SEMINAR: Below-Ground Heterogeneity: A Constraint or a Degree of Freedom?

Hydrological understanding is commonly synthesised into more or less complex mechanistic models, some of which have become “as inscrutable as nature itself” (Harte, 2002). Most of these models contain a number of unknown parameters, which have to be calibrated on past observations. This approach often leads to “equifinality” issues (Beven, 1993; Beven and Freer, 2001), which result from an ill-posed problem, where the information content in the calibration data set is insufficient to filter out a single parameter set from all possible sets. The consequence is that we cannot identify the “correct” parameter set that produces the right results for the right reasons. In addition, there is the structure problem, which results from the myriad of ways the underlying structure of the system can be organised (e.g. connected flow paths, heterogeneity in the soil, below-ground topography, vegetation properties etc.), making it practically impossible to infer the correct structure by inverse modelling.

This talk depicts soils as a substrate for vegetation, which exchanges water for free energy and CO2 at the land surface. A model is presented that simulates the dynamic adaptation of vegetation to its environment, represented by given atmospheric forcing and soil hydrological properties. One counter-intuitive result of the model is that the vertical fine root distribution can change very dynamically throughout the year, which has subsequently been confirmed by observations at the simulated site. This suggests that vertical fine root distributions should not be considered as a constraint on transpiration but a degree of freedom that plants have for their adaptation to their environment. It further suggests that some key properties of the root system may be predictable if the driving principle is known.

Following on from this, the question is raised how spatial heterogeneity in the physical soil properties would impact on hydrology and vegetation, and in how far vegetation can influence these properties itself.