Topographic influences on the scaling of groundwater-surface water exchange fluxes

English Abstract:

Exchange fluxes between groundwater systems and the land surface are controlled, in part, by depth to water table: evapotranspiration from the root zone, direct exchange via baseflow to streams, and effective recharge are all therefore strongly dependent upon topography. At higher elevations, where depth to groundwater is generally greater, a larger percentage of infiltration becomes recharge whereas in stream valleys, this infiltration is more likely to re-enter adjacent surface water systems or be taken up via transpiration, thereby reducing effective recharge in these zones. At regional scales, this topographic dependence may become very important, especially when attempting to model systems using relatively large grid cells where some sub-grid scale topographic variability or multiple surface water features are present. Via a set of modelling studies using Modflow, we have identified that areally-averaged exchange fluxes which take into account sub-grid topographic variation may be treated simple scalable power law relationships which are independent of hydraulic conductivity of the medium, and relatively insensitive to groundwater storage. These relationships reveal consistent behaviour between different topographic types and may be used to support regional scale modelling efforts, justify the use of certain simplified relationships in the surface water literature, and provide some useful insight into the phenomenon of groundwater-surface water interaction at the basin scale.