied with lime and phosphate.

The Central Arid Zone Research Institute was set up at Jodhpur in 1959 and works on sand dune stabilisation, shelter belts, dry farming, desert pasture and the spread of the desert, which is why it is the institute named in questions on the Thar.

Where the Chambal runs through the soft alluvium of Dholpur, Karauli and Sawai Madhopur, the monsoon run-off cuts deep channels that widen year after year into the badlands called bihad, taking farmland out of use and giving the area its old reputation as a hiding place for outlaws.

These, with the Sambhar basin, are the inland basins of Rajasthan: the streams that reach them have no outlet to the sea, so the water evaporates and leaves its salts behind, and the same salt is worked commercially at Sambhar, Didwana and Pachpadra.

It is the highest part of the Aravalli range and the only hill station of Rajasthan, so the monsoon winds are forced to rise over it and it receives well over a thousand millimetres a year, far more than anywhere else in the state.

The Aravalli runs from the south-west to the north-east, which is the same direction in which the Arabian Sea branch travels, so the winds slide along the range instead of being forced up over it; without that lifting there is little cooling and little rain, and the moisture is carried…