KGS Home Water Resources Index Page Water-level Program

KGS, Open-file Report 1999-15

Optimizing the High Plains Aquifer Water-level Observation Network

by Ricardo A. Olea and John C. Davis
Kansas Geological Survey

KGS Open File Report 1999-15
May 1999


The Kansas Geological Survey (KGS) and the Division of Water Resources (DWR) monitor a network of observation wells in central and western Kansas for the purpose of determining annual changes in water level in the High Plains aquifer and other aquifers. The observation network in the High Plains aquifer is intended to have a maximum uncertainty of ±10 ft in water-level elevation anywhere within the interior of the network. There are locations where this standard is not met, either because of loss of observation wells from the network over time or defects in the original network design.

We recommend that 28 new observation wells be added to the High Plains aquifer network to compensate for locally inadequate sampling. Four more observation wells should added to verify data collected from wells that exhibit aberrant changes in year-to-year water-level measurements. Readings from these wells may be the result of unusual but real fluctuations in water level in the aquifer, or they may be caused by poor mechanical conditions in the observation wells that make the measurements unreliable.

Locations where additional observation wells and verification wells should be located are shown on maps and listed in tables. One new observation well should be designated as close as possible (within a radius of 2.5 mi) to each set of specified coordinates.

List of Plates

  1. Observation wells eliminated from reliability analysis, 1999 (PDF file, 100 k)
  2. Kriging standard deviation for dependable observation wells, 1999 (PDF file, 600 k)
  3. Kriging standard deviation for proposed modified observation network (PDF file, 500 k)
(These plates are presented as Acrobat PDF files, and require the free Acrobat reader to view.)


The Kansas Geological Survey (KGS) and the Division of Water Resources (DWR) measure water levels in observation wells in central and western Kansas to monitor the water content of underground aquifers used for irrigation, domestic, and commercial water supply. Most of the observation wells penetrate the High Plains aquifer in a fairly systematic spatial pattern, forming a "network" that is intended to provide a reliable picture of the water level within the aquifer.

In theory, the best possible configuration for a water-level observation network would consist of wells located at the centers of uniform hexagons arranged in a regular pattern (Olea, 1984). Because the present observation well network was originally constructed using wells that already were being monitored, and the practical limitations on the choice of additional wells that can be used for monitoring purposes, the Kansas High Plains aquifer observation network does not conform to the ideal regular hexagonal pattern. However, over the last two decades there has been systematic progress in modifying the network to approximate the ideal sampling pattern.

This study reviews the reliability of the observation well network for estimating water levels in the High Plains aquifer and proposes modifications to improve collection of data and reduce uncertainty in the network.

The Observation Network in 1999

There were 1,272 High Plains aquifer observation wells to be measured during the January 1999 data collection program. Of these, 1,204 were successfully measured (Olea and Davis, 1999). According to Miller and Davis (1999, p. 7-8), there are 37 wells that should be removed from the network and not measured in the future because these wells have serious mechanical problems that make them difficult or impossible to measure. Of those wells, 31 tap the High Plains aquifer. These are listed in Table 1.

Table 1. Observation wells in the High Plains aquifer that should not be measured in the year 2000 for mechanical reasons.

Map Index Location
137502S 35W 34CAA 01
134603S 42W 26CCD 01
133704S 35W 06DCD 01
117607S 33W 35ADD 01
117008S 29W 01DCB 01
105008S 38W 17CDD 01
100115S 40W 26CAB 01
99016S 38W 10ABB 01
78621S 18W 32DAA 01
81421S 22W 12BCB 01
69123S 01W 19AAC 01
69623S 39W 15ADD 01
63724S 08W 04AB 01
55424S 18W 36DDC 01
56424S 26W 35CBC 01
51725S 02W 16DDB 01
52125S 33W 17DBD 01
48325S 33W 33CDA 01
40726S 01W 31CCD 01
38327S 32W 03CBB 01
34427S 38W 23CBB 01
35327S 40W 16CCC 01
24928S 19W 30CBC 01
23928S 40W 32CCB 01
22229S 15W 02CCA 01
21629S 37W 08CBA 01
13230S 34W 30ADD 02
10631S 39W 23BBB 01
9032S 38W 11ADA 01
4933S 38W 20DDB 01
5933S 43W 08BDA 01

Miller and Davis (1999, p. 8) have identified an additional 24 wells that have erratic year-to year fluctuations. Most of these wells tap aquifers other than the High Plains aquifer. The 13 erratic wells that tap the High Plains aquifer are listed in Table 2.

