The ratio $N_f/N_d$ is known as shape factor, where $N_f$ is the number of flow lines and $N_d$ is the number of equipotential drops. Flow net is always drawn with a constant $b/a$ ratio, where $b$ and $a$ are distances between two consecutive flow lines and equipotential lines, respectively. Assuming that $b/a$ ratio remains the same, the shape factor of a flow net will change if the

1. upstream and downstream heads are interchanged
2. soil in the flow space is changed
3. dimensions of the flow space are changed
4. head difference causing the flow is changed