Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates Page: 41 of 45
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4. Limited Validation Examples
Sec. 7.1.2, "Evapotranspiration and Storage," describes the validation of MASSIF against
two lysimeter facilities: Nevada Test Site and Reynolds Creek Experimental Watershed.
This validation shows that MASSIF can predict trends in evapotranspiration and soil
moisture storage reasonably well, if soil hydraulic properties are known. However, this
does not necessarily mean that MASSIF will predict infiltration correctly since small
errors in evapotranspiration, soil moisture storage, or precipitation will result in large
errors in infiltration rates.
The lysimeter validations did not use measured soil textural data. Instead, specific water
retention data measured on cores were used to generate Ksat and field capacity
parameters. The soil depth, which should have been modeled as very deep, was not, due
to the fact that the bottom boundary condition of a lysimeter is very different from a
natural soil profile. For water to seep out of a lysimeter, the bottom soil water pressure
has to become slightly positive, while in a natural soil profile, deep percolation will occur
much sooner at negative soil water pressures.
No testing of the Hanford-derived pedotransfer function was conducted. Three
parameters were calculated by using an optimization algorithm to get a good fit to
observed storage (diffuse evaporation, canopy fraction, and growth curves). This was not
a blind test, but a fitting of the predictions to observations. Optimal testing would have
used blind soils. The overall Reynolds Creek evaluation also used some optimization to
Section 7.1.3, "Run-on/Runoff," describes the validation of model results by comparison
to observed runoff events at Yucca Mountain in the 1990s as well as measured infiltration
beneath wash environments. Six stream-flow gauge sites were used, along with data from
an unsaturated borehole (UZ#4). MASSIF simulations were conducted on those
watersheds where runoff data were collected, and the predicted runoff was compared
with that observed. Using nominal values of soil hydraulic conductivity (i.e., those
chosen from the distributions used for the final net infiltration estimates), MASSIF did
not appear to predict the runoff (for example, see Figure 7.1.3-2 from Wren Wash).
Additional simulations were conducted in which the nominal value of the soil hydraulic
conductivity was reduced (to produce more overland flow). Reducing the nominal value
of soil hydraulic conductivity by a factor of -50% produced runoff that qualitatively
matched that observed. Further study was made to compare to infiltration observed at the
mouth of Lower Pagany Wash from UZ#4. To match the infiltration, the soil saturated
conductivity needed to be increased by a factor of -10 over the nominal value used in
MASSIF. The report notes that this value is still below what was measured at the site by
another order of magnitude.
Based upon these results, additional MASSIF simulations were conducted over the entire
Yucca Mountain area using lessons learned from the validation cases. It was found that
significant differences in the spatial distribution of modern recharge would occur by
changing only the soil hydraulic conductivity of soils #3 and #4. Under this condition,
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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/m1/41/: accessed May 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.