Development of A Global 5arcmin Groundwater Model (H08-GMv1.0): Model Setup and Steady-State Simulation

Abstract. Groundwater plays a critical role in regulating the global hydrological cycle and serves as the most stable freshwater resource for human daily water consumption. However, since the in-situ observations are scarce and the global modelling techniques are unmatured, many hydrological models, including H08, a global hydrological model considering human water use activities, downplay the groundwater component, i.e., the underground aquifer is often described as a simple lumped model where no lateral groundwater movement or the water table is represented. Here, we present a global H08-MODFLOW groundwater model (H08-GM), built at a five-arcmin spatial resolution, aiming to enhance the capability of the original H08 model in simulating groundwater flows. We describe the basic model setups and simulations under steady-state conditions in this paper. The sensitivity analyses are first conducted to select the best-performing model run against in-situ observations. All model runs demonstrate overall good performance, with a model-observation correlation coefficient exceeding 0.65. However, the model is more sensitive to aquifer conductivity settings compared to the two previous studies. The best-run steady-state groundwater Water Table Depth (WTD, water table below land surface) shows a consistent geographical pattern with the earlier studies in terms of the geographical pattern, where hallow WTD mainly occurs is predominantly found in humid plains and deep WTD in arid and mountainous regions. We further use the model to examine the mechanisms controlling groundwater flow dynamics and found that the groundwater head distribution generally follows topography, but its local characteristics are regulated by aquifer hydrogeological properties and river-aquifer exchanges, when surface recharge is not a dominant limiting factor. We also present the global cell-to-cell net groundwater lateral flow map and found that the magnitude in some regions is non-negligible to annual groundwater recharge. This highlights the important role of the lateral groundwater flow in maintaining the regional water budget so that it has to be considered in hydrological modeling studies in future. The steady-state simulation from this study provides the necessary initial condition for the transient simulations, which is essentially important to analyze the global groundwater decline trends and will be presented in another paper. The development of the H08-GM model therefore provides a powerful tool for large-scale groundwater studies, which enables direct comparison with other large-scale groundwater models joined the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP), and is essential to advance the development of the next-generation global hydrological models.