RuFaS: A next generation tool to support environmentally integrated dairy farm management

US dairy production provides vital food and nutrition to people, yet comes with significant environmental costs. Efficient use of nitrogen (N) on dairy farms remains a pressing concern as losses to surface water contribute to eutrophication, losses to groundwater damage human health, and gaseous losses to the atmosphere contribute to climate change. To determine optimal farm-level management that balances production with environmental impact, tools are needed that give producers options to adapt their farms while addressing the complexity and variability inherent in modern dairy systems. To meet this need, we are developing the RuFaS (Ruminant Farm Systems) model, a next-generation decision support tool that is modular, adaptable, open-source, and available to a wide range of users including dairy industry professionals, natural resource managers, and researchers. To guide the development of RuFaS, we seek the most parsimonious, process-based model to achieve our task while avoiding unneeded complexity and minimizing calibration. Thus, we base the soil and cropping components of RuFaS on simple representations of soil hydrologic and biogeochemical processes, such as the Curve Number approach for runoff and the Hargreaves method for evapotranspiration. Here we present a preliminary evaluation of RuFaS that focuses on performance of N cycling and water balance. We use an observational dataset consisting of soil N, event-based runoff and leachate N, and gas N flux measurements taken in a dairy barnyard over a 5 year period. The dataset features large periodic inputs of manure N and no vegetation, and thus a system with large N losses. We compare simulated and measured N quantities at both daily and annual timescales, and also compare to the performance of IFSM (Integrated Farm System Management), a first generation dairy farm model. As this work represents a first stage in model development, ongoing efforts focus on integration of soil and cropping systems with animals, feed storage, and manure management.