abstract: This article presents the application of the water stress index as a method of predicting the requirement to irrigate. The simple linear relationship between the crop canopy temperature minus the ambient air temperature (Tc - Ta) to the VPD is called the CWSI baseline. The data necessary to estimate the baseline and the CWSI indicator for grassland was collected at the experimental station in Sosnowica, Poland. This method of estimating the CWSI index may be helpful for identifying management decisions but the different results obtained in 1995 and 1996 and the relatively low correlation coefficients (0.54-0.73) indicate that further field research and more experimental measurements are needed.