Testing simplifying hypotheses for modelling forest throughfall and stemflow water stable isotopes

The stable isotope composition of the water entering the hydrological system is frequently used as natural tracer for plant, soil and catchment hydrology studies in forested areas. For these studies is important to know how the isotopic composition of precipitation is modified when rainfall passes through the canopies, therefore, the modelling of these processes would be very useful. However, it has been described as a complex task due to the complexity of the insufficiently known mechanisms involved. As an alternative way, we propose to test a set of hypotheses that try to simplify the main driving mechanisms. The hypotheses being tested are: i) The enriched isotopic composition of stemflow is in dynamic equilibrium with that of air moisture; ii) Dripping water has the same isotopic composition than stemflow; and iii) Throughfall isotopic composition is a mixing of those of free throughfall and stemflow. The measured event and intra-event isotopic composition of rainfall, throughfall and stemflow measured in a Scots pine plot during several years at the Vallcebre Research Catchments (South-Eastern Pyrenees), combined with Rutter and Gash models, previously tested in the studied plot, are being used to test these hypotheses.