MODIFICATION AND USE OF DRAINMOD TO EVALUATE A LAGOON EFFLUENT LAND APPLICATION SYSTEM

Abstract

Traditionally, lagoon design has considered waste inflow, sludge accumulation, individual event rainfall associated with the 25-year, 24-hour storm, and sufficient temporary storage to handle excess rainfall during non-irrigation periods. Excess rainfall was defined as the “average” or “normal” rainfall in excess of evaporation during the non-irrigation (drawdown) period. North Carolina experienced a series of tropical storms and hurricanes in 1995 that resulted in several lagoon overtoppings; however, none of the storms individually satisfied the 25-year, 24-hour criterion. These storms raised questions as to whether the 25-year, 24-hour criterion presented the appropriate design constraint to prevent lagoon overtopping or whether the cumulative impact of prolonged rainy periods (referred to herein as “chronic” rainfall) was a greater threat. To evaluate the validity of existing lagoon design criteria and emergency action measures proposed by the North Carolina Soil and Water Conservation Commission, the irrigation component of the field hydrology model DRAINMOD was modified to consider animal waste lagoon constraints of chronic rainfall, crop nitrogen utilization, and emergency lagoon operational measures. The modified DRAINMOD was used to evaluate lagoon design and operational guidelines in effect in eastern North Carolina at the time of the 1995 lagoon breaches and the proposed 1999 emergency measures. Model simulation results showed that prolonged wet periods in the winter that result in high moisture surplus are the most likely cause of excessively high lagoon stage or overflow. To minimize the occurrence of elevated lagoon stage and eliminate the risk of overflow, model results also showed that the design temporary storage criterion should be increased to account for chronic rainfall excess between drawdown periods. Intense storms with short durations, such as the catastrophic design (25-year, 24-hour) storm, mainly occurred in the summer and usually posed no risks to lagoon overflow because these events typically occurred at a time when lagoons were traditionally drawn down to their lowest allowable stage. Using a constant average nitrogen concentration for lagoon wastewater resulted in fewer irrigation applications, which in turn resulted in more frequent high lagoon stage and more overflows. Lagoon spills resulting from extreme weather conditions could be avoided by applying wastewater more frequently and temporarily suspending the crop nitrogen limit in wet years without exceeding soil hydraulic limits.