Runoff as Complexity Indicator for Terrestrial Ecosystems

Terrestrial ecosystems are often considered as among the structurally most complex systems that can be scientifically investigated. On the other hand, utilization techniques like forestry or water management, focussing on specific functional aspects, often follow relatively simple empirical rules, which led in the past to satisfying predictions. This apparent contradiction opens the question whether one could quantify the functional simplicity of such systems. We use time series analysis and information-theoretic methods to answer this question for an important type of such systems, natural watersheds, and for the obviously most important variable in these systems, water flow. Based on the distinction between randomness and complexity, three different types of data sets can be characterized: very ordered and simple, partly random and rather complex, or very random and again simple. This classification has implications for modeling in hydrology. It also demonstrates the presence of universal features characterizing watersheds over a wide range of scales and climates not seen before.