The Potential Impact of Water Reallocation on Retention and Chlorophyll a in Weiss Lake, Alabama

Abstract

ABSTRACT Water supply demand has increased in North Georgia and prompted government officials to propose a water reallocation plan that would permit two reservoirs upstream from Weiss Lake, Alabama to increase water withdrawals nearly three fold. Hydrologic modeling predicted lower flows in the Coosa River, the primary tributary of Weiss Lake, during average to below average flows (exceedences from 50 to 90%). Consequently, we predicted retention (reservoir volume/discharge) would increase in Weiss Lake under reallocation and using data from 1989 to 2000 as a baseline, we estimated the effect of longer retention on chlorophyll a concentrations (CHLA) and water clarity. CHLA measured during the growing season (April to October) was positively correlated (P < 0.01) to retention in both upstream and downstream reservoir regions and retention accounted for 24 to 28% of the variation in CHLA in linear and non-linear regression models. Under the reallocation scheme, increased retention (as predicted by HEC-5) could potentially cause an increase in CHLA of 8 to 15% compared to historic conditions if CHLA increased linearly with retention. If retention should increase 10 days with water reallocation, we estimated CHLA could increase about 20 to 30% under a “worst-case scenario”. Water reallocation would have little or no impact on water clarity as Secchi transparency did not increase with lower CHLA concentrations. If longer retention does occur in Weiss Lake with water reallocation, a greater frequency of hypereutrophic conditions is expected.