By Adi Kunarso – (Thesis M.Si Sriwijaya University, 2012), Supervised by Prof. Bart Schultz, PhD, MSc; Prof. Ir. Robiyanto H. Susanto, PhD, M.Agr.Sc; F.X. Suryadi, PhD, MSc.

Indonesia has about 30 million ha of lowlands consists of 20 million ha tidal lowlands and 10 million ha non tidal lowlands. Large scale development of lowlands in Indonesia started in 1960 when many lowlands in Sumatra and Kalimantan were reclaimed into agricultural land to increase the national food production, mainly rice, for obtaining food self sufficiency. By the 1990s opening of large scale natural lowlands, especially in Sumatra and Kalimantan mostly focused on plantation forest and oil palm plantation. The basic concept of plantation forest development is to rehabilitate degraded or non-productive land as well as to produce timber to fulfil the raw materials for forest industries. It is needed to overcome timber deficiency about 40 million m3/year (Ministry of Forestry, 2008).

Plantation forest with fast-growing species such as Acacia mangium and A. crassicarpa is expected to cover the shortage of timber supply. However, Acacia plantations are pointed as one of the sources of peatlands degradation in Indonesia. Drainage of peatlands leads to some environmental impacts such as fire, carbon release, and land subsidence. Another problem that usually occurs is low productivity of timber because basically the area is less fertile.

The future challenge related to the development of lowlands is how to deal with two contrasting needs; intensive drainage to make land suitable for crops and less intensive drainage to minimize environmental impacts. This thesis is dealing with water management at field level in one of the Acacia plantation companies in South Sumatera, Indonesia in order to propose improvement options for the water management system and the operation and maintenance of it to increase pulpwood production and reduce environmental impacts.

This research focuses on the modelling of the water balance and groundwater flow, in the peat soil of the study area. Two hydrological computer programs were used in this research. The BALANCE program was used to simulate the water balance in the field (unsaturated zone) during one year for wet, normal, and dry periods. While the MODFLOW program was used to determine optimum water levels in the canals by simulating groundwater flow in the field.

The result of the analysis shows that by using the BALANCE program, the amount of water that needs to be drained is equivalent with the excess water coming from the rainfall. In a wet year about 220 mm/month of water has to be drained during the wet season. In a normal year the average amount of water that needs to be drained is 156 mm/month, while in a dry year it is 154 mm/month. However, at the end of the dry season (August-September) almost no water in the field. So that, at the end of rainy season water from rainfall has to be controlled to maintain the groundwater table during the dry season. It means that canal system has to be designed to able to drain water during the wet season and maintain the groundwater table in the field during the dry season.

The MODFLOW program has been used to determine the optimum water level in the canals for one year-old Acacia trees in the sampling area. The result of the simulations shows that for canal spacing of 250 m, the water level would have to be set at 80 cm below surface to obtain optimum conditions in the field during the wet season. In the dry season, the optimum groundwater table in field can be reached if the water level in the canal is set at 25 cm below surface. Simulation in the wet season with a canal spacing of 125 m shows that the optimum groundwater table can be reached if the water level is set at 50 cm below surface. It means that by reducing the canal spacing, the water level in the canal can be set not too high in the dry season and not too low in the wet season to obtain the optimum groundwater table in the field.

Land subsidence occurs and affects the drainage potential of the study area. Lowering of the surface level will make gravity drainage more difficult to be applied. By considering a subsidence rate of 5 cm per year, it can be predicted that in the next 100 years from now, drainage by pumping has to be applied even though it is not suggested due to high cost. Simulation at field level shows that reprofiling for tertiary canals needs to be done to overcome flooding that may happen due to lowering of the surface level.

Efforts to manage the groundwater table in the field are needed to improve productivity and reduce subsidence rates. The canal spacing has to be adapted to the drainage requirement in order to remove excess rainfall from the field. Moreover, after some years of reclamation, canal spacing must be narrowed due to reduction of soil permeability caused by compaction of peat soil. Reduction canal spacing can be done by making new canal in between two initial tertiary canals. Meanwhile, to prevent water in the field during the dry season, stoplog structures may have to be installed in the tertiary canals to control drainage, so that too dry conditions in the field can be avoided and subsidence rates can be reduced.

Keywords: lowlands, forest, Acacia plantation, water management, land subsidence, BALANCE program, MODFLOW program