Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates Page: 40 of 45
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infiltration uncertainty on a seasonal basis, as well as possibly a significant parameter
over the duration of long-term MASSIF simulation.
In Sec. 188.8.131.52, "Input Parameters for Soil Water Balance Calculations," there is an
apparent arbitrary distribution of the nominal evaporation rate from dry soil, with a
nominal value of zero. This is a fairly weak assumption, but probably conservative. The
actual distribution of this parameter could be estimated from the lysimeter data, but this
was not done; the only characteristic determined was that long-term evaporation from
below the near surface did persist.
This section also provides an arbitrary value for the Soil Moisture Depletion Coefficient
(p), a stress parameter for evapotranspiration, without solid justification or previous
work. This may also be a conservative assumption, but it is not clear what the
The evaporation layer depth (Ze)-the mean effective depth of the surface soil layer that
is subject to drying by evaporation to dry air-has an arbitrary assignment of 0.15 m,
with a uniform distribution between 0.1 and 0.2 meters. This is rather uncertain,
particularly for coarse-textured soils. In addition, the report does not discuss how the
thickness of the surface layer affects infiltration processes; in MASSIF, a thin surface
layer will result in more infiltration than a thick layer, especially during periods when
vegetation is not active or in bare soils.
Section 6.6, "Infiltration Prediction Uncertainties," provides a lengthy mathematical
analysis of parameter uncertainties in the MASSIF simulations. In the shorter discussion
of model uncertainty, the report states that "The present study was unable to explicitly
test the accuracy of the field capacity approach for representing subsurface water flow
against field data from the Yucca Mountain site. However, a comparison was made
against HYDRUS 1-D.... This comparison demonstrates that while the field capacity
approach may not represent the transient nature of this flow accurately, it does an
adequate job of representing the cumulative net infiltration over the year." It is not
possible for the Review Panel to assess the adequacy of calculation of prediction
uncertainties when direct comparison to field infiltration or comparison to field
measurements is not presented. The report goes on to suggest that "If the spatial
distribution of neutron borehole locations is representative of the UZ modeling domain
and the net infiltration estimates from the analysis of the neutron logs is representative of
conditions away from the boreholes, this would suggest that the MASSIF model may
underestimate actual net infiltration for this area by at least a factor of 2 ... It is not clear,
however, that either of these criteria is met, and therefore it is not clear how these data
can help to estimate model uncertainty." The report concludes that "... model uncertainty
may be of a comparable magnitude to parameter uncertainty. Given the complexity of
modeling net infiltration over such a large and heterogeneous domain, such uncertainty is
not unprecedented." The panel does not have sufficient data (either in the report or
supporting documents) to assess the accuracy of these statements.
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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/m1/40/: accessed May 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.