By Yosiandi Radi Wicaksono – (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.

Nowadays most of the lowlands in Indonesia are still in an improvement process from the open canal systems, to the controlled canal systems. One of the areas that is planned to be improved in 2012 is Telang II scheme which is located in Tanjung Lago District, Banyuasin Regency South Sumatra. The study was done to analyze what is the best improvement for the water management system. Telang II will be improved by constructing water control structures in the tertiary canals and reducing the distance between two tertiary canals from 400 m to 200 m. Two canal layouts, tertiary canal with dead end and without dead end, have been introduced to be applied in the area.

The water balance was calculated by the CROPWAT program. The result showed that drainage is the most important aspects to be considered since excess rainfall occurs in almost the whole year. Rainfed can be as the best scenario to supply the crop water requirement since the highest tidal fluctuation is still below the field surface.

The CROPWAT program showed that no crop yield reduction is affected by the implementation of rainfed scenario during the whole year. No crop yield reduction occurs since the daily soil depletion as the effect of the precipitation deficit during the growing period can be covered by the daily rainfall that can keep the soil moisture always higher than the readily available moisture.

The water level in the tertiary canals is important to be maintained at the preferred level to prevent the lowering of the groundwater table. During the dry season, water level in the tertiary canals should be maintained 40-50 mm below the surface. All the scenarios from the DUFLOW model which are water control structures only in secondary canals, water control only in tertiary canals and water control structures in secondary and tertiary canals, generated the same result which can maintain the preferred water level in the tertiary canals.

From the implementation aspects the installation of water control structures in the tertiary canals is the most effective to maintain the preferred water since farmers in each tertiary canal can decide the water management strategies by themselves. The installation of water control structures in secondary canal is not effective since coordination between farmers in one secondary block is not easy.

During wet season, water layer should be maintained at 0 – 5 cm above surface during land preparation and 20 cm below surface during growing stage. The DUFLOW model showed that the water control structures in the tertiary canal can be used as stop log during the rainy day so water can be maintained 0-5 cm above the surface.

The drainage can be done without the operation of the water control structures. During the rainy day, water flows out from the tertiary canal to the primary canal through the secondary canal since the area is relatively higher than the highest tidal fluctuation. Water control structures are used to provide more stable regime by controlling the erosion and sedimentation in the canals.

The DUFLOW model was used to model each canal layout to analyze its hydraulic performance both for irrigation and drainage requirement. The result showed that tertiary canal layout without dead end is the most suitable to be implemented since it produces less sedimentation and have better ability to drain the excess water. The canal layout with dead end produces higher sedimentation especially in the tertiary canals since it has a dead end in every tertiary canal. These dead end canals also have less capacity to drain the excess water during rainfall that makes that area can be flooded.

Maintenance cost was calculated based on the Technical Guidelines on Tidal Lowland Development. The total maintenance cost that must be conducted by farmers is only 1% from the farmers’ gross revenue from one year. Modernisation is important and should be introduced to farmers since its benefit to minimizing the operation and maintenance cost.

Keywords: lowlands, water management, the CROPWAT program, the DUFLOW program