The Drainage Basin as an Open System

This lesson examines the drainage basin as an open system within the hydrological cycle, including its inputs, outputs, stores, flows and the water balance equation. It addresses Edexcel A-Level Geography (9GE0) Paper 1, Topic 5, Enquiry Question 1: What are the processes operating within the hydrological cycle from global to local scale?

---

The Drainage Basin Concept

A drainage basin (also called a catchment or watershed) is the area of land drained by a river and its tributaries. Every point within a drainage basin is connected — any rain falling within the basin will eventually reach the river channel (assuming it does not evaporate first).

The boundary of a drainage basin is called the watershed — an imaginary line following the highest points (ridges) surrounding the basin. Precipitation falling on one side of the watershed flows into one drainage basin; precipitation falling on the other side flows into an adjacent basin.

Open System vs Closed System

Unlike the global hydrological cycle (which is a closed system), a drainage basin is an open system:

Feature	Closed System (Global)	Open System (Drainage Basin)
Inputs	None (no water enters or leaves)	Precipitation (rain, snow)
Outputs	None	Channel discharge, evapotranspiration
Energy	Solar energy drives the cycle	Solar energy + gravitational energy
Total water	Constant	Variable (depends on inputs vs outputs)

graph TD
    P["INPUT<br/>Precipitation"] --> I["Interception Store<br/>(vegetation canopy)"]
    P --> S["Surface Store<br/>(puddles, lakes)"]
    I -->|"Stemflow /<br/>Throughfall"| S
    I -->|"Evaporation"| OUT1["OUTPUT<br/>Evapotranspiration"]
    S -->|"Infiltration"| SM["Soil Moisture Store"]
    S -->|"Overland Flow"| CH["Channel Store<br/>(river)"]
    SM -->|"Throughflow"| CH
    SM -->|"Percolation"| GW["Groundwater Store"]
    SM -->|"Transpiration /<br/>Evaporation"| OUT1
    GW -->|"Baseflow /<br/>Groundwater Flow"| CH
    CH -->|"Channel Flow"| OUT2["OUTPUT<br/>River Discharge"]

---

Inputs

The primary input to a drainage basin is precipitation — water that falls from the atmosphere onto the land surface. This includes rain, snow, sleet, hail and (in some definitions) fog drip and dew.

The characteristics of precipitation that matter for the drainage basin include:

Characteristic	Effect on Drainage Basin
Intensity	High-intensity rainfall exceeds infiltration capacity → more overland flow and rapid response
Duration	Prolonged rainfall saturates soil stores → increases throughflow and overland flow
Type	Snow accumulates as a store and releases water during melt; rain flows immediately
Seasonal distribution	Determines when the basin receives its water — affects river regimes
Spatial distribution	Orographic effects mean that upland areas often receive more precipitation than lowlands

---

Stores

Water within a drainage basin is held in several stores at any given time. These stores have different capacities and residence times.

1. Interception Store

Interception is the process by which precipitation is caught and held by vegetation (leaves, branches, stems) or by built structures (buildings, roads) before reaching the ground surface.

• Deciduous woodland may intercept 25–35% of annual rainfall
• Coniferous forest can intercept 30–45% due to year-round needle cover
• Grassland intercepts less (~10–20%) due to lower canopy density
• After prolonged rainfall, interception capacity is reached and all subsequent precipitation reaches the ground as throughfall (dripping through gaps) or stemflow (running down stems/trunks)

2. Surface Store

Water held on the ground surface in puddles, lakes, ponds and wetlands. In urban areas, water may also be stored temporarily on impermeable surfaces.

3. Soil Moisture Store

Water held in the pore spaces of soil. This is one of the most important stores in the drainage basin because it supplies water for plant transpiration and contributes to throughflow.

Key concepts related to soil moisture:

• Field capacity — the maximum amount of water a soil can hold against gravity after drainage
• Wilting point — the soil moisture level below which plants cannot extract water
• Soil moisture deficit — occurs when actual soil moisture falls below field capacity (common in summer in the UK)
• Soil moisture surplus — occurs when precipitation exceeds evapotranspiration and soil is at or above field capacity

4. Groundwater Store

Water stored in pores, cracks and fissures in permeable rocks (aquifers) below the water table. Groundwater is the largest freshwater store in most drainage basins and sustains baseflow to rivers during dry periods.

• The water table is the upper surface of the zone of saturation
• An aquifer is a permeable rock that stores and transmits water (e.g. chalk, sandstone, limestone)
• An aquiclude (or aquitard) is an impermeable rock layer that prevents water movement (e.g. clay, granite)

5. Channel Store

Water held within the river channel itself at any given moment. This is a relatively small but dynamic store.

---

Flows (Transfers)

Flows are the pathways by which water moves between stores within the drainage basin. The speed of these flows determines how quickly precipitation reaches the river channel — this is the lag time on a storm hydrograph.

Flows Reaching the Channel

Flow	Speed	Description	When It Dominates
Overland flow (surface runoff)	Fast (minutes–hours)	Water flows across the ground surface	When rainfall intensity > infiltration capacity; when soil is saturated; on impermeable surfaces
Throughflow	Moderate (hours–days)	Water moves laterally through soil pores, often along the boundary between permeable and impermeable layers	In most natural drainage basins; the dominant subsurface flow
Baseflow (groundwater flow)	Slow (days–months)	Water seeps slowly through rock pores/fractures towards the river	Sustains river flow during dry periods; dominant in basins with permeable geology
Stemflow	Fast	Water runs down stems and trunks of vegetation to the ground surface	In heavily vegetated basins; relatively small volume
Throughfall	Fast	Water drips through gaps in the vegetation canopy	When interception capacity is exceeded

Infiltration and Percolation

Infiltration is the downward movement of water from the surface into the soil. The rate at which water can infiltrate is called the infiltration capacity (measured in mm/hr).

Factors affecting infiltration capacity: