Nevada Water Science Center

Aquifer Tests

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Needle Point

Primary Investigators: Weiquan Dong, Southern Nevada Water Authority, and Ramon C Naranjo and Keith J. Halford , USGS

Well Data

Local Name Altitude Uppermost
Primary Aquifer Transmissivity

Aquifer Test

All Aquifer Test Files (zip)

Needle Point

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


Ranges of transmissivity and specific yield of the basin fill and carbonate rock were estimated by analyzing water-level changes in multiple observation wells that were caused by irrigation of 1,800 acres in Snake Valley, HA195, south of Garrison, Utah (Figure 1). Annual effective groundwater withdrawals were estimated between 3,500 and 5,300 acre-feet (ac-ft) during the years 2001–2003. The analysis is referred to as the Needle Point aquifer test because most of the pumpage occurs just west of the carbonate outcrop Needle Point and the observation well that is completed in carbonate rocks is on the northern tip of Needle Point (Figure 1). Hydraulic property estimates from the Needle Point aquifer test will constrain calibration of regional groundwater flow models.

Site and Geology

The aquifer test occurred near Garrison, UT in Snake Valley where water levels in more than 100 mi² of basin fill and carbonate rock were affected by pumpage for irrigation (Figure 1). The basin fill was pumped largely between 100 and 800 ft below land surface where undifferentiated intervals of silt, sand, and gravel occurred. Thicker sequences of clay were reported between 100 and 300 ft below the water table. Specific capacities less than or equal to 10 gpm/ft and limited water level data collected within the basin fill preclude differentiating the hydraulic properties of fine-grained and coarse-grained deposits.

Figure 1

Figure 1.—Location of irrigation and observation wells in Snake Valley south of Garrison, Utah (Universal Transverse Mercator projection, Zone 11, base modified from Welch and others, 2007).

The groundwater flow system was interpreted with a maximum thickness of 2,000 ft in the basin fill even though the thicknesses exceed 5,000 ft between Big Springs and Needle Point (Welch and others, 2007). Unconsolidated coarse-grained younger sedimentary rocks occur in the upper 2,000 ft and become indurated with depth. Basin fill in Snake Valley generally is impermeable deeper than 2,000 ft because Miocene sediments are predominantly clay and evaporite deposits as encountered in oil-well logs (Halford and Plume, 2011).

Transmissivity of the carbonate rocks was estimated because hydraulic conductivity is highly variable and thickness is correlated poorly with transmissivity. This finding resulted from extensive testing of the lower carbonate aquifer around the Nevada Test Site (Winograd and Thordarson, 1975, p. C20). The observations of Winograd and Thordarson (1975) were, “None of the eight holes drill-stem tested showed a uniform pattern of increase or decrease in fracture transmissibility, and open fractures were present as much as 1,500 feet beneath the top of the aquifer and 4,200 feet below land surface. In some holes the transmissibility increased markedly with depth; in others the most permeable zones were near the top of the zone of saturation.� Therefore, a nominal thickness of 1,000 ft was assigned throughout the carbonate-rock aquifer.

Transmissivity of basement rocks other than carbonate rocks were assigned a uniform value of 0.05 ft²/d because these were relatively impermeable and act primarily as barriers to flow. Intrusive granitic and siliciclastic rocks occur in the Snake Range north of Garrison and the ranges bounding Hamlin valley primarily are composed of ash-flow tuffs (Welch and others, 2007). Hydraulic conductivities of granitic and siliciclastic rocks range between 1 x 10-7 and 1 x 10-3 ft/d (Halford and Plume, 2011). An average hydraulic conductivity of 1 x 10-6 ft/d was estimated for non-welded tuff that covered more than a square mile (Halford and others, 2005). The assigned transmissivity is the product of an average hydraulic conductivity of 5 x 10-5 ft/d times the nominal thickness of 1,000 ft for non-carbonate basement rocks.


figure 2

Figure 2.—Perspective from south to north of pumping and observation wells south of Garrison, UT of seven-layer model for simulating drawdown during 2001-2003 irrigation seasons.

