Land cover over the Ogallala aquifer focus area (fig. 2) was mostly grass/pasture and cropland (undifferentiated), which accounted for 48.4 and 29.8 percent of land cover, respectively, in 2022 (National Agricultural Statistics Service, 2023; fig. 3A). The most prominent cropland-cover types in the Ogallala aquifer focus area in 2022 were winter wheat (34.9 percent), fallow/idle cropland (23.4 percent), sorghum (22.7 percent), and corn (10.1 percent). These types together accounted for 91.1 percent of cropland cover in the Ogallala aquifer focus area in 2022. However, cropland-cover types and proportions may change annually with seasonal, economic, and hydrologic factors. The Ogallala aquifer focus area is almost entirely rural; only 3.6 percent of the land cover is developed. About 0.3 percent of the land cover is combined water, wetland, or barren areas.

Land cover over the panhandle part of the Ogallala aquifer was mostly grass/pasture and cropland (undifferentiated), which accounted for 43.3 and 34.8 percent of land cover, respectively, in 2022 (fig. 3B). Winter wheat (31.9 percent) was the most common cropland-cover type in the panhandle part of the Ogallala aquifer. The northwest part of the Ogallala aquifer had a greater proportion of grass/pasture (66.6 percent) and lesser proportion of cropland cover (12.3 percent) than the entire Ogallala aquifer focus area in 2022 (fig. 3C). Winter wheat (65.2 percent) was the most common cropland-cover type in the northwest part of the Ogallala aquifer.

The climate of the Ogallala aquifer focus area is classified as humid subtropical and cold semi-arid according to the Köppen-Geiger climate classification (Kottek and others, 2006). Historical daily precipitation data from 17 climate stations (fig. 1; table 1; National Centers for Environmental Information, 2023; Oklahoma Climatological Survey, 2023) in the Ogallala aquifer study area were summarized annually for the period 1915–2022. Precipitation data from climate stations in Cimarron, Texas, and Beaver Counties, Okla., were used to represent the panhandle part of the Ogallala aquifer (fig. 4A), and precipitation data from climate stations in Dewey, Ellis, Harper, and Woodward Counties, Okla., were used to represent the northwest part of the Ogallala aquifer (fig. 5A). Daily precipitation data from years with fewer than 10 months of recorded data and from months with fewer than 25 days of recorded data were omitted from the analysis. Stations with less than 54 years of precipitation and air temperature data also were excluded, resulting in 17 climate stations (fig. 1; table1). Historical annual air temperature data from 1915 to 2022 (Vose and others, 2023) for Cimarron, Texas, and Beaver Counties, Okla., were averaged and used to represent the panhandle part of the Ogalla aquifer (fig. 4B); historical annual air temperature data from 1915 to 2022 (Vose and others, 2023) for Ellis County, Okla., were used to represent the northwest part of the Ogallala aquifer (fig. 5B). A locally weighted scatterplot smoothing (LOWESS) curve (Cleveland, 1979) was applied to the annual data to determine periods of above-mean or below-mean precipitation and air temperature. Drought periods were selected based on methods from Shivers and Andrews (2013) and Tian and Quiring (2019) (figs. 4–5).

The mean annual precipitation and air temperature for the 1915–2022 period of record, were 21.11 inches (in.) and 58.86 degrees Fahrenheit (°F), respectively, in the Ogallala aquifer focus area (National Weather Service, 2023; Oklahoma Climatological Survey, 2023). The mean annual precipitation and air temperature for the 1915–2022 period of record were 17.76 in. and 56.77 °F, respectively, in the panhandle part of the Ogallala aquifer and 23.54 in. and 60.95 °F, respectively, in the northwest part of the Ogallala aquifer (figs. 4–5).

The mean annual precipitation and air temperature for the 1998–2022 study period were 22.09 in. and 59.61 °F, respectively, in the Ogallala aquifer focus area. The mean annual precipitation and air temperature for the 1998–2022 study period were 18.75 in. and 57.47 °F, respectively, in the panhandle part of the Ogallala aquifer and 24.34 in. and 61.78 °F, respectively, in the northwest part of the Ogallala aquifer. The panhandle and northwest parts of the Ogallala aquifer had warmer air temperatures over the 1998–2022 study period than over the 1915–2022 period of record.

