Estimation and comparison of reference evapotranspiration using different methods to determine olive trees irrigation schedule in different bioclimatic stages of Tunisia

The study of olive trees water requirements allows a better water management by using more accurate methods including maximum parameters of the continuum soil-plant- atmosphere. The Penman-Monteith equations is consideredas the most rational approach and the most reliable for calculating evapotranspiration. Only this approach necessarily requires an important number of climate parameters. The use of other equations, less complicated and using less climate parameters may be a reliable and efficient alternative. This experimental study was carried out on two cultivars cv. "Meski" and cv. "Chemlali" conducted in the intensive system in different bioclimatic stages (Subhumid, Semi-Arid and Arid) in Tunisia. This work aims to estimate olive trees water needs using evapotranspiration calculation in three different bioclimatic stages. For that, we compared the Penman-Monteith formula with Blaney-Criddel, Hargreaves-Temperature, Hargreaves- Radiation and Priestley-Taylor formulas to estimate reference evapotranspiration (ET0). Results show that ET0 values calculated by Priestley-Taylor and Blaney-Criddel formulas were more or less similar to Penman-Monteith. The ET0 values found by Hargreaves-Temperature and Hargreaves-Radiation were twice the values calculated by Penman-Monteith formula. We also found good correlations between the reference evapotranspiration calculated by the Penman-Monteith equation and that calculated by Priestley-Taylor and Blaney-Criddel equations in all bioclimatic stages (R2 more than 0.85, p < 1%). The ET0 sensitivity analysis has shown that solar radiation and air temperature (energetic climatic parameters) have the dominant effect on the ET0 at the level of the different climatic regions. Accordingly, in the case of lack of some climatic parameters and in sub-humid, semi-arid and arid conditions and for the different phenological stages of the olive tree, we can use Priestley-Taylor and/or Blaney-Criddle formulas to estimate water needs.