By Derrick Schokman

Proceedings of workshop organised by Working Committee on Agricultural Science and Forestry
Published by National Science Foundation (Rs. 400)

Water is the foundation of life on earth. Surprisingly relatively little of the world’s freshwater is needed for drinking and other personal use. About 20% goes to industry and 70% more to agriculture. So it in agriculture that future water shortages will be most acute.

In Sri Lanka, water shortages for agriculture are most felt in the dry zone. This is why the ancient people, over a period of 15 centuries, constructed an intricate network of inland reservoirs (tanks), which we have inherited to conserve and put to maximum use the rains that fall during a very short period. While the large tanks have been restored and put back into operation, small tanks which are considered to be less efficient economically have received orphan treatment.

There are about 12,000 small tanks functional in the dry zone today, mostly in the Rajarata. By and large they are interconnected in a cascade system (ellangawa) where the overflow from the upper tanks is collected in tanks at a lower elevation and used to irrigate the rice fields during the "maha" season only. Even then the highly variable rainfall acts as a limiting factor on cropping intensifies and production levels which have remained well below the norm in major irrigated schemes.

Consequently on the advice of the former Minister of Science and Technology, the Hon. Batty Weerakoon, a workshop was organised by the Working Committee on Agricultural Science and Forestry (National Science Foundation) under the chairmanship of Prof. H. P. M. Gunasena to examine and report on small tank settlements.

Ten Papers

Ten papers were presented at this workshop by a wide cross-section of people with the knowledge and experience of small tank settlements.

Namely W. J. Siriweera, Vice Chancellor of the Rajarata University; M. U. A. Tennakoon, former Executive Director of the Central Bank; Vijya Jothi C. R. Panabokke, Research Fellow at the International Irrigation Management Institute; S. Somasiri, former director of the Natural Resources Management Centre in the Department of Agriculture; M. M. M. Aheeyar of the Hector Kobbekaduwa Research and Training Institute; W. M. U. Navaratne of the Mahaweli Restructuring and Rehabilitation Project; K. R. de Silva of the National Irrigation and Rehabilitation Project; P. B. Dharmasena of the Field Crops and Development Institute of the Department of Agriculture; D. D. Prabath Witharana of the Department of Agrarian Services; and H. Somapala of the N. C. P. Participatory Rural Development Project.

These papers trace the historical evolution, distribution and socio-economic aspects of small tank systems in relation to food security. They provide a necessary backdrop to understand the complexities of small tank settlement development. M. M. M. Aheeyar gives a concise account of the several institutions that managed the supply of water from these tanks, beginning with village communities (gamsabawa system) during the time of the Sinhalese kings, followed by the breakdown of this system when "rajakariya" was abolished during the early British colonial period, and the subsequent introduction of various forms of state control via provincial Government Agents in the "kachcheris", the Irrigation Department, Department of Agrarian Services and Farmer Organisations.

These changes in institutional control could have had a depressing influence on the implementation of a stabilised programme of improvement. But more importantly increasing population pressure and an expanding food front into the catchment areas of these small tanks has put a brake on improving food production.

In addition to restricting rainfall, deforestation to permit highland cropping, subsequent erosion and silting of the tanks, and the ad hoc sinking of agro-wells has created an imbalance between the water required for irrigating the rice fields and what is available.

Landmark Study

A landmark study by Panabokke of the size, form and hydrology of 230 small tanks in the Rajarata (1999) has clearly demonstrated that although 197 have adequate catchments, 190 of them have an excess of command that cannot be serviced by present tank capacities within the cascades.

It is clearly evident that any sustainable food production within these small tank cascade systems is strongly governed by the need to achieve a balance between the supply and demand for irrigation water.

The present practice of raising the level of tank bunds to increase tank capacities does not really serve this purpose. It has been questioned by Dharmasena who believes that it only creates a larger and shallower body of water that adds to the already high water loss caused by evaporation.

It also does nothing to reduce sedimentation. If this practice is allowed to continue, small tanks in time could be reduced to swamps and finally disappear. In this context Dharmasena’s alternative method of partial desilting should receive serious attention.

Planned

Improvements in small tank irrigation need to be planned on a watershed basis, considering both surface and ground water resources. Panabokke’s assessment of small tank cascades opens the door to a more valid base on which sustainable improvements can be planned and implemented. Cascades lend themselves better to quantitative analysis than individual tanks as they are closer to a natural system.

This should be considered in conjunction with the proper siting of agro wells. Senaratne, in a study of 50 cascades within the Anuradhapura District (1996) showed that a maximum of 3600 agro wells could be safely accommodated without upsetting ground water resources. But even at the time of this study the number of wells within five of these cascades had exceeded the upper critical limit. "The red signal" says Panabokke has been flashed and it is time now to take suitable control action to prevent any further expansion of agro wells in these stressed areas... well proven guidelines are now available for estimating location, spacing and density in the regolith acquifer."

No Easy Fixes

There are no easy fixes to the improvement of small tank agriculture, which is bedevilled by variable rainfall and the high risk factor involved in dry zone agriculture. Somasiri has called attention to the many less than successful attempts that have been made to increase the cropping intensive of rice in the lowlands and to stabilise upland farming.

In the light of these results there is scope to question the opportunities that would become available for a positive advancement of agricultural production within the tank cascade systems. But as small tanks constitute a very important part of the rural landscape, the participants at this workshop looked beyond the economic issue to the social and environmental aspects as well that contribute to a desirable existence.

It is the contention of Prabath Witharana that the State has failed to give adequate consideration to the social and ecological aspects of small tanks owing to a the paucity of a reliable database laid on natural resource management. This deficiency is not there any longer. The Department of Agrarian Services now has a computerised database on village irrigation schemes, consisting of 76 main attributes that could be linked to individual or cascade small tank systems with the aid of geographical information mapping systems to provide state officials with a better understanding of the real needs of small tank communities.

M. U. A. Tennakoon refers to the small tank cascades system "as a classic example of man’s ability to maintain a symbolic relationship with nature". In these circumstances there is a strong rationale for ensuring its sustainability. The information gathered in this publication should provide a launching pad towards that end.

The writer is the former information officer the Department of Agriculture and the Council for Agricultural Research Policy.