A simple and alternative approach based on reference evapotranspiration and leaf area index for estimating tree transpiration in semi-arid regions

The present work aims to develop a simple approach relating normalized daily sap flow (liters per unit of leaf area) and daily reference evapotranspiration (ET0) (mm/day). Two methods (FAO-Penman-Monteith (FAO-PM) and Hargreaves-Samani (HARG)) of the calculation of ET0 were tested in order to examine their impact on the established relationships. The data sets used for developing this approach are taken over well irrigated orchards from three experimental sites (olive trees, cv. “Olea europaea L.”, olive trees, cv. “Arbequino” and citrus trees cv. “Clementine Afourar”) conducted in the Tensift region around Marrakech (center of Morocco) and one experimental site (pecan orchard, cv. “Carya illinoinensis, Wangenh. K. Koch”) conducted in the Yaqui Valley, northwest of Mexico).The results showed that the normalized daily sap flow was linearly correlated with ET0 (mm per day) calculated by FAO-PM method. The coefficient of determination (R2) and the slope of this linear regression varied between 0.71 and 0.97 and between 0.30 and 0.35, respectively, depending on the type of orchards. For HARG method, the relationship between both terms is also linear but with more discrepancy (R2=0.7). This was somehow expected since this method is known to underestimate ET0 values in the semi-arid areas. Afterward, the validation of the developed linear relationship was performed over an olive orchard (“Olea europaea L.”) where the measurements of sap flow were available for another cropping season (2004). The scatter plot between the normalized measured and estimated sap flow based on FAO-PM method reveals a very good agreement (slope=1, and RMSE=0.14L/m2 leaf area). However, for the estimation of normalized sap flow based on HARG method, the correlation is relatively more scattered (slope=0.95, and RMSE=0.35L/m2 leaf area). A further validation was performed using the measurements of evapotranspiration (ET) by eddy correlation system and the results showed that the correlation between normalized measured ET and estimated normalized sap flow is best when using FAO-PM method (RMSE=0.33L/m2 leaf area) for estimating ET0 than when using HARG method (RMSE=0.51L/m2 leaf area).Finally, the performance of the developed approach was compared to the traditional dual crop coefficient scheme for estimating plant transpiration. Cross-comparison of these two approaches with the measurements data gave satisfactory results with an average value of RMSE equal to about 0.37mm/day for both approaches.