ASSESSMENT OF ACCEPTABLE RATES OF WITHDRAWAL TO MEET GROUNDWATER CONSERVATION STANDARDS IN THE UNCONFINED OGALLALA AQUIFER, NORTHERN TEXAS PANHANDLE

More water is pumped from the Ogallala aquifer, with its more than 70,000 irrigation wells, than from any other aquifer in Texas. Annual recharge is only a few percent of withdrawal, however, and drainage of the unconfined aquifer has lowered the water table by as much as 58 m, about 1.2 m/yr. Groundwater supplies that remain in the eastern half of the northern Texas Panhandle where the landscape is naturally less suited to large-scale irrigation are now being targeted for pumping at rates of more than 0.35 km³/yr. A proposal by one water development company, headed by a person made famous by oil-company takeover attempts in the 1980s, to pipe groundwater across the state to various water markets has attracted more media attention than have other consortia or the project by a river authority to use the groundwater to dilute its surface water being piped to High Plains cities. The groundwater conservation district and regional water planning group have a goal to have at least half of the 1998 water column in the Ogallala aquifer remaining in 2050. Traditionally, the district has granted permits allowing withdrawals at a rate of 0.3 hectare-m per year for each hectare of water right. A concern is that the conservation standard will not to be met if the new large projects pump at this rate.

A regional model of groundwater flow in the Ogallala aquifer was developed and used to evaluate what pumping rates would allow the conservation goal to be met. The model drew on as much spatially varying data as possible. The combination of mapping hydraulic conductivity using depositional systems and recharge using precipitation and soil factors was important to minimizing model error. Reaching a low model calibration error with negligible parameter adjustment gave confidence to the model's application to aquifer management.

Results of the model predict that a pumping rate of 0.15 hectare-m per year for each hectare of water right would allow about half of the projects' areas to meet the aquifer conservation goal, but rates would have to be as low as 0.08 hectare-m per year for the goal to be met if all expected projects were developed. A separate spreadsheet analysis yielded similar conclusions, showing that about half of each project's groundwater withdrawal would be drained from adjacent properties.