Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates Page: 10 of 45
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the U.S. Geological Survey (USGS) that indicated a lack of compliance with Yucca
Mountain Project Quality Assurance (QA) requirements. The emails pertained to the
infiltration model developed by the USGS and the development of the site-specific net
infiltration rate estimates for present and future climates. In response, OCRWM
conducted (1) a technical evaluation that determined the USGS net infiltration rate
estimates are corroborated by independent studies of infiltration and recharge in semi-
arid environments and (2) a programmatic evaluation that examined the QA issues but
was unable to resolve whether conditions adverse to quality had occurred. A report of
these evaluations, issued in February 2006, noted that while the science seemed sound,
QA concerns remained. To allay these concerns, OCRWM directed Sandia National
Laboratories (SNL), the Lead Laboratory for Repository Systems, to create a new
infiltration model and associated climate maps while ensuring traceability, transparency,
and full compliance with OCRWM QA requirements.
The new infiltration model has been termed MASSIF (Mass Accounting System for Soil
Infiltration and Flow). MASSIF, along with its results, has been documented in Revision
01 of the model report, which is the subject of this review. MASSIF is a collection of
Mathcad "routines" that was designed to provide an estimate of the net infiltration of
water into the fractured rock that underlies the soil at Yucca Mountain. The core of
MASSIF is a daily water balance for each 30 m x 30 m cell in the modeling domain, the
dimensions corresponding to the high spatial resolution of Landsat images.
The daily water balance for the soil in each cell is:
Roff Pain + Ron + SM AO ET NI
Roff = surface runoff
Pram = precipitation as rain
Ron = surface run-on
SM = snowmelt
AO = change in water storage in the soil
ET = actual evapotranspiration
NI = net infiltration
The water balance is written as a calculation for the runoff from the cell, in which all the
quantities to the right of the equals sign are computed from submodels (pp. 6-23 - 6-24).
Such a determination of net infiltration in arid and semiarid regions is inherently difficult
for two primary reasons. First, the infiltration depends on a water balance in which actual
evapotranspiration is almost equal to precipitation. The net infiltration is thus mainly
determined by subtracting two relatively large numbers (precipitation and
evapotranspiration) to find a small number (infiltration). So even when precipitation and
evapotranspiration can be determined with small uncertainty bounds, large uncertainty
remains for the infiltration (Gee and Hillel, 1988). The second reason is the large spatial
and temporal variability of water fluxes in arid environments (Scanlon et al., 1997). The
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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/m1/10/: accessed May 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.