Lysimeter is an important instrument to measure evapotranspiration in the field of irrigation management. The error analysis and calibration of the system is very critical for quality control of collected data. The error sources are many and different. The Kerman weighing electronic lysimeter is a complex of two cultivation tanks, load cells, data logger, and data processing weighing system with 48 m under ground building. In this study, ten-minute interval measured evapotranspiration data were analyzed. The Fourier’s series were used to model discrete measured time depended evapotranspiration data. By differentiation of the fitted model the quantity of selected time interval evapotranspiration was computed. By comparing the measured and estimated data the time depended errors were calculated. Computed errors were analyzed, and the statistical distributions of the errors were discussed.