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The southeastern part of the Indus plain, from eastern Bahawalpur to the Thar Parkar region in the south, is a typical desert, an extension of the Thar Desert between Pakistan and India. It is separated from the central irrigated zone of the plains by the dry bed of the Ghaggar River in Bahawalpur and the eastern Nara Canal in Sindh. The desert is variously known as the Cholistan or Rohi Desert in Bahawalpur and the Pat or Thar Desert in Sindh. The surface of the desert is a wild maze of sand dunes and sand ridges. Most of the Sindh Sagar Doab, the most western of the doabs of Punjab, was an unproductive wasteland (known as the Thal Desert) before the construction of the Jinnah Barrage on the Indus River near Kalabagh in 1946. The Thal canal system, which draws water from the barrage, has turned parts of the desert into fertile cultivated land.
Soils
Pakistan’s soils are classified as pedocals, which comprise a dry soil group with high concentrations of calcium carbonate and a low content of organic matter; they are characteristic of a land with low and erratic precipitation. The major soil groupings are Indus basin soils, mountain soils, and sandy desert soils. However, the very mode of soil formation gives rise to their diversification even within small areas. These soils vary in texture, chemical composition, colour, and organic content from place to place.
The Indus basin soils are mostly thick alluvium deposited by rivers and are of recent origin. Soils in the vicinity of river courses are the most recent and vary in texture from sand to silt loam and silty clay loams. They have a low organic content and are collectively known as the khaddar soils. Away from the river, toward the middle of the doabs, older alluvial soils (called bangar) are widely distributed. These soils are medium to fine in texture, have low organic content, and are highly productive under conditions of irrigation and fertilization. In some waterlogged areas, however, these soils are salinized. Strongly alkaline soils are localized in some small patches. In the submontane areas under subhumid conditions these soils are noncalcareous and have slightly higher organic content. In the delta the estuarine soils are excessively saline and barren.
Mountain soils are both residual (i.e., formed in a stationary position) and transported. Shallow residual soils have developed along the slopes and in the broken hill country. Those soils generally are strongly calcareous and have low organic content, but under subhumid conditions their organic content increases.
Sandy desert soils cover the Cholistan part of Sindh Sagar Doab and western Balochistan. They include both shifting sandy soils and clayey floodplain soils. These include moderately calcareous and eolian (wind-borne) soils.
Climate
Aridity is the most pervasive aspect of Pakistan’s climate, and its continental nature can be seen in the extremes of temperature. Pakistan is situated on the edge of a monsoonal (i.e., wet-dry) system. Precipitation throughout the country generally is erratic, and its volume is highly variable. The rainy monsoon winds, the exact margins of which vary from year to year, blow in intermittent bursts, and most moisture comes in the summer. Tropical storms from the Arabian Sea provide precipitation to the coastal areas but are also variable in character.
The efficiency of the monsoonal precipitation is poor, because of its concentration from early July to mid-September, when high temperatures maximize loss through evaporation. In the north the mean annual precipitation at Peshawar is 13 inches (330 mm), and at Rawalpindi it reaches 37 inches (950 mm). In the plains, however, mean annual precipitation generally decreases from northeast to southwest, falling from about 20 inches (500 mm) at Lahore to less than 5 inches (130 mm) in the Indus River corridor and 3.5 inches (90 mm) at Sukkur. Under maritime influence, precipitation increases slightly to about 6 inches (155 mm) at Hyderabad and 8 inches (200 mm) at Karachi.
The 20-inch (500-mm) precipitation line, which runs northwest from near Lahore, marks off the Potwar Plateau and a part of the Indus plain in the northeast; these areas receive enough rainfall for dry farming (farming without irrigation). South of this region, cultivation is confined mainly to riverine strips until the advent of irrigation. Most of the Balochistan plateau, especially in the west and south, is exceptionally dry.
Pakistan’s continental type of climate is characterized by extreme variations of temperature, both seasonally and daily. High elevations modify the climate in the cold, snow-covered northern mountains; temperatures on the Balochistan plateau are somewhat higher. Along the coastal strip, the climate is modified by sea breezes. In the rest of the country, temperatures reach great extremes in the summer; the mean temperature during June is 100 °F (38 °C) in the plains, where the highest temperatures can exceed 117 °F (47 °C). Jacobabad, in Sindh, has recorded the highest temperature in Pakistan, 127 °F (53 °C). In the summer, hot winds called loos blow across the plains during the day. Trees shed their leaves to avoid excessive moisture loss. The dry, hot weather is broken occasionally by dust storms and thunderstorms that temporarily lower the temperature. Evenings are cool; the diurnal variation in temperature may be as much as 20 to 30 °F (11 to 17 °C). Winters are cold, with minimum mean temperatures of about 40 °F (4 °C) in January.