Concerns
regarding the ability of Converse Lake to meet current and
future water demands for the city of Mobile, Alabama during
drought conditions have prompted this cooperative study between
the Mobile Area Water and Sewer System and the U.S. Geological
Survey.
The
firm yield of Converse Lake was estimated using the Massachusetts
Department of Environmental Protection’s
firm-yield-estimator (FYE) model, which recently was refined
by the U.S. Geological Survey. The model uses a mass-balance
approach to determine the maximum average daily withdrawal
rate that can be sustained during a period of record that
includes a drought of record. If the reservoir is in contact
with an aquifer, the FYE also includes routines that estimate
the volume of ground-water and surface-water exchange between
the aquifer and the reservoir.
The
average daily firm yield for Converse Lake was estimated
to be 63 million gallons per day (with an uncertainty range
of 52 to 74 million gallons per day) using the FYE routine
that does not include ground-water exchange between the reservoir
and the adjacent aquifer. If water exchange occurs between
the aquifer and the reservoir, an increase in the volume
of water available to the reservoir may occur during a drought.
Aquifer properties required by the FYE were estimated by
model calibration to observed water levels that occurred
during the drought of 2000. When ground-water exchange between
the reservoir and adjacent aquifer is included, the average
daily firm yield increased to 99 million gallons per day
with an uncertainty range of 77 to 132 million gallons per
day.
The
average daily firm yield of 99 million gallons per day, based
in part on calibrated values for aquifer transmissivity and
storage, can be used only as a guideline until these aquifer
properties can be defined better by field investigation in
the Converse Lake watershed. |
