Hi, I am not sure. But, in my opinion, it is zero on the first day of the simulation period (SWAT is a process-based model), and a 3-years warm-up period is recommended (more or less) for these issues. Then, on the first day of the 4th year (assuming that three years were warm-up), SWo is the soil moisture of the previous day (the last day of the 3rd year).

In other words, you should not consider some years at the beginning of your simulation (e.g., three years).

Mamad

thank you sir mohammed for your response still i need more answer

kindly check the reference. Tripathi MP, Raghuwanshi NS, Rao GP (2006) Efect of watershed subdivision on simulation of water balance components. Hydrol

Process 20(5):1137–1156. https:// doi. org/ 10. 1002/ hyp. 5927