Water salinity and acidity are two water quality parameters that are important for the success of crop cultivation in the Anjir Serapat Lowland Irrigation Area, Central Kalimantan. The interrupted tidal flow between Kapuas Murung River and the irrigation drainage system prevents fresh-saline water exchange. Tide is one factor determining the change in flow and water quality. This study aims to analyze the influence of tides in the Kapuas Murung River on pH and salinity in the main channel of the Anjir Serapat Lowland Irrigation Area. pH and salinity are measured simultaneously with the water level. Salinity, pH, and water level data were collected at Sta 0, Sta 10, and Sta 14 of the main channel during spring and neap tide for 27 hours. The effect of tides on pH and salinity at Sta 0 and Sta 14 is shown in the relationship between the rate of change of the water level to the rate of change of pH and salinity, and also the relationship between the slope of the water to the rate of change of pH and salinity. When heading towards the flood or the ebb, fresh water from the river can lower salinity and increase pH, and vice versa can also increase salinity and lower pH in the main channel. Changes in water level significantly influence only salinity at Sta 0, while changes in salinity at Sta 14 and changes in pH at Sta 0 and 14 are not so significant because other unmeasured variables affect pH and salinity. The pH quality of the freshwater source from the Kapuas Murung River is not much different from that of the water in the main channel. When mixing occurs, the pH change is insignificant. It is suggested to do further study of other factors that affect the pH and salinity of irrigation water in the main channel of Anjir Serapat Lowland Irrigation Area.

The Anjir Serapat lowland Irrigation Area in Central Kalimantan is an agricultural area that supports the Food Estate program. Irrigation water comes from rainwater. The existing irrigation network cannot remove excess water during the rainy season, causing inundation in low-lying areas. Stagnant water causes the pH of the water to drop so that it becomes acidic. Acid inhibits the absorption of nutrients, interfering with rice plants’ growth. This study aims to evaluate the performance of irrigation networks in removing excess rainwater. Performance evaluation was carried out with the help of flow simulation using HEC-RAS. The simulation was carried out with three scenarios, namely scenarios with existing geometric conditions, channel normalization scenarios, and normalization scenarios, along with the addition of sluice gates. The normalization scenario and the addition of the sluice gate were more effective in reducing the area of flooded land by more than 20 cm to 158.71 hectares, with a reduction of 48.27% from the existing scenario. For land with lower contours, where not all inundation can be discharged into the channel, it is necessary to add a pump. Further studies are needed to see the effect of adding pumps on the reduction of inundation in the field.