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Response to Public Comment
Listed below are summaries of comments by Jerald Anderson, followed by a response from USGS. The summary of each comment is linked to the original document submitted by Jerald Anderson.
Comments from Jerald Anderson
Complete letter from Jerald Anderson (pdf)
This statement should be referenced to source and analysis, and the mentioned flow paths should be noted with their estimated velocities as part of the characterization. This information is as significant as the quantities, and deserves a similar graphic presentation as Plate 3.
RESPONSE
The text has been revised. A synopsis of the geochemical modeling conducted by the Desert Research Institute (source of analysis) for the BARCAS study can be found in the summary report (SIR 2007-5261). The text states that “ground-water flow paths were evaluated using the geochemical model NETPATH (analysis method)”; modeling results are summarized in table 8 and presented graphically on figure 45. The flow paths modeled are referenced by initial/final path locations and flow path number in table 7. Specific and detailed information regarding the chemical sampling, NETPATH methodology, and model results are presented in DRI publication number 41230.
Some acreage previously in production was not irrigated during the years preceding 2005 (2000-2004) because of drought conditions.
RESPONSE
After further review, the authors see no inconsistency among the irrigated cropland acreage reported in the draft summary report (OFR 2007-1156) and supporting publications (DS 273 and SIR 2007-5087). Irrigated acreage based on the water use and acreage worksheets given in appendix A are, in fact, correct. The noted differences are valid, not errors. The acreage reported in the water use worksheet is defined as the acreage of irrigated cropland for 2005 regardless of whether these lands are located inside or outside of ground-water discharge areas. For example, 9,200 acres are reported for Snake Valley and 4,360 acres for Lake Valley. The acreage reported in acreage worksheet is defined as the composite acreage within ground-water discharge areas for the “recently irrigated cropland-historically mixed phreatophyte” ET unit during the year 2000 and/or 2002, and/or 2005. For example, 9,932 acres are reported for Snake Valley and 0 acres for Lake Valley indicating that in Snake Valley most irrigated acreage is within the ground-water discharge area, and for Lake Valley all irrigated acreage falls outside the ground-water discharge area. Since 9,932 acres were delineated in Snake Valley for the “recently irrigated cropland-historically mixed phreatophyte” ET unit, and only 9,200 acres were identified as actively irrigated cropland in 2005, then there are some fields within the ground-water discharge area that were not actively irrigated in 2005. A table and text was added to SIR 2007-5087, and column headings revised in appendix A to help clarify this point.
If correct, what data source can be referenced in the report to verify the data on the chart?
RESPONSE
Only 4 bar graphs illustrating the ET-unit acreage (fig. 25) and mean annual ET and ground-water discharge (fig. 31) for the sub-basins are shown for Snake Valley because subbasin 5 (southernmost subbasin) has neither ET or ground-water discharge and therefore can’t contribute to the “height” of the bar graph. A value of zero for subbasin five is listed in Appendix A, (ET and discharge worksheets) and shown on Plate 4.
If the methods are not in alignment, then the methodology must be defended relative to previous work.
RESPONSE
The text has been revised to clarify the statement. Higher recharge from the BCM, when compared to Maxey-Eakin and other methods reliant on the Maxey-Eakin approach, results in relatively higher potential for recharge in carbonate terrains that dominate the BARCAS study area than other areas in the state of Nevada.
JA6. 7th-8th line s.b. “….resulting in deposition…”
RESPONSE
Text was revised. “A Late Devonian to Early Mississippian compressive event, known as the Antler orogeny, interrupted carbonate sedimentation and resulted in the deposition of a thick sequence of siliciclastic rocks (Poole and Sandberg, 1977).”
An explanation of the use of this section and relevance to the flow characteristic conclusions should be included.
RESPONSE
The comment is correct—irrigation was not included in ground-water discharge calculations. The objective of the BARCAS study was to provide water budgets that reflect pre-development (natural) conditions. Therefore, recently irrigated cropland was replaced with a mixed phreatophytic ET unit that was assigned an ET rate which equaled the area-weighted average ET rate for all other phreatophyte units delineated in the study area. However, water-use estimates are considered important to the overall analysis to provide some understanding of the current amount of water being used and, by doing so, allows the authors to present the near zero sum. The summary report states “the remaining 80,000 acre-ft (of ground-water use) nearly equals the estimated quantity of ground-water outflow from the study area (about 90,000 acre-ft/yr)”.
This should be depicted in Figure 35.
RESPONSE
The uncertainty in water-use estimates does not effect ground-water flow directions for steady-state pre-development conditions conceptualized in the summary report. If a calibrated transient ground-water flow model is developed, then a minimum and maximum water-use estimate should be developed and incorporated into model simulations.
Uncertainty is a common thread throughout this report and should not be relegated to minor status.
RESPONSE
An uncertainty discussion for recharge and interbasin flow has been added or expanded. The uncertainty for the ground-water discharge estimates in the water budget is presented in figures and discussed in the “Limitations and Considerations of Methodology”.
JA10. Page 49: BCM discussion should include informative relative to calibration and sensitivity to assumed conditions.
