Water-readings

What is a water (drainage) basin?

The drainage basin is an area surrounded by hills or mountains, in which all surface waters flow as runoff towards a river, a lake, or eventually to the sea due to gravity and inclination. Virtually every river is inside a drainage basin: the larger the basin, the more the water drained.

The figure above shows how the watershed (red line) separates a drainage basin (red arrows) from the neighbouring ones (green arrows).

Why do hydrologists study the drainage basin around a city?

• They observe the water’s direction and estimate where it might be collected (e.g. in a dam) before escaping to the sea.
• They identify locations that can be rich in groundwater, depending on the direction of the surface water. Of course, this depends on the underground rock composition too.
• They estimate the amounts of water that the basin receives (water inflows) and whether this is sufficient for the surrounding settlements.
• They may detect the pollution paths.
• They can design the necessary flood-prevention works.

Stop and wonder

Discuss in your groups:

• In the area where you live, where does water come from?
• If water in your area comes from another basin, how far does it travel?
• Drainage basins have no national borders. Does your area’s basin belong exclusively to your country or is it shared with another country?

You can contact an expert e.g. form the Water Service Company who could answer these questions.

Group activity

Let’s make the model of the water basin

1. Crumple some sheets of newspaper or carton, making sure you create “mountains” and “valleys”.
2. Lay your folded creation inside a pan. Optionally you can colour its sides.
3. Spray the “mountain tops” with water and observe how the water flows.
4. What does the model represent?

Water-readings

The water balance inside a drainage basin

When water enters a drainage basin through precipitation (rain and snow), or transfer is considered as “inflow” while the water that escapes through evaporation, transpiration and surface runoff is “outflow”. If we think this inflow and outflow of water as a balance, then the total inflow to an area must equal the total outflow plus the net change in storage.

The water balance studies:

• The inflows: How much water precipitates into the basin every year and in which form e.g. rain, snow, hail.
• The outflows: How much water flows each year from the basin mainly from the rivers that run off.
• The storage: How much water is available in the basin, mainly found as groundwater or in dams and lakes.

Obviously, during the dry seasons the inflows and storage are decreasing. When storms occur, a substantial amount of inflow is lost to the sea, as it does not have time to percolate and enrich the  groundwater. Unfortunately, climate change accelerates both the droughts as well as the torrential downpours. To properly manage the water of a basin, experts have to monitor systematically and over time its water balance.

Water-readings

Let’s become hydrologists…

Would you like to become an hydrologist? This is an expert who studies water, the hydrological cycle, the water inflows and outflows of the drainage basins.

As an a hydrologist, you can roughly calculate the inflow in the drainage basin of your area every time it rains, if you know:

• the rainfall in millimetres (mm). You can measure the height of rain with a rain gauge. You can find instructions on how to make a rain gauge here and a short video tutorial here.
• the surface of your catchment area. Here is an interesting online tool where you can draw an area on a map and find out its enclosed area.

Estimate, the water inflow into the basin, every time it rains, is:

Rainfall (mm) x area (m2) x 1000 = ....... (cubic meters)

Knowing that an Olympic pool (50m x 25m x 2m) is filled with 2,500 cubic meters of water, how many pools could be filled with the amount of water you calculated?

Way forward

Surfing around the Internet

Visit the website of your National Meteorological Service to find out about the average rainfall in your area/drainage basin.

Dive in …

Water in the city if you want to explore where water comes from in your city.

The water cycle if you want to learn more about the water cycle .

Go back to the homepage!