UNDERLYING AQUIFERS EFFECT ON RECHARGE RATES IN LOUISIANA

It appears that average recharge rate determined from discharge data for streams/rivers throughout Louisiana is partially dependent on the hydraulic conductivity of underlying aquifers. This study includes analysis of 69 watersheds for 57 streams/rivers, which have USGS stream gagging records of over 10 years and in most cases about 25 to 36 years of record between October 1, 1965 and September 30,2001. The estimates of recharge rates are determined from analysis of average daily discharge data by ranking discharges. For the USGS ranks of between 50% and 80% have been used as estimates of recharge rate. Discharge rank is from the top down. The 50% rank is the median of the 365 average daily discharges reported for each hydraulic year (October 1 to September 30). The 80% rank is an average daily discharge that is less than 80% of a hydraulic year's average daily discharges. For this study both 50% and 80% ranks were used as possible extreme end members that are an estimate of average recharge rate.

Although average precipitation in Louisiana varies from about 1220 mm/yr to 1630 mm/yr (48 inches/yr to 64 inches/yr) the average recharge rates throughout the state vary far more from about 2.5 mm/yr to 250 mm/yr (0.1 inch/yr to 10 inches/yr) when using the 80% rank and from about 13 mm/yr to 630 mm/yr (0.5 inches/yr to 25 inches/yr) when using the 50% rank. The difference of the highest and lowest value is 2 orders of magnitude for the 80% rank data and 1.7 orders of magnitude for the 50% data. This indicates that average recharge rate must be dependent on something that varies far more than precipitation. It is possible that this variation of nearly two orders of magnitude could be a result of the underlying aqufiers varying hydraulic conductivity. The variation of hydraulic conductivities for aquifers in Louisiana is about 1.4 orders of magnitude. The most conductive Louisiana aquifer is the Mississippi Alluvial Aquifer that has a geometric mean hydraulic conductivity of 40 m/day (130 ft/day) and the least conductive aquifer is the Wilcox Aquifer that has a geometric mean hydraulic conductivity of 1.5 m/day (4.9 ft/day). The range of aquifer hydraulic conductivities is fairly similar to the range of average recharge rates that have been estimated.