Qualitative and Quantitative Assessment of Farm-Scale Runoff As Response To Grazing Operation

Due to increasing stressors such as growing urbanization and climate change, producers seek to develop innovative scientific approaches to improve the productive capacity and resilience of essential services including vital functions of hydrological cycles, improved crop or cattle production, increasing sustainability of farm systems, etc. However, overgrazing or unmanaged grazing operations lead to grassland degradation, reduced vegetation cover, degradation of topsoil, disrupted natural processes, and polluted waterways with fecal waste. Therefore, grazing management is the most effective strategy to support a sustainable ecosystem. To determine the effectiveness of management strategies, we developed a farm-scale hydrological model employing the widely used Agricultural Policy Extender (APEX) model, recently updated with features of grazing operations. This model simulates the runoff process and water quality at the farm scale as a response to prescribed grazing operations, relying on available soil, weather, and climate datasets and published databases on management practices. The objectives of this study were to: 1) demonstrate the capability of the APEX model in simulating runoff, sediment, and nutrients at the farm scale from the proper grazing operation schedules; 2) evaluate the impact of grazing operation on runoff quantity and quality at the outlet of the field and 3) assess the sensitive parameters and indicators that are essential for sustainable agroecosystems. For this purpose, we simulated the processes mentioned above on two fields: one with native prairie grass and the other with the wheat crop with and without grazing operations. We evaluated our modeling efforts by calculating performance indicators like Nash-Sutcliffe Efficiency, coefficient of determination, and percent bias using 20 years of measured runoff quality and quantity data. Such a protocol can help guide decision-makers, ranchers, and grazers with farm-scale water quality and quantity assessments.