By Evi Damayanti – (Thesis M.Si Sriwijaya University, 2013). Supervised by Prof. Charlotte de Fraiture, Ph.D, M.Sc (UNESCO-IHE), Prof. Robiyanto H. Susanto, Ph.D, M.Agr.Sc (Sriwijaya University), F.X. Suryadi, Ph.D, M.Sc (UNESCO-IHE).
To ensure the raw material for pulp and paper industries, Indonesia has developed plantation forest in production forest. However, the concession areas of plantation forests often establish on marginal soils such as peat lands. In peat land areas, water management is needed to create condition for optimum production and to keep the groundwater table at an appropriate depth for sustainability of the organic soil. The research was conducted in Muaro Jambi Regency as one of the concession areas of plantation forest company PT Wira Karya Sakti. Two modeling programs were used in this research. The behavior of groundwater flow in the field was simulated by MODFLOW program to get the optimum water level in the canal during dry and wet season also extreme rainfalls. Meanwhile, DUFLOW program was used to answer the question how to manage the canal water flow in order to achieve the water level as required by MODFLOW.
The result of MODFLOW shows that water level in the canals from late of rainy season should be set at 7.10 m + MSL or 10 cm below the surface level to obtain groundwater table 50-56 cm below the surface level in dry season which is optimum for 3 years old of Acacia crassicarpa. Meanwhile in the rainy season groundwater table in the field will rise gradually because of recharge from precipitation. Water level in the canals should be set at 6.60 m + MSL or 60 cm below the surface level. When the rainy season with this level groundwater table in the field would increase to 6.70 m + MSL or 50 cm below the surface level.
Based on DUFLOW simulation, to obtain water level of 7.10 m + MSL or 10 cm below the surface level, it is needed for controlling water level in the canals since in rainy season in April. Stop logs should be installed in the canals with height 7.00 m + MSL or 20 cm below the surface level. In the rainy season to get water level 6.60 m + MSL or 60 cm below the surface level, water will be drained gravitationally or if there was a control structure installed in the canal such as stop logs drainage could be conducted by opening the stop logs. Stop logs should be opened until the height of stop logs 6.50 m + MSL or 70 cm below the surface level.
In extreme rainfalls, water level in the canals should be lowered at 6.40 m + MSL or 80 cm below the surface level to get optimum groundwater table in the field. Groundwater table in the area then will go up into 6.70 m + MSL or 50 cm below surface level. It means that inundation will not occur in the area if water level in the canal is maintained at 80 cm below the surface level in extreme rainfalls condition. To obtain water level 6.40 m + MSL or 80 cm below the surface level water in the canal will be drained gravitationally or if stop logs have installed in the canal, drainage could be conducted by full opening the stop logs.
In pulp wood plantations the canal is not only having function for controlling water table and draining excess water from the land but also for transportation. One of alternatives of water control structure for maintaining water level in primary canal as well as for transportation function is by navigation lock. Meanwhile for the secondary and tertiary canals stop logs is suggested to be installed because of its simple operation and relatively low cost.
Keywords: forest plantation, peat lands, water management, MODFLOW program, DUFLOW program, groundwater table, water level.