Observation wells were located northeast and northwest of the primary irrigated area (Table 1, Figure 1) and were completed within 150 ft of land surface. Well O1 was drilled to 65 ft below land surface which is inferred to be the contact between basin fill and carbonate rock. All four wells were completed in basin fill. Observation well completions were shallower than the pumping well completions.


Table 1.—Well location and construction data for observation wells that were used in the Needle Point irrigation analysis aquifer test (See figure 1 for well locations).

[Latitude and longitude are in degrees, minutes, and seconds and referenced to North American Datum of 1983; ft amsl, feet above North American Vertical Datum of 1929 (NAVD 29) except as noted; ft bgs, feet below ground surface.]









Well name








surface elevation, ft amsl


Hole depth, ft bgs





(C-24-20) 1 Needle P Spr Box







5,450 ª








(C-23-19)20bcd- 1







5,416 ª







195 N11 E70 35BA USGS-MX (Hamlin












Valley N.)










195 N11 E70 35AD USGS-MX (Snake V.


















ª Elevation is in feet above North American Vertical Datum of 1988 (NAVD 88)

Period and Area of Analysis

Water level declines during the 2001–2003 irrigation seasons were analyzed because crops had been inventoried and drought conditions existed during this period. Irrigated acreage in Snake Valley was inventoried during 2000, 2002, and 2005 (Welborn and Moreo, 2007) so irrigated areas were known or could be interpolated within a year. Irrigated crops covered 1,200, 1,800 and 1,800 acres during 2001, 2002, and 2003, respectively. Irrigated crops covered less than 700 acres during 2000. Most of the irrigation water was assumed to be groundwater because of the drought conditions during the period of analysis.

Annual groundwater pumping ranged between 2,400 and 5,300 ac-ft assuming application rates of 2 or 3 ft/acre during a 200-d irrigation season (Figure 3). Pumpage varied annually during 2001, 2002, and 2003 because irrigation periods were 214, 215, and 177 d, respectively. Annual application rates of 3.5 ft/acre were measured in Smith Valley where the altitude and latitude are similar to Snake Valley (Rush, 1976). Groundwater pumping rates were assumed to be less than the application rates because of annual precipitation of less than 0.5 ft, return flow, and limited surface-water diversions. Annual application rates for irrigation were estimated to the nearest foot given the uncertain effects of return flow on water-levels (Figure 3).


figure 3

Figure 3.—Annual simulated pumpage from the Needle Point aquifer test model assuming average application rates of 2 and 3 ft during 2001, 2002, and 2003.

Water Levels and Drawdowns

The effects of pumping prior to 2000 were assumed to be compensated by increased recharge during the wet years of 1997 and 1998, when annual precipitation totaled 8.5 and 12.6 inches, respectively, at Eskdale, Utah. These totals are relative to an average, annual precipitation of 6.4 inches between 1966 and 2005. The effects of increased recharge and reduced pumpage are reflected in relatively steady water levels between 1996 and1999 (Figure 4). Water-level changes in well O1 provide the best integrated response to irrigation and climatic conditions because the well is completed just above relatively transmissive carbonate rock.

Water-level changes were observed in four wells that were affected by groundwater 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 were made during the period of analysis (Figure 4). Water levels in the observation wells ranged between 4 and 145 ft below land surface.


figure 4

Figure 4.—Water level changes in observation wells O1, O2, O3, and O4 where depths to water were 4, 16, 142, and 69 ft below land surface, respectively, during April 2004.

Drawdowns were estimated by subtracting water levels measured during the test period from water levels that were measured March 2001. Water-level declines from prior pumping were minor or uncertain enough that further corrections were not justifiable. Drawdown rates ranged between 0.1 and 0.7 ft/yr and these extremes were observed in wells O4 and O1, respectively (Figure 4).

