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Village Tank Agriculture in the Dry Zone

2012

Sustainability of the traditional tank-village irrigation system had been maintained in the past simply not only from structural maintenance. Each and every component of the eco-system was given due consideration. The attention was paid not only on macro-land uses such as paddy land, settlement area, chena lands, tank bed, etc. but also on micro-land uses such as goda wala, iswetiya, gasgommana, perahana, kattakaduwa, tisbambe, kiul-ela, etc.

Special note prepared for the Training Workshop for District Planning Directors and Samurdhi Assistant Commissioners – 28th June 2012 VILLAGE TANK AGRICULTURE IN THE DRY ZONE P.B. Dharmasena, [email protected] INTRODUCTION The dry zone accounts for nearly 60 percent of the total land area (6.54 million hectares) of Sri Lanka. The land that can be used for agricultural purposes in Sri Lanka is about 2.71 million hectares, of which about 48 percent is in the dry zone. From a long time, the dry zone settlers had realized that the pre-requisite to life and culture in any form, in Sri Lanka's dry zone is the water reservoirs and the intricate network of canals, which water the land. Hence, the basic feature of the traditional farming system has become the man-made small reservoirs (village tanks), which are constructed by damming river tributaries or streams and linking the large reservoirs, through anicuts and canals, with perennial source of water in the wet zone. TRADITIONAL VILLAGE ECO-SYSTEM The farming system of the dry zone communities is characterized by its three-fold pattern of land use. Rice, the Sri Lankan's staple food crop is grown in the irrigable lowland mainly in maha season and perhaps in yala season depending upon the water availability in the village tank. The village hamlet (gan-goda) is in either side of the rice grown area (wel-yaya) usually below the tank with perennial crops and vegetables. With the influence of tank water and because of lower elevation and imperfectly drained soils, most of the fruit crops such as mango, jak etc., coconut and some vegetables are easily grown in the home garden. `Chena' the third component, is the oldest farming practice of dry zone villagers, a form of shifting cultivation in the upland using direct rainfall. Traditional wisdom in agriculture and the living has not been developed within few decades. It is a long time-tested knowledge, which created an environmentally adapted, disaster tolerant and sustainable living system. Their agriculture had been adjusted to absorb any weather vagaries by shifting the cultivation time and selecting farming practices. They cultivated chena and paddy lands according to the seasonality of rains thus; at least they could get a successful harvest from one cultivation. ‘Kekulama (dry sowing), Bethma (shared cultivation), Thaulu govithena (tank bed cultivation) etc. are the best examples showing how they could avert the drought effects on their farming. Traditional communities made every attempt to conserve soil, water, and natural habitat. Food security was one of the in-built aspects of their culture. Use of groundwater for agriculture was never practiced by them, and it ensured the water security. An adequate dead storage was found in tanks to be utilized during dry period for all purposes and had 1 Special note prepared for the Training Workshop for District Planning Directors and Samurdhi Assistant Commissioners – 28th June 2012 been the only source of water for cattle and wild animals. There was a broad diversity in flora and fauna in and around and the availability of water in the tank during the dry period assured their survival. Sharing resources equally and the equity of ownership were the most striking features of their culture, which led to build up a peaceful and sustainable rural society. Environmental pollution was not a topic for discussion. With the disappearance of the features discussed above, the whole system was subjected to deteriorate socially, physically and economically leaving vulnerability to disasters with them. Sustainability of the traditional tank-village irrigation system had been maintained in the past simply not only from structural maintenance. Each and every component of the ecosystem was given due consideration. The attention was paid not only on macro-land uses such as paddy land, settlement area, chena lands, tank bed etc. but also on micro-land uses such as goda wala, iswetiya, gasgommana, perahana, kattakaduwa, tisbambe, kiulela etc. Geographical setting of these land uses, and descriptions and importance of them are discussed below (Fig. 1). Gasgommana (tree belt) - It is the upstream land strip above the tank bed, accommodating water only when spilling. Large trees such as kumbuk, nabada, maila, damba etc. and climbers such as kaila, elipaththa, katukeliya, kalawel, bokalawel etc. are found in this area. This vegetation is natural and seeds are floating on water. The gasgommana acts as a wind barrier reducing evaporation from the tank and lowering water temperature. It gets closure to the bund from either side where roots of large trees make water cages creating breeding and living places for some fish species. This strip of tree demarcates the territory between human and wild animals. Perahana (meadow) – It is the meadow developed under gasgommana and filters the sediment flow coming from upstream chena lands. Iswetiya