How to determine the direction of water level boundary?

Like Figure (a), I want to set a water level boundary condition in a specific grid. For example, the red grid in Figure (b). Assume that it is a river, whose flow direction is from southwest to northeaset. How can I choose the direction (S, E, W, N) in Figure (a), that is, the option “3,4,5,6”?

In such a case the correct direction can be determined by looking at the cell map, as shown here: In your case it is likely to be from the northern edge of the cell, so option 6, but referring to the cell map and checking the I, J value can help confirm this when unclear. You can also see more information about open boundaries here: you can look at an example model such as this one: Section 4, Figure 46, shows an example of setting an open boundary.