What this lesson teaches
I can explain how a drainage basin's inputs and outputs set its water balance, and why it varies.
Syllabus 9173, Topic 2.1, 2. Drainage Basin Hydrology. How does the balance between input and output in the drainage basin system vary in the tropics?
Make a guess
A river keeps flowing through a month with no rain at all. Where does its water come from?
- From evapotranspiration back into the river.
- From rain that falls in the same month.
- The river creates its own water.
- Water stored earlier in soil and rock.
Show the answer
Water stored earlier in soil and rock.
In the wet season, rain refills soil and groundwater. In dry months, groundwater feeds the river as baseflow, so storage falls.
Water coming in minus water going out equals the change in storage, so a river can keep flowing by using water stored earlier.
A drainage basin is the area of land that drains into a river and its tributaries. A watershed, the high ground around it, separates it from other basins. Water flows downstream from the source to the mouth. Groundwater may cross the surface boundary.
Inputs are water entering the basin. The main one is precipitation. Snowmelt from high mountains can feed some tropical rivers. But if the snow is already counted as stored in the basin, melting is an internal move, not new water. Do not count it twice.
Outputs are water leaving the basin. The main ones are evapotranspiration and river discharge at the mouth. Potential evapotranspiration (PET) is how much could evaporate if water were plentiful. Actual evapotranspiration (AET) is how much really does. In dry seasons, AET falls below PET because soils are dry.
In a simple basin: precipitation = AET + runoff + change in storage. Use the same period and units, often mm. A negative change means storage fell. Discharge is a rate, often in cubic metres per second, so convert it to a volume or depth before putting it in a monthly budget.
Budgets vary with rain timing, heat, plants and rock. Wet-season rain refills soil and groundwater. In the dry season, groundwater keeps rivers flowing. Humid does not mean water supply is the same in every month or every basin.
- Budget
- Precipitation = AET + runoff + change in storage.
- PET vs AET
- Possible evaporation vs actual evaporation.
- Negative storage change
- The basin is drawing on stored water.
Step by step
Set the boundary
Basin, time period and units.
Count transfers correctly
Separate real inputs from internal moves like snowmelt.
Work out storage change
Rain minus AET minus runoff.
Worked example: A monthly budget
A basin gets 120 mm of rain, loses 70 mm through AET and sends 80 mm out as runoff. Storage change is 120 - 70 - 80 = -30 mm. Water stored earlier made up the difference.
Watch out for this
When PET is high, actual evapotranspiration must also be high.
PET assumes plenty of water. AET is lower when soils and plants run dry.
Check your understanding
In a simple budget, rainfall is 50 mm, AET is 40 mm and runoff is 35 mm. What happens to storage?
- It falls by 25 mm overall.
- It cannot be worked out without PET.
- It rises by 25 mm.
Show the answer
It falls by 25 mm overall.
Yes. 50 - 40 - 35 = -25 mm.