IHACRES-3S - A 3-store formulation for modelling groundwater-surface water interactions

A streamflow regime can be broadly categorised as perennial, intermittent, or ephemeral, with ephemeral streams having relatively short-duration flow after rainfall and no baseflow, and intermittent streams having a more sustained flow during the wet season, but no baseflow during extended dry periods. Most existing rainfall-runoff models have been developed for humid catchments, in which streamflow is perennial. Thus the assumptions on which they are premised are often inappropriate for capturing the dynamic interactions between stream and groundwater in the more variably-connected systems in semi-arid and arid catchments. This paper presents IHACRES-3S, a new formulation of the IHACRES rainfall-runoff model, which has been developed for modelling streamflow in variably-connected groundwater-surface water catchments or catchments at risk of a change in flow regime due to groundwater extractions or climate change, such as the Messara catchment in Crete. The linear routing module has been reformulated as a 3-store model, with the new store behaving as a seasonal perched water table. Variations in the recharge between the two subsurface stores and the introduction of a streamflow evaporation term are shown to improve model performance, with the latest formulation able to capture the timing of the switch from a perennial stream to an ephemeral stream, as well as the low flow behaviour between wet seasons (Figure 1). Including a streamflow evaporation term is also shown to contribute to improved performance in modeled baseflows in the Messara catchment. Model complexity has increased due to the addition of new parameters and assumptions and more sensitivity testing is required to determine the final formulation, and level of complexity, as well as its wider applicability.