Irrigation and water management

Rainfall in the dry zone of Sri Lanka is limited to 50-75 inches. Under these conditions, rain fed cultivation was difficult, forcing early settlers to develop means to store water in order to maintain a constant supply of water for their cultivations. Small irrigation tanks were constructed at village level, to support the cultivations of that village. The earliest medium-scale irrigation tank is the Basawakkulama reservoir built by Pandukabhaya. Nuwara wewa and Tissa wewa reservoirs were constructed a century later. These reservoirs were enlarged in subsequent years by various rulers.

Construction of large scale reservoirs began in the first century AD under the direction of Vasabha. The Alahara canal, constructed by damming the Amban river to divert water to the west for 30 miles (48 km), was constructed during this period. Among the reservoirs constructed during the reign of Vasabha, Mahavilacchiya and Nocchipotana reservoirs both have circumferences of about 2 miles (3.2 km). During the reign of Mahasen, the Alahara canal was widened and lengthened to supply water to the newly constructed Minneriya tank, which covered 4,670 acres (18.9 km²) and had a 1.25 miles (2.01 km) long and 44 feet (13 m) high embankment. He was named Minneri Deiyo (god of Minneriya) and is still referred to as such by the people in that area. The Kavudulu reservoir, Pabbatanta canal and Hurulu reservoir were among the large irrigation constructions carried out during this period. These constructions contributed immensely to the improvement of agriculture in the northern and eastern parts of the dry zone. Reservoirs were also constructed using tributaries of the Daduru Oya during this period, thereby supplying water to the south western part of the dry zone. This conservation and distribution of water resources ensured that the water supply was sufficient throughout the dry zone.^[130] James Emerson Tennent described the ancient irrigation network as:

... there seems every reason to believe that from their own subsequent experience and the prodigious extent to which they occupied themselves in the formulation of works of this kind, they attained a facility unsurpassed by the people of any other country. The water resources of the dry zone were further exploited during the times of Upatissa I and Dhatusena. The construction of the Kala wewa, covering an area of 6,380 acres (25.8 km²) with an embankment 3.75 miles (6.04 km) long and 40 feet (12 m) high, was done during Dhatusena's reign. A 54 miles (87 km) canal named the Jayaganga carries water from the Kala wewa to the Tissa Wewa and feeds a network of smaller canals. The construction of this network is also attributed to Dhatusena. The Jayaganga supplied water to 180 square kilometres of paddy fields. By the end of the fifth century AD, two major irrigation networks, one supported by the Mahaweli river and the other by Malvatu Oya and Kala Oya, were covering the Rajarata area. The Mahavamsa records that many other rulers constructed a number of irrigation tanks, some of which have not yet been identified. By the eighth century, large tanks such as Padaviya, Naccaduva, Kantale and Giritale had come into existence, further expanding the irrgiation network. However, from the eighth century to the end of the Anuradhapura Kingdom, there wasn't much activity in construction of irrigation works.

Technology

Advanced technology was required for the planning and construction of large reservoirs and canals. When constructing reservoirs, the gaps between low ridges in the dry zone plains were used for damming water courses. Two different techniques were used in construction; one method involved making an embankment using natural rock formations across a valley and the other involved diverting water courses through constructed canals to reservoirs. All the reservoirs and canals in an area were interconnected by an intricate network, so that excess water from one will flow into the other. The 54 miles (87 km) long Jayaganga has a gradient of six inches to the mile, which indicates that the builders had expert knowledge and accurate measuring devices to achieve the minimum gradient in the water flow. The construction of Bisokotuva, a cistern sluice used to control the outward flow of water in reservoirs, indicates a major advancement in irrigation technology. Since the third century, these sluices, made of brick and stone, were placed at various levels in the embankments of reservoirs.