Scalability of Grid- and Subbasin-Based Land Surface Modeling Frameworks for Hydrologic Simulations
Water models talk topography, land surface models talk grids. Choosing a common language was important, according to researchers at Pacific Northwest National Laboratory, because land surface models must also represent hydrology in Earth system modeling. To simulate water runoff and river flow, the team showed that dividing the area into units based on subbasins defined by topography-the computational unit used by water models-makes land surface models more robust across different spatial resolutions versus the uniform grid-based units.
"By comparing the two methods in a widely used land model coupled with a river transport model, the subbasin approach more realistically simulated the water systems through the land," said principal investigator and PNNL atmospheric scientist Dr. Ruby Leung. "This research reveals how important it is to restructure land models to represent spatial heterogeneity for more robust runoff and streamflow simulations in Earth system models."
Water, land, air. Scientists develop numerical models to simulate how these systems operate and use them to predict how the systems respond to natural and human-caused changes. Testing "what ifs" is complicated by modeling results that change with the size of the computational grids, which is limited by the available computational horsepower. Researchers are striving for the most system complexity possible. Faithfully simulating water system processes requires allegiance to topography, those divisions of landscape such as mountains, valleys, and land cover. These are the natural boundaries that influence the water system's response to climate change.
By adopting computational units based on the natural boundaries scientists tag as subbasins the researchers solved an important challenge and enabled hydrologic processes to be more robustly simulated across a range of model resolutions. Their findings also facilitate understanding of the many processes that control freshwater supply. This research shows the way to more robust water system simulations at regional and global resolutions.
Topography has a dominant control on processes that determine freshwater supply. Traditionally, hydrologic models are designed to represent the variety of hydrologic processes within computational units defined by topographic boundary of watersheds or subbasins. However, land surface models generally define their computational units using simple rectangular grids to facilitate exchanges with atmospheric models that share the same grids.
To understand the scalability of both, the PNNL researchers implemented the subbasin approach in the Community Land Model (CLM) coupled with the Model for Scale Adaptive River Transport (MOSART) for comparison with the model's current grid-based approach.
They performed simulations in the large river basins with diverse climate and hydrology using model resolutions ranging from 1/8 to 1 degree. For each approach, they evaluated the differences between the coarser resolution hydrologic simulations and the simulations at the highest resolution of 1/8 degree. They found the coarser resolution simulations using the subbasin approach approached the high-resolution simulations faster and more consistently as resolution increases than simulations using the grid-based approach.
Their in-depth analysis showed that the scalability advantages of the subbasin approach are related to a combination of improved atmospheric forcing and consistency of runoff generation and the routing processes across spatial resolutions.
Traditionally, land surface models used in Earth System Models compute land surface processes on a structured grid that follows the latitudes and longitudes of the globe. However, hydrologists have more commonly employed catchments, or subbasins, as the computational units because hydrological processes are strongly influenced by topography that defines the subbasin boundaries. A team of Department of Energy scientists at Pacific Northwest National Laboratory investigated the relative merits of grid- and subbasin-based land surface modeling approaches for hydrologic simulations, with a focus on their ability to perform consistently across spatial resolutions in simulating runoff generation. They produced simulations in the grid- and subbasin-based Community Land Model at 0.125°, 0.25°, 0.5°, and 1° spatial resolutions over the U.S. Pacific Northwest. For the first time, the team found the importance of spatial structure for multi-scale modeling of hydrological processes, which may improve modeling of coupled processes in Earth System Models. The significant scalability advantage for the subbasin-based approach compared to the grid-based approach for simulations of runoff, affects other processes such as soil moisture, evapotranspiration, and streamflow. They found the source of runoff scalability is related to the atmospheric forcing and land surface parameters of runoff generation, both sensitive to surface elevation and more scalable in the subbasin framework.
The U.S. Department of Energy Biological and Environmental Research supported this research as part of the Earth System Modeling Program, Regional and Global Climate Modeling Program, and Integrated Assessment Research Program. Development of input data and the CLM and MOSART simulations received support from the PNNL's Platform for Regional Integrated Modeling and Analysis (PRIMA) initiative, a Laboratory Directed Research and Development program.