The principle aim of this study was to develop a model to predict the water processes under growing grass. This paper includes: the GRAGRO model simulates the vertical flow in an unsaturated homogeneous soil, changes in the water table as a result of precipitation and the calculated daily evapotranspi ration. The object of this study was to complete the GRAGRO model with a subroutine IRRDEC, which simulates the prediction of the irrigation, and calculates the amount of water which must be supplied to carry out this irrigation. This model was tested and model results were compared with measured values observed in several lysimeters with different ground water tables on a peat soil located in Sosnowica, Poland.