HYPORHEIC DYNAMICS DURING STORMS ARE PRIMARILY GOVERNED BY PRE-STORM GROUNDWATER CONDITIONS: INSIGHTS FROM A FULLY COUPLED NUMERICAL MODEL UTILIZING THE BRINKMAN-DARCY EQUATION

The interaction of groundwater and surface water in the hyporheic zone has been observed to contribute to biogeochemical cycling and metabolism in stream networks. Groundwater flux is a primary control on these biogeochemical processes within the hyporheic zone, governing contact time between surface water and the reactive subsurface domain. In this research we quantified annual and storm-driven groundwater controls on hyporheic zone size and exchange flux using a numerical model that couples the Navier-Stokes and Brinkman-Darcy equations in the surface and subsurface, respectively. Comparing steady-state solutions of this fully coupled model against other published solutions indicated an order of magnitude fit and similar conceptual behavior. Next, representative transient scenarios of annual groundwater responses were constructed to simulate winter-spring recharge and summer-fall baseflow recession. Expansion of the hyporheic zone was quantified by increases in surface water flux into the subsurface and increases in depth of the groundwater-surface water transition. Next, representative storm scenarios were constructed to simulate the transient response of the hyporheic zone to both surface and subsurface forcing, superimposed on the annual scenarios. Results indicated that the hyporheic zone expands and contracts primarily due to annual scale groundwater fluctuations. The pre-storm conditions induced by the annual groundwater fluctuations determined both the duration and magnitude of hyporheic zone expansion. As a consequence of these temporal patterns, storm induced flushing of dissolved oxygen and other dissolved constituents into and through the hyporheic zone will vary depending on the antecedent conditions.