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Developing and demonstrating a Kilometer-Scale Land Simulation Capability for Modeling the Terrestrial Water Cycle over the Contiguous U.S.

Product Definition

Kilometer-scale (k-scale) modeling allows for explicit modeling of physical processes that are poorly represented in climate models with typical resolutions of 25-100 km, thus providing opportunities to significantly improve the accuracy of climate simulations (Slingo et al. 2022). The recent advancements in computing power are making k-scale regional and global simulations using Land Surface Models (LSMs) and Earth System Models (ESMs) increasingly feasible (Condon et al. 2020, Caldwell et al. 2021). Groundwater is a vital human water resource that provides 20-30% of global freshwater withdrawals (Döll 2009). Previous k-scale LSM studies for specific watersheds or basins have demonstrated the impacts of fine scale-structures on terrestrial hydrologic processes including groundwater dynamics (Maxwell and Kollet 2008, Fan et al. 2013).

However, the parameters of LSMs within ESMs being run at the k scale are typically derived from coarse-resolution data sets or outdated data sets. Consequently, k-scale modeling may not accurately represent fine-scale land surface heterogeneity unless high-resolution land surface parameters at the kilometer or finer scales are used. Additionally, LSMs will have to be recalibrated at k scale to accurately simulate terrestrial processes (Ruiz‐Vásquez et al. 2023) and the recalibration is expected to have a significantly large computational cost at such high spatial resolution.

In this report, we develop a first-of-a-kind k-scale global land simulation capability and demonstrate this for modeling the terrestrial water cycle over the contiguous U.S. in ESMs. To enable this capability, we first develop a new set of global land surface parameters at 1-km resolution by using the newest high‑resolution data sources for multiple years by combining the latest and most accurate available global data sets to provide input data for ESMs, including the U.S. Department of Energy’s flagship Energy Exascale Earth System Model (E3SM) (Leung et al. 2020). Second, we produce an initial 5-year simulation using E3SM Land Model version 2 (ELMv2) over the contiguous United States (CONUS) at 1-km resolution using the newly developed land surface parameters. A spatial scaling analysis was performed to underscore the value of the high-resolution land surface parameters, and AI/ML methods were used to identify the most important land surface parameters and climate conditions that drive the spatial variability at k scale and hence, spatial information loss as resolutions were coarsened in ELMv2 simulations. Using the newly developed 1-km input data, the k-scale simulations show significant spatial heterogeneity of soil moisture, latent heat, emitted longwave radiation, and absorbed shortwave radiation, which is expected to have important effect on modeling of land-atmosphere interactions. Third, we demonstrate significant improvements in the ELM-simulated water table depth over CONUS by calibrating ELMv2’s subsurface drainage parameterization for k-scale modeling.

Product Documentation

This report documents a k-scale land simulation capability, which includes the development of global k-scale land surface parameters for LSMs (2.1), the application of the newly developed surface parameters in k-scale ELMv2 simulation over CONUS and analysis that demonstrates the importance of the k-scale surface parameters data set (2.2), and the calibration of ELMv2’s subsurface hydrologic processes to improve the prediction of groundwater at a 1-km resolution over CONUS (2.3).