Development and evaluation of a variably saturated flow model in the global E3SM Land Model (ELM) version 1.0
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Published:2018-10-11
Issue:10
Volume:11
Page:4085-4102
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ISSN:1991-9603
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Container-title:Geoscientific Model Development
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language:en
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Short-container-title:Geosci. Model Dev.
Author:
Bisht GautamORCID, Riley William J.ORCID, Hammond Glenn E.ORCID, Lorenzetti David M.
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.
Publisher
Copernicus GmbH
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