Table 2. Observation wells in the High Plains aquifer that are erratic in their year-to-year behavior.

Map IndexLocation
81321S 39W 07CBA 01
68323S 33W 26ABB 01
66823S 33W 28CDC 01
69423S 34W 17CCC 01
5133S 35W 23CBB 01
7832S 36W 21AAC 01
9731S 26W 30BBB 01
11331S 28W 10BCB 01
12130S 26W 32DDD 01
33027S 23W 28AAA 01
48425S 34W 34DBD 01
68223S 35W 25BBB 03
68423S 34W 21DDC 01

If these 44 wells are removed, there will be 1,228 dependable wells in the High Plains observation network. Plate 1 is a posting showing the locations of all wells listed in Tables 1 and 2.

Kriging Standard Deviation for Dependable Wells

The kriging standard deviation is used in geostatistics as a measure of reliability in spatial estimation (Olea, 1999, ch. 10). The larger the kriging standard deviation, the greater the uncertainty in the associated estimate, or stated in another way, the less reliable is the estimate.

In 1997, when the KGS assumed responsibility for measuring observation wells previously measured by the U.S. Geological Survey, it was agreed that a desirable and achievable level of reliability for the High Plains aquifer network could be set at a maximum kriging standard deviation of 10 ft in those areas of the major aquifer that were free from border effects (Olea, 1997a). Plate 2 is a map showing the kriging standard deviation that would be obtained if the 1,228 observations wells that are considered measurable and reliable were measured.

Proposed Modifications to the Network

Based on the standard that the maximum kriging standard deviation inside the central portions of the High Plains aquifer should not exceed 10 ft, all of the isolated areas shown in orange or green on Plate 2 are undersampled. Each year since 1997, locations where additional observation wells are necessary have been proposed (Olea, 1997a; Olea, 1997b; Olea, 1998). Selecting these well locations is straightforward--it has been theoretically determined and confirmed in practice that, for the semivariogram of the residuals of the water-level elevation, it is sufficient to have one well in each hexagon of radius 4.5 km, arranged in a regular hexagonal pattern, to achieve a kriging standard deviation of less than 10 ft. Figure 1 shows the appropriate hexagonal network drawn at the same scale as the maps in Plates 1-3.

Figure 1. Regular hexagonal network of observation wells in the High Plains aquifer required to reduce the maximum uncertainty in water level to ±10 ft.

Hexagonal pattern is the most efficient way to collect data

An advantage of using the kriging standard deviation as the measure of uncertainty is that its value depends on the semivariogram and the spatial configuration of the observation points, and not on the individual measurements of water level (Olea, 1999, p. 24-25). Plate 3 is a map of the kriging standard deviation that would be obtained if 32 wells were added to the network at the locations specified in Table 1. Note that wells do not have to be placed precisely at the positions specified. Depending on the locations of neighboring observation wells, usually any spot within a hexagon circumscribed by a circle 2.5 mi (4 km) in radius and centered at the listed position will result in a kriging standard deviation that is less than 10 ft. Of course, the closer a well can be found to the specified location, the greater and more uniform the reduction in kriging standard deviation.

In Table 3, identification numbers less than 70 are the same well locations that were proposed previously. Identification numbers below 50 are locations originally proposed in 1997 (Olea, 1997b), and numbers between 50 and 70 represent locations first proposed in 1998 (Olea, 1998). This year for the first time we suggest two types of new observation wells. Locations with identification numbers between 70 and 90 indicate where wells are needed to compensate for original inadequate control or to replace one of the 31 wells to be deleted from the network because of serious mechanical problems.

The four locations with identification numbers in the 90s are needed to verify anomalous water-level measurements from wells where the kriging standard deviation would exceed 10 ft if the erratic wells were dropped from the network. At the present, we recommend that all 13 anomalous observation wells listed in Table 2 continue to be measured, because their unusual readings may represent true behavior of the aquifer and not the result of poor mechanical conditions in the observation wells. New observation wells at locations 91-94 will provide a check on these readings; if the new wells also behave in an unusual manner, this will confirm that the behavior is real and not a sampling artifact.

Table 3. Centroids of proposed locations for new wells required to reduce the kriging standard deviation below 10 ft in the High Plains aquifer.