The OWRB permits and regulates all nondomestic groundwater use and some domestic groundwater use (Oklahoma Statute § 82-1020.1[2] [Oklahoma State Legislature, 2021b]); domestic groundwater use of 5 acre-ft/yr or less is exempt from permitting (Oklahoma Statute § 82-1020.3 [Oklahoma State Legislature, 2021c]). OWRB (2025, p. 1) explains domestic use as follows:

Groundwater permits issued by the OWRB are divided into nine categories: irrigation; public supply; industrial; power; mining; commercial; recreation, fish, and wildlife; agriculture; and other (OWRB, 2024d). For the purposes of this report, the categories of industrial; power; mining; commercial; recreation, fish, and wildlife; and other were combined and are hereinafter referred to as “other.” Most of the groundwater permits in the Ogallala aquifer focus area were for irrigation in the panhandle part of the Ogallala aquifer (fig. 6); of the more than 4,100 irrigation permits in the Ogallala aquifer focus area, more than 3,800 are in the panhandle part of the Ogallala aquifer (OWRB, 2024d).

Permitted groundwater users have been required to submit annual groundwater-use reports to the OWRB since 1967 (Oklahoma Statute § 82-1020.12 [Oklahoma State Legislature, 2021d]). Reported annual groundwater-use data from the panhandle and northwest parts of the Ogallala aquifer were summarized for the period of record (1967–2022), the study period (1998–2022), and a recent drought period (2010–15; figs. 4–5 and 7–8; tables 2–3).

Most of the reported groundwater use in the panhandle part of the Ogallala aquifer was for irrigation, which accounted for 346,988 acre-ft/yr or 96.9 percent of mean annual groundwater use during the 1967–2022 period of record (fig. 7A). During the same period, 4,967 acre-ft/yr or 1.4 percent of groundwater use was for public supply (fig. 7A), primarily for the towns of Beaver, Boise City, Goodwell, Guymon, and Hooker, Okla. (fig. 6). The remaining groundwater use was for agriculture (1.3 percent), and other (0.4 percent) uses (fig. 7A). Annual groundwater use during the 1967–2022 period of record in the panhandle part of the Ogallala aquifer ranged from a minimum of 251,781 acre-ft/yr in 1981 to a maximum of 568,066 acre-ft/yr in 2014 (table 2; fig. 7B). From 2010 to 2015, most of Oklahoma was in a period of drought (figs. 4A–5A), which coincided with an increase in annual groundwater use (National Integrated Drought Information System, 2024). Mean annual groundwater use in the panhandle part of the Ogallala aquifer for the 2010–15 drought period was 522,143 acre-ft/yr, or 163,978 acre-ft/yr greater than mean annual groundwater use in the panhandle part of the Ogallala aquifer during the 1967–2022 period of record (table 2). Precipitation events which occurred in the summer of 2015 ended the 2010–15 drought period, but groundwater use continued to be elevated compared to pre-2010 levels through 2022 (fig. 7B). During the 1998–2022 study period, mean annual groundwater use in the panhandle part of the Ogallala aquifer was 422,054 acre-ft/yr, which was 63,889 acre-ft/yr greater than mean annual groundwater use during the 1967–2022 period of record but less than the mean annual groundwater use during the 2010–2015 drought period (table 2; Codner and others, 2026).

Like in the panhandle part of the Ogallala aquifer, most of the reported groundwater use in the northwest part of the Ogallala aquifer was for irrigation, which accounted for 28,805 acre-ft/yr or 83.3 percent of mean annual groundwater use during the 1967–2022 period of record (fig. 8A). During the same period, 5,376 acre-ft/yr or 15.6 percent of groundwater use was for public supply (fig. 8A), primarily for the towns of Shattuck and Woodward, Okla. (fig. 6). The remaining groundwater use was classified as other (0.7 percent) and agriculture (0.4 percent). During the 1967–2022 period of record, annual groundwater use ranged from a minimum of 2,680 acre-ft/yr in 1967 to a maximum of 65,386 acre-ft/yr in 2001 in the northwest part of the Ogallala aquifer (table 3; fig. 8B); mean annual groundwater use was 34,579 acre-ft/yr. The pattern of increased groundwater use in the panhandle part of the Ogallala aquifer beginning in 2010 was repeated in the northwest part of the Ogallala aquifer where years with the highest groundwater use were all prior to 2010. The 2010–15 drought period was more severe in terms of the persistence of below average precipitation and above average temperature in the panhandle part than in the northwest part of the Ogallala aquifer, which could explain why groundwater use was not as elevated in this part of the aquifer when compared to the panhandle part of the Ogallala aquifer (figs. 4–5). Mean annual groundwater use in the northwest part of the Ogallala aquifer during the 2010–15 drought period was 30,231 acre-ft/yr, or 4,348 acre-ft/yr less than mean annual groundwater use for the 1967–2022 period of record (table 3). During the 1998–2022 study period, mean annual groundwater use in the northwest part of the Ogallala aquifer was 39,645 acre-ft/yr, or 5,066 acre-ft/yr greater than the mean annual groundwater use during the 1967–2022 period of record (table 3).

