River discharge is defined as the volume of water passing a measuring point or gauging station in a river in a given time. It is measured in cubic metres per second (cumecs). The overall discharge from the drainage basin depends on the relationship between preciptiation and storage factors and can be summarised as follows:
Drainage basin discharg= precipitation – evapo-transpiration + or – changes in storage
Discharge can be illustrated using hydrographs. These can show annual patterns of flow ( the river regime) in response to climate. Short-term variations in discharge are shown using a flood of storm hyrdograph.
The storm hydrograph
The storm hydrograph (shown to the left) shows variations in a river’s discharge over a short period of time, usually during a rainstorm. The starting and finishing level show the base flow of the river. As storm water enters the drainage basin the discharge rises, shown by the rising limb, to reach the peak discharge, which indicates the highest flow in the channel. The receding limb shows the fall in the discharge back to the base level. The time delay between maximum rainfall amount and peak discharge is the lag time.
Factors affecting a river’s discharge
Rock and soil type
· Permeable rocks ad soils (such as sandy soils) absorb water easily, so surface run-off is rare
· Impermeable rock and soils (such as clay soils) are more closely packed. Rainwater can’t infiltrate, so water reaches the river more quickly
· Pervious rocks (like limestone) allow water to pass through joints, and porous rocks (like chalk) have spaces between the rock particles
· In urban areas, surfaces like roads are impermeable – water can’t soak into the ground. Instead, it runs into drains, gathers speed and joins rainwater from
other drains – eventually spilling into the river
· In rural areas, ploughing up and down (instead of across) hillsides creates channels which allow rainwater to reach rivers faster increasing discharge
· Deforestation means less interception, so rain reaches the ground faster. The ground is likely to become saturated and surface run-off will increase
· The amount and type of rainfall will affect a river’s discharge
· Antecedent rainfall is rain that has already happened. It can mean that the ground has become saturated. Further rain will then flow as surface run-off towards the river
· Heavy continual rain, or melting snow, means more water flowing into the river
· Steep slopes mean that rainwater is likely to run straight over the surface before it can infiltrate. On more gentle slopes infiltration is more likely.
· Hot dry weather can bake the soil, so that when it rains the water can’t soak in. Instead, it will run off the surface, straight into the river.
· High temperatures increase evaporation rates from water surfaces, and transpiration from plants – reducing discharge
· Long periods of extreme cold weather can lead to frozen ground, so that water can’t soak in