What factors cause variation in river discharge levels?

River discharge levels can vary due to factors such as precipitation, evaporation, transpiration, and human activities.

Precipitation is a primary factor that influences river discharge levels. When it rains, the water that falls on the land surface can either infiltrate into the ground or flow over the surface into rivers and streams. The amount of rainfall and its intensity can significantly affect the volume of water that reaches the river. For instance, heavy rainfall can cause a rapid increase in river discharge, leading to flooding. Conversely, during periods of low rainfall or drought, river discharge levels can decrease significantly.

Evaporation and transpiration also play a crucial role in determining river discharge levels. Evaporation is the process by which water changes from a liquid state to a gaseous state, while transpiration involves the movement of water within plants and its evaporation from aerial parts, such as leaves. Both processes can reduce the amount of water available in a river system. High temperatures and strong winds can increase the rate of evaporation, leading to lower river discharge levels. Similarly, areas with dense vegetation can experience high rates of transpiration, which can also reduce river discharge.

Human activities can also significantly impact river discharge levels. For instance, the construction of dams and reservoirs can alter the natural flow of rivers, often reducing downstream discharge levels. Similarly, water extraction for irrigation, industrial use, or domestic consumption can significantly decrease the amount of water available in a river system. On the other hand, urbanisation can increase river discharge levels due to increased surface runoff from impermeable surfaces like roads and buildings.

Lastly, the physical characteristics of a river's catchment area, such as its size, shape, and slope, can influence discharge levels. Larger catchment areas can collect more water, leading to higher discharge levels. Steeper slopes can increase the speed at which water flows into the river, potentially increasing discharge. Conversely, flatter areas may allow more water to infiltrate into the ground, reducing the amount that reaches the river and thus lowering discharge levels.