During the Chilean summer of 2008 in the Laja-Diguillín irrigation system, a study was conducted to estimate irrigation water loss due to infiltration and evaporation in two sections of the Polcura and Laja rivers, which are used for channeling these waters. To evaluate losses due to infiltration, a monthly water balance model with specific equations was used which included infiltration and evaporation processes in the sections of the river. This model was parameterized using data for infiltration velocity in the riverbed, measured during a field campaign and calibrated using historical flow data. Losses in channeling the irrigation water ranged from 4.5 to 2.8% of the flow discharged into the system.