Calibrating Agro-Hydrological Model under Grazing Activities: Challenges and Implications

Authors: Nelson, Amanda; MASKEY, MAHESH - Oak Ridge Institute For Science And Education (ORISE); Northup, Brian; Moriasi, Daniel

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2024
Publication Date: 3/22/2024
Citation: Nelson, A.M., Maskey, M.L., Northup, B.K., Moriasi, D.N. 2024. Calibrating Agro-Hydrological Model under Grazing Activities: Challenges and Implications. Journal of Hydrology. https://doi.org/10.3390/hydrology11040042.
DOI: https://doi.org/10.3390/hydrology11040042

Interpretive Summary: The hydrological/crop model, Agricultural Policy Extender (APEX), was recently updated with a grazing module, APEXgraze, that provides a robust method for recommending sustainable livestock farming practices. This study developed an approach to calibrating runoff-related parameters for that new module. In this context, we calibrated the APEXgraze model to examine the impact of grazing operations on native prairie and cropland containing winter wheat and oats for study sites located in central Oklahoma. Using existing model performance criteria, we found that there are four multiple solutions in the hydrological model. Because of the non-unique solutions, different metrics revealed distinct changes in response variables. This required us to use the least objective function value to compare the behavior of two watersheds under different management and grazing intensities. According to our findings, water yield, deep percolation, sediment yield, phosphorus and nitrogen levels, and temperature stress were reduced when grazing activities were imposed, particularly in native prairies. As expected, grazing reduced forage yields in native prairies and increased crop grain yields in cropland. This study offers several research avenues in designing farm management strategies and elucidating different scenarios under different climate and management conditions.

Technical Abstract: The Agricultural Policy Extender (APEX) model was recently updated with a grazing module, APEXgraze, that provides a robust method for recommending sustainable livestock farming practices. This study developed a combinatorial deterministic approach to calibrating runoff-related parameters, assuming each parameter follows a normal distribution in space. In this context, we calibrated the APEXgraze model to examine the impact of grazing operations on native prairie and cropland containing winter wheat and oats for study sites located in central Oklahoma. Using existing model performance criteria, we found that there are four multiple solutions in the hydrological model, showing an inherent equifinality. The calibrated results showed that runoff representations had coefficients of determination and Nash-Sutcliffe efficiencies of greater than 0.60 in both watersheds regardless of grazing. Because of non-unique solutions, different metrics revealed distinct changes in response variables. For brevity, we concentrated on the least objective function value to compare the behavior of two watersheds under different management and grazing intensities. According to our findings, water yield, deep percolation, sediment yield, phosphorus and nitrogen levels, and temperature stress were reduced when grazing activities were imposed, particularly in native prairies, although these changes were less pronounced in croplands. This observation could be that cropland receives less frequent grazing than native prairies. As expected, grazing reduced forage yields in native prairies and increased crop grain yields in cropland. We envision that this study offers several research avenues in designing farm management strategies and elucidating different scenarios under different climate and management conditions.