This research was conducted to determine water-yield functions of Cuore (Lactuva Sativa L. var. Longifolia) lettuce cultivars under open field conditions at research station of Konya Soil, Water and Deserting Control Research Institute during 2017-2018 growing season. In the study, four different irrigation treatments were constituted as 60 % (I₆₀), 80% (I₈₀), 100% (I₁₀₀) and 120% (I₁₂₀) of the cumilative evaporation measured from the Class A Pan within 4 days period. The experiment was conducted according to randomized block design with three replications.

According to research results, maximum evaportanspration rate was measured in I₁₂₀ treatment as 441.9 mm while minimum evapotranspiration was measured in I₆₀ treatment as 317.8 mm. The highest average marketable head weight was obtained at I₁₂₀ treatment with 865.85 g and the lowest average marketable head weight was obtained at I₆₀ treatment with 475.68 g. Irrigation water use efficiency and water use efficiency varied between 14.6-18.3 kg m^-3 and 12.5-17.0 kg m^-3, respectively, depending on the treatments. The yield response factor (ky) was found as 1.38 100 % of the cumilative evaporation of the Class A Pan is recommended as a optimal irrigation program for drip irrigated lettuce crop under field condition to get higher and quality lettuce yield for the Konya Region.