Modelling the Water Budget in a Beech Forest (Water Budget Model)
From 01/1995 to 12/1997
Principal Investigator: Michael Hauhs Staff: Ralf Callenberg Grant: 0339476 B Vorhersage und Erklärung des Verhaltens und der Belastbarkeit von Ökosystemen unter veränderten Umweltbedingungen
The aim of this project was to develop a water budget model for trees. As in many phenomenological models of tree water use, the overall control is placed at the level of stomata. So the main focus is the dynamics of stomatal conductance.
The opening of stomata as such is due to the uptake demand for CO2, the loss of water being just a price the plant has to pay for its carbon assimilation. Here it is assumed that the actual control is a result of the plant’s adaptation to its environment. The underlying hypothesis is that the plant maximises photosynthesis under a given water supply and uses stomatal conductance and thus transpiration as a control parameter. Within this view it would not be necessary to consider the actual processes of water uptake and the underlying physiological processes of stomatal opening. However, the existing mathematical descriptions of the optimisation problem appeared to be limited to artificial environments and hardly to generalise to arbitrary boundaries. As an alternative to existing analytical approaches to this problem an evolutionary algorithm is used to find the optimal control of stomatal conductance. With this technique it was possible to enlarge the limited applicability of the conventional investigation. A program, called opTree, was written, which can be confronted with arbitrary time series in precipitation and climatic data. As a response to this training set the program will find an optimised feedback function, i.e. stomatal conductance as a function of environmental conditions. The model for the soil part could be kept very easy and was fitted to the observed runoff. The first application was a beech forest in Panola, due to the fact that at this catchment transpiration is very high. Thus the dynamic of the runoff is sensitive to the modelled transpiration. This first experience with opTree showed the applicability of the optimisation hypothesis to describe the water budget of tree stands. (final report 1998)