The Unit Tamban is one of the tidal irrigation areas in the Central Kalimantan Mega Rice Project (MRP). The Unit Tamban is an irrigation system consisting of three primary canals with a settling pond or reservoir at each end. After being developed in 1970 by the Tidal Rice Field Development Project (P4S), the three reservoirs no longer function because they are full of sediment. Sedimentation also occurs in primary and secondary canals, impacting tidal attenuation and decreasing canal capacity. The reservoir at the end of the primary canal assists water regulation for irrigation purposes and leaching acid sulfate water. Three scenarios can be used to increase the capacity of the irrigation system, namely reservoir revitalization, secondary canals normalization, and combining two previous scenarios. The performance of both options was assessed by hydraulic simulation modeling using the HEC-RAS software. The hydraulic simulations show that reservoir revitalization can increase the volume of incoming water during high tide by about 90% more than without a reservoir. If the secondary canal normalization is carried out, the fresh water volume entering the secondary canal exceeds 19% before normalization.

Unit Tamban is one of the tidal swamp irrigation areas located in the ex-PLG area, precisely in Block D of Central Kalimantan. The productivity of Unit Tamban is very low, with only one planting every year due to the low water quality in the canal and the unfinished land reclamation process. The volume of freshwater from the tide is only 229,788 m3, while the leaching of acidic sulphate soils requires 248,000 m3. Unit Tamban has a dominant hydro-topography of types B and C, so some agricultural land uses rainwater as an irrigation water supply. The study aimed to examine the effect of rain discharge on the existing irrigation network as an additional water supply in the leaching of acidic sulphate soils in the swamp irrigation area of Unit Tamban. This study used dependable rainfall for calculating rain discharge and one-dimensional hydraulic modelling using HEC-RAS. The water volume in rainy conditions can meet the needs of acid sulphate soil leaching, while there is a deficit of 7% in existing conditions. The availability of dependable rain can meet the need for acid sulphate soil leaching so that the reclamation process can run well.