ANALYSIS AND MODELING OF JULY 2003 HYDROGRAPHS FROM THE EDWARDS AQUIFER RECHARGE ZONE

Several days of rain swept across the Edwards Aquifer recharge zone in July 2003 and provided an opportunity to investigate the hydrologic response of the aquifer. The Edwards Aquifer Authority maintains a network of water level recorders on the recharge zone in the San Antonio area as part of its Optimization Technical Studies program. Hydrographs from the recorders were analyzed and modeled to interpret the nature of aquifer in which the wells are completed (i.e., matrix or conduit) and he relationship between water quality and groundwater flow.

For each hydrograph, the Peakfit® software was used to separate the baseline conditions from the curve to isolate the change due solely to the recharge event. The resulting curve was matched to one or more pulses to calculate the lag time between the start of the precipitation and the water-level response. The hydrographs are asymmetrical pulses with long recession curves. Several hydrographs show a single pulse and relatively small response, indicating little or no connection to conduits, and five hydrographs showed a moderate response. Three showed strong responses with three pulses of recharging water moving past the well that originated from different locations in the recharge zone and arrived at different times.

EPANET2 is software that simulates hydraulic and water quality behavior in pressurized pipe networks. It calculates friction head loss using the Hazen-Williams, Darcy-Weisbach, or Chezy-Manning formulas, which is appropriate for modeling karst systems. It was used to model selected hydrographs for both hydraulic head and water quality. By simulating karst flow paths as pipelines, the models show the relationship between diameter, roughness, length, water quality, and other characteristics.