or potawetiya (soil ridge) - An upstream soil ridge constructed at either side of the tank bund to prevent entering eroded soil from upper land slopes. Godawala (water hole) - A manmade water hole to trap sediment and it provides water to wild animals. This might had been a strategy to evade man-animal conflict. Kuluwewa - A small tank constructed above relatively large reservoirs only to trap sediment and not for irrigation purpose. It provides water for cattle and wild animals. Tisbambe – It is a fertile land strip found around the settlement area (gangoda) and does not belong to anybody. Tree species such as mee, mango, coconut etc. are grown in 2 Special note prepared for the Training Workshop for District Planning Directors and Samurdhi Assistant Commissioners – 28th June 2012 scattered manner. Mostly this area was used for sanitary purposes as the resting place of buffaloes. Buffaloes were used as a protection mechanism from wild animals and malaria. Fig. 1. Tank-village ecosystem Kiul ela – This is the old natural stream utilized as the common drainage. Tree species such as karanda, mee, mat grass, ikiri, vetakeya etc. and few rare small fish species are also found in water holes along the kiul ela. Most importantly it removes salts and iron polluted water and improves the drainage condition of the paddy tract. Kattakaduwa (downstream reservation) – This is a reserved land below the tank bund. It consists of three micro-climatic environments: water hole; wetland; and dry upland, therefore, the vegetation developed is rich in diversity. This land phase prevents entering salts and Ferric ions into the paddy field. The water hole referred to as ‘yathuruwala’ minimizes bund seepage by raising the groundwater table. Villagers plant vetakeya along the toe of the bund to strengthen the bund stability. In fact it is a common village garden, where people utilize various parts of the vegetation for purposes such as fuel wood, medicine, timber, fencing materials, household and farm implements, food, fruits, 3 Special note prepared for the Training Workshop for District Planning Directors and Samurdhi Assistant Commissioners – 28th June 2012 vegetables etc. Specifically they harvest row materials from this vegetation for cottage industries. TANK CASCADE SYSTEMS Small irrigation tanks do not exist as individuals. Natural drainage system in a watershed is blocked by earth bunds in appropriate locations to store water forming a series of tanks along the drainage. The drainage pattern formed in the undulating topographic formation in the dry zone landscape can be classified as dendritic drainage pattern. This ramifying nature of the drainage system has led to form clusters of small tanks found in series, which are connected to form a system known as ‘tank cascades’ (Fig. 2). Fig. 2. Tank cascade systems in the dry zone Existence of small tanks in a cascade pattern is an advantageous feature in many ways. Surface water bodies spread over an area can maintain the groundwater level closer to the land surface at least in lower portions of the minor basins. It can be stipulated that absence of such a branched system of tanks could lead to rapid depletion of groundwater due to natural gradient of the drainage system. Therefore, in the absence of tank cascade systems natural vegetation seeing now would have not been in the same composition with deep-rooted large tree species found in the various positions along the catenary slope. 4 Special note prepared for the Training Workshop for District Planning Directors and Samurdhi Assistant Commissioners – 28th June 2012 Upper tanks in a tank cascade system act as buffer reservoirs to absorb flood-generating rainfall, which would otherwise bring the risk of breaching lower tanks. Similarly, these upper tanks are buffer reservoirs to supply water to the lower tanks when they are in short of water to save the crop. Since the tanks exist not in isolation but as clusters, and they are hydrologically inter-related, planning for individual tanks could create conflicts in water resource management among them. ‘STATUS QUO’ OF SMALL IRRIGATION SCHEMES When compared rice yield data in small irrigation with that in major irrigation schemes, it is clear that the rice yields are always lower in small irrigation schemes. This situation has emerged due to several reasons. Most important factors are low level of crop and water management, lack of proper weed pest and diseases management, poor tillage operations and lack of proper drainage. Cultivable extent from small tanks decreases gradually due to tank sedimentation and high tank water losses. Sedimentation of tanks not only causes reduction of storage capacity but also leads to alter the tank bed geometry. Subsequent rehabilitation works, where the capacity has been improved by raising the spill and the tank bund have created a shallow water body spreading over a larger surface area. This makes the situation more complicated creating several other problems. They are: a) inundation of upstream paddy lands; b) development of salinity conditions in the upper area; c) increase of tank water losses; d) disappearance of the tree strips in the high flood region (Gasgommana) and the grass cover (Perahana) underneath; and e) disappearance of some indigenous fish species, which cannot survive in shallow waters or do not find a favourable breeding environment. Water losses from small tanks are very high. Within a period of 2 – 3 