What mechanisms drive changes in stream flow over time?

Changes in stream flow over time are primarily driven by precipitation, evaporation, and human activities such as damming and irrigation.

Precipitation is the most direct factor affecting stream flow. When it rains, water is added to the stream, increasing its flow. The amount of rain, its intensity and duration, and the area over which it falls all influence the stream flow. For instance, a heavy downpour over a large area will cause a more significant increase in stream flow than a light drizzle over a small area. Snowfall also contributes to stream flow, but its effect is delayed until the snow melts.

Evaporation also plays a crucial role in stream flow. When water in the stream is heated by the sun, it turns into vapour and rises into the atmosphere, reducing the amount of water in the stream. The rate of evaporation depends on several factors, including temperature, humidity, wind speed, and the surface area of the water. In hot, dry, windy conditions, evaporation can significantly decrease stream flow.

Human activities can also have a significant impact on stream flow. For example, the construction of dams can dramatically alter the flow of a stream by storing water in a reservoir and releasing it in a controlled manner. This can lead to changes in the timing and magnitude of peak flows, as well as reductions in low flows. Irrigation, another human activity, can also affect stream flow by diverting water from the stream for agricultural use. This can reduce the amount of water in the stream, particularly during dry periods when irrigation demand is high.

In addition to these primary factors, there are also secondary factors that can influence stream flow. These include the type and amount of vegetation in the watershed, the geology and topography of the land, and the characteristics of the soil, such as its permeability and water-holding capacity. These factors can affect how quickly and how much water reaches the stream after a rain event, as well as how much water is lost to evaporation and transpiration.