Groundwater use for irrigation was used to estimate irrigation return flows, further explained in the “Recharge” section of this report. Mean annual groundwater use for irrigation during the 1998–2022 study period in the panhandle part of the Ogallala aquifer was 405,541 acre-ft/yr (Codner and others, 2026). Mean annual groundwater use for irrigation during the 1998–2022 study period in the northwest part of the Ogallala aquifer was 32,075 acre-ft/yr (Codner and others, 2026).

The USGS installed two continuous groundwater-level recorders in wells in the Ogallala aquifer focus area: GW15 installed in 1956 and GW16 installed in 2012 (fig. 1; table 1; USGS, 2024a). The OWRB installed 18 continuous groundwater-level recorders in wells in the Ogallala aquifer focus area as part of their Groundwater Monitoring and Assessment Program starting in 2022 (fig. 1; table 1; OWRB, 2024c). Tomlinson and others (2024) provide a map showing groundwater-level changes in the panhandle during 1982–2022 which could be used as a reference for future tracking of groundwater levels in the aquifer. Groundwater-level data from the USGS continuous-recorder wells (GW15 and GW16) indicate seasonal patterns, with declines starting around June through September and increases in October through May (figs. 9, 10). Groundwater levels at GW15 have a decreasing pattern, and those at GW16 are relatively steady through time. These patterns are consistent with the groundwater-level change data presented in McGuire and Strauch (2024b).

Daily streamflows measured at USGS streamgages consist of two primary components: base flow and runoff. Base flow is the component of streamflow discharged by surrounding groundwater systems. Runoff is the component of streamflow supplied by overland flow and stormflow (interflow) of precipitation into a stream channel (Barlow and Leake, 2012).The Base-Flow Index code (Wahl and Wahl, 1995) in the USGS Groundwater Toolbox (Barlow and others, 2015) was used to separate daily streamflow data from the USGS National Water Information System (USGS, 2024a) into base-flow and runoff components. The Base-Flow Index code divided daily streamflow data into n-day-long windows (where n is the user-defined number of days) and identified the minimum streamflow within each window. Turning points were then identified by comparing the minimums of each window below a certain fraction (the user-defined f-statistic, in this case 0.9 or 90 percent) to immediately adjacent minimums. If 90 percent of a given minimum was less than the preceding and succeeding minimums, that minimum was considered a turning point. These turning points were then used to construct a base-flow hydrograph with semilogarithmic interpolation. By using the turning points, the base-flow index (BFI), or the ratio of base flow to total streamflow, was calculated. Multiple n-day windows were tested by plotting mean BFI against different n-day values (from 2 to 15 days), and a slope change was evident around 3 days in most daily streamflow data analyzed. For consistency, a 3-day window and an f-statistic of 0.9 were used for base-flow analysis in this study.

The Beaver River and Cimarron River are the two main drainage features in the panhandle part of the Ogallala aquifer (fig. 1; Luckey and Becker, 1999). USGS streamgage 07234000 Beaver River at Beaver, Okla. (map identifier S21, fig. 1), and USGS streamgage 07156900 Cimarron River near Forgan, Okla. (map identifier S22, fig. 1), were selected for base-flow analysis (table 4). Although S22 is outside of the Ogallala aquifer, it is 5 miles upstream of the aquifer and was therefore considered an important part of the historical streamflow analysis. S21 and S22 had a large gap in data during 2022, so a base-flow analysis could not be completed for that year. Therefore, the period 1998–2021 was used for the base-flow analysis.

