[A-HW21] Surface and subsurface hydrologic models: Technical advances and applications for water management

[AHW21-05] Improved representation of runoff processes using only groundwater model

★Invited Papers

*Yosuke Miura^1,2, Kei Yoshimura^1,3,4 (1.Institute of Industrial Science, The University of Tokyo, 2.Meteorological Research Institute, 3.Department of Civil Engineering, School of Engineering, University of Tokyo, 4.Earth Observation Research Center, Japan Aerospace Exploration Agency)

Keywords:Parameterization, Variably saturated flow equation, Coarse grid, Steep topography, Whole Japan

We developed a groundwater flow model as one component of Earth System Model (ESM). This groundwater flow model is based on the variably saturated flow equation and aims to be applied on a global scale. For global application, a coarse grid size must be used due to the large number of computational grids. However, the coarse grid size reduces the representativeness of the hydrological process. In particular, runoff processes in areas with steep topography are considered to be significantly degraded. The simplest solution is to make the computational grid size finer, but this reduces its applicability to the global scale. Therefore, it is important to parameterize the model while using a coarse grid, but without reducing the expressiveness of the hydrologic process. The groundwater flow model used in this study does not explicitly solve for surface flow. Surface flow is a two-dimensional phenomenon, but it is a fast flow compared to subsurface. Therefore, when coupled with subsurface flow, which is a three-dimensional, slower flow, the computation time may be limited by the surface flow. In mountainous areas, the surface flow is thought to flow down slopes and into rivers, while in plain areas, most of the surface flow are considered to be river and flood flows. Since models that can handle river flow and flooding phenomena on a global scale (e.g., CaMa-Flood) have been developed, we developed a model that can be coupled with these models.
In this study, we focus on runoff among hydrological processes and present a parameterization method and its results using topographic information within 1-minute grid (about 2km). The parameterization method is as follows. First, a cumulative density function is constructed by ascending order of all elevations (30m grid size) contained within the 1-minute grid. Next, parameters are obtained for each 1-minute grid by fitting this cumulative density function using the least-squares method. This parameter is input into the model to obtain the surface moist area in the model for each 1-minute grid according to the groundwater level. From this area and the difference between the surface and subsurface water levels, the runoff volume is determined. The water level at the surface is set to the groundwater level one step before in a nonlinear loop, and it is assumed that the surface water level is almost linked to the groundwater level. 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