A digital model was used to provide a quantitative description of the Ogallala aquifer in Texas County, Oklahoma, and to predict saturated thickness and water in storage from the aquifer at specified future times. The Ogallala aquifer, which consists of unconsolidated sand, gravel, and clay, is the principal source of ground water in Texas County. Saturated thickness ranged from 0 feet to over 600 feet. The estimated value used for specific yield in most of the areas was 0.15 but 0.05 was used in some places. Hydraulic conductivity ranged from 0 to more than 200 feet per day, and recharge ranged from 0.2 to 2.2 inches per year.
Irrigation pumpage was estimated using crop acreage and estimate of irrigation requirements. For projection simulations with large stress, a reasonable maximum stress using a minimum of 4 wells per square mile and 1972 pumping rate per well, if saturated thickness was more than 38 feet, was used.
Four types of boundaries were used in the model. They are:
- A zero-flux (impermeable) boundary on the perimeter of the modeled area.
- A constant-head boundary for a reach of the Cimarron River.
- A boundary which is a constant-head boundary initially but converts to an impermeable boundary (depending on the potentiometric gradient at the boundary) for a reach of Beaver River, Palo Duro Creek, and south of Palo Duro Creek.
- A boundary which is a partially penetrating stream with leaky-stream bed for parts of Beaver River and Coldwater Creek.
The base period for calibration was 1966. The model was calibrated by a simulation from 1966 to 1968 in which pumpage was modified until the 1968 calculated heads matched closely the 1968 observed heads. The model was verified by a simulation from 1966 to 1972, using the 1966 to 1972 pumpage stress, in order to determine the degree of conformity between 1972 calculated heads and 1972 observed heads. The agreement was acceptable.
