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

Due to rapid development, there were a lot of land use changes especially in Java Island, Indonesia, which was the central of paddy production. Nowadays, the paddy field development is reaching out the lowland area based on the food security program which required more production to ensure the supply stability.

Based on that issue, a lot of project had been established to answer the challenge and so called ‘one million hectare project’ (Proyek Lahan Sejuta Hektar) in Kalimantan Island was one of the example that finally failed with left some environmental problems beside social and economic.

Regarding to that failure, some research projects had occurred to study the development strategy in that area in order to bring more paddy field which could increase the production. One of the recent projects which run in 2007 was Ex Mega Rice Project (EMRP). The project was conducted in Central Kalimantan. Then, the research of Kapuas River Potential was conducted in 2010.

However, there is one of crucial parameter that necessary to study in order to get good sustainable development of paddy field which is salinity intrusion. Every development, either in downstream or upstream areas, will influence the salinity intrusion which paddy cannot resist at some value of concentration.

The smaller values of upstream discharge the more extend salinity intrusion length, but what the relationship of this occurrence in the river system which influenced by tidal dynamic and for what extent that the paddy field can be developed in the upstream and downstream area regarding the salinity intrusion length.

In order to answer the questions, this research tried to see the relation between salinity intrusion length and upstream discharges for a certain areas by using numerical modelling using MIKE 11. The modelling contained 5 scenarios which divided into 2 major which were calibration and salinity intrusion scenarios. The salinity scenarios were contained 4 scenarios which represented correlation between development area and upstream boundary discharges.

In calibration process, all of the boundaries were using water level fluctuation data because only this type of data that measured simultaneously for approximately 14 days which were measured by Swamp Research and Development Agency under Ministry of Public Works, Indonesia. Further, the schematization was built using the topography survey results which contained location of the bench-mark and control-point in x-y coordinates. In these locations, there were also measured the cross-sections and became the references for measuring the water level. The results in the calibration process was compared to 3 observed water level data which locations are different with the boundaries but still in the streams. The results were the calculated water levels were nearly approaching the observed data. Thus, the model was ready to simulate the salinity intrusion.

To proceed to salinity intrusion simulation, there was 1 branch built in the downstream to represent a development area. The location was built to approach the strategic development of the local government which planned to open 20 thousands ha in the downstream area. However, the simulation was using 100 and 500 thousands ha development areas with 100 and 2000 m3/s for each upstream boundary discharges and development area simulations while the downstream discharge still the same with the one in calibration simulation.

To have a good representative of the development area, a value of discharge was calculated as there would be a paddy field cultivation area with also considered the losses along the canals in general values.

At salinity intrusion simulation, there were 2 types of simulation conditions being done, hydrodynamic and advection-dispersion simulation. However, the simulation were connected each others. Hydrodynamic module was producing the water level and discharge values along the research river streams for each simulation. Advection-dispersion yielded the salinity intrusion fluctuations in every simulation based on the hydrodynamic simulation.

By combining the results for each development area, it yielded the relationship graph of salinity intrusion length-upstream discharges. Based on the relationship graph of relationship, in extreme condition when the upstream discharges at 2 * 100m3/s, the intake for 100 thousands hectare should not be put less than 23 km from the river mouth and 28 km for 500 thousands hectare.

Based on the results, it can be seen that it is important to include and predict the salinity intrusion in the master plan of development. However, it is also necessary to consider developing further the research in order to see the salinity intrusion movement in the ground level. In addition, it is need to put into consideration to make good measurement plans based on the modelling software that will be used.

Keywords: discharge, Kapuas River, numerical modelling, salinity intrusion, MIKE 11