Welcome to Hydrology: The Drainage Basin System
Hello! Welcome to your first step in mastering Physical Geography. Today, we are looking at the drainage basin system. Think of a drainage basin as nature's own plumbing system. It is an area of land drained by a river and its tributaries. It acts like a giant funnel, collecting all the water within a certain area and moving it toward the sea.
Don’t worry if some of these terms seem new or a bit "sciencey" at first. We will break everything down into simple pieces. By the end of these notes, you’ll see that the drainage basin is just a series of inputs, stores, flows, and outputs.
Geography Pro-Tip: In the exam, always remember that a drainage basin is an open system. This means it has energy and matter (water) entering and leaving it.
1. The Outputs: Where does the water go?
Outputs are simply the ways water leaves the drainage basin system. If the basin is a bathtub, these are the ways the water disappears from the tub.
Evaporation: This is when liquid water from the ground or from puddles turns into water vapor (gas) because of the sun's heat.
Evapotranspiration: This sounds like a big word, but it’s just a combination of evaporation and transpiration (where plants "breathe" out water through their leaves). It is the total loss of water from the basin into the atmosphere.
River Discharge: This is the most obvious output. It is the volume of water flowing out of the basin through the main river channel. It is usually measured in "cumecs" (cubic meters per second).
Quick Review:
Water leaves the basin by going up (evapotranspiration) or out (discharge).
2. The Stores: Where is the water kept?
Stores are places where water is held stationary for a period of time. Imagine these as different "waiting rooms" for water.
Interception: This happens when plants, trees, or even buildings catch the rain before it hits the ground.
Analogy: Think of a thick tree canopy like an umbrella. It stops you from getting wet immediately during a light rain shower.
Surface Water Storage: Water held on the surface of the earth in puddles, ponds, or lakes.
Soil Water Storage: This is water held in the spaces between soil particles. It's what keeps your garden plants alive!
Groundwater Storage: This is water held deep underground in the cracks and spaces of rocks. This is the largest store of fresh water in the basin.
Channel Storage: This is the water that is currently "sitting" in the river channel itself as it moves along.
Key Takeaway:
Water can be stored above ground (interception, surface water), in the ground (soil water, groundwater), or in the river (channel storage).
3. The Flows: Moving Water Above Ground
Flows (or transfers) are the movements of water between the different stores. First, let's look at what happens on or above the surface.
Throughfall: This is rain that either falls directly through gaps in leaves or drips off leaves to reach the ground.
Stemflow: This is water that trickles down the stems of plants or the trunks of trees to reach the soil.
Memory Aid: Think of water "hugging" the stem to get to the bottom.
Overland Flow (Surface Run-off): When water flows across the ground surface. This happens if the ground is too hard (like concrete) or too saturated (already full of water) to soak up any more rain.
Channel Flow: The movement of water specifically within the river's banks.
4. The Flows: Moving Water Below Ground
This is where many students get confused, but the trick is to follow the path of the water from the surface, downwards, and then sideways.
Step 1: Infiltration
Water soaks into the soil surface from above.
Common Mistake: Don't confuse infiltration (entering the soil) with percolation (moving deeper into the rock).
Step 2: Throughflow
Once water is in the soil, it starts moving sideways (laterally) towards the river. This is usually quite slow.
Step 3: Percolation
This is the downward movement of water from the soil into the underlying rock. Think of it like coffee percolating through a filter.
Step 4: Groundwater Flow
Water moving sideways through the rocks deep underground. This is the slowest movement of all.
Step 5: Baseflow
This is the groundwater that eventually seeps into the river bed. This is why rivers keep flowing even when it hasn't rained for weeks!
Did you know?
In a desert, overland flow is very fast because the ground is baked hard. In a rainforest, infiltration is high because the soil is soft and protected by trees!
5. Underground Water Features
To understand how water behaves deep underground, we need a few specific terms.
Water Table: This is the "top level" of the underground water. Below this line, the rocks are completely soaked (saturated).
Ground Water: As we learned, this is the water stored below the water table.
Recharge: This is the "refilling" process. When it rains and water percolates down to the water table, we say the groundwater is being recharged.
Springs: A spring occurs where the water table meets the land surface. Water naturally flows out of the ground here.
Quick Summary Checklist
Before you move on to the next chapter, make sure you can answer these:
1. Can I name three outputs of the system? (Evaporation, Evapotranspiration, Discharge)
2. Do I know the difference between infiltration and percolation? (Soil vs. Rock)
3. Can I explain stemflow? (Water running down the tree trunk)
4. Do I understand what a water table is? (The upper limit of saturated rock)
Keep going! You’ve just mastered the fundamental "map" of how a river basin works. Next, we will look at how these flows change when it storms!