DECIPHeR-GW v1: a coupled hydrological model with improved representation of surface-groundwater interactions

Groundwater is a crucial part of the hydrologic cycle and the largest accessible freshwater source for humans and ecosystems. However, most hydrological models lack explicit representation of surface-groundwater interactions, leading to poor prediction performance in groundwater-dominated catchments. This study presents DECIPHeR-GW v1 (Dynamic fluxEs and ConnectIvity for Predictions of HydRology and GroundWater), a new surface-groundwater hydrological model that couples a model based on hydrological response units (HRUs) and a two-dimensional gridded groundwater model. Using a two-way coupling method, the groundwater model component receives recharge from HRUs, simulates surface-groundwater interactions, and returns groundwater levels and groundwater discharge to HRUs, where river routing is then performed. Depending on the storage capacity of the surface water model component and the position of the modelled groundwater level, three scenarios are developed to derive recharge and capture surface-groundwater interactions dynamically. Our coupled model was set up at 1 km spatial resolution for the groundwater model, and the average size of the surface water HRUs was 0.31 km2. The coupled model was calibrated and evaluated against daily flow time series from 669 catchments and groundwater level data from 1804 wells across England and Wales. The model provides streamflow simulation with a median Kling-Gupta efficiency (KGE) of 0.83 across varying hydro-climates, such as wetter catchments with a maximum mean annual rainfall of 3577 mmyr-1 in the west and drier catchments with a minimum of 562 mmyr-1 in the east of Great Britain, as well as diverse hydrogeological conditions including chalk, sandstone, and limestone. Higher KGE values are found in particular for the drier chalk catchments in southeast England, where the average KGE for streamflow increased from 0.49 in the benchmark DECIPHeR model to 0.7. Furthermore, our model reproduces temporal patterns of the groundwater level time series, with more than half of the wells achieving a Spearman correlation coefficient of 0.6 or higher when comparing simulations to observations. Simulating 51 years of daily data for the largest catchment, the Thames at the Kingston River basin (9948 km2), takes approximately 17 h on a standard CPU, facilitating multiple simulations for model calibration and sensitive analysis. Overall, this new DECIPHeR-GW model demonstrates enhanced accuracy and computational efficiency in reproducing streamflow and groundwater levels, making it a valuable tool for addressing water resources and management issues over large domains.