For S21, the mean annual BFI for the period of record (1938–2021) was 34.26 percent, which was 21.51 percent less than the mean annual BFI for the study period (1998–2021) of 55.77 percent (table 4). For S22, the mean annual BFI for the period of record (1966–2021) was 84.80 percent, which was 5.90 percent less than the mean annual BFI for the study period (1998–2021) of 90.70 percent. S21 and S22 display a pattern of decreasing annual total streamflows and base flows throughout their respective periods of record (fig. 11). The annual BFI 5-year moving average displays an increasing pattern for most of the period of record for sites S21 and S22; however, the annual BFI shows less variation for S22 than for S21. The annual BFI 5-year moving average for S21 also shows a decreasing pattern starting in 2001 that is not present in the annual BFI 5-year moving average for S22. The decreasing pattern in annual streamflows and base flows and the increasing pattern in annual BFI for S21 and S22 coincide with the 1998–2022 climatic period during which the mean annual precipitation (18.75 in/yr) for the panhandle part of the Ogallala aquifer was greater than the historical (1915–2022) mean annual precipitation (17.76 in/yr) for the panhandle part of the Ogallala aquifer (fig. 4).

Wolf Creek is the main drainage feature in the northwest part of the Ogallala aquifer (fig. 1). USGS 07235600 Wolf Creek near Gage, Okla. (map identifier S32), and USGS 07236000 Wolf Creek near Fargo, Okla. (map identifier S23), have data that together cover most of the period between predevelopment and the study period (table 4; fig. 12). S23 covers the period 1943–75 and has a mean annual BFI of 53.53 percent (fig. 12A). Available data from S32 only partially cover the 1998–2022 study period (fig. 12B) but still were used to represent the study period (table 4). S32 had a mean annual BFI of 82.69 percent (fig. 12B). If data from S23 and S32 are viewed as one record representing Wolf Creek, the record shows an increasing pattern in annual BFI that is similar to those for annual BFI at S21 and S22.

The increases in the annual BFI for Beaver River and Wolf Creek over the streamgage periods of record indicate that the proportion of annual total streamflow that comes from annual base flow is increasing relative to annual runoff. Annual base flows and runoff have both decreased, but annual runoff has decreased by a greater percentage than annual base flows (Wahl and Tortorelli, 1997; Esralew and Lewis, 2010).

In the eastern part of the study area in Kansas and Oklahoma, the Crooked Creek Fault displaced Permian through Tertiary sediments (fig. 13; Luckey and Becker, 1999). The Crooked Creek Fault creates an offset in the base of the Ogallala aquifer and causes abrupt changes in saturated thickness in northern Beaver County, Okla. The Cimarron Arch is a local feature mapped in Cimarron and Texas Counties, Okla. (Northcutt and Campbell, 1995). The northwest part of the Ogallala aquifer is predominantly in the Anadarko Basin, which is the primary basin of the Ogallala aquifer focus area where sediments were deposited (Luckey and Becker, 1999). Surficial geologic units in the Ogallala aquifer focus area dip to the southeast at an average rate of 1 degree or less (Hart and others, 1976). Additional structural features that affect the Ogallala aquifer focus area are described in Hart and others (1976) and Luckey and Becker (1999).

Quaternary alluvium, dune sand, and terrace deposits overlie the Ogallala Formation in some parts of the Ogallala aquifer focus area. Alluvium and terrace deposits overlie the Ogallala Formation along streams (figs. 13, 14) and consist of unconsolidated sand, gravel, clay, and silt (Hart and others, 1976). Dune sands are present on the northern side of the Beaver River and were deposited by prevailing southerly winds (fig. 13; Gutentag and others, 1984). The terrace deposits were formed when sediments from streambed erosion were carried downstream and eventually deposited onto terraces (Gutentag and others, 1984). Some of the high terrace deposits were derived from the erosion of the Ogallala Formation; however, most of the terrace deposits were formed later, during the Quaternary Period. Alluvium and terrace deposits east of Beaver County (but within the study area) contain the Beaver-North Canadian River alluvial aquifer in areas where these deposits do not directly overlie the Ogallala Formation (Ryter and Correll, 2016). Alluvium deposits and dune sands that overlie the Ogallala Formation are considered part of the Ogallala aquifer because they have similar appearance and hydraulic properties to the Ogallala Formation (Hart and others, 1976).

The Black Mesa Basalt in the northwest corner of Cimarron County, Okla., is an outcrop of Pliocene igneous rock that is composed of a dense, olivine basalt (figs. 13–14; Hart and others, 1976; chart D of Fay, 1997). Isolated areas of volcanic ash are present in the study area (Hart and others, 1976). The volcanic ash deposits are not distinguished from other geologic units in the Ogallala aquifer focus area (fig. 13), nor are they expected to have a substantial hydraulic contribution to the hydrogeology of the Ogallala aquifer focus area.

