Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates Page: 38 of 45
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temporal distributions of net infiltration, including their extremes across the model
domain. Subsurface lateral flow, including any preferential or funnel flow during
saturated/unsaturated conditions, may occur at finer scale than 30 m x 30 m scale,
depending on the local heterogeneity in soil and bedrock properties. As a result, the
natural flow path should be better characterized (e.g., in a triangular irregular network
rather than a regular square grid). Also, mean hydraulic properties of the bedrock (UZ),
in comparison to the overlying soil layers, does not ensure that water will always move
vertically downward through the bottom boundary in all soil cells. The panel agrees that
subsurface lateral flow will provide further opportunity for extraction by ET in
downstream soil cells; however, soil layers with contrasting hydraulic properties in the
bedrock/soil interface, including the geometry of convergent/divergent flow zones with
hit and miss fractures will dictate the ultimate subsurface flow routing. So there is
concern about the accuracy of spatio-temporal variability and net infiltration extremes
under saturated/unsaturated conditions during different projected climatic scenarios. The
evidence of flow in the washes counters the suggested argument of no field evidence that
subsurface lateral flow is important (unless the authors suggest this is only due to surface
lateral flow). Only site-specific representative experimental/modeling studies will
provide more insight on this unresolved issue.
In summary, not considering subsurface lateral flow resulting from topography, soil
layering, and anisotropic hydraulic conductivity can distort the predicted spatial
distribution of net infiltration at the soil-bedrock interface in the region. This may
underestimate the upper bound for net infiltration to the top boundary of the UZ model in
areas where subsurface pathways cause funneled or focused recharge. This hydrologic
behavior will not be captured with the 30 m x 30 m uniform-grid, one-dimensional model
used in this effort. A multi-dimensional model that is resolved spatially and temporally
and captures the directional and interconnected subsurface flow, with supporting field
data, would better serve the purpose of predicting net infiltration in the Yucca Mountain
3. Incompletely Defined Bounds of Uncertainty
In the uncertainty analysis, the choice of parameters to vary was based on their impact on
net infiltration; parameters determined to vary insignificantly were eliminated from the
analysis. Parameters that were varied in the simulation include daily rainfall, plant height,
maximum rooting depth, soil depth class 4, bedrock conductivity for two units, readily
available water, minimum transpiration coefficient, evaporation depth layer, and slope of
the NDVI/crop coefficient function. The uncertainty ranges were based upon literature
values in general, and distributions were taken to be, in general, limited to the range of
distributions reported in the literature only. In this modeling effort, uniform distributions
were used for most of the parameters, rather than the more traditional normal
distributions for ecological parameters. This approach may result in an unrealistic
sampling of outlier processes and prediction of the range of calculated infiltration.
Additionally, in the net infiltration modeling, most hydraulic transport properties
(including hydraulic conductivities) were assigned log-uniform distributions rather than
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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/m1/38/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.