Hydrologic Setting

Map

This map shows the extent and variability in aquifer thickness across central Oklahoma, with the thickest saturated zones occurring in parts of Oklahoma and Cleveland Counties. (Mashburn et al., 2019).

Regional Climate

The Central Oklahoma (Garber-Wellington) aquifer lies within a continental subhumid climate zone, characterized by hot summers and cool winters. In Oklahoma County, which encompasses a large portion of the aquifer, the average monthly temperature is about 60°F, with a gradual increase in temperature from north to south. Seasonal variation is significant: the average maximum temperature typically occurs in July at 93°F, while the average minimum temperature is recorded in January at 26°F. These temperature extremes influence evaporation rates and indirectly affect groundwater recharge processes during different times of the year (Mashburn et al., 2019).

Long-term precipitation records from NOAA Cooperative Observer Program (COOP) stations indicate a shift toward wetter conditions in the Central Oklahoma (Garber-Wellington) Aquifer region. The 1950–1979 period, representing a relatively dry historical baseline, had an average annual precipitation of approximately 33.36 inches and included two significant multi-year droughts (Mashburn et al., 2019). In contrast, the 1991–2020 climate normal for Oklahoma County shows an increase to 36.22 inches annually (“Oklahoma County Climate Summary,” 2023). May consistently receives the highest monthly precipitation, averaging 5.18 inches, while January remains the driest month at 1.33 inches. These trends suggest increased potential for groundwater recharge in recent decades (Mashburn et al., 2019).

Figure 1. Monthly average precipitation for the Central Oklahoma (Garber-Wellington) Aquifer across three climate periods: 1950–1979, 1980–2009, and 1991–2020. Precipitation increased in most months during the latter two periods, suggesting a shift toward wetter conditions that may influence groundwater recharge. All data are based on NOAA Cooperative Observer Program (COOP) stations, compiled from Mashburn et al. (2019) and OCCS (2023).

These climatic patterns—especially precipitation timing and intensity—play a vital role in aquifer behavior. Cooler months allow more effective recharge, while hot summers can increase losses to evaporation and transpiration before water infiltrates. Understanding these dynamics is essential to modeling and managing groundwater resources in the region.

Recharge

Recharge to the Central Oklahoma (Garber-Wellington) Aquifer occurs primarily through two pathways: direct infiltration of precipitation and seepage from streams and lakes into the aquifer. These processes deliver water to the saturated zone and are influenced by precipitation patterns, soil and rock permeability, soil moisture storage, and land surface slope (Mashburn et al., 2019).

The U.S. Geological Survey estimated recharge across the region by analyzing streamflow in basins without regulated flow, major surface withdrawals, or wastewater discharge. In these settings, natural groundwater discharge to streams (base flow) was used as an indicator of recharge. Estimates based on streamflow measurements showed recharge rates ranging from 0.19 to 3.17 inches per year during 1987–1989, and from 0.20 to 2.02 inches per year in 2009 (Mashburn et al., 2019).

Additional estimates were made using long-term streamflow trends to assess how much water enters the aquifer over time. This approach produced higher recharge values, ranging from 1.3 to 4.1 inches per year in different parts of the aquifer. Differences between sites reflect natural variation in soil texture, land use, and aquifer characteristics like sand content and thickness (Mashburn et al., 2019).

Taken together, these findings show that both rainfall and surface water contribute to recharge, and that rates vary by location due to differences in landscape and subsurface properties. Quantifying these inputs is essential for understanding how the aquifer responds to climate and land use changes over time (Mashburn et al., 2019).

Surface Water Bodies

Several primary rivers in the Central Oklahoma (Garber-Wellington) Aquifer study area—including the Cimarron River, North Canadian River, and Canadian River—play an important role in the region’s hydrology. However, the natural flow of these rivers has been significantly altered by dams, diversions, and wastewater discharges. For example, the North Canadian River below Lake Overholser is influenced by releases from Canton Lake, Lake Overholser, and diversions into Lake Hefner Canal. The Canadian River near Bridgeport is regulated by Lake Meredith in Texas, and flow in the Deep Fork River and Little River is affected by Arcadia Lake and Lake Thunderbird, respectively (Mashburn et al., 2019).

Due to these modifications, many of the larger rivers are not ideal for estimating natural groundwater recharge. Instead, smaller, unregulated streams such as Captain Creek and Deer Creek were used in the USGS investigation to measure base flow—the portion of streamflow sustained by groundwater discharge during dry periods. These measurements are typically taken during winter months when runoff, evaporation, and transpiration are minimal. In January 2009, base flow in the study area ranged from 0.23 ft³/s at Deer Creek near Newalla to 4.46 ft³/s at Captain Creek near Wellston (Mashburn et al., 2019).

These gaining streams demonstrate that the aquifer actively contributes to surface water flow in parts of the region. While many larger rivers are hydrologically regulated, these smaller streams offer valuable insight into the natural connection between the aquifer and the surface water network.

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