A New Approach to Water Management in Direct Tidal Swamps with Acid Sulphate Soils

This report concerning the development of a new water management system in direct tidal swamps with acid sulphate soils with the Pulau Petal area in South Kalimantan as an example. The most important characteristic of acid sulphate soils is that the soil contains the mineral pyrite that oxidizes when the soil is aerated and produces sulphuric acid. To prevent the soil from acidification it must be kept waterlogged and if this fails or acid or toxic elements are already present they must be leached out, which requires large amounts of water. The state of the art of lowland development in Indonesia and the formation, ripening and classification of acid sulphate soils are discussed briefly.In relation to this alternative reclamation strategies are laid out. The Pulau Petak area is an island formed by the rivers Barito, Pulau Petak and Kapuas and the Java sea and is a lowland area with acid sulphate soils. Existing research programs in this area are used to picture the general situation. Data on tidal movements in the area are scarce and had to be calculated. Transmissivities are extremely high varying from 90 to 600 m^/day. After these general chapters the study focuses on the particular boundary conditions of the area with direct tidal influence i.e. gravity irrigation is possible at least during springtide and drainage is always possible.This lead to the objective to design a new water management system that provides the irrigation and drainage facilities for one local rice variety per year with the restrictions that drainage water must not be reused for irrigation as it can be acid and interflow of acid ground water must be intercepted. The new water management system will have a completely separated irrigation and drainage system where a one-way-flow is induced by installing an inlet sluice at the entrance of the secondary irrigation canal that closes when the flow direction reverses and an outlet sluice which allows only water to flow out is installed in the secondary drainage canal. The water level in the irrigation canal will be kept as high as possible and the water level in the drainage canal will be as low as possible. The strategy on tertiary level during the cropping (wet) season is to supply water for the plant and if possible to leach the soil. If a shortage of irrigation water to fulfil the needs for both these practices occurs the water level in the tertiary drain is set up to reduce the leaching component. In the dry season the water level in the tertiary drain is set in order to keep the soil water logged and irrigation is only applied when the water table tends to drop below the soil surface. There is an intermediate period of a month that proceeds the cropping season where the objective is to maximize the leaching flow within the boundary condition that the soil stays water logged. From a control point of view the above can be translated in an open irrigation system to minimize the headless and upstream control water level regulators in the tertiary drain that can maintain two setpoints. For this purpose a Vlugter gate was modified in such a way that a switch can be made in the setpoint that is maintained by increasing or decreasing the counterweight. The design of this gate and the size of the counter weight that should be added to switch from the low to the high setpoint are given. Finally a feasibility study of the project was executed where the internal rate of return was used as indicator of feasibility . With a project lifetime of 40 years and a construction time of two years the internal rate of return appeared to be 15 %. The construction costs per net arable hectare have a present value of $1771. Conclusion is that it appears to be a feasible project and should therefor be carried out. In similar areas the system could be used to reclaim new lands. Further study into the quality of the irrigation water in the dry season is recommended in order to investigate the potential for growing two crops per year,and an analysis of the effects of the counter weight adjustments on the gate behaviour of the modified Vlugter gate is advisable.