Assessing the food availability and food security of countries is a critical exercise in which crop simulation models are essential. Application of crop models has been limited often to estimate yield per unit area of one or a few important field crops, whereas what is really required is the total national production of diverse crops including forages, vegetables and fruit trees that compete for limited resources of land and water. In this study a simple crop model (SSM-iCrop2; Simple Simulation Models) was set up for an entire country using a bottom-up approach such that it provides representative estimates of potential yield and other crop properties at provincial level as influenced by climate, soil, management and cultivar. The information is then used to calculate total plant production at province and country levels, as influenced by available land and water resources and by the efficiency of utilizing the resources using the concepts relative yield gap and irrigation efficiency. Iran was used as a case study to develop the modeling framework and illustrative outputs. Development of the framework resulted in accumulation of large bodies of valuable geospatial information and statistics across disciplines that are critical for analysis of plant production at a country level. The framework allows different scenarios of national plant production to be evaluated. This includes assessing the possibility of increasing national plant production via intensification, optimizing water allocation across plant species at province and country levels by changing the cropping pattern, and assessing and prioritizing possible ways of adapting a country's agriculture to limited land and water resources and climate change.