The SWAT Model

SWAT model
The SWAT model, a physically based model developed by the United States Department of Agriculture (USDA), operates at a daily time step. This model has been developed on a physical, semi-distributive, scale-basin basis for continuous time and with emphasis on soil surface processes. SWAT's sub-watersheds are divided into hydrological response units (HRUs) that have unique combinations of slope, land use, and soil type within the sub-basin and form the basic land segment for computing flow and transport. The spatial relationship between the sub-basins and their output is established continuously and ultimately ends in the outlet of the main basin. (Arnold et al. 2012).
The simulation of the karstic basin and the development of the
To enable this section, the ICRK value should change from zero to one in the basins.bsn files, then, by inserting the Sol_CRK parameter into the calibration of the model and applying changes to the sub-basins of the karstic structure, the model was set for the karstic basins. These settings are cosidered for areas of the world with Vertisol soil. In this approach, it has been assumed that these types of soils behave like karst areas, with the difference that their seams and gaps are large and small during the wet and dry periods.( SWAT IO). The function of this section is that fractures on the surface of the earth, have deep up to 50 cm in dry days, the seams and gaps are wider than the lower parts. This model is considered the seams and gaps variables with respect to the season of precipitation and temperature. In order to increase accuracy in simulating surface runoff values, penetration and time variations are important in this part of the model. Different conditions of fractures are shown in Figure 4 with respect to soil moisture variations. By entering the Bypass Flow in the model, SWAT calculates the seam volume for the soil matrix per day for each layer. the influence and runoff are initially calculated using the SCS or Green Empty method On days when precipitation occurs. If a runoff is formed, it is allowed to enter into the seams and gaps. The volume of water entering the seams and the gaps is equal to the volume of seams and gaps, and then the runoff is flowing to the surface (SWAT IO). The estimation of seams and gaps volume depend on the maximum possible gap in the soil, crop capacity, soil moisture, along with the Sol_CRK coefficient. This coefficient is estimated in the calibration step. This process occurs in daily model calculations. In this study, the SWAT model is close to the real conditions of the karstic