Hydraulic analysis of rainfall utilization on acid sulphate soil leaching in lowland irrigation Area of Unit Tamban Central Kalimantan

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.