Threshold behaviour in hydrological systems as (human) geo-ecosystems: manifestations, controls, implications

摘要

In this paper we review threshold behaviour inenvironmental systems, which are often associated with the onset of floods,contamination and erosion events, and other degenerative processes. Keyobjectives of this review are to a) suggest indicators for detectingthreshold behavior, b) discuss their implications for predictability, c)distinguish different forms of threshold behavior and their underlyingcontrols, and d) hypothesise on possible reasons for why threshold behaviourmight occur. Threshold behaviour involves a fast qualitative change ofeither a single process or the response of a system. For elementaryphenomena this switch occurs when boundary conditions (e.g., energy inputs)or system states as expressed by dimensionless quantities (e.g. the Reynoldsnumber) exceed threshold values. Mixing, water movement or depletion ofthermodynamic gradients becomes much more efficient as a result.Intermittency is a very good indicator for detecting event scale thresholdbehavior in hydrological systems. Predictability of intermittent processes/system responses is inherently low for combinations of systems states and/orboundary conditions that push the system close to a threshold. Post hocidentification of "cause-effect relations" to explain when the systembecame critical is inherently difficult because of our limited ability toperform observations under controlled identical experimental conditions. Inthis review, we distinguish three forms of threshold behavior. The first oneis threshold behavior at the process level that is controlled by theinterplay of local soil characteristics and states, vegetation and therainfall forcing. Overland flow formation, particle detachment andpreferential flow are examples of this. The second form of thresholdbehaviour is the response of systems of intermediate complexity – e.g.,catchment runoff response and sediment yield – governed by theredistribution of water and sediments in space and time. These arecontrolled by the topological architecture of the catchments that interactswith system states and the boundary conditions. Crossing the responsethresholds means to establish connectedness of surface or subsurface flowpaths to the catchment outlet. Subsurface stormflow in humid areas, overlandflow and erosion in semi-arid and arid areas are examples, and explain thatcrossing local process thresholds is necessary but not sufficient to triggera system response threshold. The third form of threshold behaviour involveschanges in the "architecture" of human geo-ecosystems, which experiencevarious disturbances. As a result substantial change in hydrologicalfunctioning of a system is induced, when the disturbances exceed theresilience of the geo-ecosystem. We present examples from savannahecosystems, humid agricultural systems, mining activities affecting rainfallrunoff in forested areas, badlands formation in Spain, and the restorationof the Upper Rhine river basin as examples of this phenomenon. Thisfunctional threshold behaviour is most difficult to predict, since itrequires extrapolations far away from our usual experience and theaccounting of bidirectional feedbacks. However, it does not require thedevelopment of more complicated model, but on the contrary, only models withthe right level of simplification, which we illustrate with an instructiveexample. Following Prigogine, who studied structure formation in openthermodynamic systems, we hypothesise that topological structures whichcontrol response thresholds in the landscape might be seen as dissipativestructures, and the onset of threshold processes/response as a switch tomore efficient ways of depleting strong gradients that develop in the caseof extreme boundary conditions.