A subbasin-based framework to represent land surface processes in an Earth system model

Abstract

Abstract. Realistically representing spatial heterogeneity and lateral land surface processes within and between modeling units in Earth system models is important because of their implications to surface energy and water exchanges. The traditional approach of using regular grids as computational units in land surface models may lead to inadequate representation of subgrid heterogeneity and lateral movements of water, energy and carbon fluxes. Here a subbasin-based framework is introduced in the Community Land Model (CLM), which is the land component of the Community Earth System Model (CESM). Local processes are represented in each subbasin on a pseudo-grid matrix with no significant modifications to the existing CLM modeling structure. Lateral routing of water within and between subbasins is simulated with the subbasin version of a recently developed physically based routing model, Model for Scale Adaptive River Transport (MOSART). The framework is implemented in two topographically and climatically contrasting regions of the US: the Pacific Northwest and the Midwest. The relative merits of this modeling framework, with greater emphasis on scalability (i.e., ability to perform consistently across spatial resolutions) in streamflow simulation compared to the grid-based modeling framework are investigated by performing simulations at 0.125 • , 0.25 • , 0.5 • , and 1 • spatial resolutions. Comparison of the two frameworks at the finest spatial resolution showed that a small difference between the averaged forcing could lead to a larger difference in the simulated runoff and streamflow because of nonlinear processes. More systematic comparisons conducted using statistical metrics calculated between each coarse resolution and the corresponding 0.125 • -resolution simulations showed superior scalability in simulating both peak and mean streamflow for the subbasin based over the grid-based modeling framework. Scalability advantages are driven by a combination of improved consistency in runoff generation and the routing processes across spatial resolutions.

Scalability of grid- and subbasin-based land surface modeling approaches for hydrologic simulations

“…• Readme • Figure S1 • Figure S2 • Figure S3 • Figure S4 • Figure S5 Correspondence to: T. K. Tesfa, teklu.tesfa@pnnl.gov 2000] and river transport [e.g., Goteti et al, 2008], which, despite important advances, have several limitations in their routing approach and model inputs [Tesfa et al, 2014]. As another attempt on the use of subbasinbased representation in a land surface model, Tesfa et al [2014] introduced a new subbasin-based approach built upon version 4 of the Community Land Model (CLM4) [Oleson et al, 2010;Lawrence et al, 2011] with some important technical advances including meteorological and land surface inputs derived from highresolution data sets coupled with a new physically based river routing model [Li et al, 2013].…”

“…The subbasin-based CLM (SCLM) refers to the new CLM modeling framework introduced by Tesfa et al [2014], where the study domain is divided into a number of irregular subbasins following the natural watershed divides. To be consistent with the grid-based modeling approach, the subbasins are organized into a two-dimensional matrix of subbasins, each being treated as a single node for the resolution of interest.…”

“…As another attempt on the use of subbasinbased representation in a land surface model, Tesfa et al [2014] introduced a new subbasin-based approach built upon version 4 of the Community Land Model (CLM4) [Oleson et al, 2010;Lawrence et al, 2011] with some important technical advances including meteorological and land surface inputs derived from highresolution data sets coupled with a new physically based river routing model [Li et al, 2013]. The Community Land Model version 4 (CLM4) [Lawrence et al, 2011] has a large user community, and its use of the TOPMODEL approach for parameterizing runoff may allow it to take more advantages of the subbasinbased land surface representation.…”

“…In this study, two different land surface spatial representations of CLM4 are applied: the standard grid-based CLM (hereafter denoted as CLM) and the subbasin-based CLM (hereafter denoted as SCLM) described in Tesfa et al [2014]. CLM4 is the latest version of the land component of the Community Earth System Model (CESM) [Collins et al, 2006;Gent et al, 2010;Lawrence et al, 2011], which has been designed and used for studies of interannual and interdecadal variability, paleoclimate regimes, and projections of future changes of the global earth system.…”

“…The subbasin-based approach offers important advantages over the standard grid-based approach from a hydrologic perspective ( [Tesfa et al, 2014] and references therein) including: (1) conceptually, parameterizing runoff generation is more straightforward when the computational units follow topographic boundaries as opposed to the grid-based approach where grids often encompass areas from several natural subbasins, challenging the very concept of runoff generation formulation in parameterizations such as TOPMODEL in which topographic variation has major control in runoff generation; (2) a one-to-one correspondence between the subbasins and the river network structure, which makes it easy to parameterize runoff routing as opposed to the grid-based approach where the grids often cross over multiple river network reaches [Tesfa et al, 2014]; and (3) a platform to share advances made by the land surface modeling and hydrologic science communities to improve hydrologic simulation in land surface models. Despite these distinct advantages, in the last decade, only few land surface modeling efforts have attempted to apply land surface models using the subbasin-based approach to improve parameterization of spatial variability of soil water [e.g., Koster et al,…”