Runoff parameterization for global scale hydrology based on a variably saturated flow model

Groundwater is the world's essential water resource, and groundwater flow is strongly linked with surface water. In some cases, groundwater can even affect precipitation through evapotranspiration. In the current Earth System Model (ESM), a fixed and constant one-dimensional vertical grid is used in the unsaturated zone. The thickness of the unsaturated zone is expected to differ in a given region under a future climate. Therefore, the representation of groundwater flow in the ESM may be insufficient. In particular, on steep slopes such as mountainous areas, it is considered that there are limitations to the runoff process. In this study, we developed a three-dimensional variably saturated flow model that was parameterized for the runoff process and validated in the mountainous area.

In mountainous areas where topographic terrain is severe, calculations at hundreds to tens of meters are necessary to better performance for groundwater flow. However, it is unrealistic to calculate the groundwater dynamics at that scale over global lands. Therefore, it is necessary to calculate on a coarse grid with parameterization. We developed the runoff parameterization using a 1-minute grid with topographic information within the grid. The parameterization validation was performed for the whole of Japan, which has a large elevation distribution. A part of the results of the land surface model, MATSIRO, was passed to the developed model to calculate runoff. The calculated runoff was input into the river routine model, CaMa-Flood, and compared to observed river discharge. As a result, the reproducibility of the river discharge was improved compared to the case without the parameterization and the MATSIRO’s results.