In a study to investigate water content distributions inside a lysimeter in a noninvasive manner, we used ground penetrating radar (GPR) tomography. Our main objective was to evaluate the temporal changes and spatial distributions of the volumetric water content after a short but intensive irrigation of part of the lysimeter. High frequency GPR antennas of 1-GHz nominal frequency were used because of the small spatial dimensions of the investigated lysimeter (cylinder of 1.5-m height and 1.2-m diameter) and the desired spatial resolution in the range of decimeters. To ensure a relatively steady distribution of water inside the lysimeter for the time-consuming tomographic survey, simple parallel transmission measurements were used to track the water dynamics. Water contents and water content changes were calculated by means of a mixing formula describing the relation between electromagnetic wave propagation velocity and the water content. The transmission measurements indicate a diffusive process following the irrigation for a duration of several hours. The tomographic measurements clearly show the area of increased water content associated with the irrigation.