The surficial geology in the Ogallala aquifer focus area is dominated by the Miocene to Pliocene Ogallala Formation (fig. 13; Luza and Fay, 2003; Stoeser and others, 2005). The Ogallala Formation is a heterogeneous mixture of gravel, sand, silt, clay, and caliche, which can be difficult to differentiate from overlying unconsolidated materials (Hart and others, 1976). The Ogallala Formation consists of several hundred feet of coarse sand and fine gravel in certain areas and hundreds of feet of clay with lenses of very fine sand in others (Hart and others, 1976). Well yields from the Ogallala aquifer, except where it is very thin, are at least 500 gallons per minute and can be as much as 2,000 gallons per minute (Luckey and Becker, 1999).

In the western half of Cimarron County, Cretaceous bedrock units underlie the Ogallala Formation (Hart and others, 1976). In parts of Cimarron and Texas Counties, Triassic to Jurassic geologic units underlie the Ogallala Formation (Hart and others, 1976). Well yields from the hydrogeologic units contained in the Mesozoic geologic units that underlie the Ogallala aquifer are generally less than well yields from the Ogallala aquifer; well yields can be used to identify the base of the Ogallala aquifer (Hart and others, 1976).

The Cretaceous geologic units contain three different hydrogeologic units that are hydraulically connected to the Ogallala aquifer (fig. 14). The Upper Cretaceous Greenhorn Limestone of the Colorado Group contains an aquifer in areas outside the study area (fig. 14; Barkmann and others, 2021). The Greenhorn Limestone is a mix of gray limestone and calcareous shale with some bentonite (fig. 14; Hart and others, 1976). The Greenhorn Limestone is underlain by the Upper Cretaceous Graneros Shale of the Colorado Group, which is a confining unit and is a gray to black shale (fig. 14; Barkmann and others, 2021). The Dakota aquifer underlies the Graneros Shale and is contained in the Upper Cretaceous Dakota Sandstone and the Lower Cretaceous Glencairn Formation and Lytle Sandstone (Macfarlane, 1995). The Dakota Sandstone is mostly fine- to medium-grained sand, and is buff to light-brown (Hart and others, 1976). The Dakota Sandstone is present in the western part of Cimarron County but has been eroded away in the remainder of the Ogallala aquifer focus area (fig. 13; plate 1 of Hart and others, 1976). The Glencairn Formation and Lytle Sandstone directly underlie the Ogallala Formation in western Cimarron County where the Dakota Sandstone has been eroded away (plate 1 of Hart and others, 1976). The Glencairn Formation and Lytle Sandstone were previously grouped and called the Purgatoire Formation, but for this report they are mapped separately (Luza and Fay, 2003). The Glencairn Formation is a fossiliferous shale with interbedded sandstones (Luckey and Becker, 1999). The Lytle Sandstone is a fine- to medium-grained, white to buff sandstone (Luckey and Becker, 1999).

Two hydrogeologic units are contained in Jurassic geologic units in the study area. The Upper Jurassic Morrison Formation contains a confining unit identified outside of the study area, which is a varicolored shale, fine- to very coarse-grained sandstone, limestone, dolomite, and conglomerate, but variations in lithology are common (Luckey and Becker, 1999). The Morrison Formation is considered a lower confining unit for the Ogallala aquifer in Colorado and western Cimarron County, Okla. (Hart and others, 1976; Barkmann and others, 2021). The color is highly variable, with greenish gray being the dominant color (Hart and others, 1976). The Middle Jurassic Exeter Sandstone contains an aquifer located outside the study area (Robson and Banta, 1995). The Exeter Sandstone is disconformable with the overlying Morrison Formation (fig. 14; Hart and others, 1976) and is sometimes called the Entrada Sandstone (Fay, 1997). The Exeter Sandstone is a fine- to medium-grained sandstone, with some gravels found locally (Luckey and Becker, 1999).

The Dockum aquifer, which is recognized as a minor aquifer in Texas but not Oklahoma, is contained in the Upper Triassic Dockum Formation (Dutton and Simpkins, 1986), which underlies the Exeter Sandstone. The Dockum Formation is a sandstone that grades upward to a shaly sandstone (Luckey and Becker, 1999). The Dockum aquifer directly underlies the Ogallala aquifer in eastern Cimarron County and western Texas County, Okla., (Hart and others, 1976) but yields less groundwater than the Ogallala aquifer (Hart and others, 1976). The difference in geochemistry of the groundwater in the Ogallala and Dockum aquifers has been used to determine the base of the Ogallala aquifer (Dutton and Simpkins, 1986).

