Dairy lagoon design and management under chronic rainfall

An important feature of the ASAE revised standard practice EP403.3 DEC98 fo r the design of anaerobic lagoons for animal waste management is the additi on of normal runoff and normal precipitation on the lagoon surface between drawdown events in the calculation of runoff volume (ROV). In the previous version of this standard (EP403.2 AUG93), only the runoff and precipitation to the lagoon surface from a 25 yr/24 h-storm event were considered in ROV Lagoon systems under both standards are designed to contain runoff from a 25 yr/24 h-storm event, although significant rainfall events rarely occur a s a single wet day For Dublin, Texas, the design 24-h, 25-yr rainfall is 18 .5 cm. From 86 years with complete precipitation records, the return interv al of an 18.5 cm event from consecutive wet days is about 6 years rather th an 25. Beyond the additional runoff volume from multiple wet-days, chronic rainfall also considers the preclusion of dewatering due to wet weather con ditions. To evaluate the risk of overflow from a one-stage lagoon sized for a 1,000-head dairy considering the recurrence of chronic rainfall events, a simple lagoon water balance model was developed Actual daily rainfall for 86 years was used to estimate runoff into and precipitation on the lagoon. Drawdown management It as based on lagoon level, daily rainfall, and satur ated ground. When normal runoff and precipitation was considered in the ROV , the frequency of lagoon overflow events was within expected frequency for the 86 years of record for most sensitivity scenarios, When normal runoff and precipitation was not considered in the ROV the frequency of overflow e vents often exceeded the expected frequency emphasizing the importance of c onsidering more than just the 25 yr/24 h-Storm event in determining the run off volume of a lagoon.