Lesson video

Hello geographers.

My name's Mrs. Hormigo and I'm looking forward to teaching you today.

I hope you're going to enjoy the lesson and learn lots.

Let's get started.

Today's lesson looks at a storm hydrograph, which helps to understand how a river responds to rainfall.

By the end of the lesson, you'll be able to explain the characteristics and factors affecting the shape of a storm hydrograph.

There are four keywords for today's lesson, discharge, infiltration, lag time, and drainage density.

Discharge is the volume of water flowing through a river channel at any given time, and it's measured in cumecs.

Infiltration is the process by which water on the ground surface enters the soil.

Lag time is the time between the peak rainfall and the peak discharge.

And drainage density is a measure of how many rivers and streams there are within a given area.

A high drainage density indicates many streams and rivers in the given area.

There are two parts to today's lesson.

The first, what is a storm hydrograph and the second, what affects the shape of a storm hydrograph.

Let's begin our first learning cycle.

A storm hydrograph is a graph that shows how the discharge of a river or stream changes over time in response to a rainfall event.

Let's have a look at each part in a little bit more detail.

On the x-axis or the bottom axis, we have time.

On the y-axis, we have two different scales.

The first shows us how much rain has fallen.

Rainfall is measured in millimetres and it is shown as a bar graph.

We also have river discharge, which is measured in cumecs.

The river discharge is shown as a line graph.

The tallest bar chart shows the peak rainfall.

So this is when the maximum amount of rain had fallen.

The highest point on the curved line graph showing us discharge shows the peak discharge.

So the highest discharge that was experienced during this or following this rainfall event.

Let's have a quick chew or false then.

The purpose of a storm hydrograph is to present data on rainfall.

Think carefully about this hydrograph that you've just looked at and make your decision.

I hope you said false, but can you tell me why? Maybe have a chat to the person next to you.

I hope you said something like this, although a storm hydrograph does present rainfall data, its purpose is to show how river discharge changes over time in response to rainfall.

So let's have a little look now.

The peak discharge is not at the same time as the peak rainfall because it takes time for the precipitation to reach the river.

So it takes time for the precipitation that's fallen within the drainage basin to flow into the river via various different routes.

The lag time is the time between the peak rainfall, so we can see there it starts at the highest bar chart and the peak discharge.

So the highest point on the curve of the line graph and the lag time tells us how long it takes for the rainwater to reach the river.

So during a rainfall event, water enters the drainage basin as precipitation.

We can see that identified on the diagram.

Some of this water will then be lost.

That might be via evaporation or via transpiration.

So water that evaporates from the leaves of tree leaves and branches of trees.

Other parts of this rainfall will flow as surface runoff into the river.

So it will hit the ground and it will run straight down into the river.

Some of it will infiltrate, which means moving downwards into the soil.

If it flows a surface runoff, it flows much more quickly than if it is infiltrated.

You can see there precipitation, transpiration, evaporation.

And then in addition to that, we've got infiltration.

The water moving down through the soil or surface runoff the water moving across the surface and into the river.

Once the water has infiltrated, has two options.

It may travel through the soil as throughflow towards the river, or it may percolate further.

So go down further this time into the rocks and travel as groundwater.

Groundwater takes the longest time to reach the river.

So if we think surface runoff is the quickest, then throughflow and then groundwater taking the longest time.

So those three processes are the way in which water reaches the river once it has fallen as precipitation.

Let's just check you've understood that.

Can you complete the annotations around this diagram? Number one is water enters the drainage basin via? Number two, water may then? Into the soil and flow as? Number three, or flow back into the river as? And number four, water may percolate into the rock and return to the river as? There are five key words to add in.

Pause the video and have a go.

Hopefully you added these correctly.

So we have water entering the drainage basin via precipitation.

Water may then infiltrate into the soil and flow as throughflow.

Or flow straight back into the river as surface runoff.

And number four, water may percolate into the rock and return to the river as groundwater.

Well done if you manage to get those five keywords.

Let's look now back at the storm hydrograph.

The base flow, so we can see there it's highlighted in pink is the normal discharge of the river.

And this base flow comes from sources as we've just seen, like groundwater.

It has, it is a steady flow before and after the storm event.

Storm flow, so we can see there, it's highlighted in blue is the part of the river discharge that results directly from a storm or a rainfall event.

So it's in addition to the base flow.

Check for you now, the vertical axis of a storm hydrograph has two different scales on it.

Is that true or false and why? Hopefully you said true and you remembered that it has a scale to measure the river discharge, which is a line graph and a scale for the rainfall event.

So showing us how much rain has fallen, and this is a bar chart.

Let's go back to the storm hydrograph.

We've got here rising limb labelled.

So the rising limb shows how the discharge is increasing after the rainfall event.

And the steeper the rising limb, the quicker the water is returning to the river.

So if we remember surface runoff, throughflow, and groundwater flow means that the rain returns to the water at different speeds.

The steeper the rising limb, the more quickly the water is returning.

