Modeling the age of subsurface runoff at the catchment scale – what makes it younger or older?

Whether subsurface flow is relatively young or old when it passes by the catchment outlet is a strong indicator of weathering processes, 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 (or better even: webs), we employed the physically-based, spatially explicit 3D model HydroGeoSphere in a virtual catchment running 100 scenarios with different combinations of catchment, climate and vegetation properties. One result showed, e.g., that streamflow in forested areas appeared to become older on average compared to a non-vegetated site. Upon closer inspection, this was not necessarily only caused by subsurface runoff becoming slower/older due to lower hydraulic conductivities of drier soils when there was active root water uptake. Another component of this increase in stream water age was the different partitioning of precipitation into subsurface runoff and groundwater flow. Relatively more water was transported in the slower groundwater domain and less within the soil at the bedrock-soil interface.

This is to show that, in order to make meaningful predictions about the age of hydrologic fluxes, it may not be the best approach to single out specific catchment and climate properties. Instead, 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.