months since the seasonal rains cease, most of the tanks appear as somewhat marshy lands infested with aquatic weeds. Total tank water loss through evaporation and percolation varies from 35 to 90 percent depending upon geometry of the water body. Water losses are higher from tanks with shallower water bodies than those with deep water. Therefore, it is clear that tank bed geometry determines more the water storing efficiency of a tank than other factors do. These results indicate that about half the storage stored in a tank would not remain to irrigate the downstream command area. Soils in most of these tank-village areas are Reddish Brown Earths (RBE) occupying on upper portions of the land catena and Low Humic Gley soils on lower or bottom lands. Rainfed cultivation is taking place in the upper part, where RBE soils are found. Soil erosion hazard is significant in this soil on account of (a) the low water stability of the soil aggregates, (b) easy slaking of the soil macro-aggregates following sudden wetting, 5 Special note prepared for the Training Workshop for District Planning Directors and Samurdhi Assistant Commissioners – 28th June 2012 (c) rainfall intensities often exceeding the infiltration rates of the soils and (d) the undulating nature of the catenary landscape systems. Not only soil erosion but the sedimentation of small irrigation tanks could also develop several problems such as (a.) reduction of the tank storage capacity consequently decreasing the extent of irrigable area, (b.) reduction or retardation of subsurface inflow to the tank, (c.) blocking the sluices and channels with sediments, (d.) filling the dead storage entirely with silt so that no more water available in the tank during the dry season for use of people, cattle and wild animals and (e.) spreading the tank water towards upstream area which would cause inundation of upper paddy fields and high evaporation losses from the tank. PRODUCTIVITY IMPROVEMENT STRATEGIES Following strategies can be recommended to regain the sustainability of tank-village production system. 1. Crop diversification supported by supplementary groundwater. 2. Intensive agriculture using less water consuming and drought resistant crops integrated with livestock 3. Desilting tanks followed up with a strong effort to protect upstream tank catchments to prevent subsequent sedimentation. 4. New and effective institutional mechanism to small tank system management. 5. Feeding the tank cascade systems wherever possible than destroying them as attempted earlier in the Mahaweli Project area. In tank rehabilitation at present, the tank bund is strengthened, structures repaired or replaced, and the capacity lost due to deposition of sediment is regained by raising the spill and the tank bund. This has come out with the common belief that the desiltation of minor tanks would result in very low economic returns. However, scientists, planners and engineers cannot escape from the challenge of disappearing minor tanks from the dry zone landscape during next few decades. Desiltation of small tanks should aim not only at increasing storage potential and reducing tank water loss but also at protecting the tank eco-system. As desiltation is an expensive task as well as a must to undertake, it is important to develop a technical concept, which generates a low cost and effective desiltation process. The partial desiltation concept was first introduced with this background (Dharmasena, 1994) on the basis of findings from hydrological research studies conducted by the Field Crops Research and Development Institute, Mahailluppallama (Fig. 3). Sedimentation studies indicate that half of the sediment deposited in small tanks is found within one third of the tank bed area closer to bund. Thus, the same capacity can be 6 Special note prepared for the Training Workshop for District Planning Directors and Samurdhi Assistant Commissioners – 28th June 2012 maintained by removing sediment in this area and heap up in the upstream area. These soil heaps must be formed at safe gradient and stabilized with trees and grasses to prevent washing down to the tank. These mounds would appear as micro-islands, where productive plant species could be grown. These soil mounds must not block the natural drainage, which supply water to the tank. Further, to prevent sediment coming into the tank there is a need to construct a soil bund along the periphery of the desilted area except in places where natural streams enter into the tank. Fig. 3. Partial desilting concept Partial desiltation of a tank would provide various benefits to the community, some of which are difficult to assess with an economic analysis. It is quite obvious that the return to investment from desiltation is not economical if the purpose of desiltation is to increase the storage. The concept of partial desiltation is not meant merely to increase the storage unless there is a demand from the community or an additional storage potential in the system. The economic analysis should therefore, be based on consideration of following benefits in order to determine the return to investment of partial desiltation. Partial desiltation reduces the water-spread area. More than half the land inundated with tank water would be free of surface water after a successful desiltation. Water body would be confined to the portion closer to tank bund. The land area freed from water spread can be covered with perennial vegetation. This