River discharge is the volume of water in cubic metres that flows in a river per second. River discharge is affected by precipitation (the more precipitation, the higher the discharge), hot weather (higher temperature means lower discharge as evaporation rate is high) and removal of the water from the river (abstraction) reduces river discharge.
Hydrographs show river discharge over a period of time. Hydrographs show the volume of water flowing at a certain point in a river changing over a period of time. Storm hydrographs show river discharge around the time of a storm event and only cover a short time period.
Peak discharge - the highest point on the graph, when the river discharge is at its greatest
Lag time - the delay between peak rainfall and peak discharge which happens because it takes time for the rainwater to flow into the river. A shorter lag time can increase peak discharge because more water reaches the river during a short period of time.
Rising limb - part of the graph that leads up to the peak discharge. The river discharge increases as rainwater flows into the river.
Falling limb - part of the graph after the peak discharge. Discharge is decreasing because less water is flowing into the river. A shallow falling limb shows water is flowing in the from store long after it has stopped raining.
The exact shape of a hydrograph varies with each river drainage basin and each individual storm event. A flashy hydrograph is when the it was steep, symmetrical rising and falling limbs. This is because the drainage basin has a rapid run off and not much storage capacity giving a hydrograph with a short lag time and high peak discharge.
The storm hydrograph is affected by physical factors:
Drainage basin characteristics - the physical features of the drainage basin affect lag time and peak discharge, for example: