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In most bucket-type hydrological models, water can only flow from the groundwater to the stream and the flux is based on the groundwater storage. However, many catchments have losing stream sections, where streamflow recharges the groundwater. We developed a formulation to represent groundwater recharge by streamwater in a bucket-type model and tested this formulation for the Panola Mountain Research Watershed to demonstrate its function and assess its performance. The upper reach of the Panola catchment is often dry and highly affected by flow from a bedrock outcrop; further downstream the stream is perennial. We simulated streamflow with the fully-lumped version of a bucket-type model and compared it to 1) a variant with sub-catchments to more realistically represent the low storage and quick response from the bedrock outcrop and 2) a variant that also includes the bi-directional exchange between groundwater and the stream. For all three model variants, we compared simulated and observed streamflow and groundwater dynamics. Although the gain in overall model performance by including the bi-directional exchange between the groundwater and the stream was small, the explicit representation of this exchange led to better streamflow simulations during drying-down and wetting-up periods. For Panola the fluxes along the stream appeared less important than subsurface drainage from the upper sub-catchment to the downstream sub-catchment. We recommend considering the bi-directional fluxes between groundwater and stream in bucket-type hydrological models where these processes are important, and the focus of the simulations is on low flow conditions.

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