| A computer simulation of the yield response of grass/legume mixture pastures to soil water, temperature, and nitrogen was developed. The model operates on a daily time step over a growing season and compiles harvest and season totals. The model consisted of seven submodels: soil water content, soil temperature and heat flow, root mass growth and distribution, soil nitrogen dynamics, grass growth, legume growth, and grass/legume interactions. All seven submodels were necessarily coded into a single computer program because of their interactions with each other on a daily basis. Much of the model complexity is due to the large number of inputs required for each submodel, and due to the interactions and interdependencies between the various submodels. Despite this, flexibility and ease of use was built into the model operation, and the user interface.; Most of the model components were created using algorithms that were verified by previous research. Crop coefficients were based on modeled leaf area index. The modeled competition between grasses and legumes was based on their competition for light, which is based on leaf area index, leaf orientation, leaf transmission coefficients, and crop height; soil water, which is based on root distribution and growth; and nitrogen, which is based on the legumes' ability to fix nitrogen and the gasses' ability to benefit from fixed and mineralized nitrogen.; The model was shown to give reasonable results and a sensitivity analysis on several key parameters was performed. The model was calibrated with data collected from a rotationally grazed pasture in Cache County, Utah, and validated against data collected from a harvested high-elevation site in Rich County, Utah. As with all natural systems models, although the model generally matched measured results, the measured data showed significant variability when compared to the modeled results. |
