Comparison of Almond Tree Transpiration Determined by Sap Flow Measurements and Lysimetry

We compared half-hourly transpiration data of a single adult almond tree given by a weighing lysimeter (Ely) to that supplied by compensation heat pulse (CHP) sap flow probes (trunk, F-t, and branches, F-bi, sap flow). The lysimeter was covered with a plastic film to minimize soil evaporation. The tree was subjected to alternate periods of full irrigation and shortage of water imposed during a period of high climatic demand (June-August, Cartagena, Southern-Spain). F-t was measured on the north and south sides of the trunk (F-ts and F-tn, respectively) and F-bi was determined on the three main branches. It was observed that the position of sap flow probes influenced the values of F-t, with higher rates (similar to 35%) occurring at the south location. Using a wounding width of 3.4 mm, a good agreement was obtained at a daily scale between Ely and the mean value of trunk sap flow, F-tm = (F-ts + F-tn)/ 2. Total branch sap flow overestimated Ely by approximately 40%, suggesting that the wounding width to be used for branches would be somewhat lower than that of the trunk. It was concluded that, once calibrated, the CHP technique is able to supply accurate and detailed information for the study of the short-term dynamics of almond tree water status and its response to abiotic stresses.