The impact of precipitation and potential evapotranspiration on water flux partitioning and transit times at the catchment scale – a modeling study

In order to investigate how annual precipitation amount (P) and potential evapotranspiration (ET_p) influence the partitioning of water fluxes into flow, evaporation and transpiration, we employed the physically-based spatially explicit 3D model HydroGeoSphere in a virtual catchment running 100 scenarios with different combinations of catchment and climate properties. In addition, we looked at the changes in the transit times of the different fluxes.

Unsurprisingly, the fraction of flow increases with larger P and decreases with stronger ET_p. Both transpiration and evaporation fractions generally decrease with larger P and increase with stronger ET_p. However, the increase in the evaporation fraction ends for dryness indices (ET_p/P) larger than 1 while the increase in the transpiration fraction continues. With regard to the transit times we found that on the one hand transpiration becomes younger in catchments with more P while it becomes older when ET_p increases (vegetation has to resort to all potential water sources, also the ones deeper down and older). Evaporation and flow on the other hand become younger with larger P but become older with weaker ET_p (basically, the decrease in transpiration leaves water longer in the system which is then available for older evaporation and streamflow).

This also means that an acceleration of the hydrologic cycle can be caused both by an increase or decrease in the dryness index – depending on whether this change is caused by a change in P or ET_p. This can have significant impacts for predicting catchment response and solute transport in light of future climate variability.