The Aryan Sinhalese civilization that emerged in Sri Lanka with migration from Bengal, East India in 543 BC took root in the Dry zone, the rolling plains of North and North Central (Rajarata) and the South Eastern (Ruhunu) of ancient Sri Lanka.Though the land was excellent for agriculture, the difficulty in diverting enough water to their cultivation was the main problem. Hence the magnificent irrigation schemes were born. Aryan Sinhalese were dependent on an agriculture-based economy which prospered in their glorious past until the advent of the European colonizers.
For more than a millenum and a half, the populace of the Sinhalese Buddhist civilization was concentrated in the region which is generally termed Dry Zone. Even today Dry Zone is clearly distinguishable from the rest of the land area of Sri Lanka by the variance in the density of population. The greater part of this region receives a mean annual rainfall of fifty to seventy-five inches, which though not meager in a comparative sense, is limited to a few months of the year. In this whole region, which covers about seventy percent of the total land area of the Island, only the northernmost peninsula and a stretch of land in the north-western coastal belt with water-retentive limestone rock-strata afford scope for extensive irrigation by means of wells.
Rice being on the staple food, the cultivation under submerged conditions requires an abundant supply of water. The rice fields in the dry zone of Ceylon receive an annual rainfall of less than 75 inches most of which is brought about by short spells of heavy precipitation. Very often this quantity represents only a fraction of the water requirements for a double-cropped rice and the rice fields of these areas. Therefore heavily dependent on supplementary irrigation from tanks and reservoirs. The intermediate zone and the wet zone however receive higher rainfall, which is fairly evenly distributed throughout the years.
This condition permits the growth of rice in certain area without supplementary irrigation. Nevertheless, there are many rice fields in the wet zone, which are irrigated from streams and channels flowing from the adjoining residual lands.