Numerical Analysis

Hydraulic properties of the basin fill and carbonate rock 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 193 rows and 161 columns (Figure 2, Figure 5). Rows and columns were assigned widths of 500 ft where observation wells and the majority of pumping wells existed (Figure 5). Row and column widths were multiplied by 1.1 from the area of 500-ft on a side cells to the edges of the model that extended laterally 330,000 ft away. All lateral model boundaries were specified no-flow boundaries. The model grid was oriented north-south in UTM, zone 11 projection for convenience where the lower, left model corner was easting and northing, 651,614 and 4,187,459 m.


figure 5


Figure 5.—Numerical model grid, pumping wells, observation wells, Spring Run south of Garrison, UT.

The vertical extent was discretized into 7 layers between 0 and 2,000 ft below the water table, which was within 10 ft of land surface near many irrigation wells. Layers ranged between 5 and 1,400 ft thick. Layers 1, 2, 3, and 4 had uniform thicknesses of 5, 10, 20, and 30 ft, respectively. Thin layers occurred nearest the water table to better simulate drainage and capture of water from the Spring Run (Figure 5). The combined thickness of layers 5 and 6 varied between 5 and 1,935 ft thick (Welch and others, 2007) so the maximum thickness of the basin fill was 2,000 ft. Layer 5 contained 25 percent of the combined thickness so the base of layer 5 corresponded with the typical depth of the irrigation wells. Layer 7 was a uniform 1,000-ft thickness that simulated carbonate-rock aquifer. 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.

The period between April 1, 2001 and March 31, 2004 was simulated with six stress periods during the Needle Point aquifer test. Each stress period was divided into 50 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. Irrigation pumping occurred during stress periods 1, 3, and 5 which were 214, 215, and 177 d, respectively. All pumpage was from layer 5 which ranged between 70 and 550 ft below the water table around the irrigation wells. 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. This was consistent with the assumption that water-level declines had largely ceased during the wetter 1997-1998 period prior to the 2001-2003 irrigation seasons.

The observation wells were simulated as piezometers where simulated observation well depths were assigned discretely to model layers. Observation well O1 was assigned to layer 3 in the basin fill near the simulated contact between basin fill and carbonate rock. Observation wells O2, O3, and O4 in the basin fill were assigned to layer 5. A distinction between simulating the observation points as piezometers or well screens of finite length were minor because the period investigated was multiple years, rather than less than a day. Uncertainties in well construction caused some misfit between simulated and measured drawdowns.

Observations from well O1 were considered best because more than 30 water levels were measured during the period of analysis. Water-level changes also were regional because well O1 was completed adjacent to more transmissive carbonate rock and was affected greatly by all nearby pumping (Figure 4). Drawdowns were affected by a larger volume of aquifer between well O1 and the pumping wells and were much less sensitive to errors in pumping well locations.


Hydraulic Property Estimates

Hydraulic conductivity of basin fill, transmissivity of carbonate rock, specific yield of basin fill, specific yield of carbonate rock, and vertical–to-horizontal anisotropy were estimated. Specific storage could not be estimated with limited drawdowns and annual pumpage estimates so specific storages of 1.5 x 10-6 and 1.5 x 10-7 1/ft were assigned throughout the basin fill and carbonate rock, respectively. A uniform hydraulic conductivity of 5 x 10-5 ft/d was assigned to non-carbonate basement rocks, granites and siliciclastics because drawdowns were not sensitive changes in assigned hydraulic conductivity.

Simulated drawdowns matched measured drawdowns with an unweighted RMS error of 0.15 ft (Figure 6, Figure 7). This is a small error relative to maximum simulated drawdowns of 3 and 4 ft in observation wells O2 and O1, respectively. RMS errors of 0.05 and 0.15 ft for observation wells O3 and O4, respectively were a more significant fraction of maximum simulated drawdowns of 0.4 and 0.6 ft. Simulated and measured rates of seasonal and annual decline agreed except in well O2 during March 2002 (Figure 7). This might have been the effect of an individual well being pumped.


figure 6


Figure 6.—Simulated and measured drawdowns in observation wells O1 and O3 during 2001-2003 irrigation seasons with an annual application rate of 3 ft.


figure 7

Figure 7.—Simulated and measured drawdowns in observation wells O2 and O4 during 2001-2003 irrigation seasons with an annual application rate of 3 ft.

