Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates Page: 13 of 45
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significant recharge in those areas. For example, significant net infiltration beneath
washes failed to be predicted under the monsoon climate scenarios, when significant net
infiltration would likely occur. The inability of MASSIF to adequately capture infiltration
under washes is likely due to lack of site-specific soil data for parameterization, the
omission of subsurface lateral flow in the model, the "resetting" of soil moisture to dry
conditions at the end of each water year, and the overestimation of soil depth in upper
wash environments that leads to overestimation of evapotranspiration in the upper wash
Figure 18.104.22.168-1 in the model report shows that the majority of soil depths assumed for
MASSIF are larger than those from reported Yucca Mountain measurements. Soil depth
class 2 seems to be very poorly predicted; this is the depth class likely to be in the
washes. In particular, soil depths for soil depth class 2 show the most overestimation
(only two MASSIF depths are less than the observed soil depths). This may explain the
inability of MASSIF to accurately model runoff. Many of the boreholes are in soil depth
class 2, where MASSIF consistently over predicts soil depth and therefore likely under
predicts net infiltration.
In addition, the model results show an apparent lack of upland and interfluve infiltration
variability. This lack of upland variability likely results from the simplification, or
"lumping," of input parameters, such as the spatial aggregation of soil and geologic units
and a constant rooting depth assumed for each soil type, and from the omission of the
effect of subsurface lateral flow.
a. Was the methodology used appropriate and effectively applied?
The mass-balance approach used by MASSIF is appropriate for the time and space scales
considered. MASSIF was competently applied, especially considering the limited amount
of site-specific data used by the modeling team for parameterizing the model.
However, the characterization of the geologic inputs (hydraulic properties of soils, soil
depths, and saturated hydraulic conductivity of bedrock) is questionable, given that the
values for these parameters were derived without using measurements from Yucca
Mountain either in the field or in the laboratory, with the exception of a few soil depth
and bedrock Ksat measurements. The near absence of soil depth and soil hydraulic
measurements from the site for validation is one of the primary weaknesses of the
For example, the Hanford-derived pedotransfer function is of questionable value for use
at Yucca Mountain without extensive testing and comparison. Yet, the soil hydraulic
properties calculated by the pedotransfer function have not been compared to site-specific
Yucca Mountain soil properties measured directly as part of site-characterization
activities. The panel reviewed the comparison of the Hanford-derived soil hydraulic
properties to the measured water content/capillary pressure measurements from USDA,
Soil Survey of Nye County, Nevada, Southwest (2004), as well as the reported
measurements of hydraulic conductivity and data from Istok et al., both reported in Data
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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/m1/13/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.