Watersheds as complex dissipative geochemical systems: stability, variability and resilience. Theory and practical applications

The PhD research project was aimed to investigate watersheds as complex dissipative systems characterized by peculiar properties as self-organization. The knowledge of watersheds dynamics has significant implications in water management, given the increasing need of solutions facing the multiple challenges in the preservation of such resource (Baker et al. 2008). From this point of view, the study focused on the development of tools able to deal with relevant issues concerning the watersheds management, such as the lack or scarcity of information, the complex features of aqueous systems and the compositional nature of geochemical data (i.e. concentration values). The proposed methodologies have been applied both to surficial and ground waters, mainly coming from GEOBASI (the geochemical repository of Tuscany Region), by using an interdisciplinary statistical–geochemical approach, based on Compositional Data Analysis (CoDA). Results pointed out how the CoDA approach is essential in order to 1) solve a common problem in great geochemical databases, such as the presence of missing data and, simultaneously, values below different detection limits; 2) provide an enhanced compositional version of the Langelier-Ludwing plot, thus permitting not only the water classification but also the implementation of robust statistical tools inside it; 3) contribute to the understanding of the complex features of watersheds. Regarding to this latter, the work concerned the presence of intermittency in chemical values, by applying methods (i.e. fractal analysis) which consider the possibility of spatial irregularities and not smoothly distributions in geochemical data. Actually, the intermittency is a peculiarity of fractal variability which, in turn, is an emergent property of complex dissipative systems. Hence, the fractal analysis allowed to reacha twofold result. On one hand, fractal modelling is necessary in multilay- ered aquifers when spatial continuity is scarce, due to complex dynamics and heterogeneity within the underground. Moreover, it provides a probe about how the chemical composition reflects the organization of the underground system. On the other hand, chemical reactions in surficial waters are able to produce entropy and dissipate energy according to the intensity of the mixing processes with the higher TDS (total dissolved solids) groundwater along the river course. The strongest dissipative behaviours are associated with those variables showing a spatial progression towards the thermodynamic equilibrium and a propensity to deviate from the lognormal to the power-law distribution.