By Rini Yulianti – (Thesis M.Si Sriwijaya University, 2013). Supervised by Prof. Charlotte de Fraiture Ph.D., M.Sc. (UNESCO-IHE), Prof. Ir. Robiyanto H. Susanto, M.Agr.Sc., Ph.D (Sriwijaya University), F.X. Suryadi, Ph.D., M.Sc. (UNESCO-IHE).

Timber plantations in peat lowlands cultivate crops for supporting pulp wood industries such as Acacia Crassicarpa. To get high productivity, peat lowlands have to be drained to reach specify requirement. Peat land is vulnerable to external annoyance especially due to drained and excessive drainage that will lead to irreversible drying. Drainage not only has an impact on land subsidence, but also causing the peat swamp became vulnerable to fire, especially in dry season. In peat land, the priority of water management should be to stabilize water levels within a certain target range.

This research focuses on the modelling of groundwater flow on the peat soil timber plantation in Tanjung Jabung Timur District area. Processing MODFLOW as groundwater flow software has been used to simulate water level in canal and groundwater table in the field as required for achieve optimum growth. To measure fire risk status in the study area has been calculated using Keetch Byram Drought Index modified with groundwater table. Land subsidence rate is estimated based on linear correlation regression based on groundwater table depth in Acacia Crassicarpa plantation.

The simulated groundwater flow using MODFLOW program gives the adjustment of calibrated hydraulic conductivity value 5.25 m/day and recharge 4% (76 mm/year). The simulated MODFLOW shows water level in canal should be in range from 8.18 m+MSL or groundwater table 0.80 m-surface to 8.46 m+MSL or groundwater table 0.50 m-surface as required for optimum tree growth. This requirement can be applied in three type of year: dry, normal, and wet year.

The Keetch Byram Drought Index modified by groundwater table has been applied to measure fire risk status in study area. Based on Keetch Byram Drought Index calculation for three type of year e.g. dry, normal, and wet year, with four level of groundwater e.g. 0.50 m, 0.60 m, 0.70 m, and 0.80 m below the surface indicate that in the dry and normal year status of fire risk is increasing at a groundwater table 0.70 m and 0.80 m below the surface.

The existing condition of groundwater table in study area gives the average value of 0.74 m below the surface. Assuming that for one cycle of Acacia Crassicarpa cultivation is 6 year, the rate of subsidence is equal to 0.31 m/6 year. The rate of subsidence will give the lower rate than the initial condition if the groundwater table is adjusted to optimum growth requirement (0.50 m – 0.80 m below the surface) to be 0.26 m/6 year.

The design of hydraulic structures has to be appropriated with the soil condition, operational purposes and feasible maintenance. It is recommended to install weir, lock, and stop log.

Keywords: water management, groundwater, peat, lowlands, forest, Acacia plantation, Keetch Byram drought index, Processing Modflow