Water Management System
The water supply and drainage system of the rice fields depend mainly on the tidal water levels in the adjoining canals. A relatively high land level in relation to the high tide water level in the canals is a main limitation for water supply for most of the swampland. Rainfall is not sufficient to provide sufficient water for a percolation rate of 8 mm/day. (See also Web page Thesis for reasoning for the need of a high percolation rate). Pumped irrigation is not yet part of the system. Usually the maintenance of the canal system is the main problem for good drainage.
Problems encountered:
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Water supply. For most rice fields in the swamplands it is difficult to provide sufficient water when depending on rainfall and irrigation supply at high tide. Fields are located too far away from the canals to receive water in time in the field ditches and or on the land. In most areas the land levels are equal or higher than the high tide water levels in adjoining canals. This makes the water supply system even more critical, as in this case the water can be supplied to the field only via a very intensive distribution network of shallow field ditches. These ditches should be filled with freshwater at high tide. |
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Pump irrigation. Pump irrigation is still in its early stage of development. The farmers tend to use the irrigation water to flood the fields and to retain the water as much as possible. However water applied to a field very quickly disappears by the high permeability of the surface soil and farmers feel they have applied the water in vain. Based on practical experience the best solution is to apply irrigation water during land preparation and puddling of the surface layer, making use of a two-wheel tractor, followed by irrigation during early growth of the rice plant. In this period leaching is most effective for good growth and farmers have less a feeling they waste the water. For the rest of the rice growth the rice plant should rely completely on rainfall, except for extreme dry periods. The implementation and acceptance of this technique needs also time. |
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Drainage system. Maintenance of the whole drainage system is of paramount importance to maintain the drainage capacity. That means frequent cleaning from plants and re-digging from main canal up to quaternary drain. During high rainfall periods, recommended for drainage and percolation, the average water level in the adjoining canal should be at least 30 cm below field level to provide sufficient drainage potential in the field. Usually the design is correct to provide sufficient drainage depth potential in the field, but it is maintenance of the drainage infra-structure that is the problem. Often farmers lack also interest in maintenance because, by the absence of an on-farm water management system, they also have no idea what could be the positive effect of canal maintenance. See also the negative influence of the ditch wall effect. |
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Percolation. In China tile drainage proved to be the most effective sub-surface method for percolation followed by mole drainage. Least effective in improving percolation were open ditches and surface drainage. While open ditches are not effective because of the ditch wall effect, they also have to be re-dug deeply for every crop and often they are full of weeds. This contributes to a very high workload for the farmers. Therefore farmer's lack interest to maintain them. |
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Water control. The irrigation and drainage system is controlled by control structures at secondary or tertiary level. Active Water User's Associations are extremely important. Farmer's groups must decide and agree on Operation & Maintenance and the cropping pattern. These farmer's groups are still weakly organized, also because there are no clear instructions how to operate the system. The present system at field level is not adequate to provide sufficient supply and percolation. |
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Dense supply system. The construction of an intensive canal system which enables supply within a few hours at high tide to all rice fields is essential.That means that the supply system should be extended by shortening the distance from canal to each particular rice field.The shorter the distance the better. Most ideal would be the situation that each field is bordering a canal. The field distance to the tertiary canal should be not more than 100 m to enable sufficient supply or drainage in the fields during the tides. That means the distance between two tertiary canals should be not more than 200 m. In practice ISDP proposed so-called sub-tertiary canals of not more than 60-80 cm wide, situated in between two larger tertiary canals at 400 m distance. See on Web page Design Macro the drawing of the lay-out. |
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Sub-surface drainage. The swamplands are characterized by soft and pyritic subsoil's. In the subsoil often also large roots and tree-trunks are still present. In these conditions it would be advisable not to use mole drainage or tile/pipe drainage. The best solution might be subsurface drainage, using narrow trenches filled with wood-twig bunches, covered by soil material. In Holland good experience exists with this sub-surface drainage method in comparable conditions as the swamplands of Indonesia. Sixty to seventy year ago they were used for the new polders in the former Zuiderzee. These polders were characterized by soft soils in the early years after reclamation. In later stages tile drainage can be recommended after the soils become more mature. |
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Double connected canals. No dead-ended canals are essential to avoid stagnant water conditions. Stagnant water conditions promote acidification of soil and canal water. Dramatic improvements of acidity can be reached after establishing a double connection: from pH 3.0 to pH 6 in just one month. |
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One-way flow in main canals far away from river. In main canals, situated far away from the river, it is important to promote one-way flow in the main system to avoid slackwater conditions and acidity accumulation in the canal water. One-way flow can be obtained by the construction of main floodgates in the canals with only one-way flow objectives. At one side of the main canal system tidal water is running only into the scheme at high tide, at the other side of the main canal system the canal water is only drained out at low tide. |
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One-way flow in main canals bordering peat domes. In areas bordering peat domes, the water running from the peat dome should be collected into a collector drain and further used for flushing the main canal system. It is very important that the run-off from the peat dome is controlled with drop structures to avoid excessive water losses and keeping water levels high enough in the adjoining agricultural land. |
Proposition :
- The construction of an intensive water supply system up to field level for areas with land levels less than 20 cm above mean high tide level in the adjoining canal during springtide.
- The construction of a sub-surface drainage system using wood-twigs. This system could be used to improve percolation, but also could be used as a sub-surface infiltration system for relatively high areas to prevent a deep drop of groundwater tables during the growing season of rice.
- Training for farmers to encourage the optimum use of these systems for water management.
- Encouraging use of tractor and applying pump irrigation during land preparation and early growth of the rice plant
Design has a major effect on potentials in the area. There are a number of important factors to be taken into account when preparing a design for a new project or upgrading existing Swamp Schemes.
Design options and Soil Surveys. There are five types of hydrological/soil characteristics distinguished in the swamps with a major effect on the design options.(See WebPage Design Macro) Using conventional soil survey data are apparently not sufficient to determine potentials in the area and to make an evaluation and recommendations for swamp development. It is obvious, based on the experience in Indonesia, that design of a scheme, in combination with hydrological/soil characteristics, determines these potentials. Conventional soil surveys ignore the topography, the hydrology and design lay-out of the swamp scheme system. It should be realized that only a combination with design and hydrology will make soil characteristics feasible for land evaluation purposes. (See WebPages Yields and Information) It also appears that presently recognized soil types are often too complicated and are difficult to map and will have little relation with the real potentials.
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See also Table of Contents.