Development and evaluation of a variably saturated flow model in the global E3SM Land Model (ELM) version 1.0

Abstract

Abstract. Improving global-scale model representations of near-surface soil moisture and groundwater hydrology is important for accurately simulating terrestrial processes and predicting climate change effects on water resources. Most existing land surface models, including the default E3SM Land Model (ELMv0), which we modify here, routinely employ different formulations for water transport in the vadose and phreatic zones. Clark et al. (2015) identified a variably saturated Richards equation flow model as an important capability for improving simulation of coupled soil moisture and shallow groundwater dynamics. In this work, we developed the Variably Saturated Flow Model (VSFM) in ELMv1 to unify the treatment of soil hydrologic processes in the unsaturated and saturated zones. VSFM was tested on three benchmark problems and results were evaluated against observations and an existing benchmark model (PFLOTRAN). The ELMv1-VSFM's subsurface drainage parameter, fd, was calibrated to match an observationally constrained and spatially explicit global water table depth (WTD) product. Optimal spatially explicit fd values were obtained for 79 % of global 1.9∘ × 2.5∘ grid cells, while the remaining 21 % of global grid cells had predicted WTD deeper than the observationally constrained estimate. Comparison with predictions using the default fd value demonstrated that calibration significantly improved predictions, primarily by allowing much deeper WTDs. Model evaluation using the International Land Model Benchmarking package (ILAMB) showed that improvements in WTD predictions did not degrade model skill for any other metrics. We evaluated the computational performance of the VSFM model and found that the model is about 30 % more expensive than the default ELMv0 with an optimal processor layout. The modular software design of VSFM not only provides flexibility to configure the model for a range of problem setups but also allows for building the model independently of the ELM code, thus enabling straightforward testing of the model's physics against other models.

New Representation of Plant Hydraulics Improves the Estimates of Transpiration in Land Surface Model

“…Deep soil moisture is strongly related to the soil water table. Most previous land surface models simulate the water table diagnostically, based on either pressure head profile or a parameterization scheme [38,39]. A numerical framework, which explicitly tracks the soil water table, has opened a novel avenue for soil hydrology modeling [39] and, therefore, improved the quality of the deep soil moisture modeling.…”

Representing Intercell Lateral Groundwater Flow and Aquifer Pumping in the Community Land Model

“…Furthermore, the small difference in river discharge among different simulations highlights the fact that: (1) inclusion of FELFELANI ET AL. lateral flow and groundwater pumping does not substantially alter river discharge simulations over large basins (e.g., Mississippi River, Ohio River), especially at the current model grid of ∼13 km, (2) the impact of pumping was minimal because pumping occurs mostly in semiarid and dry regions where the water table is relatively deep and has low impact on river flow (again, at the current model grid and spatial extent) (Winter et al, 1998), and (3) the subsurface runoff parameters (i.e., q drai,max and f drai ) are not specifically calibrated for the selected basins and hence are not highly sensitive to groundwater table depth that characterized the primary difference between the four simulations (Bisht et al, 2018). It is expected that by increasing the FELFELANI ET AL.…”

An Introduction to the E3SM Special Collection: Goals, Science Drivers, Development, and Analysis

“…Building from CLM4.5, E3SM Land Model (ELM v1) includes new options for representing soil hydrology and biogeochemistry, featuring a variably saturated flow model (Bisht et al, 2018) and a new module for phosphorus dynamics (Yang et al, 2019). To understand the effects of nutrient limitations on carbon-climate feedbacks and their sensitivity to model structural uncertainty, two biogeochemistry approaches are included in ELM v1 to contrast a mechanistic (Equilibrium Chemistry Approximation or ECA; Tang, 2015;Zhu et al, 2016Zhu et al, , 2017) and a conceptual (Converging Trophic Cascade or CTC; Duarte et al, 2017;Mao et al, 2016;Raczka et al, 2016) framework for representing nutrient competition between microbes, plants, and abiotic processes.…”