Understanding the
Catchment Area
A canyons catchment area or watershed gives you an idea about the volume of water you can expect to find inside the canyon. Below is an in depth look at what a catchment area means and the factors to take into account.
Measuring the catchment.
To measure a canyons catchment area you need to predict where the water feeding the canyon will land, this is usually easier when a catchment area is steeper.
The measurement of a catchment area is in kilometres squared (km²), this makes working out the effect of rainfall easier.
A measurement of a catchment area – Gaia GPS
Lag Time
The lag time is the amount of time it takes for a canyons water level to react to precipitation or rainfall. A canyon with short lag time will see a rise in levels shortly after rainfall.
How to Predict a canyons lag time.
The main factors that influence a canyons lag time are:
- Vegetation – Forested catchments tend to see longer lag times.
- Gradient – Steeper catchments tend to see shorter lag times.
- Permeability – permeable catchments tend to see longer lag times.
- Size – Small catchments tend to see shorter lag times.
- Shape – Fan shaped catchments tend to see shorter lag times.
A example of a catchment area with little vegetation, steep sides, impermeable ground covering and small area results in a shorter lag time – Gaia GPS
A example of a catchment area covered in vegetation, shallow sides, permeable ground covering, large area and fern shaped results in a much longer lag time – Gaia GPS
Rainfall
Once you know the size of the catchment area you can work out what sort of effect rainfall will have on the canyon.
When a weather forecaster predicts rainfall (e.g. 1mm) they are suggesting, that depth of rain will fall in 1m². knowing the catchment area size tells us how much water we could expect to fall on our catchment.
If a catchment area is 5km² and rainfall is predicted to be 1mm over 1hr then we can assume a total of around 5000m³ of water will be falling on our catchment in that hour. Below are factors to consider which reduce this volume in the canyon.
- Infiltration
- Evaporation
- Evapotranspiration
- Overland flow speed.
- Distance
A visual guide to water movements in a catchment area, with the canyon section included. Notice how different areas manage water movements differently.
Shape
The shape of the catchment influences the time it takes for levels to rise after precipitation. Here are the two different shapes of catchment areas.
Fern
The fern shaped catchment is defined by a main river (the stem) with several streams joining along the way (the leaves).
The fern shape catchment reacts with slow rise in levels over a longer period
Fan
The fan shaped catchment is defined by a main river with several tributaries of similar length.
The fan shape catchment reacts with a fast rise in levels over a shorter period.
Using these two examples of catchment shapes, we can make a more accurate prediction to the expected flow rate after rain fall.