Rivers are highly dynamic and variable systems. The regime of a river is one way that we can measure its activity over time and space. There are many factors that can affect the river regime, including changes in the water cycle, temperature and the drainage basin’s characteristics.
Definition of a River Regime
The river regime refers to the changes in a river's discharge over the course of a year in response to a number of factors.
The factors affecting river regime include precipitation, temperature, evapotranspiration and the physical properties of the drainage basin. Let's have a quick look at how these factors have an impact:
River discharge
River discharge is a measurement of the amount (volume) of water passing through the river channel at any one time. From the river discharge, we can find out the rate of flow of a river. To calculate the river discharge, the following calculation is used:
This equation makes it clear that the components of river discharge are the volume of water (and other materials) passing through a river and the speed at which it travels. Like the river regime, river discharge is affected by the variability of the climate and the properties of the river.
Figure 1: Average discharge of the world's largest rivers, Ashersea/Wikimedia
Difference Between River Regime and Hydrograph
The river regime measures the differences in a river over the course of a year. A hydrograph, usually called a storm hydrograph, measures the amount of water and rate of flow in a specific part of a river, usually over a short(er) period of time. This is often used after a storm when the flow is much higher than normal.
Factors Affecting River Regime
By definition, there are multiple factors that can impact the river regime over time and space.
Drainage basin properties
A drainage basin is an area drained by a river. They vary in many ways, including in size, shape, rock type, vegetation coverage and more.The shape of the drainage basin can significantly impact the river regime. For example, if the sides of the drainage basin are steep, then water can get to the river more quickly because of the faster flow over the land (overland flow).
If the shape of the drainage basin is more circular, then the time taken for water throughout the drainage basin is, on average, shorter. Again, this would influence the river discharge to be faster. This is because, in a more rectangular basin, some water from the furthest points would take a much greater amount of time to reach the river channel.
As the river regime refers to changes in the river discharge, the drainage basin properties only really contribute if they change. This can occur as a result of processes such as erosion, weathering and deposition. All of these processes can impact drainage basin properties by either facilitating or slowing the movement of water from around the drainage basin to the river. For example, erosion and weathering have the potential to alter the gradient of the sides of the drainage basin. If erosion or weathering happens to make the gradient steeper, it will impact the river regime.
Figure 2: The water budget of (parts of a) watershed. Source: USGS.
Land use & vegetation
Another characteristic of drainage basins that influences the river regime is land use and the presence or absence of vegetation. This is because of the relationship between these factors and the movement of water from around the drainage basin to the river itself. Let’s explore how this works:
Land use – changes in the use of land around the drainage basin can impact the amount of water and the speed at which it reaches the river. For example, if the permeability of the land is altered through construction, then surface runoff is increased, and more water can reach the river more quickly. It is important to note that land use typically does not change as often as some of the other factors discussed here. As such, it is perhaps of lesser influence in the case of many rivers.
Runoff describes water simply flowing directly over the surface, as opposed to being absorbed into the ground.
Vegetation – one of the major ways that vegetation can impact the river regime is through its interception of precipitation. This means that there is a barrier (e.g. leaves) preventing, reducing or slowing the movement of water from the sky to the ground. The more vegetation there is, the more interception occurs. Interception can impact the river regime by reducing the speed at which water reaches the river or reducing the amount of water. As river discharge is a volumetric measurement, a decreased volume of water available is a critical component. Changes in vegetation over the year by natural seasonal fluctuations or human interference are therefore a driver of annual variation in river discharge.
River properties
The main property of the river that affects the river discharge and, by extension, the river regime is the cross-sectional area. The cross-sectional area of a river can be changed over time for many reasons. This can be through the erosion of the banks or river bed or through the deposition of rocks or other matter that can obstruct the flow of water. These changes can alter the river discharge over the course of a year by affecting the volume of water that can pass through the channel at any one time.
Figure 3: Cross-section of a river, USGS
Precipitation and evapotranspiration
The balance between precipitation and evapotranspiration alters the river regime. Precipitation and evapotranspiration are key controls of river discharge because they directly impact the amount of water in the river system. Where precipitation adds water, and so has the potential to increase river discharge, evapotranspiration does the opposite and can decrease the river discharge. Precipitation and evapotranspiration are variable over the course of the year.
Figure 4: Evapotranspiration = evaporation + transpiration. Source: USGS.
In the UK, typical patterns include higher levels of precipitation in the winter months and higher levels of evapotranspiration in the summer months. However, extreme weather events can cause heightened precipitation and/or evapotranspiration throughout the year. For example, a river's peak discharge often coincides with a storm event. This balance between the two processes significantly influences the river regime. This also relates to and affects the water balance because it alters the inputs and outputs of water in the system.
The water balance (also known as the water budget) is a measure of the balance between the inputs, outputs and stores of water within a system. Changes in the river regime can alter the water balance because factors affecting it (e.g. precipitation and evapotranspiration) also alter the water balance if changed. As well as this, a regime with a slower river discharge will alter the water balance by keeping water within the system for longer.
Temperature
Another element of the climate that impacts the river regime is temperature. Temperature is a key determinant in the levels of evapotranspiration. Higher temperatures are associated with higher levels of evapotranspiration. This is because the associated increased amount of energy allows water to change its state from a liquid to a gas. As we have discussed already, evapotranspiration and its annual variability is a key factor affecting a river regime. Therefore, the temperature is also an indirect influence.
Impact of Climate Change on River Regimes
Climate change is seeing long-term changes in the temperature and precipitation patterns found across the world. While we hear lots about how this is causing glaciers to melt and islands to flood, we don't hear much about how this affects rivers and their river regimes. We have just discussed how changes in precipitation, evapotranspiration and temperature all have the potential to significantly alter the river regime. Now imagine that these changes are not anomalies but are long-term changes. This will mean that the river regime will also change on a long-term basis. In extreme cases, this can result in flooding and droughts and can have a severely negative impact on the surrounding environment and populations.
Importance of a River Regime
It is important to understand the river regime for many reasons. The main reason is that it can help with river management and ensure that the catchment area and its inhabitants are protected from damage. For instance, if the river regime suggests that severe flooding could happen in the winter months, strategies ought to be implemented to reduce the damage done.
River Regime - Key Takeaways
- River regime is the changes to river discharge over a year.
- The river regime is influenced by many factors, including but not limited to temperature, precipitation, evapotranspiration and drainage basin properties.
- Changes in the river regime often coincide with changes in the water balance/water budget.
- Climate change is altering river regimes on a long-term basis, which can result in negative impacts for the surrounding environment and population(s).
- It is important to understand the river regime to facilitate river management and ensure protection for the catchment area and its population.
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