Transmissivity of the basin fill varied little, 1,200 to 1,300 ft²/d, regardless of annual application rates (Table 2). Specific capacity for a transmissivity of 1,200 ft²/d would be about 6 gpm/ft which is in the reported range of 1 to 10 gpm/ft. Specific yield of the basin fill varied little, 0.12 and 0.13 (Table 2). Vertical–to-horizontal anisotropy also varied little, 0.06, which suggests that this estimate is more representative of the basin fill than the carbonate rock.


Table 2.—Hydraulic properties of basin fill and carbonate material estimated with the numerical model.


[Specific storage of 0.0000015 1/ft was assigned.; Hydraulic conductivity of
0.00005 ft/d was assigned to non-carbonate basement rocks.]


  Hydraulic property




Hydraulic conductivity of Basin Fill, ft/d   0.6 0.6
Transmissivity of Basin Fill, ft²/d        1,200  1,300

Specific yield of Basin Fill, d'less




Hydraulic conductivity of Carbonate, ft/d




Transmissivity of Carbonate, ft²/d




Specific yield of Carbonate, d'less




Vertical-to-horizontal anoisotropy, d'less





Transmissivity of the carbonate rock varied greatly, 7,000 and 16,000 ft²/d, for annual application rates of 2 and 3 ft, respectively (Table 2). Specific yield of the carbonate rock varied more than fivefold, 0.001 and 0.006 in response to a 50 percent increase in pumpage. Hydraulic diffusivity, transmissivity divided by storage coefficient, of the carbonate rock was less variable than transmissivity or storage coefficient, but still ranged between 3,000,000 and 6,000,000 ft²/d.

Hydraulic property estimates are integrated values for an area of about 100,000 acres. Simulated drawdowns exceeded 0.5 ft across 130,000 acres at the end of the 2003 irrigation season (Figure 8). About 80 percent of the water pumped during the simulation period was released from storage where drawdowns exceeded 0.5 ft. Transmissivity and specific yield estimates were affected by differences in annual application rates, but the area affected by pumpage did not change. Maximum extent of drawdowns and captured volumes of surface water were not affected by differences in net annual application rates of 2 or 3 ft.


figure 8

Figure 8.—Simulated drawdowns on October 29, 2003 at the end of stress period 5, the end of the 2003 irrigation season. Annual application rate was 3 ft.

The contributing area of the Needle Point aquifer test was much greater than the contributing area of conventional aquifer tests because the pumped volume ranged between 10,000 and 15,000 ac-ft. This is more than 100 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.


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.

Halford, K.J., R.J. Laczniak, and Galloway, D.L., 2005, Hydraulic characterization of over pressured tuff in central Yucca Flat, Nevada Test Site, Nevada: U.S. Geological Survey Scientific Investigations Report 2005-5211, 55 p.

Halford, K.J., and Plume, R.W., 2011, Potential effects of groundwater pumping on water levels, phreatophytes, and spring discharges in Spring and Snake Valleys, White Pine County, Nevada, and adjacent areas in Nevada and Utah: U.S. Geological Survey Scientific Investigations Report 2011-5032, 52 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.

Rush, F.E., 1976, Water requirement and efficiency of sprinkler irrigation of alfalfa, Smith Valley, Nevada—A case history: Nevada Division of Water Resources, Information Report 24, 14 p.

Welborn, T.L., and Moreo, M.T., 2007, Irrigated acreage within the Basin and Range carbonate-rock aquifer system, White Pine County, Nevada, and adjacent areas in Nevada and Utah: U.S. Geological Survey Data Series 273, 18 p.

Welch, A.H., Bright, D.J., and Knochenmus, L.A., 2007, Water resources of the Basin and Range carbonate-rock aquifer system, White Pine County, Nevada, and adjacent areas in Nevada and Utah: U.S. Geological Survey Scientific Investigations Report 2007-5261, 96 p.

Winograd, I. J. and Thordarson, W., 1975, Hydrogeologic and hydrochemical framework, south-central Great Basin, Nevada-California, with special reference to the Nevada Test Site: U.S. Geological Survey Professional Paper 712-C, 125 p.



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