Dr. P.Vidanapathirana
Senior Lecturer
Postgraduate Institute of Archaeology

Agricultural activities in the settlements in Dry Zone of ancient Sri Lanka seem to have been mainly dependent on a man-made irrigation system. The general condition in the Dry Zone necessitated a continuous maintenance of a systematic irrigation pattern. The environment of the Dry Zone was naturally water poor due to annual and prolonged droughts. The tank system was a positive response to the challenge demanded by the natural phenomenon. The tank, the dam and the canal bear witness to the hydraulic engineering ingenuity of ancient Sri Lanka. Within this irrigation system there flourished an agricultural pattern resulting in a self sufficiency in food in a dry yet fertile soil (Farmer 1951:3-16; Murphey 1957:181-200; Thambyahpillay 1964:88; Gunawardana 1971:170).

In Dry Zone of ancient Sri Lanka, primitive methods of water control and the original use of irrigation started from the storage of flood water in small tanks constructed in streams and river valleys. The rich and fertile soil in river valleys and stream beds was in the peneplain of the Dry Zone ideal for rice cultivation. The territorial communities were dependent on small irrigation tanks and rainfall agriculture was controlled by the natural gravity flow in the landscape. With the introduction of irrigation technology and consequent expansion in large scale cultivation and production of rice, changes occurred in the village based economic landscape morphology in the first, second and the third order in dendrite pattern of the catchment areas (Vidanapathirana 2007:199). In time rice cultivation flourished along the tributary flood plains.

The regional irrigation systems at the beginning seem to have been initiated and managed by families, family groups or small community groups. These irrigational units were either private properties or small scale properties under different households. Thus, according to Leach and Murphey, the history of irrigation systems in Sri Lanka is the result of a long relationship or even a struggle between villagers and the ruling elite (Murphey 1957:187; Leach 1959:17).

Number of the tanks in the Dry Zone of Sri Lanka

On several occasions efforts have been made by various researchers to gather statistical data about the number of tanks in the Dry Zone of Sri Lanka. Thus, according to the 1855 administration report of Flanderk there had been 2000 tanks in the Nuvarakalaviya (CSP 1855). Ievers in his administration report mentions that there had been 2877 tanks in the Nuvarakalaviya alone (CPS 1886:138). Kennedy, an irrigation engineer estimated the number of small tanks in the Dry Zone to be 15,000 (Kennedy 1933:229). According to Cook, the number of small village tanks marked on the one inch survey map of Sri Lanka was about 12,000 (Cook 1950:75). Ceylon Survey Department, survey maps published in 1922-32 count up to about 11,200 tanks (Brohier 1945:2-7).

These statistics may have been restricted to the number of tanks already in use during the period of the survey. Scholars at different stages attempted a statistical survey regarding the number of tanks in the entire Dry Zone of Sri Lanka. Madduma Bandara states that there are 20,000 small and large tanks in the area of the entire Dry Zone (Madduma Bandara 1985:101). Mendis in his research says that there are 30,000 irrigation tanks spread over the same area (Mendis 1986:22). Panabokke restricted his research to the Anurādhapura district in the Dry Zone and the number of tanks are 4000 (Panabokke 2000:124). In Dry Zone where there is an annual rainfall of less than 1500mm, there nearly 18,000 small irrigation reservoirs scattered over the region (Weerakoon & Herath 2001:19). Based on a cartographic plotting of the Malvatu Oya and Kalā Oya basins from the one inch topographical sheets, it is noted that there are 3431 major, medium and village tanks spread out within the main river basins and their tributaries (Vidanapathirana 2007:191).

Catchment morphometry and tank distribution pattern

The river basin is a systematic natural open hydraulic system accepting and rejecting the incoming and overflowing water freely without restriction. The catchment ecosystem includes geological structure, soil profile, climate, gradient and shape of the basin, vegetation and finally human action. The above factors influence the formation of the stream network as a single component. The distribution pattern, the density and the nature of the tanks depend on the geological structure, soil profile, the amount of seasonal rainfall, gradient and also the nature of the geographical formation of the valley.

The lithology of the underlying basement rock of the Dry Zone’s upper basins is the Highland Series of Precambrian rock consists of chanockitic gneisses and some metasediment of the khondalite rock (Cooray 1995:5). This formation of rock provided the construction material for dams across streams and canals for the storage and carriage of water. Origin of parent material of soils weathered gneisses rock large amount of the stone gravel of the khondalites rock of Highland series. Thus, favourable ground formation was also used as a foundation layer in constructing large reservoirs (Kularatnam 1978:219).