UTM Coordinates Location County
Easting, m Northing, m
4251813.884387669.465S 41W 23Cheyenne
5366500.4400000.4S 29W 3Decatur
6362000.4391500.4S 29W 32Decatur
9350000.4206000.24S 31W 3Finney
13352092.754180125.8026S 31W 26Finney
92331000.4207500.24S 33W 3Finney
93317500.4210000.23S 34W 29Finney
94324000.4214000.23S 34W 13Finney
18293505.494147498.9730S 37W 11Grant
71294000.4170000.27S 37W 35Grant
72298000.4161000.28S 36W 32Grant
21379130.004165221.0828S 28W 10Gray
22360000.4157000.29S 30W 3Gray
25324079.644146795.0430S 34W 12Haskell
76268000.4137000.31S 40W 13Morton
77269000.4129000.32S 39W 7Morton
29310500.4397000.4S 35W 14Rawlins
61310000.4385500.5S 35W 23Rawlins
54576500.4207000.238 7W 31Reno
56598000.4210000.23S 5W 21Reno
33343000.4138303.3331S 32W 2Seward
34347000.4111000. 33S 31W 32Seward
75264000.4157000.29S 40W 14Stanton
57283000.4138000.31S 38W 9Stevens
58283000.4129000.32S 38W 9Stevens
73296000.4136000.31S 37W 14Stevens
74276000.4136000.31S 39W 14Stevens
78312500.4111000.33S 35W 34Stevens
79303000.4130000.32S 36W 3Stevens
80295000.4118000.33S 37W 11Stevens
81295000.4108000.34S 37W 11Stevens
91303000.4121000.32S 36W 34Stevens

The uncertainty near the five wells listed in Table 4 exceeds the minimum standards, but because these wells are located on or near the boundaries of the High Plains aquifer, this is not surprising. Accurately sampling the edges of the High Plains aquifer is not a priority at present, so no action is recommended to investigate the reliability of water level measurements in areas surrounding these wells.

Table 4. Observation wells in the High Plains aquifer that are erratic but are located near the aquifer boundary.

Map IndexLocation
81321S 39W 07CBA 01
33027S 23W 28AAA 01
11331S 28W 10BCB 01
12130S 26W 32DDD 01
9731S 26W 30BBB 01

Conclusions and Recommendations

Field reports on the mechanical conditions of wells, combined with a year-toyear analysis of changes in water level at individual wells and geostatistical analysis, supports the following conclusions and recommendations:

  1. There are 31 High Plains observation wells with mechanical problems so serious that they probably cannot be successfully measured in the future. We concur with Miller and Davis (1999), that these wells should be dropped permanently from the observation well network.
  2. There are 13 observation wells with unusually variable year-to-year water levels. We propose that they continue to be measured, but that four new observation wells in their neighborhoods also be measured to determine if the behavior is real or the result of poor measurement conditions.
  3. We recommend that 28 wells be added to the High Plains network to achieve a maximum kriging standard deviation of no more than ±10 ft. throughout the central part of the High Plains aquifer. The number of additional wells proposed is a 44% drop from the 50 new wells proposed last year, indicating that progress in filling the gaps in the network was made in 1999.


Miller, R.D., and J.C. Davis, 1999, 1999 Annual Water Level Raw Data Report for Kansas: Kansas Geological Survey Open-File Report No. 99-5, 271 p. and 1 compact disk.

Olea, R.A., 1984, Sampling design optimization for spatial functions: Mathematical Geology, vol. 16, no. 4, p. 369-392.

Olea, R.A., 1997a, Sampling Analysis of the Annual Observation Water-Level Wells in Kansas: Kansas Geological Survey Open-File Report No. 97-73, 44 p.

Olea, R.A., 1997b, Modifications to the High Plains Aquifer Observation Network Expansion in Open-File report 97-73: Kansas Geological Survey Open-File Report No. 97-84, 3 p., 1 plate.

Olea, R.A., 1998, Proposed Expansion of the High Plains Aquifer Observation Network for Reliability Enhancement: Kansas Geological Survey Open-File Report No. 98-39, 3 p., 2 plates.

Olea, R.A., 1999, Geostatistics for Engineers and Earth Scientists: Kluwer Academic Publishers, New York, 303 p.

Olea, R.A., and J.C. Davis, 1999, Sampling Analysis and Mapping of Water Levels in the High Plains Aquifer of Kansas: Kansas Geological Survey Open-File Report No. 99-11, 35 p. and 9 plates.

Kansas Geological Survey, Water Level Project
Comments to
Placed online Feb. 2004
Available online at URL =