By Andri Tri Atmojo – (Thesis M.Si Sriwijaya University, 2013). Supervised by Prof. Charlotte de Fraiture, PhD, MSc (UNESCO-IHE), Prof. Robiyanto H. Susanto, PhD, MAgrSc (Sriwijaya University), Laszlo Hayde, PhD, MSc (UNESCO-IHE).

In the last 10 years, timber plantation in Indonesia develops rapidly parallel with the dramatic growth of pulp industries. Currently timber plantation can be found at 22 provinces and covers total area of about 10 million hectares. In Sumatra, the development of timber plantation mostly took place in lowland peat and can be found broadly in Riau, Jambi, and South Sumatra Provinces. In South Sumatra itself, timber plantation is concentrated in lowland peat in the eastern part of the area. Due to the characteristic of peatland, timber plantation has to be managed properly especially related to the water management and peat oxidation issues.

Sumber Hijau Permai Corporation Limited is one of the companies that manage lowland peat for acacia plantation in Banyuasin District, South Sumatra Province. To optimize their area for pulp wood production, drainage system with canal network, dikes, and drainage structures were developed. However, due to the various condition of the field including soil types, topography, and influence of the tidal, improvement is still needed in some part of the area.

The selected study area is located in the southern part of the concession area. Tidal influence, topographic condition, and peat soil are the elements that have to be taken into account in developing this area. With the existing drainage structures, inundation and too dry are the main problem in this area.

To overcome those problems, analysis of DUFLOW simulation was conducted. The result shows that the existing drainage structures still work properly under the extreme rainfall condition (136 mm in a day). The inundation occurred only in short period +11.5 hours (in two days) with the maximum water level up to 4.06 m.

The remaining problem i.e. too dry condition can be solved by installation of weirs in the connector canal. Further study by using MODFLOW program shows that in order to achieve optimum growth of 2 years old of acacia, the water level in the canals have to be maintained at 3.25 m+MSL. As the consequence, the height of the weirs has to be set also to that level.

The performance of the drainage structures is also influenced by the land subsidence in the field. With the 5 cm/year of land subsidence rate, the performance of drainage structures was observed in the next 10, 20, 30, and 40 years. The result of DUFLOW simulation showed that the deeper the land subsidence, the deeper and the longer inundation occurred in the field.

To eliminate the duration of inundation and also the effect of land subsidence, further study is needed. Therefore the optimum growth of acacia in this area can be achieved.

Keywords: lowland, peat, drainage structures, acacia, water table, inundation, land subsidence, DUFLOW program, MODFLOW program