Calibration of Apex Model To Assess Farm-Scale Runoff For Grazing Operation and Uncertainty Analysis

Like crop production, expansion of livestock production is also crucial to meet increased food demands to cope with human population growth and ongoing climate change. As such, the use of natural plant communities for grazing herbivores has increased immensely with or without proper management in rangeland or cropland. With the advancement of science and technology, ranchers, grazers, scientists, and governments recently recognized proper grazing management for the conservation of natural resources to maximize productivity without harming the socio-ecological long-term sustainability of croplands. This study concentrates on developing a methodology to preserve natural processes such as runoff at the farm outlet while maximizing the biomass during scheduled grazing operations. This study validates the applicability of this approach for quantitative and qualitative assessment of seasonal and interannual hydrology of the humid area, South Central United States, influenced by grazing operations in grassland and cropland. For this, we used the framework established in the recently modified crop simulation model, Agricultural Policy Extender (APEX) model relying on available soil, weather, and climate datasets and published databases on management practices for multiple grazing operations. Specifically, the main objectives are to: 1) calibrate the modified APEX for continuous grazing operation based on the available runoff datasets; 2) quantify and compare the model's performance in terms of runoff, biomass, temperature, water, and nutrient stresses with and without grazing operations and two kinds of land use; and 3) perform the uncertainty analysis to elucidate the sensitive parameters that impact the field scale hydrology. This research highlights the need for more adaptive grazing strategies to allow the sustainability of the cropland ecosystem. It will also inform potential interactions of livestock management with the cropland's climate, weather, and hydrology. Further, we will discuss potential research avenues that generate scenarios under different climates, land use, and other humid, semi-arid, arid areas.