the basic equations used in the swap model and practical guide of model

nowadays, it’s very usual to use these models in almost all sciences. use of these models has provided quick, appropriate and economic answers to many questions. modeling is developing in different fields of agriculture, including irrigation and drainage. field tests are useful to determine and analyze the different irrigation management but there are significant limitations. the most important limitation is the validity of the experiment that is limited by the physical conditions of the area where the experiment performes there. one of the models that is used in agricultural science is the swap model that is a model for simulating of soil, water, atmosphere and plant. this model is able to analyze interactions between water movement, growth of plant and transfer of solutions, prediction of yield under different regimes of water and salt, long-term simulation and irrigation scheduling. although swap model is able to simulate relations between soil, water, atmosphere and plant under different management field conditions completely but it has limitations too. one of these limitations is many numbers of input data. so far many researches have been done using this model but by searching on the scientific articles, there is no complete source that offers the basic equations used in the model and also the practical guide is not available. in this article, according to the model guide, the equations that used for model calculations, has been described. also a useful and practical guide for users who want to use this model has been presented.