soil is fertile with nutrients and 7 Special note prepared for the Training Workshop for District Planning Directors and Samurdhi Assistant Commissioners – 28th June 2012 high level of organic matter (5 - 8 %) and also has an easy access to groundwater. In a cottage industry improvement programme, this land may best be utilized to grow Bamboo (Bambusa spp.), Rattan (Calamus spp.), Mat grass (Cyperus pangorei), Vetakeya (Pandanus spp.), Patabeli (Hibiscus tiliaceus), Palmaira (Borassus flabellifer), Kithul (Caryota urens) etc. all of which provide various row materials for cottage industries. Water storing efficiency of the tank would be increased with improvements on tank geometry by partial desiltation. Any water remaining in the tank after `maha' cultivation can be kept with minimum losses for yala cultivation. Further, this tank storage can raise the groundwater in the command area and yala cultivation can be supplemented by well water with a great assurance. Both these reasons would result in increasing the cropping intensity of the command area. Crop diversification is an appropriate option to optimize the income level of farmers and increase the land and water productivity. Field crops, perennials, and vegetables can be introduced according to the land suitability for different crops. As regolith aquifers are limited groundwater reserves, and their depletion would cause environmental hazards, only 25 percent of the potential groundwater storage in an aquifer is recommended for abstraction. In selecting location of abstraction it is recommended that imperfectly drained area is the most suitable area for construction of agro-wells. Mostly the need for an integrated approach is felt a must in the downstream area where tank irrigation and agro-well irrigation are concentrated. Soil moisture has become a constraint for rain-fed farming while water shortage is the issue of downstream farming. Therefore, integration of tank irrigation with groundwater supply is essential after given due consideration for effective use of seasonal rains. Nevertheless, conservation of soil moisture at any possible stage for any possible crop by any means would definitely increase the productivity of water as well as efficiency of water resource use. However, our experience indicates that farmers engaged in cultivation with well irrigation do not gain much benefits from this water resource and consequently receive less income. Following short-falls are associated with their farming. i. Lack of understanding on the availability of well water. ii. Farmers do not plan their cultivation to obtain maximum benefit of rainfall. iii. Moisture conservation techniques are not adopted by farmers even if their crops are suffered with water shortage. iv. Most of the agro-well farmers are still cultivating according to seasons. 8 Special note prepared for the Training Workshop for District Planning Directors and Samurdhi Assistant Commissioners – 28th June 2012 v. Farmers have no experience on irrigation layouts for conjunctive use of water resources. Therefore, on-farm water management has to be given priority as the farmer alone cannot face the challenge of avoiding conflicts in integrated water management. Future studies with respect to on-farm water management may need to concentrate on: (a) irrigation layout and scheduling; (b) reduction of water conveyance losses; (c) moisture conservation methods; and (d) crops and cropping patterns aiming at achieving maximum benefits from the integrated management effort. In integrated water management planning one should not overlook the fact that the sustainability of management would rest on farmer participation and their ensured leadership in decision making. Finally, it is noteworthy that water management planning should not be targeted at keeping water resources unused, but using them while maintaining the hydrological balance to become the eco-system sustainable in every aspect. It has been spelled out in various policy documents that rural development programs particularly land, water and agricultural developments should be based on watershed concept. This is with no exceptions applicable to the dry zone small tank cascade systems. Development programs launched in these cascades are somewhat similar and the cascade management committees formed are in operation to certain extent. Nevertheless, sustainability of such management approach even at present is questionable since institutional mechanisms and linkages with existing service agencies are not fully established. Thus, the works undertaken in the past two decades in identifying the cascade development concept warrant follow up actions. This calls for developing proper planning guidelines to identify rehabilitation and development priorities, prepare development plans for various aspects such as irrigation, agriculture, environment, livestock, forestry, cottage industry etc., and organize institutional mechanism for implementation of the said plan for sustainable water resource management. REFERENCE Dharmasena, P.B., 1994. Sedimentation and desiltation of minor tanks. IRMU Newsletter, Irrigation Research Management Unit, Irrigation Department of Sri Lanka. Vol. 1, No. 2(6). Dharmasena, P.B., 2010. Essential Components of Traditional Village Tank Systems, Paper presented at the Seminar on Cascade Irrigation Systems for Rural Sustainability heldon 9 th December 2010 at SLFI, Colombo, organized by Plan Sri Lanka. 9