For most of the Ogallala aquifer focus area, undifferentiated Permian geologic units, which are commonly referred to as red beds, unconformably underlie the Ogallala Formation (fig. 14; Gutentag and others, 1984). The Permian geologic units consist of predominantly red or orange shale, silty mudstone, and anhydrite; limestone and halite are locally present (Luckey and Becker, 1999). These undifferentiated Permian geologic units generally function as a lower confining unit to the Ogallala aquifer (Luckey and Becker, 1999). The Permian units unconformably underlie Mesozoic units in the western part of the Ogallala aquifer.

The types and concentrations of dissolved chemical constituents in groundwater vary based on the lithologic and hydrologic properties of the geologic material through which the water flows. Other factors that affect groundwater quality include recharge rates, residence times within the aquifer, variations in temperature and pressure, land use (particularly agriculture), and pumping which can induce upward migration of saline waters from deeper parts of the aquifer or underlying geologic units. The range of the age of the groundwater in the Ogallala aquifer focus area is from less than 1,000 years to 9,000 years (Andrews and others, 2000).

Groundwater-quality data of the Ogallala aquifer focus area presented in this report were measured by the USGS and the OWRB as part of their Groundwater Monitoring and Assessment Program (OWRB, 2018). The USGS sampled groundwater from 25 wells in the panhandle part of the Ogallala aquifer during the period of November 20, 1958, to April 20, 2021, and 5 groundwater wells in the northwest part of the Ogallala aquifer during the period of November 19, 1951, to May 11, 2015 (USGS, 2024a). The OWRB sampled groundwater from 93 wells in the panhandle part of the Ogallala aquifer in July–October 2016 and June 2021 and 24 wells in the northwest part of the Ogallala aquifer in August–September 2013 and July–August 2019 (OWRB, 2024e).

Descriptions of groundwater quality in this report focus on total dissolved solids (TDS) concentrations and specific conductance values as well as concentrations of major cations and anions. TDS and specific conductance values were similar between the panhandle and the northwest parts of the Ogallala aquifer. TDS concentrations in the panhandle part of the Ogallala aquifer ranged from 223.0 to 1,559.0 milligrams per liter (mg/L), with a mean of 454.5 mg/L (OWRB, 2024e). TDS concentrations in the northwest part of the Ogallala aquifer ranged from 265.0 to 724.0 mg/L, with a mean of 393.1 mg/L. The U.S. Environmental Protection Agency (2022) has established a secondary drinking water standard of 500 mg/L for TDS; however, the State of Oklahoma specifies a beneficial use designation for groundwater, with TDS concentrations less than 3,000 mg/L for all purposes, and a limited beneficial use designation for treated groundwater, with TDS concentrations between 3,000 and 5,000 mg/L (Legal Information Institute, 2022). Groundwater samples from 37 wells in the panhandle part of the Ogallala aquifer and from 1 well in the northwest part of the Ogallala aquifer exceeded the secondary drinking water standard for TDS (OWRB, 2024e; USGS, 2024a). However, no groundwater samples from the Ogallala aquifer focus area had TDS concentrations greater than 3,000 mg/L in Oklahoma. Specific conductance values for groundwater samples from the panhandle part of the Ogallala aquifer ranged from 334.5 to 2,078.0 microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 °C), with a mean of 648.9 µS/cm at 25 °C (OWRB, 2024e; USGS, 2024a). Specific conductance values for groundwater samples from the northwest part of the Ogallala aquifer ranged from 355.0 to 1,050.0 µS/cm at 25 °C, with a mean of 552.6 µS/cm at 25 °C.

