Adjust the slider to vary the steepness of the ground and watch how the water reacts.
The flow speed or velocity of the water is mainly determined by the slope of the terrain the rain falls on. The steeper the slope the faster the water flows. Try rolling marbles over a plank: the steeper the plank, the faster they will roll.
Next to flow velocity you also have rise velocity: the speed with which the water level rises. The unit to describe the rising speed is usually centimeter per hour or centimeter per day or something similar.
Rise speed is determined by a combination of factors
The infiltration capacity of the surface: rocks but also streets and buildings have a very low infiltration capacity. Nearly all of the precipitation (rain, snow) will then run off or will stay and form puddles and ponds (ponding).
Slope of the terrain: on slopes water will not form a layer. Water will always flow down and will gather at the lowest point or points and there it will rise.