This report describes a study of the hydrology, hydrogeological framework, numerical groundwater-flow models, and results of simulations of the effects of water use and drought for the Beaver-North Canadian River alluvial aquifer, northwestern Oklahoma. The purpose of the study was to provide analyses, including estimating equal-proportionate-share (EPS) groundwater-pumping rates and the effects of projected water use and droughts, pertinent to water management of the Beaver-North Canadian River alluvial aquifer for the Oklahoma Water Resources Board.

The Beaver-North Canadian River alluvial aquifer consists of unconsolidated sand, gravel, silt, and clay in varying proportions that underlies the Beaver and North Canadian River Valleys for approximately 175 miles (mi) from the Oklahoma Panhandle to the western edge of Oklahoma City in central Oklahoma. The aquifer as delineated for this study varies from 4 to 12 mi wide and is as thick as 308 feet (ft) in the northwest where the aquifer includes the Ogallala Formation.

There are two distinct but in most areas hydraulically connected alluvial units that compose the Beaver-North Canadian River alluvial aquifer: a Quaternary-age topographically higher terrace deposit and a topographically lower, younger alluvium along the active river channel that includes active and Quaternary-age alluvium. The Beaver River composes the headwaters of the North Canadian River, which begins at the confluence of the Beaver River and Wolf Creek. The aquifer is divided for water management into two geographic areas: Reach I upstream from Canton Dam and Reach II downstream from Canton Dam. Reach I covers an area of approximately 874 square miles (mi²), and Reach II covers an area of approximately 371 mi². The Beaver-North Canadian River alluvial aquifer crosses several climatic zones, from semiarid in the west to continental subhumid in the east. Mean annual precipitation varies from 23.5 inches (in.) in the western part of this aquifer to 35.7 in. in the east.

Surface-water demands were met through numerous temporary and permanent surface-water diversions from the Beaver and North Canadian Rivers during the period of study. During the study period, seven diversions removed a mean annual 2,000 acre-feet (acre-ft) of water from Reach I. There were 14 diversions from Reach II with a mean annual permitted volume of approximately 81,000 acre-ft, including diversion into the Lake Hefner Canal for the Oklahoma City public water supply. During the period of this study, 17 temporary surface-water diversion permits were active in Reach I, with total permitted volumes of 2,000 acre-ft, and 41 diversions were active in Reach II, with total permitted volumes of 38,000 acre-ft. The total water use for each temporary permit was assumed to be taken over the 3-month period allotted to temporary withdrawal permits.

The groundwater-use analysis full period of record, 1967–2011, was divided into two sub-intervals because of varying water use, 1970–80 and 1981–2011. Groundwater use in Reach I and Reach II was substantially greater from 1970 to 1980 compared to the rest of the period, and the sub-period 1981–2011 was used because this period includes recent population growth and modern irrigation methods. The total mean annual groundwater use in Reach I was 15,309 acre-feet per year (acre-ft/yr) during 1967–2011; 20,724 acre-ft/yr during 1970–80, and 13,739 acre-ft/yr during 1981–2011. Total mean annual groundwater use in Reach II was similar but slightly less than in Reach I, with 14,098 acre-ft/yr during 1967–2011; 19,963 acre-ft/yr during 1970–80; and 12,285 acre-ft/yr during 1981–2011.

Irrigation composed 72 percent of groundwater use in Reach I and 48 percent of groundwater use in Reach II during the 1967–2011 period. Public water supply was a much smaller proportion of total groundwater use in Reach I (15 percent) than in Reach II (39 percent). The proportion of groundwater use for power was 10 percent in Reach I and 5.2 percent in Reach II. All other water-use categories in Reach I only composed 2.2 percent of groundwater use in Reach I. In Reach II, industrial, mining, and commercial categories combined accounted for 4.4 percent of groundwater use; recreation, fish, and wildlife groundwater use accounted for 2.3 percent; and nonirrigated agriculture accounted for 1.5 percent of groundwater use.

Permian-age bedrock underlies the Beaver-North Canadian River alluvial aquifer. In the east, the Dog Creek Shale, the Duncan Sandstone, and the Blaine and Chickasha Formations, none of which are notable sources of groundwater in the study area, underlie the Beaver-North Canadian River alluvial aquifer. In the northwestern part of Reach I, bedrock is composed of the Rush Springs and Marlow Formations, which are productive aquifers in some areas. The Cloud Chief Formation is not a source of groundwater.

One hydrogeological unit was delineated in the Beaver-North Canadian River alluvial aquifer, composed of the terrace deposits and alluvium, with limited flow between this unit and bedrock units. Groundwater in this aquifer generally flows from northwest to southeast and across the aquifer toward the Beaver and North Canadian Rivers.

