Olive tree is one of the oldest crops in the Earth, is the symbol of peace, life, strength, loyalty, abundance, health, eternity. It grows on barren ground, almost without water, but with plenty of sunshine and with that characteristics becoming the symbol of the Mediterranean region. All humans are witnesses of increasingly frequent droughts, so very resistant and durable olive tree is necessary to irrigate if we want high-quality fruit. The aim of this work is to determine the lack of water during the growing season in hydrologically unfavorable (dry) years in the cultivation of olive trees in the area of the Istrian peninsula, specifically in the area of city Buzet. Air-conditioning components for the town of Buzet in terms of irrigation were determined using 30-year series of meteorological data from Weather Statin (WS) Pazin and phenological growth of olive trees. Reference evapotranspiration (Eto) is calculated according to Penman-Monteith method. The values of effective rainfall calculated method USBR from the relationship of mean monthly average precipitation and amount of monthly precipitation that delimited the lower quartile. The results show that the total lack of water depends on the amount and seasonal distribution of rainfall. Calculated by the lack of water for many years of average rainfall and the dry years. Deficit of water in the average perennial of years is not determined, but in the dry years it amounts 38.2 mm. From the analyzed data, it is concluded that irrigation is a necessary measure in the cultivation of olive trees in the city of Buzet.