Percolation theory addresses the spatial and temporal scaling properties of disordered networks, both for topological and transport-related processes. The critical zone is, effectively, a random network. The ability to predict the dimensions of the critical zone as a function of time, as well as its carbon storage properties, are both critical to the understanding of its dynamics. In addition, combining the scaling relationships with the principal that represented ecosystems will tend to optimize net primary productivity, generates a basic understanding of the water cycle. When the future of hydrology is discussed, these facts should be of central importance. Furthermore, although the definition of complex systems is not necessarily generally agreed on, it is usually understood that phase transitions, such as described by percolation theory, are emergent phenomena within the range of topics encompassed. Thus, this topic is of fundamental relevance to the present conference.