Real and Apparent Daily Springflow Fluctuations during Drought Conditions in a Karst Aquifer, Barton Springs Segment of the Edwards Aquifer,Central Texas

摘要

Flow from Barton Springs is one of the most important measures of the overall condition of the Barton Springs segment of the Edwards Aquifer. Resource management rules (e.g., drought declaration triggers) have been developed that depend on accurate springflow data. In 1978, the U.S. Geological Survey (USGS) established a stagedischarge relationship between a USGS monitor well near the main spring, and manual discharge measurements taken primarily below Barton Springs pool in Barton Creek near its confluence with Lady Bird Lake. The stage-discharge relationship is very sensitive to fluctuating water levels in the reservoir. During the summer of 2011,fluctuations of up to 1 ft from Lady Bird Lake appeared to influence the stage-discharge relationship and manual measurements, which varied up to 30% during low-flow conditions. This is an influence previously undocumented. To understand this influence the USGS and others manually measured flow over a 24-hr period beginning on September 2,2011, using acoustic Doppler velocimeters. Nineteen manual measurements collected over a 24-hr period ranged from 15.6 to 21.8 ft~3/s (cfs), while lake levels fluctuated about 0.6 ft. Results indicate that the lake stage influences the depth of the profile in Barton Creek with high lake levels producing higher apparent discharge values. Rapid flux in lake stage also influences the velocity along the profile such that a rapid rise generally reduced the natural velocity, producing smaller apparent discharge values, and vice versa. Measurements can now be planned to occur during steady-state conditions (low lake level), usually in the early morning hours. In addition to influences on manual measurements, lake stage may be providing some backpressure on the pool and aquifer, producing daily fluctuations in the USGS well, and affecting the stage-discharge relationship in the monitor well.