How climate, catchment and vegetation characteristics impact water flux partitioning and transit times at the catchment scale – a modeling study

Whether flow is relatively young or old when it passes by the catchment outlet is a strong indicator of weathering processes, biogeochemical reactions, nutrient availability, pollution susceptibility and the hydrologic response of a catchment. It depends not only on individual catchment, climate, event and vegetation properties, it is also the result of a multitude of interactions between different processes and catchment states within the hydrologic system.

In order to begin to disentangle the cause-effect chains, we employed the physically-based, spatially explicit 3D model HydroGeoSphere in a virtual catchment running 270 scenarios with different combinations of catchment, climate and vegetation properties. For example, we looked at the influence of vegetation density and rooting depth while also considering different soil moisture conditions that modify the relationships between water ages and vegetation properties. In the same way, we varied the hydraulic conductivity of the soils and observed the water age relationships conditional on antecedent soil moisture. It became clear very quickly that simple, straightforward dependencies between individual catchment, vegetation, event and climate properties do hardly exist.

This is to show that, in order to make meaningful predictions about the age of hydrologic fluxes, it is inevitable to consider more than one variable when predicting biogeochemical responses at the catchment scale. Thus, it can be extremely helpful to look at the individual properties and the processes they control, their potential interactions and interdependencies, in a bottom-up approach within the framework of a hydrologic model.