The Physical Environment

Topography

The Himalaya and its associated mountain ranges form a physical and cultural divide between South Asia and the lands to the north and east. They rise in a series of roughly parallel ranges northward from the plains of Bangladesh, India, and Nepal, with each range progressively higher.

South of the world’s highest mountain system is the densely populated alluvial plain of Ganga in India and Bangladesh, and the Indus Plain, mostly in Pakistan. Both these plains have extensively de­veloped systems of irrigation. Canal irrigation is particularly well developed in the Indus Plain and the western Ganga Plain. These plains are subject to periodic flooding that is especially heavy and periodi­cally devastating in the eastern part of India and Bangladesh.

The Thar Desert, south of the Indus Plain, is shared by India and Pakistan. Here is a vast arid region hemmed in from the east by the low Aravalli Range. This thinly populated desert region is charac­terized by shifting sand dunes, large playa lakes, salt pans, and, to the southwest, marshlands.

South of the Ganga Plain is peninsular India. Com­posed of several plateaus, this region, except for the black soils of the Deccan Plateau and the river del­tas, is primarily an area of thin, reddish soils of medium to poor quality. Part of the peninsula that is still forested and of marginal quality for agriculture is home to many tribal people. Skirting the penin­sula on the west is the narrow but intensively culti­vated and densely populated Malabar coast. On the east the coastal plain is broader. The island nation of Sri Lanka is composed of central highlands sur­rounded by a coastal plain.

Climate

South Asia is very hot during the summer; the high­est temperatures are found in the Thar Desert. Hot desiccating winds in May and June are frequent in the entire Indus-Ganga Plain and cause substantial problems of heat stress (Planalp 1971). Tempera­tures of over 90^oF are commonplace. In the moun­tainous regions elevation modifies the tempera­tures. During winter there is a general temperature gradient from north to south. Temperatures are in the 40^oF range in the north, reaching well over 75^oF in the south.

South Asia is characterized by a high degree of rainfall seasonality, with very uneven geographic distribution, and great variability from year to year. In much of India, Bangladesh, Nepal, Bhutan, and Pakistan, most of the rainfall is received from June through September. Over the southern part of penin­sular India, autumn rainfall is significant. On the southeastern coastal area the predominant season of rains is autumn/Most of peninsular India and the Thar region suffer from high variability of rainfall. In general, the seasonality of rainfall in the predomi­nantly agrarian societies of South Asia has meant that farming is very dependent upon rainfall despite major developments in irrigation during the twenti­eth century. Variability of rainfall, therefore, means that farming communities may suffer from frequent seasonal food shortages, undernutrition, lowered re­sistance, and higher morbidity.

Convincing arguments have been made about the relationship of climatic seasonality to seasonal mor­bidity, especially among the poor. Evidence from di­verse regions of the world, including India,and Ban­gladesh, shows that seasonal fluctuations bear more heavily on the poor than on other social groups (Chambers, Longhurst, and Pacey 1981). High- rainfall regions, especially where humidity is also high (e.g., the Malabar coast, Eastern India, and Bangladesh) are the major endemic areas of infec­tious diseases such as cholera, typhoid, filariasis, and malaria.