Excess Runoff Contribution to Daily Discharge Frequency of Channel Catfish Farms in Alabama

Abstract

Abstract Aquaculture facilities producing more than 45,454 kg/yr and discharging 30 d or more annually, excluding excess runoff, are designated concentrated aquatic animal production facilities by the United States Environmental Protection Agency (USEPA), and they require National Pollutant Discharge Elimination System permits. Three individual ponds and three of four channel catfish farms in Alabama discharged 30 d or more annually. USEPA did not define excess runoff or explain how to adjust for it. There is normally at least 7.6 cm storage depth in ponds when rainfall begins. Runoff that causes overflow should be considered excess runoff. The year in Alabama can be divided into three soil moisture epochs. The 24‐h rainfall amounts necessary to cause ponds to overflow under normal conditions for each epoch were estimated by water budgeting. For watershed ponds, daily rainfall greater than amounts given below could cause excess runoff: Period Blackland Prairie region (cm) Other regions (cm) December to April 2 3.3 May to August 3.8 5.8 September to November 5.6 7.5 Daily rainfall above 7.5 cm could cause overflow during any soil moisture epoch from embankment ponds in all regions. Ponds discharged for 2–5 d after some rainfall events, and discharge for up to 4 d after an excess rainfall day should be excess runoff.