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Response to Public Comment
Listed below are summaries of comments by John Schinkle, P.E., followed by a response from USGS. The summary of each comment is linked to the original document submitted by John Schinkle.
Comments from John Schinkle, P.E.
Complete letter from John Schinkle, P.E. (pdf)
Uncertainties are acknowledged in report but may be much greater than perceived. The modeling involves assumptions such as the saturated hydraulic conductivity of bedrock, and computations rely on measurement limited to a 1-year period taken at a limited number of sites. Uncertainties may be huge and can only be improved with a quantum increase in data and analyses.
RESPONSE
The authors recognize the need for more data and analyses to improve estimates particularly because “recharge to a basin occurs through a number of processes, including mountain-block, diffuse, mountain-front, and stream-channel recharge.” However, only empirical transfer methods (such as the Maxey-Eakin method) and watershed modeling (such as BCM) provide spatially distributed estimates. An advantage of the BCM is the incorporation of geologic and soil information to allow spatial differences in the potential for recharge within the same elevation zones unlike the Maxey-Eakin method. The BCM results are nonunique particularly with respect to assignment of hydraulic properties and the percentage of runoff that becomes recharge to the ground-water system. The percentage of runoff that becomes recharge was set at 15 percent even though it has been shown to range from 10 (Death Valley) to 90 (Humboldt) percent. Using a single, low-end value is a conservative approach because data is not available throughout the study area to adjust for each HA. The analyses are not based on 1-year of data but uses long-term (112-year) precipitation data. Text has been added to the summary report to describe the uncertainty and limitations for each of the water-budget components.
A paramount consideration in this study should be the potential consequences of continuous withdrawal of substantial volumes of groundwater during periodic draught cycles, sometimes having a 20-year duration or longer. Consequences would be most severe during these recurring draught events, and this should be paramount in any decision involving withdrawl of large fractions of ground-water in perpetuity.
RESPONSE
The authors recognize the variance of rainfall. The influence of this variability is somewhat lessened by presenting recharge and discharge estimates that reflect long-term average hydrologic conditions. While long-term average values do not accurately reflect any single place or single moment in time, presenting average values is considered an appropriate approach by smoothing out seasonal and year-to-year fluctuations and providing a conservative estimate. Providing an evaluation of the potential consequences of proposed ground-water withdrawals was not an objective of the BARCAS study. The objective of the study was to develop a conceptual understanding of the aquifer systems but not to numerically simulate current or anticipated future stresses on the ground-water system. Numerical models are best suited to depict an integrated three-dimensional calibrated flow field and subsequent hypothesis testing to evaluate the system response to many types of stresses such as drought, and locations and volumes of ground water pumped from the system.
The latter characterization is probably more typical.
RESPONSE
Figure 16 has been modified. Figure 16 is a conceptualization of a multi-scale aquifer system and does not represent all of the possible flow paths.
JS4. An acronym list would be helpful.
RESPONSE
An acronym list was added to the summary report.
JS5. Format of the final Glossary item should be edited.
RESPONSE
The glossary has been revised.
