Historians believe that the Indus Valley civilization agriculture must have been highly productive; after all, it was capable of generating surpluses sufficient to support tens of thousands of urban residents who were not primarily engaged in agriculture. It relied on the considerable technological achievements of the pre-Harappan culture, including the plough. Still, very little is known about the farmers who supported the cities or their agricultural methods. Some of them undoubtedly made use of the fertile alluvial soil left by rivers after the flood season, but this simple method of agriculture is not thought to be productive enough to support cities. There is no evidence of irrigation, but such evidence could have been obliterated by repeated, catastrophic floods.
The Indus Valley civilization appears to contradict the hydraulic despotism hypothesis of the origin of urban civilization and the state. According to this hypothesis, cities could not have arisen without irrigation systems capable of generating massive agricultural surpluses. To build these systems, a despotic, centralized state emerged that was capable of suppressing the social status of thousands of people and harnessing their labor as slaves. It is very difficult to square this hypothesis with what is known about the Indus civilization. There is no evidence of kings, slaves, or forced mobilization of labor.
It is often assumed that intensive agricultural production requires dams and canals. This assumption is easily refuted. Throughout Asia, rice farmers produce significant agricultural surpluses from terraced, hillside rice paddies, which result not from slavery but rather the accumulated labor of many generations of people. Instead of building canals, Indus civilization people may have built water diversion schemes, which—like terrace agriculture—can be elaborated by generations of small-scale labor investments. In addition, it is known that Indus civilization people practiced rainfall harvesting, a powerful technology that was brought to fruition by classical Indian civilization but nearly forgotten in the 20th century. It should be remembered that Indus civilization people, like all peoples in South Asia, built their lives around the monsoon, a weather pattern in which the bulk of a year's rainfall occurs in a four-month period. At a recently discovered Indus civilization city in western India, archaeologists discovered a series of massive reservoirs, hewn from solid rock and designed to collect rainfall, that would have been capable of meeting the city's needs during the dry season.
Even though, the nature of the Indus civilization's agricultural system is still largely a matter of conjecture due to the paucity of information surviving through the ages, the possibility of speculation remains.