Nevada Water Science Center


Aquifer Tests

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Email:pgardner@usgs.gov

 

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Penoyer Valley

Primary Investigators: Weiquan Dong, Southern Nevada Water Authority, and Ramon Naranjo and Keith Halford

Well Data

USGS Site ID
Local Name Altitude Uppermost
Opening
Lowermost
Opening
Primary Aquifer Transmissivity
(ft2/d)
373955115490201 O-02 4850 238 ALLUVIAL FILL 10000-17000

 

Aquifer Test

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Penoyer Valley

Aquifer Test (pdf) || Groundwater Levels (NWISweb)

 

Ranges of transmissivity and specific yield of the basin fill were estimated by analyzing water-level changes in multiple observation wells that were caused by development of irrigated fields in Penoyer Valley, HA170, near Rachel, NV (Figure 1). Irrigated acreage totaled 900 acres during 1978 and increased to 2,300 acres during 1986-1989. Annual effective ground-water withdrawals were estimated between 2,600 and 11,800 acre-feet (ac-ft) during the 1978-1989 irrigation seasons. Hydraulic property estimates from the Penoyer aquifer test will constrain calibration of regional ground-water flow models.

Site and Geology

The aquifer test occurred near Rachel, NV in Penoyer Valley where an area of more than 100 mi2 was affected by pumpage for irrigation (Figure 1). The basin fill was pumped largely between 100 and 300 ft below land surface where undifferentiated intervals of sand and gravel occurred. Insufficient information exists to differentiate the hydraulic properties of basin-fill which will be cited as homogeneous, coarse-grained fill (Appendix A).

The groundwater flow system was interpreted with a thickness of 1,000 ft even though the thickness of basin fill exceeds 10,000 ft (Belcher, 2004). The thickness of more permeable sediments is unknown, but a thickness must be assigned in the numerical flow model used to analyze the aquifer test. Hydraulic conductivity of the basin fill in Penoyer Valley was not estimated or reported because of the uncertainty in the thickness of the aquifer.

figure 1

Figure 1. Location of pumping and observation wells in Penoyer Valley near Rachel, Nevada (Universal Transverse Mercator projection, Zone 11).

Observation wells generally surrounded the pumping wells and irrigated areas (Table 1, Figure 1) and were completed between 30 and 540 ft below land surface. Observation well completions generally were shallower than the pumping well completions.

Period and Area of Analysis

Water level declines during the 1978-1989 irrigation seasons were analyzed because intensive irrigation began during 1978, crops had been inventoried, and sufficient water-levels were measured during this period. Irrigated acreage and groundwater withdrawals by well were estimated in Penoyer Valley during 1960-1998 (Moreo and others, 2003). Irrigated crops covered between 900 and 2,300 acres during the period of analysis. The irrigation water was entirely ground-water except for local precipitation because surface-water supplies are absent. Annual ground-water pumping for irrigation ranged between 2,600 and 11,800 ac-ft assuming application rates of 3 and 5 ft/yr (Figure 2). These application rates range from the minimum to most likely (Moreo and others, 2003).

figure 2

Figure 2. Annual pumpage from Penoyer Valley for application rates of 3 and 5 ft/yr.

 

Water Levels and Drawdowns

Water-level changes were observed in twelve wells that were affected by ground- water pumping for irrigation (Table 1, Figure 1). Water levels have been measured as frequently as weekly and as infrequently as annually. Annual and quarterly measurements occurred during the 1978-1989 period of analysis Figure 3). Water levels in the observation wells ranged between 20 and 240 ft below land surface.

Drawdowns were estimated by subtracting water levels measured during the test period from water levels that were measured during October 1978 prior to most pumping (Moreo and others, 2003). Drawdown rates ranged between 0.5 and 1 ft/yr and these extremes were observed in wells O-12 and O-02, respectively (Figure 3). Well O-12 was more than 2 mi from the nearest pumping well and well O-02 was within 2,000 ft of the nearest pumping well (Figure 1).

figure3

Figure 3. Water level changes in selected observation wells and period of analysis.

Numerical Analysis

Hydraulic properties of the basin fill were estimated by minimizing differences between simulated and measured drawdowns. Drawdowns were simulated with a three-dimensional, MODFLOW model that simulated seasonal, spatially distributed pumping (Harbaugh and McDonald, 1996). Parameter estimation was performed by minimizing a weighted sum-of-squares objective function with MODOPTIM (Halford, 2006).