Peak discharge, as we've looked at before, is the highest point on the hydrograph.

And that shows the maximum river discharge after the storm.

So the highest that the river will get following this rainfall event.

Then we have the falling limb.

And the falling limb shows where the discharge decreases after the storm and the river returns to its base flow.

So normal levels.

Let's see if you've been listing, can you complete the missing labels on this hydrograph? There are five things pointed out.

Pause the video and have a go.

Did you manage to do it? So we have peak rainfall that points to the highest bar chart, highest bar on the bar chart.

We have our rising limb, which is the section of the discharge line graph that's in, shows an increase in their river discharge following the storm event.

We have peak discharge, which is the maximum amount of discharge in the river following the storm event.

Then we have the falling limb as the river returns to its base flow and its normal level.

And then we have the lag time, which is the time between the peak rainfall and the peak discharge, the time it takes for water to reach the river after the rainfall event.

Well done if you manage to label all of those correctly.

Let's have a go at first task.

What does a hydrograph measure? Can you have a go at answering that as your first question? And then secondly, looking at the storm hydrograph again, can you label but also explain the meaning of each term? Think carefully and have a go.

Come back when you are ready.

For your first answer, you might have put a hydrograph measures the discharge, so the flow of a river over time, and it shows how the volume of water in the river changes in response to a rainfall event.

Let's see how you got on with your labels.

They may have included this.

So a rising limb, which shows how the discharge increases after rainfall.

Peak rainfall, which points to the highest bar in the bar chart.

And it's the maximum amount of rainfall that fell during this event.

Peak discharge, the highest point on the hydrograph showing the maximum river discharge after the storm.

Lag time, the time between the peak rainfall and the peak discharge.

And the falling limb, which shows where the discharge decreases after the storm and it returns back to its base level.

Well done if you manage to label and explain each of those terms correctly.

Let's move now to our second learning cycle.

What affects the shape of a storm hydrograph? So there'll be a range of factors that affect the shape.

So firstly, the characteristics of the drainage basin.

The size, is it a big or a small drainage basin? Drainage density, so the number of streams and rivers on the surface.

The rock type, is it permeable or impermeable? Soil, is it saturated or is it dry? And the slope, is it a steep or a gentle slope? All of these will have an impact on how quickly water returns to the river following an event.

The precipitation event itself.

So the intensity of the rainfall.

The time, so how long did the rain come for? Was it a short storm or was it a longer period of rainfall? And how are humans using the light drainage basin? The types of land use, this can affect whether water is infiltrating or flowing a surface runoff, which will have an impact on the speed at which water returns to the river and how much does.

This hydrograph shows an example of a short lag time, and we call it a flashy hydrograph.

It's very distinct in its shape, and it indicates a quick rise and fall of the river's discharge after a rain event.

And that's because the river is responding quickly.

It has a steep rising limb, a high peak discharge, and quite a steep falling limb, which means it returns back to normal fairly quickly.

Because we have a steep rising limb, we must have a shorter lag time.

And because we have a shorter lag time, there is an increased risk of flooding.

This is the opposite.

A low flat hydrograph, which indicates a longer lag time.

It shows that the river has a slow response to rainfall, so water is not returning to the river quickly.

The river discharge remains relatively low and rises and falls gently.

Let's have a check now.

Which of these phrases describe a flashy hydrograph? Is it A, a steep rising limb? Is it B, a low discharge, or is it C, an increased flood risk? Pause the video and make your decisions.

Hopefully you looked at the image on the slide and remembered it was a steep arising limb.

And because of this steep rising limb, we have a shorter lag time, which increases the risk of flooding.

So what factors lead to a flashy hydrograph? A small drainage basin, a high drainage density, impermeable rocks, steep slopes, saturated soil, prolonged and intense rainfall, and urban areas.

So if you think back to those three categories of factors we discussed at the start, these are how they apply and create a flashy hydrograph.

We'll look at these in a little bit more detail now.

So in smaller drainage basins, water does not have as far to travel to the river.

So it returns to the river more quickly.

In steep sided drainage basins and we can see an image there of an OS map, the contour lines being very close together, indicating a steep slope.

The water flows much more quickly down the slopes and back into the river.

Therefore, both of these factors lead to a shorter lag time because water is reaching the river more quickly.

And so we get a steep rising limb, which leads to a flashy hydrograph.

A higher drainage density means that water can flow into the streams and rivers more quickly.

So if you remember, the drainage density is describing how many rivers and streams are on the surface.

The more of these that there are for water to flow into, the more quickly the water returns to the main river.

Therefore, this means that there is a shorter lag time.

So water is reaching the river more quickly and we end up with a flashy hydrograph.

Saturated soils, so when their soils are saturated, it means that water can no longer infiltrate into them and instead the water will flow as surface runoff.

Now water returning via surface runoff does so much more quickly than if it infiltrates and returns as throughflow.

So this too leads to a shorter lag time and a steeper rising limb and a flashy hydrograph.

Impermeable rock types means that water cannot infiltrate into the soil and the rocks.

