HESS Opinions: Are soils overrated in hydrology?

Traditional hydrological theories are based on the assumption that soil iskey in determining water's fate in the hydrological cycle. According tothese theories, soil hydraulic properties determine water movement in bothsaturated and unsaturated zones, described by matrix flow formulas such asthe Darcy-Richards equations. They also determine plant-available moistureand thereby control transpiration. Here we argue that these theories arefounded on a wrong assumption. Instead, we advocate the reverse: theterrestrial ecosystem manipulates the soil to satisfy specific watermanagement strategies, which are primarily controlled by the ecosystem's reaction toclimatic drivers and by prescribed boundary conditions such as topographyand lithology. According to this assumption, soil hydraulic properties arean effect rather than a cause of water movement. We further arguethat the integrated hydrological behaviour of an ecosystem can be inferredfrom considerations about ecosystem survival and growth without relying oninternal-process descriptions. An important and favourable consequence ofthis climate- and ecosystem-driven approach is that it provides a physicaljustification for catchment models that do not rely on soil information andon the complexity associated with the description of soil water dynamics.Another consequence is that modelling water movement in the soil, ifrequired, can benefit from the constraints that are imposed by the embeddingecosystem. Here we illustrate our ecosystem perspective of hydrologicalprocesses and the arguments that support it. We suggest that advancing ourunderstanding of ecosystem water management strategies is key to buildingmore realistic hydrological theories and catchment models that arepredictive in the context of environmental change.