The model domain was discretized into 233 rows and 169 columns (Figure 4). Rows and columns were assigned widths of 1,000 ft where observation wells and the majority of pumping wells existed. Row and column widths were multiplied by 1.1 from the area of 1000-ft on a side cells to the edges of the model that extended laterally 330,000 ft away. All lateral model boundaries were specified as no-flow boundaries. The model grid was oriented north-south in UTM, zone 11 for convenience where the lower, left model corner was easting 590,567 m and northing 4,148,254 m. The basin fill was simulated with a single 1000-ft thick layer. The base of the aquifer system was specified as a no-flow boundary. Changes in the saturated thickness of the aquifer were not simulated because the maximum drawdown near the water table was small relative to the total thickness.

figure 4

 

Figure 4. Numerical model grid, pumping wells, and observation wells in Penoyer Valley.

 

The 1978-1989 irrigation seasons were simulated with twenty-four stress periods and the first period started April 1, 1978. Each stress period was divided into 25 time steps, the initial time step was about 0.0001 d, and successive time steps were 1.3 times greater than the previous time step. Recovery from each irrigation season occurred during the even numbered stress periods when no pumpage was simulated. Initial conditions of zero drawdown were assigned throughout the model domain.

Hydraulic Property Estimates

Transmissivity and specific yield of the basin fill were estimated as homogeneous, isotropic properties throughout Penoyer Valley to match measured drawdowns. Simulated drawdowns matched measured drawdowns with a weighted RMS error of 0.4 ft (Figure 5). This is a small error relative to drawdowns of 20 ft after 12 years of irrigation.

Simulated and measured rates of annual decline agreed well in representative wells O-2 and O-12 (Figure 5) and all other wells (Appendix B). This suggests that the hydraulic properties of the basin fill can be interpreted reasonably with homogeneous units where the hydraulic diffusivity, transmissivity divided by storage coefficient, is 100,000 ft2/d. The hydraulic diffusivity estimate is relatively constant for annual application rates between 3 and 5 ft.

Transmissivity of the basin fill ranged between 10,000 and 17,000 ft2/d for annual application rates of 3 and 5 ft, respectively (Table 2). Specific yield ranged between 0.10 and 0.17 for annual application rates of 3 and 5 ft, respectively. Transmissivity andspecific yield estimates increased proportionally with increased application rates.

figure 5

Figure 5. Simulated and measured drawdowns in representative observation wells O-02 and O-12 during 1978-1989 irrigation seasons with an annual application rate of 5 ft.

table 2


Simulated drawdowns between 0.2 and 1 ft affected between 210,000 and110,000 acres, respectively, after the 1989 irrigation season (Figure 6). Hydraulic property estimates are integrated values for an area of about 110,000 acres if the detection threshold for drawdown is assumed to be 1 ft. Between 68 and 90 percent of the pumped volume was released from storage between simulated drawdowns of 4 and 1 ft, respectively. The affected areas and percentages of pumped volumes were not affected by differences in annual application rates of 3 or 5 ft.

The contributing area for the Penoyer aquifer test was much greater than the contributing area of conventional aquifer tests because the pumped volume ranged between 64,000 and 106,000 ac-ft. This is about 2,000 times greater than the volume pumped during a conventional aquifer (pumping) test. For example, continuously pumping 3,000 gpm during a 3-d period removes less than 40 ac-ft from an aquifer.

figure 6

Figure 6. Simulated drawdowns on March 31, 1990 at the end of stress period 24 after recovering from the 1989 irrigation season.

References

Belcher, W.R., ed., 2004, Death Valley regional ground-water flow system, Nevada and California--Hydrogeologic framework and transient ground-water flow model: U.S. Geological Survey Scientific Investigations Report 2004-5205, 408 p.

Halford, K.J., 2006, MODOPTIM: a general optimization program for ground-water flow model calibration and groundwater management with MODFLOW: U.S. Geological Survey Scientific Investigations Report 2006-5009, 62 p.

Harbaugh, A.W., and McDonald, M.G., 1996, Programmer's documentation for MODFLOW-96, an update to the U.S. Geological Survey modular finite difference ground-water flow model: U.S. Geological Survey Open-File Report 96-486, 220 p.

Moreo, M.T., Halford, K.J., La Camera, R.J., and Laczniak, R.J., 2003, Estimated ground-water withdrawals from the Death Valley regional flow system, Nevada and California, 1913-98: U.S. Geological Survey Water-Resources Investigations Report 03-4245, 28 p., on-line at http://pubs.usgs.gov/wri/wrir034245/

 

 

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