Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates Page: 33 of 45
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saturated hydraulic conductivity. Without further measurements, it is not possible to
quantify the uncertainty in soil depths and bedrock conductivity.
A standard dew point offset is used for Yucca Mountain based upon Allen et al. (1998).
However, actual values of the dew point offset can be derived from observed weather
data. Relative humidity and temperature have been routinely collected at Yucca
Mountain, and the offset could easily have been calculated for the years of data
developed. Real data should be used when available.
2. Subsurface Lateral Flow Excluded by the Modeling Approach
While most processes are included that preserve the spatio-temporal variability of
hydrologic fluxes/states, some hydrologic processes were assumed to be negligible
because of the use of a simple vertical model. A significant process excluded by this
approach is subsurface lateral flow that can occur as a result of vertical heterogeneity in
soil hydraulic conductivity, conductivity differences along the soil-bedrock interface, and
as a result of a lateral head gradient across model cells under saturated or unsaturated
conditions on slopes as well as flat landscapes. In addition, variability and anisotropy in
soil hydraulic properties, soil depth, rooting depth, and bedrock topography contribute to
the lateral flow process. Subsurface topography (e.g., the soil-bedrock interface) may
route the water laterally before it can build up vertically in a grid cell. This subsurface
lateral flow ultimately affects the amount of surface runoff from each grid cell as well as
the available soil water for root uptake and evapotranspiration.
Subsurface lateral flow can occur under both saturated and unsaturated conditions; the
latter, of course, is more common in arid environments. The physical conditions that
cause lateral flow under saturated conditions are completely different from those under
Saturated lateral flow is caused by perched groundwater that develops at the interface
between two layers where the top one has a higher hydraulic conductivity than that
below; for example, a coarse sand overlying a finer-textured layer. A characteristic of
perched systems is that they are underlain by unsaturated sediments. The development of
perched groundwater and water table mounds in stratified alluvium occurring during flow
in the Santa Cruz River, an ephemeral stream near Tucson, Arizona, contributed to
groundwater recharge (Schmidt, 1995). Two saturated layers were clearly separated by an
unsaturated zone with lower water content. Nevertheless, the hydraulic conductivity for
this unsaturated transmission zone was sufficient to transmit vertical leakage from the
perched system to the water table mound. Where wells are screened through a perched
layer or where water leaks through casing joints at the perched layer, cascading water
occurs. This is a common observation in arid alluvial basins and is evidence of saturated
lateral flow in the vadose zone.
Unsaturated lateral flow is caused by perched water that develops at the interface
between two layers of contrasting hydraulic conductivities or by anisotropy of the
unsaturated hydraulic conductivity. Contrary to the case of saturated lateral flow in
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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/m1/33/: accessed May 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.