Therefore, we have less throughflow and groundwater flow and more surface runoff, which again means that the lag time will be shorter.

A surface runoff is the quickest way for water to return to the river.

And again, we have a steep rising limb, which can be seen on this flashy hydrograph.

Let's have a check now.

How do drainage basin characteristics lead to a flashy hydrograph? Can you fill the gaps to complete the explanation? Pause the video and come back when you've had a go.

Hopefully you spotted these words were for the gaps.

A small drainage basin, high drainage density, impermeable rocks, steep slopes, and saturated soil all lead to a shorter lag time, which means water reaches the river more quickly, which leads to a steep rising limb and a flashy hydrograph.

So let's think now about the rainfall event itself.

If we have prolonged rain or heavy rainfall over a short period of time, we'll have a much greater rate of surface runoff.

Because of this, the lag time will be shorter and we'll have a flashy hydrograph.

And then thinking about land uses, drainage basins with urban areas have more impermeable surfaces.

This means that water cannot infiltrate, and so more of it flows a surface runoff, which leads to a shorter lag time and a flashy hydrograph.

Now this is the opposite shape, a low flat hydrograph.

And if you remember here, this was when the river had a slow response to a rainfall event and a much lower discharge.

The lag time is much longer.

So let's think which factors might lead to this.

It will be a large drainage basin.

The water has much further to travel before it returns to the river.

A low drainage density, there's not many rivers of streams on the surface, so it takes longer for the water to find alternative routes back to the river.

Permeable rocks which encourage infiltration and percolation.

So more water returns via throughflow and groundwater flow.

Gentle slopes, the water won't travel as quickly down gentle slopes.

The soil is dry, it's ready to receive rainfall, and so it can infiltrate into it.

The rainfall is very light so the soil doesn't become quickly saturated or overwhelmed, and infiltration rates remain high.

And forests and vegetation as a main land use.

These can intercept rainfall so that less reaches the ground and it will mean that some of it is absorbed by the roots, so it reduces the amount of rain returning to the river and slows it down as well.

So if we have a look at that, these factors, the large drainage basin, the low drainage density, permeable rocks, gentle slopes, dry soil, light rain, and forests and natural vegetation, slow and reduce the water reaching the river, which leads to a longer lag time and a lower peak discharge, which is shown on this low flat hydrograph.

Right, let's have a check for you now.

These statements are all factors that influence the shape of a hydrograph, which of them are not in the correct column? So read through them carefully and circle the ones which are not in the correct column.

Hopefully you looked through these.

For a flashy hydrograph, steep slopes is correct, prolonged and heavy rainfall, saturating the soil is good, impermeable rocks is correct, but a low drainage density is not right.

We, in a for a flashy hydrograph, water is returning quickly to the river.

So we want to have rivers and streams on the surface to help this to happen.

For the low flat hydrograph, gentle slopes and permeable rock is correct, but saturated soils would encourage surface runoff, which would mean that the water returned very quickly and would lead to a flashy hydrograph.

As with high drainage density, lots of rivers and streams on the surface would mean a speedy return of the water to the river.

So I hope you managed to correctly identify those three factors.

Well done.

Your second task then.

What term is used to describe each hydrograph? What have we been using to describe these two different shaped hydrographs? And then for both of the hydrographs, choose one factor and explain how it influences the shape.

Pause the video when you've had it and come back when you've had a go at these two questions.

Hopefully your first question was an easy one.

You were able to remember that hydrograph on the left is a flashy hydrograph with a short lag time.

And the hydrograph on the right is a low flat hydrograph with a much longer lag time.

Well done.

And then for number two, you had to choose one factor and explain how it influences the shape.

So steep slopes can lead to a flashy hydrograph, and this is because water moves down the steeper slopes via surface runoff rather than infiltrating into the soil.

Therefore, more water reaches the river more quickly after the rainfall event, which causes a steeper rising limb and a shorter lag time, characteristic of a flashy hydrograph.

Well done.

Permeable rock leads to a low flat hydrograph as rainfall can infiltrate into the soil and then percolate into the rock flowing as groundwater into the river.

This means water takes much longer to reach the river leading to a more gentle rising limb and a longer lag time, and quite often a lower peak discharge.

Well done if you manage to correctly choose a factor and explain how it influences the shape.

Let's have a look now at the summary for today's lesson.

A storm hydrograph is a graph that shows how the discharge of a river or stream changes over time in responses to a rainfall event.

If you remember, there are two scales on the vertical axis showing us the discharge in cumecs, but also the rainfall in millimetres.

Lag time is the time between the peak rainfall and peak discharge.

A flashy hydrograph has a short lag time and a low flat hydrograph has a longer lag time.

Drainage basin characteristics such as the size of it, the rock type, the slope, the rainfall event itself, and human land uses all influence the shape of a hydrograph.

Well done.

I hope you now understand a little bit more about the storm hydrograph, what it shows, its purpose, and why its shape changes.

I look forward to seeing you all again soon.