Groundwater recharge from precipitation was estimated for the entire Beaver-North Canadian River alluvial aquifer and then itemized for both reaches by using a soil-water-balance (SWB) model. At two locations in Reach I, a water-table fluctuation method was used to estimate local recharge. Total mean annual groundwater recharge from the soil-water-balance method was estimated to be approximately 136,400 acre-ft in Reach I and 82,400 acre-ft in Reach II; the mean annual recharge for both reaches combined was approximately 218,800 acre-ft. Two sites in Reach I located at observation wells with continuous water-level measurements and nearby streamflow-gaging stations with precipitation gages were used to estimate the percentage of precipitation that becomes groundwater recharge. The Woodward site was located at observation well OW-4 near the Woodward, Okla. (07237500), streamflow-gaging station. Total precipitation and recharge for the Woodward and Seiling sites were calculated for the water year 2013. The Woodward site had a total of 14.18 in. of precipitation and 6.3 in. of recharge was calculated, equaling 44 percent of precipitation. The mean percentage of precipitation that was estimated to become recharge in the SWB model for the period 1980–2011 at that location was 9.2 percent, although adjacent SWB-model cells were as high as 20 percent of precipitation. The Seiling site had a total of 26.84 in. of precipitation during the water year 2013, and a total of 6.9 in. of recharge was estimated, equaling 25.9 percent of precipitation. At the Seiling site, the mean percentage of precipitation that became recharge in the SWB model for the period 1980–2011 was 23.0 percent.

The principal inflow to the Beaver-North Canadian River alluvial aquifer was estimated to be surface recharge from precipitation, and plant evapotranspiration was estimated to be the greatest discharge, followed by stream and lake base flow, groundwater pumping, and flow to seeps and springs along the eastern margin of the aquifer. Reach I also included inflow from the High Plains aquifer as lateral inflow of groundwater, though this flow was estimated to be a very minor component of the total water budget. Most of the Beaver and North Canadian Rivers were determined to be gaining streamflow from groundwater, but several reaches in Reach I upstream from Wolf Creek were determined to be losing streamflow through infiltration to the aquifer.

Aquifer hydrogeologic characteristics were estimated from borehole lithologic logs, well-construction information, and published aquifer tests and during numerical model calibration. The maximum saturated aquifer thickness in Reach I was estimated to be 308 ft, and the mean thickness was estimated to be 36 ft. The maximum saturated thickness in Reach II was estimated to be 86 ft, and the mean thickness was estimated to be 29 ft. Mean hydraulic conductivity of Reach I was estimated to be 70 feet per day (ft/d) with a range of 7–279 ft/d. Mean hydraulic conductivity in Reach II was estimated to be 92 ft/d with a range of 4–279 ft/d.

Both reach models were calibrated manually by using trial-and-error adjustment of recharge, hydraulic conductivity, specific yield, and conductance of boundary conditions. The Reach I model used 28 head observations during the steady-state period of 1980 and 487 head observations during the transient period of 1981–2011. The root-mean-square error of head residuals (observed minus simulated head) was 3.86 ft, and 83 percent of head residuals were between -5 and 5 ft. The Reach II model was calibrated to 75 steady-state head observations and 134 head observations during the transient period. The root-mean-square error of head residuals for that reach was 3.58 ft, and similar to Reach I, 85 percent of residuals were between -5 and 5 ft.

Several analyses were performed by using the numeric groundwater-flow models as predictive tools, including estimating the EPS pumping rate for both reaches. The EPS is defined by the Oklahoma Water Resources Board as an annual per-acre groundwater-pumping rate that will reduce saturated thickness in half of the aquifer to 5 ft or less over a period of 20 years; additional estimates were made for periods of 40 and 50 years. Other analyses included using models to estimate the effects of groundwater pumping and a prolonged drought on groundwater in storage and streamflow and lake storage of water.

The EPS pumping rate was found to be approximately 0.57 acre-feet per acre per year ([acre-ft/acre]/yr) in Reach I and 0.73 (acre-ft/acre)/yr in Reach II for a 20-year period. For a 40-year period, the annual EPS pumping rate was determined to be 0.54 (acre-ft/acre)/yr in Reach I and 0.61 (acre-ft/acre)/yr in Reach II. For a 50-year period, the EPS pumping rate was determined to be 0.53 (acre-ft/acre)/yr in Reach I and 0.61 (acre-ft/acre)/yr in Reach II.

Groundwater pumping at the 2011 rate for 50 years resulted in a 3.6-percent decrease in the amount of water in groundwater storage in Reach I and a decrease of 2.5 percent in the amount of groundwater in storage in Reach II. A cumulative 32-percent increase in pumping greater than the 2011 rate over a period of 50 years caused a decrease in groundwater storage of 4.0 percent in Reach I and 3.3 percent in Reach II.

A hypothetical severe drought was simulated by using aquifer recharge flow rates during the drought year of 2011 for a period of 10 years. All other flows including evapotranspiration and groundwater pumping were set at estimated 2011 rates. The hypothetical drought caused a decrease in water in aquifer storage by about 7 percent in Reach I and 7 percent in Reach II. Another analysis of the effects of hypothetical drought estimated the effects of drought on streamflow and lake storage. The hypothetical drought was simulated by decreasing recharge by 75 percent for a selected 10-year period (1994–2004) during the 1980–2011 simulation. In Reach I, the amounts of water stored in Canton Lake and streamflow at the Seiling, Okla., streamflow-gaging station were analyzed. Streamflow at the Seiling station decreased by a mean of 75 percent and was still diminished by 10 percent after 2011. In Reach II, the effect of drought on the streamflow at the Yukon, Okla., streamflow-gaging station was examined. The greatest mean streamflow decrease was approximately 60 percent during the simulated drought, and after 2011, the mean decrease in streamflow was still about 5 percent. Canton Lake storage decreased by as much as 83 percent during the simulated drought and did not recover by 2011.