RESPONSE
Additional text has been added to the BCM discussion in the summary report including a more detailed description of the uncertainty. The following is a portion of the text added to the report. “Data are limited for some input parameters used in the BCM. As a result, the uncertainties associated with these parameters may be a significant source of potential error for estimates of ground-water recharge, particularly for two parameters—the saturated hydraulic conductivity of bedrock and the associated volume of runoff that becomes recharge. The hydraulic conductivity estimates for bedrock are uncertain because of limited data on hydraulic properties in recharge areas, particularly on the properties and spatial distributions of fractures, faults, and fault gouge. Hydraulic conductivities used in the BCM span up to five orders of magnitude for the least permeable units (table 2 in Flint and Flint, 2007). BCM-estimated recharge is relatively sensitive to changes in saturated hydraulic conductivity of bedrock because this parameter determines the partitioning of water between in-place recharge and runoff (Flint and Flint, 2007). Although the portion of runoff that becomes recharge varies significantly across Nevada (Flint and others, 2004), an assumed value of 15 percent is considered reasonable for areas of central Nevada dominated by in-place recharge; however, the uncertainty likely is greater in runoff-dominated areas. Previous investigations used percentages of recharge from runoff ranging from a low value of 10 percent in the Death Valley regional flow system in southern Nevada to a high value of 90 percent for the Humboldt regional flow system in northern Nevada (Flint and Flint, 2007). These percentages were therefore chosen as the endpoints representing the range of uncertainty for the recharge estimates shown in figure 22 (confidence limit)—10 percent for the low end and 90 percent for the high end. As a result, the uncertainty in recharge estimates increases for basins such as Lake, Snake, and Spring Valleys where the potential runoff recharge exceeds the potential in-place recharge (fig. 21 and 22).“
JA11. Page 86: Some indication of the time-phasing of recharge should be included.
Local experience indicates that time lags between significant snowfall and groundwater level increases range from 1 to 3 years, depending on location. The impact of 2005 as a record-precipitation year should be noted, since it was the primary time period for the BARCASS data collection.
RESPONSE
The estimated annual recharge is for the period from 1895-2006 and does not reflect the hydrologic condition during any single year such as higher than average precipitation in 2005. The authors recognize that there is time lag between snowfall and the occurrence of peak annual ground-water levels and the absolute magnitude of the peak from year-to-year also is influenced by antecedent precipitation volumes. The purpose of the ground-water pumping discussion on page 86 was “to evaluate the significance of water withdrawals to ground-water discharge under pre-development conditions”.
JA12. Page 2: Assumption that equal amounts are pumped from carbonate and basin-fill must be justified.
South Snake Valley experience would not support this assumption, as production depth and yield makes basin-fill locations preferable.
RESPONSE
Text has been revised to clarify the intent of the analysis of storage capacity in the basin fill and carbonate aquifers. For equivalent volumes of aquifer material, the capacity of the basin-fill aquifer to store water is significantly greater than that of the carbonate-rock aquifer. The evaluation of aquifer storage assumes ground-water is pumped from equivalent volumes of basin fill or carbonate rock. For example, about 36 million acre-feet of water would be released from storage from a 100-foot drop of the water table in the basin-fill aquifer; whereas, less than a million acre-feet of water would be released from storage from a 100-foot drop of the potentiometric surface in the carbonate aquifer. The analysis of aquifer storage does not consider the potential impacts to changes in storage resulting from ground-water pumping, such as declining water levels in wells, decreasing spring discharge, diminished water quality, or loss of native vegetation.
JA13. Plate 1: No geologic sections are available through the potential interbasin flow zone areas, such as between Spring and Snake Valleys.
They would be helpful in understanding the characteristics which might support or impede flow between basins.
RESPONSE
Correct—there are no cross-sections on Plate 1 that show the interbasin flow segments. However, several of the “important” segments are shown in figure 42 including between Spring and Snake Valleys. The general distribution of hydrogeologic units is shown rather than the geologic units. The distribution (cross-sectional area), gradients (from the potentiometric-surface maps), and interbasin flow estimates were used to evaluate the reasonableness of hydraulic properties estimated.
JA14. Plate 3: There is a flow arrow pointing eastward from Snake Valley toward the Confusion Range……
RESPONSE
The flow across the Confusion Range is not supported by the DSC results. Uncertainty of the magnitude of interbasin flow has been added to the summary report and flow arrows have been removed from plate 3.
JA15. Plate 4: The totals for the categories in Steptoe Valley do not equal the sum of the sub-basin.
The entire plate should be checked for accuracy.
RESPONSE
Plate 4 values for Steptoe Valley and subbasins have been corrected. All of the other values are correct.
JA16 through 19. Pages 4-8: A table of values associated with the graphical representation of the findings should be included which displays the values and the range of uncertainty associated with the selected values.
RESPONSE
The uncertainties in the water budget estimates are presented in the various sections of the report. The color-coded hydrographic areas shown in figures in the summary of major findings have large ranges as presented in the corresponding explanations. The large ranges should not give the impression of accuracy. See response to your comment 10 above concerning water budget estimates for specific years.
