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a Jackson School of Geosciences, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX 78758
b USDOE, Sandia Natl. Lab., Albuquerque, NM 87185
bridget.scanlon{at}beg.utexas.edu
The review comment by Gee et al. (2006) presents examples of failures in ET covers at different semiarid sites having design attributes similar to those described in our paper; however, it fails to point out the differences in seasonal precipitation patterns. Precipitation in the examples given throughout their comment (Ogden, UT; Altamont and Marina, CA) occurs primarily in the winter, when vegetation is dormant, whereas precipitation at the New Mexico and Texas sites evaluated in our study occurs primarily in the summer, when vegetation is active and ET is high. For example, 90% of precipitation occurs in winter (November–April) at the California sites, whereas 15 to 23% of precipitation occurs in the winter at the Texas and New Mexico sites, respectively. Seasonal distribution of precipitation is very important for performance of ET covers.
In their comment, Gee et al. (2006) suggested that our conclusion that "a 1-m-thick ET cover underlain by a capillary barrier should be adequate to minimize drainage to 
1 mm yr–1 in these arid and semiarid settings" appears to be unqualified. This is a data-specific conclusion, and "these arid and semiarid settings" refer to the New Mexico and Texas sites described in our paper. The "Southwestern USA" was included in the title to show the general location of the sites, but there are no statements in the paper that the results of our study should apply throughout this entire region. The conclusion related to appropriate cover thickness for these sites is one of several conclusions in the study, and each conclusion should not be considered in isolation but as part of the entire study. Another conclusion specifically refers to the suitability for ET covers of the climate at these sites with predominantly summer monsoonal precipitation that is readily removed by ET. Gee et al. (2006) referred to failure of ET covers at sites in Altamont and Marina, CA, but the Mediterranean climate at these sites is characterized by predominantly winter precipitation, when vegetation is dormant.
It is not clear whether the comment related to the importance of soil type and related soil water storage refers to our study. Available water storage was analyzed in detail for the different soils in the covers in our study. Calculated available water storage for the covers (Table 5 in Scanlon et al., 2005), based on monolithic soil properties and thickness, exceeded winter precipitation (November–April; Sierra Blanca 59 mm; Albuquerque 86 mm) by factors of 1.4 to 2.7.
We agree with Gee et al. (2006) that there is no generic design standard for ET covers. The examples from our study show the detailed characterization, field testing, and modeling analyses required to evaluate a proposed design. We agree that a large factor of safety is required because of uncertainties in cover-system performance based on monitoring and modeling analyses. The cover designs in our study provide a large factor of safety, based on available water storage relative to winter precipitation, as noted previously. The addition of a capillary barrier provides an additional factor of safety because calculated available water storage for an ET cover underlain by capillary barrier systems at both sites is approximately 2.5 greater than that of an ET cover alone. Gee et al. (2006) did not address the fact that available water storage estimates for ET covers monitored using a drainage lysimeter include capillary barriers because drainage lysimeters behave like capillary barriers. Monitoring results for the New Mexico ET cover and ET covers in the Alternative Cover Assessment Program (ACAP) (Albright et al., 2004) basically represent ET covers underlain by capillary barrier systems. Therefore, there have been no true performance tests of monolithic ET covers, which is what most understand to be the definition of an ET cover. The effect of the lysimeter (i.e., the seepage face lower boundary condition) on estimated water storage of an ET cover should be evaluated using modeling or other approaches.
We did not test cover performance at our sites under conditions that would emulate climate change. The covers at the New Mexico and Texas sites were irrigated to assess performance under stressed conditions. Enhanced precipitation testing using irrigation was also used at the Hanford site (Ward and Gee, 1997).
In their comment, Gee et al. (2006) did not seem to support the use of capillary barriers because of their unpredictable performance and cost of installation. Unpredictable performance is related to unstable flow; however, our findings indicate that the capillary barrier system at the Texas site did not show any drainage even after excessive irrigation applications (1881 mm in 3 d). Cover slopes at the New Mexico and Texas sites were negligible; therefore, comments by Gee et al. (2006) related to sloping capillary barriers should not apply to these sites. Although the economics of capillary barriers are not addressed in our study, the conclusion related to the use of capillary barriers in our paper is qualified by practical considerations including economics. Cost analyses were conducted for different cover designs at the New Mexico site (Dwyer, 1998). The costs per square meter were $93 for the capillary barrier and $74 for the ET cover. Differences in construction costs for these two designs should consider the costs for the increased thickness of monolithic ET covers to achieve the increased storage provided by a capillary barrier. There is also a limit to how thick a monolithic ET cover can be that is related to how deep the vegetation roots can extend. Khire et al. (2000) indicated that the underlying coarse layer in a capillary barrier can be thin, which should reduce costs. Economic issues should be evaluated at a site-specific level relative to availability of different materials and such evaluations should examine cost differences between thicker, monolithic covers without a capillary barrier versus thinner monolithic covers underlain by a capillary barrier.
Gee et al. (2006) suggested that vegetation dynamics should be known a priori to reduce uncertainty in model simulations of drainage. Ideally, vegetation parameters should not be input into the model but should be simulated by the model in response to climate and soil water storage changes. Dynamic vegetation is being incorporated into land–atmosphere models to better predict the water cycle response to global climate change (Foley et al., 2000; Bonan et al., 2003). Similar approaches should be developed for ET covers because vegetation is an essential component of cover-system performance and prescribing it in the input parameters is too restrictive and cannot be done in simulations of future conditions. The importance of hysteresis may be problem specific; however, there is little information in the literature on comparisons of simulation results with and without hysteresis, and few studies have measured hysteresis in soils.
In the concluding remarks, Gee et al. (2006) specified a number of factors that should be considered in designing and testing ET covers. These factors were generally considered in our evaluation of the New Mexico and Texas covers, including evaluation of site-specific conditions, detailed calculations of water storage capacity, and large safety factors provided by capillary barriers. Detailed field measurements of water storage were also described. Although Gee et al. (2006) suggested that more effort should be expended in model calibration, previous studies by Fayer et al. (1992) indicate that calibrated models do not necessarily provide improved predictions of cover-system performance outside of the calibration period. Because model predictions will always have uncertainty, regardless of how much calibration is done, models should be used with caution. Modeling analyses may be more useful in assessing the relative importance of different design attributes, vegetation dynamics, and long-term climate forcing.
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