Symbols, SI Units and Physical Quantities within the Scope of Sap Flow Studies

Some confusion was observed during the 7 International Workshop on Sap Flow (Seville, Spain, 21-24 October 2008) with respect to the use of symbols, units and physical quantities in presentations and during discussions. It was therefore decided at the end of the workshop that a technical note would be very useful with the aim to summarize the basics of the “Système International des Unités (SI)” and to focus on particular aspects that are relevant for sap flow studies. Starting from a number of standard papers that have been published in the past, an extra effort was made to put sap flow nomenclature against the physical background of plant-water relationships, of water flow in the soil-plant-atmosphere continuum and of transport equations for mass, heat and hydraulic flow. Information is given on the application of SI units and the main SI conventions. Tables are included for practical use of basic and derived SI units and for common abbreviations encountered in sap flow studies. An overview of a coherent nomenclature for sap flow studies, together with definitions of sap flow quantities, symbols and units is given as well. INTRODUCTION During the 7 International Workshop on Sap Flow (Seville, Spain, 21-24 October 2008) it became clear that participants frequently did not exactly speak the same language or used the same nomenclature when they were discussing their studies on sap flow phenomena and related topics. Some confusion could be observed when symbols, units and even physical concepts were used during their presentations. Scientific communication was sometimes difficult, especially during discussions held after a presentation. At the end of the Seville meeting it was therefore concluded that a technical note would be useful with the aim to summarize the basics of the “Système International des Unités” (official English translation: “International System of Units”, or referred to as the standard system of SI units), and to focus on some particular aspects that are relevant for sap flow studies. A decimal system of weights and measures was already proposed as early as 1670 by Gabriel Mouton, a French mathematician and vicar of the St Paul’s church at Lyon (France). The metric system was finalized during the French revolution (from 1789 till 1799). At that time this system stirred considerable controversy as it illustrated the