Thus, soil profile in the upper basins of the Malvatu Oya, Kalā Oya south of the Matalē hill slopes belong to the area consists of dark reddish brown sandy loams to sandy clay loam surface soil as thick as twenty four inches (Haunting Survey Corporation [HSC] 1963:68). The layer beneath, with a thickness of twenty four to thirty inches has a high concentration of fine quartz, gravel and clay. The marked increase in clay in the ‘B’ horizon has a prolonged retaining effect on the internal drainage of the soil. Though there is less possibility of construction of tanks in the gradient of this area, small tanks were constructed within the valley bottoms in the upper basins of the catchment area.

The annual rainfall in the upper basin to the lower basin is the Dry Zone area of around 1100mm to 2300mm. Seventy percent of this rainfall occurs during the months of October to February. The annual run off in the island is about five million hectares and sixty five percent of this run off is wasted into the sea (Manchanayake & Madduma Bandara 1999). It was out of this percentage that water was utilized by the ancient hydraulic technicians to irrigate the entire Dry Zone. Under the prevailing climatic conditions, the soil of the Dry Zone has a tendency to encourage the rapid destruction of its organic matter and due to seasonal torrential rains this soil is subject to heavy erosion. Varying agricultural practices have contributed to the maintenance of a fertile soil structure favourable for agriculture. Changing agricultural practices from the earliest times have added to the improvement of the soil (Jayasena 1995:87; Jayasena & Dessanayake 1995:12).

The condition of the geological formation of this bed rock is such that there is a possibility of storing a high percentage of ground water in large quantities. The ground water storage in a layer of this nature is high in percentage. Thus during the rainy season water remains stored in this particular ground water table. As the dry season approaches, the water table declines and towards the end of the dry season water is found only in isolated pockets of crystalline rock, or in places where water is stored in artificially formed tanks or canals (Vitanage 1958:13-18; Sirimanne 1974:115).

A continuous water table occurs only for a short period after the rainy season and as the dry season approaches, the water table declines towards the end of the dry season. Finally, water remains only in pockets of decomposition in the crystalline rock floor in the Dry Zone areas where the water table is artificially maintained by a tank irrigated through a canal (Sirimanne 1974:113). Thus, the ancient inhabitants expertly utilized this particular geological structure for their irrigational engineering feats with the construction of major, medium, village tanks and canals. The ancient irrigational engineers were skilful enough to protect the river from floods by constructing major tanks at points were the flood water gathered from several streams and rivers.

The gradient in rolling topography and the slope ranging varies in the Dry Zone peneplain. The slope gradient thus, commences from zero and goes up to four percent. In the second successive area of undulating topography the slope ranging gradient occurs at four to eight percent. In the third rolling topography, the slope ranging gradient is from eight to fifteen percent. In the balance hilly topography with slopes, the gradient always exceeds fifteen percent (HSC 1963:117-122). Within the above geo-ecological formation, water could be stored in tanks only in a topographical setting where the gradient is from zero to four percent. Thus, the tank in the zero to four percent gradient is limited only to river valleys (Vidanapathirana 2007:195).

The spread of tank density is less in the Matale foothills due to the steep gradient in the hills. In the eastern boundary of the upper basin, the density of the spread of tanks is more as the gradient in the land is less, providing a favourable background for the constructed of tanks. There is a low density of tanks as the underlying subdivision is both extensive and intermixed with areas of rock knob plains, ridges and a marked feature of the soil catena. Soil catena is a sequence of soils derived from similar parent material and climatic conditions but having different characteristics due to variation in relief and drainage, the presence of rolling hilly subdivisions. The natural drainage pattern, the shape of the valley, the zone of flowage has also contributed to the density of village tanks in the area. The geographical area discussed above is the upper catchments of rock belonging to the khondalite group. There is also the appearance of quartzite, crystalline limestone and gneiss of the khondalite group within the same formation (Cooray 1984:87; Almond 1995:9). The quartzite and gneiss stand out as ridges while the limestone occupies valley bottoms. Natural springs or ulpota or bubula occur from both these layers (Vitanage 1958:161; Sirimanne 1974:115).

The central and lower catchment areas of the Malvatu Oya and Kalā Oya basins are mainly underlined by a rock of a geological transition zone and also by the granite and granulate gneisses of the Vijayan series (Cooray 1984:108). Yet, in the area around the river mouth are seen deep deposits of sub recent alluvial deposits. Below the alluvial deposits along a large area of the north-western coastal belt is believed to be a Miocene limestone layer consisting of a Pliocene and Holocene deposit of a large thickness (Sirimanne1974:113; Swan 1983:10; Jayasena 1993:85; Dessanayake 1995:12). It was on this particular thick layer of alluvial soil that a number of tanks were constructed by the ancient settlers in the mouth of the Aruvi Āru (lower basin of the Malvatu Oya), Mōdaragam Āru and Kalā Oya. In the Vilpattu and Murunkan in western coastal area the number of ancient settlements are limited of Occappukallu, Sinadiyagala, Galgē vihāra, Valevehera, and Tōnikallu as resources are available only among the Precambrian erosional remains in the area. Thus, the sellers had constructed small scale tanks with available and limited resources.

