Evaluation of rainfall and epic simulations for estimating probabilities of nutrient loss from dairies

Abstract

Site-specified information about climates, soil, and management is needed to estimate the probability of nutrient release from large livestock operations in central Texas. Actual weather information and field conditions and management were analyzed and the probability of nutrient transport in runoff was estimated for two dairies during 1995 and 1996. The primary objectives were to compare the probabilities of actual and simulated rainfall for a long period in central Texas and evaluate the use of the Erosion Productivity Impact Calculator (EPIC) for estimating water runoff and nutrient transport from individual fields and whole dairies in central Texas. A secondary objective was to write a routing program that would quantify runoff and nutrient transport between individual fields. The estimates of probabilities of nutrient loss from EPIC simulations were evaluated in relation to similar estimates from probabilistic risk assessment (PRA) on dairies. Water runoff and soluble nitrogen and phosphorus transport in runoff were generated on daily basis using EPIC for each individual field. The routing and supporting programs calculated the quantities and probabilities of water runoff and nitrate and phosphorus release for each discharge point and the entire dairy. The probability of nutrient release in runoff was 10.98% for one dairy and 11.32% for another dairy. The probabilities calculated by empirical methods in PRA fit within the range of probabilities calculated by EPIC and the routing model during a 93-year simulation that used actual rainfall records. The agreement between methods indicated both may be helpful for the purpose of evaluating agricultural impacts on water quality. The EPIC and PRA methods are useful sources of information for dairy waste management, but both methods need to be validated through site-specific monitoring of nutrient discharge from dairies.