Major cation and anion concentrations in groundwater samples were examined by using the Piper (1944) method (figs. 15, 16). For the use of the Piper (1944) method, samples were required to include calcium, magnesium, sodium, potassium, bicarbonate, carbonate, chloride, sulfate, and fluoride concentrations. Concentrations in milligrams per liter were converted to units of milliequivalents per liter, and samples were excluded from analysis if the difference in the cation-anion balance for the sample was greater than 10 percent. In total, 174 out of the 181 groundwater-quality samples met the aforementioned criteria; 141 samples were from the panhandle part of the Ogallala aquifer, and 33 samples were from the northwest part (OWRB, 2024e; USGS, 2024a). Most groundwater samples in both parts of the aquifer were bicarbonate type, with calcium having the highest relative cation concentration and then magnesium in most samples (figs. 15, 16). Groundwater samples collected from the northwest part of the Ogallala aquifer were less mixed than groundwater samples from the panhandle part and had a larger milliequivalent concentration of calcium bicarbonate. The calcium bicarbonate likely indicates high amounts of caliche, a calcium carbonate hardpan, that interacts with rainwater during recharge (Sanford, 2023).

The geographic extent of the Ogallala aquifer focus area (fig. 1) was established by the OWRB (2024b). Because the Ogallala aquifer is an unconfined bedrock aquifer, the aquifer extent was generally defined by the surficial exposure of the Ogallala Formation as shown on 1:250,000-scale geologic maps from Stoeser and others (2005). The top of the Ogallala aquifer in the focus area was defined as the land-surface altitude obtained from a 10-meter (horizontal resolution) digital elevation model (DEM) (USGS, 2015). The altitude of the base of the Ogallala aquifer in the focus area (fig. 17) was generally considered to be the top of the underlying Permian or Cretaceous rocks. The altitude of the base of the aquifer was calculated by subtracting the depth to the top of these underlying bedrock units from the altitude of the top of the Ogallala aquifer.

Well completion reports containing drillers’ lithologic logs obtained from the OWRB (2024b) were used to modify the base of the Ogallala aquifer from Luckey and Becker (1999). Approximately 330 wells were considered fully penetrating; therefore, the depths of those wells were assumed to be depths to the base of the aquifer (top of the underlying Permian and Cretaceous units). The drillers’ lithologic logs were searched for terms representing the Permian and Cretaceous bedrock units (for example, redbed, blue clay, and yellow clay). The depth of the first occurrence of these terms was subtracted from the altitude of the top of the aquifer to calculate the altitude of the aquifer base. The altitudes of the aquifer base were then used to modify the aquifer-base contours from Luckey and Becker (1999), primarily in Texas, Beaver, and Ellis Counties, Okla., to incorporate data from the newer well logs.

A potentiometric-surface map is a contour map of the altitudes of the water surface in tightly cased wells at a specified time (Fetter, 2001). The potentiometric surface is contoured from synoptic water-table altitude measurements in many groundwater wells across an aquifer extent. Potentiometric-surface maps can be used to show the general directions of groundwater flow in the aquifer. Groundwater flows perpendicularly to the contours towards decreasing contour altitudes.

In March 2021, the OWRB made synoptic water-table altitude measurements (Codner and others, 2026) at groundwater wells (fig. 18) throughout the Ogallala aquifer focus area. The depth to water below the land surface was measured at 311 wells (Codner and others, 2026) to determine the altitude of the water table referenced to the North American Vertical Datum of 1988. Five measurements were not used because there was evidence of pumping during or immediately before the levels were measured. For the purposes of the potentiometric-surface measurements, the OWRB combined the Dakota and Dockum aquifers and used them as a part of the Ogallala aquifer for this analysis (Codner and others, 2026). Because the Ogallala aquifer is an unconfined aquifer, the groundwater-level altitude measurements were calculated by subtracting the depth-to-water measurements from the land-surface altitude obtained from a 10-meter (m; horizontal resolution) DEM (USGS, 2015). The groundwater-level altitude measurements were gridded to make a potentiometric surface for the Ogallala aquifer focus area by using the inverse-distance-weighted interpolation method in the ArcGIS Topo to Raster tool (Esri, 2024d) and contoured by using the ArcGIS Contour tool (Esri, 2024a). The potentiometric-surface contours from Luckey and Becker (1999) were used as control points for areas with little to no groundwater-level altitude data. Additional control points were created along streamlines in the Ogallala aquifer focus area and were assigned groundwater-level altitudes from the 10-m DEM minus 15 ft to ensure that the potentiometric contours were below the land surface near streams. Based on the potentiometric surface, groundwater within the Ogallala aquifer focus area generally flows from west to east and begins flowing toward streams in eastern Texas County (fig. 18).