Ancient settlers made use of coastal fluvial and delta plains and associated water ways in the area to construct tanks for the storage of water. The major river system often stored a considerable amount of ground water. The area covered here is about 40-50 ft. in thickness, resulting in a major storage of ground water. In the topography of the area are seen limestone outcrops at intervals (Sirimanne 1974:113-115; Jayasena 1995:89).The plain here is notably lacking in surface drainage and there are many circular water field depressions locally known as villus. In the limestone belt in the area fresh water occurs above sea level. This water had to be conserved carefully for the human consumption of fresh water and as a result the technique of ‘ūra-kata-lin’ is seen in place like Kollankanatta and Kudiramalai in western coastal area from ancient time (Brohier 1929:394). The ūra-kata lin were constructed by setting together earthen rings made by potters. This technique was developed for obtaining fresh water from the streams running in between the sandstone boulders found below the Miocene limestone layer. There is a lack of artificial irrigation in this region for the purpose of agricultural although the soil may have been suitable for agricultural activities. The reason is that the land does not have the proper topographic nature for gravity flood irrigation and the land remains at the same level with a slope ranging gradient from zero to two percent (HSC 1963:67). This geographical formation has barred the area from having a developed irrigation culture.

The subdivision in the middle river basin is often interrupted by several tracks of rock knobs plains, erosional remnants and mantled plain quartzite shaped in narrow strips with a north-south trend that confirms to the appearance of the underlying bedrock and also by strips of flood plains along some of the larger river. There is a thicker density of village tanks in the middle basin of the Kalā Oya and Malvatu Oya as the area has a gradient between four and five percent. Here, an extensive area of the level topography is ideally suitable for large scale irrigation, cascade tank systems and gravity flow irrigation systems. The micro-relief and the shallow valley nature in the catchment areas along with the underlying rock structure running both towards the northern and the southern directions had been a geological structure which, immensely facilitated the construction of tanks in the area ( Panabekke 2000:126). Thus, in the general surface geo-structure of the middle river basin is seen at times rock knob plains, erosional remains, ridges, rolling hills and shallow valleys. The composition seen in Anurādhapura, Seppukulama and the east of Maha Kanadarāva, Kattiyāva, Maradankadavala and Nägama also contributed to the tank density in the area.

River ordering system and tanks distribution pattern

In geomorphology, it is customary to classify stream canals in a hierarchy, in which behavioural pattern of a river and a catchment came under scrutiny for the first time by Horton in 1945. His hypotheses could not be accepted as practical sense due to his limiting the study only to arid and semi-arid areas. Attention was later drawn to a broader analysis by Hewlett’s hypotheses in 1961 by expanding the study to cover more countries in the world. Strahler introduced a hypothesis in 1964, after the study of the stream ordering system which, was more applicable to a tropical environment (Schulz 1974; Brutsaert 2006:442). In this system, tributaries without branches are called first-order streams; two branches are designated as second-order streams; third-order streams can only be formed by the joining of any two second-order segments. The Strahler method as it is illustrated in figure 1, in this example, there are eight first-order streams, three second-order streams and one third-order stream, where the shape and the catchment ecosystem always the divisions in river ordering.

Fig. 1 Stream order system (Strahler hypothsis)

In analysing the stream order and the tanks distribution pattern in Malvatu Oya and Kalā Oya basins, through the geographical information system, the hypothesis of Strahler being applied as a base as his analysis seems to be ideally applicable to the stream ordering system in the Dry Zone of Sri Lanka. Maps1 & 2 indicate the tank density in the Malvatu Oya and Kalā Oya basins and its tributaries and also the hydraulic regime and the spatial distribution pattern and its interaction with the river ordering system and the tank distribution pattern. Thus, the village tank density is more in the first and the second order while major and medium tanks are seen in the third, fourth and the fifth order stream. Village settlements and village tanks are more frequent in the first, second and third order stream. In examining the location of ancient settlements, one could easily find a scattered landscape with a host of minor tanks in the Dry Zone. The village settlements in ancient Sri Lanka were mainly based with village tanks associated with micro catchment areas as the nucleus.

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