Components and Processes in Drainage Basins
- Imagine you’re hiking in the mountains after a rainstorm.
- As you look around, you notice water trickling down rocks, pooling in small depressions, and eventually joining a rushing stream.
- Have you ever wondered how all these small flows connect to form rivers and, ultimately, reach the ocean?
- This interconnected journey of water is orchestrated by the drainage basin system, a dynamic network of inputs, stores, flows, and outputs.
Drainage Basin
Drainage Basin is an area of land drained by river and its tributaries. It includes water found on the surface, in the soil and in near-surface geology.
A system is an assemblage of interrelated parts that work together by way of some driving process. They are a series of stores or components that have flows or connections between them. There are three types of property:
- Elements: Things that make up the system of interest
- Attributes: The perceived characteristics of the elements
- Relationships: Descriptions of how the various elements (and their attributes) work together to carry out some kind of process
Systems can be classified as:
- Isolated systems: These have no interactions with anything outside the system boundary. There is no input or output of energy or matter. Many controlled laboratory experiments are this type of system and they are rare in nature.
- Closed systems: These have transfers of energy both into and beyond the system boundary but not transfer of matter. The Earth can be considered as a closed system as the exchange of matter with the outer space is negligible.
- Open systems: These are where matter and energy can be transferred from the system across the boundary into the surrounding environment. Most ecosystems are examples of open systems.
The Drainage Basin is An Open System
- A drainage basin is like nature’s giant funnel, collecting precipitation from the land and channeling it into rivers that eventually flow to the sea.
- What makes this system fascinating is its open nature: water enters (inputs), moves through various pathways (flows), is held temporarily (stores), and exits (outputs).
- This continuous movement is a key part of the hydrological cycle.
- Think of a drainage basin as a giant sponge that absorbs water, moves it around, and eventually releases it.
- The sponge’s capacity and pathways determine how quickly and efficiently this happens.
Inputs: Precipitation as the Starting Point
- The journey begins with precipitation, which transfers water from the atmosphere to the Earth's surface.
- Precipitation can take many forms, rain, snow, hail, or sleet and its characteristics (type, intensity, and duration) significantly influence how water moves through the system.
- Rainfall intensity: During heavy rain, the soil’s infiltration capacity can be overwhelmed, leading to surface runoff.
- Snow and ice: Water stored in frozen forms (the cryosphere) is released gradually as it melts, providing a steady supply to rivers.
- Seasonal variation: In some regions, precipitation patterns shift with the seasons, affecting water availability and river flow.
Keep an eye on precipitation intensity, it’s a critical factor in determining whether water infiltrates the soil or flows overland as runoff.
Outputs: Water Leaving the System
Water exits the drainage basin through two main processes:
1. Evaporation
- Evaporation transforms water from a liquid to a gas, returning it to the atmosphere.
- Factors like temperature, wind speed, and surface characteristics influence evaporation rates.
Bare soils and rocky surfaces lose water quickly, while vegetated areas retain moisture longer.
2. Transpiration
- Transpiration occurs when plants release water vapor through tiny pores (stomata) in their leaves.
- Together, evaporation and transpiration form evapotranspiration, a major output of water from the drainage basin.
Evapotranspiration
Evapotranspiration is the total output of water from the drainage basin directly back into the atmosphere.
In tropical rainforests, high temperatures and dense vegetation lead to significant water loss through evapotranspiration, reducing the volume of water that reaches rivers.
Flows: The Movement of Water Within the Basin
- Once precipitation enters the drainage basin, it travels through various pathways.
- These flows determine how water moves from the vegetation and land surface to rivers and groundwater.
Water that enters a drainage basin will interact with vegetation in various ways impacting the flow of water:
- Interception: The precipitation that falls on the vegetation surfaces (canopy) and is temporarily stored (interception storage) on these surfaces.
- Stemflow: The portion of precipitation intercepted by the canopy that reaches the ground by flowing down stems, stalks or tree boles.
- Throughfall: The portion of the precipitation that reaches the ground directly through gaps in the vegetation canopy and drips from leaves, twigs and stems.
1. Infiltration
- Sandy soils allow rapid infiltration, while clay-rich soils slow it down.
Infiltration
Infiltration is the process by which water penetrates the soil surface. Factors like soil type, vegetation cover, and rainfall intensity influence infiltration rates.
Infiltration capacity is the maximum amount of water soil can absorb, while infiltration rate measures how quickly water enters the soil.
2. Throughflow
- After infiltrating the soil, water may move laterally through soil layers.
- This is called throughflow, and it often follows natural pathways like root channels or cracks in the soil.
3. Overland Flow (Surface Runoff)
Overland flow
Overland flow occurs when water cannot infiltrate the soil and instead flows across the land surface.
- This often happens during intense rainfall, when soil is saturated, or in urban areas with impermeable surfaces.
- Students often confuse overland flow with river flow.
- Remember, overland flow refers to water moving across the land surface before it enters a river.
4. Base Flow
Base flow
Base flow is the slow movement of groundwater into rivers.
- This process ensures that rivers continue flowing during dry periods.
Imagine base flow as a steady drip from a faucet, it may be slow, but it keeps the river alive even when rain is scarce.
Stores Are Temporary Holding Places for Water
- Water is temporarily stored at various points in the drainage basin, regulating how quickly it reaches rivers.
- It includes:
- Vegetation: vegetation intercepts rainfall, with some water evaporating directly from leaves and the rest dripping to the ground (throughfall) or flowing down stems (stemflow).
- Soil Moisture: soil moisture refers to water held in the pores of soil particles. This store supports plant growth and maintains throughflow.
- Aquifers: aquifers are underground layers of permeable rock that store water. They provide a long-term water supply and maintain river flow during dry periods.
- Cryosphere: the cryosphere includes water stored as ice or snow. For example, glaciers act as reservoirs, slowly releasing water into rivers as they melt.
Can you think of two ways melting glaciers impact river flow and water availability?
Why Does This Matter?
- Understanding the drainage basin system is crucial for managing water resources and mitigating risks.
- For instance:
- Flood management: Knowing how precipitation and soil saturation influence overland flow helps predict and prevent floods.
- Water supply: Aquifers and soil moisture are vital for agriculture, industry, and domestic use.
- Climate change: Shifts in temperature and precipitation disrupt the balance of inputs, flows, and stores, with significant consequences for ecosystems and human livelihoods.
How might deforestation or urbanization alter the balance of water flows and stores in a drainage basin?
Reflection and Broader Implications
The Drainage Basin is a System
- Inputs: Precipitation is the starting point.
- Outputs: Evaporation and transpiration return water to the atmosphere.
- Flows: Water moves via infiltration, throughflow, overland flow, and base flow.
- Stores: Water is temporarily held in vegetation, soil, aquifers, and the cryosphere.
Key Processes in System Connectivity
- Water Flow: Water moves through the drainage basin via precipitation, surface runoff, infiltration, and river flow.
- Any disruption to this flow such as damming a river can alter the volume and timing of water reaching downstream areas.
- Sediment Transport: Rivers carry sediment (sand, silt, and clay) from upstream to downstream.
- This sediment is crucial for maintaining riverbanks, deltas, and ecosystems.
- Activities like deforestation or dam construction can disrupt this transport, leading to erosion or sediment buildup.
- Flooding Risks: Changes in water flow and sediment transport can increase or decrease the risk of flooding downstream.
