The Katingan tidal irrigation area frequently experiences high salinity due to its coastal position and acidity due to the acid sulphate soil type and water discharge from the neighboring peat forests. This study aims to investigate the dynamic of pH and Electrical Conductivity (EC) caused by the interaction of peat water and brackish water, which is impacted by tides in an acid sulphate soil environment. Field measurements were conducted to collect data on water level, pH, and EC in both channels and fields. The dynamic of acid water from peat forests and acid sulphate soils with brackish water in the irrigation network was investigated using HEC-RAS simulations. The simulation incorporates water level, EC, and pH recorded at the Katingan River from December 22, 2022, to January 5, 2023. The results of simulations are calibrated with observed data at the main channel. The influence of brackish water on increasing pH and EC (pH = 5 - 7, EC = 0 - 5.94 dS/m) is more significant in hydrotopography Type A, but it is less significant (pH = 4 - 5.5, EC = 0 - 2.25 dS/m) in hydrotopography Type B, which is less affected by brackish water. The water pH is more acidic in hydrotopography Type C, which is not impacted by tides. These simulation results may serve as a foundation for developing and operating sluice gates to improve water quality by allowing or preventing the entry of water into the irrigation network.
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