Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates Page: 15 of 45
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alternative model was not used because (1) it is unable to simulate runoff between cells
and (2) the previous model used for net infiltration was a mass-balance model and the
available data were more compatible with that approach. However, the report
acknowledges (p. 6-15) that the inability to simulate runoff could have been overcome by
examining two- or three-dimensional versions of HYDRUS, but "other models and
methods were easier to implement." The report also acknowledges that "the strength of a
Richards' equation approach is that it can simulate the spatial and temporal details of
unsaturated water movement in soil" and that "appropriate properties could have been
estimated and developed for a Richards' equation approach..." This was not pursued
because it requires "substantial and detailed information" and "the available soil property
dataset was limited."
HELP incorporates a quasi-two-dimensional water balance model to simulate water
movement in the unsaturated zone. HELP was not used "primarily because it was
developed for a different type of application" (landfill systems). To be used in this
modeling effort, "HELP would require substantial modifications."
The reasons given for not using such alternative modeling approaches are not sufficient.
A more realistic two- or three-dimensional model that can simulate the details of
unsaturated water movement in soil is what is needed to capture the subsurface lateral
flow that likely affects the spatial distribution of the net infiltration or, at least, to develop
simple but more realistic routines for improved simulation of subsurface lateral flow. For
example, results from a 3-D Richards' equation are more realistic than those from a 1-D
Richards' equation, even when the same hydraulic properties (or data) are used. Lack of
data or the need for substantial modifications of existing numerical simulators is not
sufficient justification for using the one-dimensional MASSIF model.
c. Are bounds of uncertainty adequately defined?
The uncertainty reported at the aggregated domain level for the spatially averaged, mean
annual results may be adequately defined; but given the lack of field measurements and
the omission of subsurface lateral flow, this is impossible to confirm. The reported
uncertainty is based on parameter uncertainty; however, the discussion in Section 6.6.3
suggests that model uncertainty "may be of a comparable magnitude to parameter
uncertainty" (p. 6-219). Whether the two uncertainties may be additive, multiplicative, or
otherwise, is not addressed.
Discussion of spatial distribution uncertainty is limited. Spatial uncertainty is not
characterized in the report by soil depth class, bedrock type, or other key-variable spatial
aggregation. The color differences on the net infiltration maps suggest there are
significant differences in infiltration by location on the landscape, but data that support
this are not presented. The different map colors shown may not be statistically
significant, since the discussion of spatial uncertainty in Section 6.6.2 suggests that
variability within map colors may be greater than the differences between map colors. If
it is unrealistic to characterize uncertainty within a map color for an area as large as
Yucca Mountain, then it makes little sense to show such a delineation. The report states
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Oak Ridge Institute for Science and Education. Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates, report, August 30, 2008; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc897028/m1/15/: accessed May 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.