Climate changes lead to a rise in air temperature, which significantly increases the water needs of plants. Maintaining crop productivity will increasingly require the use of plant irrigation. The aim of this study was to assess the water needs of grapevines cultivated in the western provinces of Poland. The calculations were made on the basis of temperature and precipitation measurements collected at three meteorological stations in the period 1981–2010. Water needs were calculated as crop evapotranspiration, which was estimated by crop coefficients and reference evapotranspiration, determined using the Blaney–Criddle formula. The rainfall deficit was assessed by Ostromęcki’s method. The tendency to increase the water needs was observed in each subsequent decade of the thirty-year period, both in the whole growing season (May–October), as well as in June–August and July. The highest values of the linear correlation coefficient for the trend of time variability in water needs occurred from June to August. An analysis of water needs and rainfall deficits indicates the need for the additional irrigation of vineyards in western Poland, especially in very dry years and in June–August. Current research results are helpful in designing vineyard irrigation systems and allow an economical and efficient planning of grapevine irrigation.