Discharge & Hydrographs

By Alex Jackson

The discharge of a river (or stream) is the volume of water that flows past a point in the river’s course per second. The volume is measured in cubic metres (m3) and it’s per second so the units of discharge are cubic metres a second or m3s-1. Coincidentally, 1m3s-1 is the same as 1 cumec so the discharge of a river is often measured in cumecs because it’s a bit easier to say. Discharge is normally measured at gauging stations that are situated at different points along the river.

The discharge of a river changes over time depending on a few factors. The most influential factor is the weather. After heavy rainfall the discharge of the river will be higher because there’s more water entering the river. The weather affects discharge so much that there’s a special graph that we can draw called a hydrograph which shows precipitation and discharge on the same graph and makes it easy to see how quickly precipitation affects the discharge of a river. A storm hydrograph is a specific type of hydrograph that, surprise, surprise, shows precipitation and discharge during and after a storm. The main difference between a normal hydrograph and a storm hydrograph is that a storm hydrograph is over a much shorter period of time. Below is a storm hydrograph for the fictional River Shui:

There’s a curve showing the discharge of the river and there’s a series of bars showing some (fairly heavy) precipitation. There’s a few things to note on this graph. First is the lag time. The lag time is the time difference between the peak precipitation and and the peak discharge. A long lag time indicates that it’s taking a long time for precipitation to enter the river. Conversely, a short lag time indicates that the precipitation is entering the river fairly quickly. The rising limb is the steep part of the discharge line that has a positive gradient, indicating that the discharge is increasing. The falling limb is the opposite showing that the discharge is falling.

Factors Affecting a Storm Hydrograph

The Drainage Basin

The shape of a hydrograph is altered by a few different things. One factor is the shape of the drainage basin. Drainage basins come in a wide assortment of shapes. (Roughly) Circular shapes are common as are more elongated and narrow shapes. For a circular drainage basin, the river’s hydrograph can often be described as “flashy” because it will have a fairly steep rising limb and a high peak discharge. This is because all points in the drainage basin are (again, roughly) equidistant from the river so all the precipitation reaches the river at the same time.

The size of the drainage basin obviously has an impact on the hydrograph. Large basins will have high peak discharges because they catch more precipitation but at the same time they’ll have longer lag times than small basins because the water takes longer to reach the rivers.

Basins with steep slopes will have a high peak discharge and a short lag time because the water can travel faster downhill. Finally the drainage density of a basin will affect the lag time and the steepness of the falling limb. Basins with lots of streams and rivers (a high drainage density) will have a short lag time and a fairly steep falling limb because water will drain out of them quickly.

Soil & Rock Type

If a river is surrounded by non-porous and impermeable rocks (e.g., mudstone) it’s going to have a high peak discharge and a short lag time. Impermeable rocks won’t let water percolate through them, forcing the water to travel via overland flow. This is much faster than groundflow, interflow and throughflow so the lag time is reduced. Furthermore, non-porous rocks can’t store water so the peak discharge of a river is increased as more water enters the river rather than being stored in the drainage basin.

The soil’s ability to let water infiltrate has a similar effect to the dominant rock type in a drainage basin. Unconsolidated soils allow water to infiltrate and so act as a store in a drainage basin. In addition, water travels slowly through soil via throughflow. This reduces the peak discharge while increasing the lag time of a river. On the other hand, extremely fine clay soils don’t allow water to infiltrate. As a result, water travels quickly as overland flow, reducing the lag time of a river.

Weather & Climate

The intensity of a storm will obviously impact the peak discharge of the river. More rainwater = more water in the river so a higher discharge. Not immediately obvious is the type of storm (or precipitation) that takes place. A winter storm (i.e. snow) will result in an increase in the river’s discharge when the snow melts but this often won’t be for a long time, so the lag time will be huge.

If it’s been raining heavily previously, the ground may be waterlogged so the lag time will be reduced because water will be unable to infiltrate and will instead travel via overland flow. Similarly, if the climate’s been hot & dry or freezing cold the ground will be hard and water will once again be unable to infiltrate and will instead travel as overland flow, reducing the lag time and increasing the peak discharge.

Vegetation Cover

If the area surrounding the river has thick vegetation cover then lots of precipitation will be intercepted, greatly increasing the lag time. In addition, the peak discharge will decrease because vegetation will absorb the water and lose it through transpiration and evaporation.

Human Activity

Humans will normally cover soil in impermeable materials like tarmac or concrete which will increase surface run off and reduce the amount of water being stored, increasing the peak discharge and reducing the lag time. As water doesn’t infiltrate easily in urban areas humans often build storm drains that run directly into a river, reducing the lag time and increasing the river’s peak discharge.

Analysing Storm Hydrographs

It’s very common for questions about hydrographs to come up in the exam but that’s OK, these are normally 2 or 4 mark questions that ask you to read something off a graph or describe the graph. Easy marks. The ever so slightly harder questions are the ones that ask you to explain a hydrograph because these require you to think. Let’s look at the storm hydrograph for the fictional River Shui again:

If asked to describe the hydrograph you could quote the lag time, peak discharge and comment on the steepness of the rising and falling limbs (remember, state values off of the graph). Instead, you could be asked to explain the hydrograph’s shape. These reasons are essentially all the factors we discussed previously. You need to state a factor and then explain how and why it affects the shape of the hydrograph.

For the River Shui’s hydrograph, we could say that the high peak discharge and the steep rising limb suggests that the drainage basin is circular because if it was, the precipitation will land at points equidistant from one another and reach the river at roughly the same time, producing the high peak discharge.