Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates Page: 8 of 45
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lack of site-specific data and the use of an oversimplified model, the review panel
was unable to confirm whether the model uses parameter values, assumed ranges,
probability distributions, and bounding assumptions that are technically defensible,
reasonably account for uncertainties and variabilities, and do not result in an
underestimation of the long-term net infiltration.
The spatially averaged net infiltration estimates that result from the modeling effort
(summarized in Table 188.8.131.52-1 in the model report), along with their uncertainty ranges,
may or may not accurately capture the value of net infiltration at Yucca Mountain for the
modeling domain as a whole. The fact that the results are generally consistent with other
regional estimates for mean net infiltration is not proof they are correct for Yucca
Mountain. More importantly, in the opinion of the review panel, calculated spatial
averages may be less relevant than identification of areas where sustained infiltration may
exceed the averages, along with characterization of the likely distribution of such areas
over the modeling domain.
Statements in the report indicate a general lack of confidence in the spatial distribution of
net infiltration values produced by the model. Maps of mean annual net infiltration
suggest distinct spatial patterns of infiltration, but they do not compare well with limited
field observations, and the limited discussion of spatial variability suggests the variance
within map colors may greatly exceed the difference between colors. Not enough field
data are available to adequately constrain and characterize the spatial variability and
patterns. It is not possible to confirm with existing information that there are not areas for
which infiltration is underestimated. Even with a wide uncertainty range, the infiltration
at a given location could be underestimated because of soil and root depths, subsurface
lateral flow, and other heterogeneities such as fracture properties and fracture distribution
that vary differently than the assumed (but largely unknown/untested) values. For
example, the modeling simulation does not show significant infiltration beneath
ephemeral stream channels; the report's authors caution that "this result raises some
important questions about the predicted spatial distribution of net infiltration produced by
the model" (p. 6-203).
The ORISE Review Panel concludes, along with the report authors, that "more field work
would have to be performed in order to evaluate the accuracy of the spatial distribution of
net infiltration in the current maps" (p. 6-203). For quantification of the uncertainty range
of the spatial distribution of the net infiltration, collection of sufficient site-specific data
will be required to better characterize such critical parameters as soil hydraulic
properties, soil depth, fracture distributions, and vegetation rooting depth and to test the
underlying assumptions of the modeling effort, most notably, the assumed negligible
effect of subsurface lateral flow. The report's authors are in agreement: "Sensitivity
analyses ... suggest that there may be insufficient characterization of soil properties
(depth, holding capacity, and hydraulic conductivity) over the modeling domain to obtain
accurate and detailed maps of net infiltration" (p. 8-11). Only with adequate field data
can the modeling effort accomplish its purpose of credibly describing the spatial
distribution and associated uncertainty bounds for net infiltration of water at Yucca
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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/m1/8/: accessed May 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.