Development and Verification of a Three‐Dimensional Variably Saturated Flow Model for Assessment of Future Global Water Resources

Abstract

Abstract Global water resource assessment has been conducted primarily for surface water and shallow groundwater (unconfined aquifers). Groundwater is a major water resource worldwide. Nevertheless, future water resource assessment integrating surface water and groundwater has not been carried out at the global scale. Large grid sizes are used in global‐scale models due to computer resource constraints. Underground, the unsaturated zone that connects surface water and groundwater is expressed using a vertically one‐dimensional grid of fixed size. However, the thickness of the unsaturated zone varies greatly between dry and humid areas, and future unsaturated zone conditions may differ from those at present due to climate change. In addition, for mountainous areas with steep slopes, representing the unsaturated zone in one vertical dimension is a major limitation. We believe that such a representation causes difficulty in adequately expressing underground water flow and thus introduces uncertainties into future prediction results. In this study, we developed a code to represent variably saturated flow with groundwater storativity in three dimensions based on physical equations, which can be used as references for parameterization in the future. To confirm the accuracy of this code, we verified it using vertical one‐ and two‐dimensional infiltration problems, a three‐dimensional groundwater pumping problem, hillslope problem, and water balance. The code achieved reliable results for each problem.