By Ahmad Saifullah – (Thesis M.Si Sriwijaya University, 2013). Supervised by Prof. Charlotte de Fraiture Ph.D., M.Sc. (UNESCO-IHE), Prof. Ir. Robiyanto H. Susanto, PhD., M.Agr.Sc. (Sriwijaya University), Laszlo Hayde, PhD, MSc (UNESCO-IHE).

Lowland in Indonesia is distributed across Sumatra, Kalimantan, and Papua with the total area is around 20 million hectares. This great amount of lowland is suitable for rice fields, oil palm and acacia plantations. Nowadays, more than 0.8 million hectares of plantation forests are located in lowlands of South Sumatera Province.

Developing pulpwood plantation forest in lowland area is facing several problems for example inundation in wet season, water scarcity in dry season, and pyrite oxidation in acid sulphate soil. Meanwhile, Acacia tree is a fast growing species which need relatively sufficient groundwater table to get optimum growth. When the pulpwood plantation forest cannot solve the problem and provide sufficient groundwater, failure and damage of Acacia growth is occur because water stress and water logged that cause financial losses. To gain a high yield from lowland plantation forest, a well-managed water management in lowlands is an important measurement.

This research is based on modelling simulation which is MODFLOW and DUFLO software. MODFLOW program will be used to analyze the groundwater fluctuation related to the drainage spacing and water level in the drainage canals. The drainage canal performance in the wet and dry season is modelled by using DUFLOW program. These two programs are applied to know the best water management system in the study area. There are 2 scenarios that will be conducted with MODFLOW modelling to determine the optimum drainage spacing, while DUFLOW program simulates the water level in the drainage canal.

Optimum growth of acacia trees requires optimum groundwater table depth. MODFLOW simulates the required groundwater table depth by adjusting the water level in the tertiary canal. The results show that for the existing drain spacing 125 meter in wet season, the water level in the tertiary canal is maximum 0.6 meter below the surface and minimum 1.1 meter below the surface to fulfill the required groundwater table depth for optimizing the Acacia growth. Then, in the dry season the water level in the tertiary canal is maximum 0.2 meter below the surface and minimum 0.6 meter below the surface. Meanwhile, the result for the optimized drain spacing 62.5 meters is that the maximum water level in tertiary canal is 0.4 meter below the surface and minimum 0.8 meter below the surface in the wet season, while n the dry season the water level should be set to maximum 0.3 meter and minimum 0.8 meter below the surface.

It is also important to provide an optimum groundwater table that equal for every acacia tree in the field. This study simulates the effect of the drain spacing to the groundwater fluctuation in the field by using MODFLOW software. The results shows that the optimized drain spacing 62.5 meter give smaller difference between water level and groundwater table is different between than the existing drain spacing 125 m. In the wet season, the difference is 0.31-0.36 m for 125 meter drain spacing while for 62.5 meter drain spacing is around 0.04 -0.09 meter. Meanwhile in the dry season, the difference is 0.15-0.22 meter for 125 meter drainage spacing and 0.05-0.12 meter for 62.5 meter drainage spacing. It is recommended to use 62.5 meter drainage spacing in order to get equitable groundwater depth distribution in the field. By using MODFLOW program, the leaching process is simulated by increase the water level 0.2 meter above the pyrite line. The result shows that groundwater level will be above the pyrite line at the 7th days for drainage spacing 125 meter, while drainage spacing 62.5 meter need 3 days.

Based on the DUFLOW simulation analyses, the existing drainage canals scheme cannot fulfil the water need by the tertiary canal. The highest water level in the drainage is 3.0 meter which is below the required water level for the tertiary canal. However the existing drainage scheme can cope with the rainfall intensity of 40 years return period. It causes 0.2-0.7 m of inundation for 5 hours which is still tolerated by the Acacia trees. Installation of stop logs in primary and secondary canals can increase and maintain water level in the canals at 3.3 meter +MSL in the dry season and can reach 2.4 meter above mean sea level or 1.1 meter below the surface in the wet season which is required by groundwater table to get optimum growth of Acacia trees.

The DUFLOW simulation result shows that installing the stop logs as weirs to maintain the water level at certain level can meet the required water level in the dry season and avoid inundation due to extreme rainfall. The stop logs installation can ensure the water management scheme can provide optimum groundwater table for optimum growth of Acacia trees.

Keywords: pulpwood plantation, lowland, water management, DUFLOW, MODFLOW.