The land that a river drains is called the catchment, and a river of this size could collect the rain that falls on an area of several hundred square kilometres. The fate of each drop of rain depends on where and when it lands. Some of the rain evaporates back into the atmosphere from the surface of plants.
Water hitting the ground can also evaporate, as well as soak into the soil.
Once in the soil it can be used by plants before being passed back into the atmosphere. The amount of rainfall that becomes run-off depends on how much rain there is and when during the year it falls. It also depends on the type of land on which it falls. This doesn t happen, so there must be some other source of water. To understand we need to follow the path of raindrops which reach the ground surface and pass into the soil.