Models are important in science to help us conceptualize better the interactions in systems. In the case of agriculture, first thing is to identity the components of the system, some of which are: soil (depth, organic and mineral content), rain (annual average, trends), sunlight (duration during growing season), temperature (average, trends), crops (hardiness to drought, floods; effect on the soil), fertilizer (source, cost), harvest window, etc. These need to be related in the model to aspects of food security, such as, population size, calorie and nutrition requirements of the population, transportation costs, storage costs, distribution, etc. I suggest you take these variables and others you decide are important and create a diagram or concept map relating them. That would be your model.