by Erry Koriyanti (Dissertation, 201 5) Supervised by Prof. Dr. Ir. Robiyanto H. Susanto, M.Agr.Sc, Dr. Dedi Setiabudidaya, Dr. Ngudiantoro, M.Si., and Dr. Ir. F .X. Suryadi, M.Sc.
Lowlands are some of natural resources which are very potential to be used as agricultural lands, yet lowlands utilization for agriculture is still not optimal. Based on a data from the Directorate of Lowlands and Coastal, Directorate General of Water Resources, Ministry of Public Works, 2009, from a total of 33.4 hectares, only about 1.8 hectares of lowlands in Indonesia is developed by the government. This limited utilization is due to some constraints found when developing lowlands as agricultural land. One of the constraints is highly fluctuative water table that is not suitable for the plants. Because of that, water management is the key to agricultural development at wetlands. Subsurface drainage is one of water management techniques that is effective for lowlands.
Drainage systems on lowlands should function both as a drainage system and an irrigation system. Other than draining the land to match the needs of the plant, the drainage system should also keep the soil moisture so that the water table level is not too far from the ground. This is to prevent the pyrite layer to be oxidized and to keep the peat from drought. Pyrite and peat are common in lowlands. Oxidized pyrite will cause the soil to become acidic and damaged, while dried peat will be damaged and easily catch fire. These effect in turn will cause land subsidence. Thus, a good water table management will not only increase productivity and cropping index, but also help preserving the land.
Subsurface drainage system used in this study is constructed using perforated pvc pipe with sand as an envelope material. Subsurface drainage canals are planted at a depth of 0.7 m under the soil surface with some variations in the distance between canals.
Drainmod model simulation is used to determine the most optimal drainage system design. Drainmod model validation is done by using the correlation coefficient between the water table fluctuations from drainmod simulation results with the water table fluctuations from direct field observation. The validation showed an excellent 0.81 correlation coefficient, concluding that drainmod model simulation can be used to determine the distance needed between the canals to obtain water table fluctuations that ranged from 0.6 to 0.7 m under the soil surface.
The research results showed that the subsurface drainage system is able to keep the water table level around the root zone of plants. The most optimal subsurface drainage system to keep water table fluctuations around 0.6-0.7 m below the soil surface is the subsurface drainage system design that uses perforated PVC pipes with a diameter of three inches and a 2.80 o cross-section of the pore. Pipes are planted at a depth of 0.7 m under the soil surface with a canal spacing of 25 m.
Keywords: Lowlands, subsurface drainage, Drainmod, water table management