The saturated thickness of the Ogallala aquifer within the Ogallala aquifer focus area (fig. 19) was determined by subtracting the gridded altitude of the aquifer base from the gridded potentiometric-surface altitude (figs. 17, 18). The saturated thickness ranged from less than 20 ft in some areas in western Cimarron County to 517 ft in the downthrown parts of Crooked Creek Fault in Beaver County (fig. 19). The mean saturated thickness of the Ogallala aquifer within the focus area was 125 ft in March 2021 (table 5).

The distribution and variability of textural and hydraulic properties of aquifer materials, especially the horizontal hydraulic conductivity, are the primary controls on groundwater flow in the Ogallala aquifer focus area. Hydraulic properties were previously studied throughout the Ogallala aquifer focus area, but this investigation focused on studies that were completed in Oklahoma and Texas. Hart and others (1976) performed 10 multiwell pumping tests in the panhandle part of the Ogallala aquifer to determine transmissivity, storage coefficient (which is roughly equivalent to specific yield in unconfined aquifers), and hydraulic conductivity values. The transmissivity values ranged from 500 to 11,800 feet squared per day (ft²/d), with a mean of 5,438 ft²/d; the storage coefficient values ranged from 0.002 to 0.11, with a mean of 0.05; and the hydraulic conductivity values ranged from 2.1 to 55 feet per day (ft/d), with a mean of 25.7 ft/d. Teeple and others (2021) estimated hydraulic conductivity and specific yield values in the Ogallala aquifer in Texas by using single-well aquifer tests, borehole electromagnetic flowmeter measurements, and borehole nuclear magnetic resonance (NMR) measurements. Those hydraulic conductivity values ranged from 0.55 to 350 ft/d, with a median of 5.7 ft/d, and specific yield values ranged from 0.03 to 0.13, with a median of 0.09.

Multiple methods were used in this study to estimate the range and central tendency of hydraulic conductivity values in the Ogallala aquifer focus area. These methods included borehole NMR measurements, slug tests, and analysis of lithologic descriptions from wells completed in the Ogallala aquifer. Even though these methods were consistently used across the Ogallala aquifer focus area, hydraulic properties between the panhandle and northwest parts of the Ogallala aquifer could not be differentiated because of limitations on the amount of testing that could be completed. Therefore, hydraulic properties are presented for the Ogallala aquifer focus area as a whole.

Groundwater storage, in acre-feet (table 5), was calculated for the Ogallala aquifer focus area for the year 2021 by using the following formula, which was modified from Fetter (2001) for an unconfined aquifer:Groundwater storage = S_y × b_a × A,(2)where

A specific yield value of 0.09 was assigned to the Ogallala aquifer focus area based on the NMR measurements described in the “Hydraulic Properties Estimated from Nuclear Magnetic Resonance” section. This value is comparable to the specific yield of 0.10 determined from the aquifer tests of Hart and others (1976). Total groundwater storage for the panhandle and northwest parts of the Ogallala aquifer were calculated by multiplying the mean estimated specific yield of 0.09 by the aquifer area and the mean saturated thickness (fig. 19; table 5)

Hydrologic boundaries in the conceptual model represent real-world sources (inflows) and sinks (outflows) of water to and from the aquifer. Boundaries that act as both inflows and outflows may be referred to as “net inflows” or “net outflows” depending on which flow component dominates. Hydrologic boundary flows considered in this report include recharge, irrigation return flows, well withdrawals, saturated zone evapotranspiration, streambed seepage, change in groundwater storage, lateral groundwater flows, and vertical leakage (fig. 21; table 8).

A conceptual-model water budget estimating mean annual inflows to and outflows from the panhandle and northwest parts of the Ogallala aquifer, together called the Ogallala aquifer focus area, for the 1998–2022 study period (fig. 21; table 8) was developed from the conceptual model. The balanced total inflows and outflows for the Ogallala aquifer focus area were estimated 499,143 acre-ft/yr for the 1998–2022 study period. The components of the conceptual-model water budget were estimated from analyses of available data; however, where data were unavailable or inadequate, assumptions were made from previous investigations as described in the “Hydrologic Boundaries” section. The inflow percentages for the Ogallala aquifer focus area are 44.97 percent for recharge, 1.75 percent for recharge due to irrigation return flow, and 53.28 percent for net change in groundwater storage. The outflow percentages for the Ogallala aquifer focus area are 6.39 percent for net lateral groundwater flow, 1.11 percent for net streambed seepage, and 92.50 percent for well withdrawals. Vertical leakage and saturated-zone evapotranspiration were assumed to be negligible outflow components of the conceptual-model water budget.