Drainage Basins and Hydrology

The Drainage Basin System

Definition and Boundaries

A drainage basin (catchment or watershed) is the area of land from which all precipitation flows to a common outlet, typically where a river enters a lake, sea, or ocean. The boundary of a drainage basin is the watershed (drainage divide), a line of high ground separating one basin from another. The drainage basin is an open system: energy and matter enter, are transformed within the system, and leave.

System Components

Component	Description	Examples
Inputs	Water entering the system	Precipitation (rainfall, snow, hail, sleet), intercepted water, groundwater inflow from adjacent basins
Outputs	Water leaving the system	Evaporation, transpiration, river discharge to the sea, groundwater outflow
Stores	Water held within the system	Vegetation canopy interception, surface storage (puddles, lakes, ponds), soil moisture, groundwater (aquifers, unconfined and confined), channel storage, snowpack and glaciers
Transfers (flows)	Movement of water between stores	Throughfall, stemflow, overland flow (Hortonian flow), infiltration, percolation, throughflow (interflow), pipeflow, groundwater flow (baseflow), channel flow

The Hydrological Cycle at Basin Scale

The hydrological cycle operates at the drainage basin scale through a series of interconnected flows. Precipitation falling on the basin is intercepted by vegetation (interception store) or reaches the ground surface. Water on the surface may infiltrate into the soil (soil moisture store) or flow over the surface as overland flow. Infiltrated water moves through the soil as throughflow or percolates downward to recharge groundwater (groundwater store). Groundwater moves slowly toward the river channel as baseflow. Water is returned to the atmosphere through evaporation from open water and soil surfaces, and transpiration from vegetation.

The relative importance of each flow pathway depends on the physical characteristics of the basin (climate, geology, soil type, slope, vegetation cover) and human modifications (urbanisation, deforestation, drainage, irrigation).

The Water Balance Equation

The water balance of a drainage basin over a specified time period is:

$P = Q + E \pm \Delta S$

where $P$ is total precipitation, $Q$ is total runoff (river discharge), $E$ is total evapotranspiration, and $\Delta S$ is the change in all storage components combined. Over a long period (typically the hydrological year), $\Delta S$ approaches zero, yielding:

$P = Q + E$

This identity states that all precipitation is ultimately partitioned between runoff and evapotranspiration. The ratio $Q/P$ is the runoff coefficient, which indicates the proportion of precipitation that becomes streamflow. In arid regions, $E/P$ approaches 1 (almost all precipitation returns to the atmosphere). In humid regions with impermeable geology, $Q/P$ may exceed 0.6.

Storm Hydrograph Interpretation

Components of a Storm Hydrograph

A storm hydrograph (flood hydrograph) plots river discharge (typically in m$^3$/s on the y-axis) against time (typically hours on the x-axis) for a specific rainfall event. The key features are:

• Baseflow: the sustained, relatively constant flow maintained by groundwater discharge into the channel between storm events.
• Rising limb: the period during which discharge increases following the onset of rainfall, as water from various sources reaches the channel.
• Peak discharge: the maximum instantaneous discharge during the storm event.
• Lag time: the interval between the centroid of rainfall (or the peak rainfall intensity) and the peak discharge. Short lag times indicate rapid runoff generation; long lag times indicate slow, delayed response.
• Falling limb (recession curve): the period during which discharge declines as the river drains water stored in the channel, on the floodplain, and in the subsurface. The recession curve is often approximately exponential.
• Bankfull discharge: the discharge at which water overtops the channel banks and begins to flow across the floodplain.

Hydrograph Shape and Runoff Generation

The shape of the storm hydrograph reflects the relative contributions of different runoff sources. A flashy hydrograph has a short lag time, steep rising limb, high narrow peak, and rapid recession, indicating that overland flow dominates. A damped hydrograph has a long lag time, gentle rising limb, broader and lower peak, and extended recession, indicating that throughflow and baseflow dominate.

The unit hydrograph technique, developed by Sherman (1932), is a standard method for predicting the hydrograph response of a drainage basin to a given rainfall input. The unit hydrograph is the hydrograph resulting from 1 unit (e.g., 1 cm) of effective rainfall (rainfall that contributes to runoff, after accounting for losses to infiltration and interception) falling uniformly over the basin within a specified duration. The principle of superposition allows the unit hydrograph to be scaled and combined to predict the hydrograph for any storm.

Factors Affecting Storm Hydrographs

Physical Factors

Factor	Effect on Hydrograph	Mechanism
Rainfall intensity	Shorter lag time, steeper rising limb, higher peak discharge	Intense rainfall exceeds infiltration capacity rapidly, generating overland flow (Hortonian overland flow)
Rainfall duration	Longer lag time possible, broader peak, higher total runoff volume	Prolonged rainfall saturates the soil progressively, reducing infiltration capacity over time
Antecedent soil moisture	Wet antecedent conditions: short lag time, high peak; dry conditions: long lag time, lower peak	Saturated or near-saturated soil has limited capacity to absorb additional water, so a greater proportion becomes runoff
Geology	Impermeable rock (clay, granite): flashy hydrograph; permeable rock (chalk, sandstone): damped hydrograph	Permeable rock promotes deep infiltration and groundwater storage, delaying runoff
Soil type	Clay soils: flashy; sandy/loam soils: damped	Clay soils have low infiltration rates; sandy soils drain rapidly, promoting infiltration
Slope angle	Steep slopes: short lag time, high peak discharge	Gravity accelerates overland flow on steep slopes, reducing the time for infiltration
Drainage density	High density: short lag time, flashy response	A dense network of channels collects water rapidly and delivers it to the main channel
Basin size and shape	Large basin: longer lag time; elongated basin: longer lag time; circular basin: shorter lag time	Water from distant parts of large or elongated basins takes longer to reach the outlet
Vegetation cover	Dense vegetation: long lag time, lower peak discharge	Interception reduces effective rainfall; root systems increase infiltration; transpiration reduces soil moisture

Human Factors

Urbanisation. Urbanisation profoundly alters the hydrological response of a drainage basin. The replacement of permeable vegetated surfaces with impermeable surfaces (concrete, asphalt, rooftops) reduces infiltration, increases overland flow, and accelerates the delivery of water to drainage channels. Stormwater drainage systems (gullies, pipes, culverts) further concentrate and accelerate flow. The net effect is a shorter lag time, higher peak discharge, and more frequent and severe flooding. Studies in the UK have found that urbanisation can increase peak discharge by 2--5 times compared to equivalent rural basins.

Deforestation. Removing trees reduces interception (which can account for 10--40% of gross rainfall in dense forests), decreases transpiration, and reduces root-mediated soil structure and infiltration capacity. Research in the Himalayas and Amazon basin has demonstrated that deforestation increases overland flow, reduces baseflow, and makes streamflow more seasonal (higher peaks in the wet season, lower flows in the dry season).

Agricultural drainage. Land drainage (under-drainage with perforated pipes, open ditches) is installed to lower the water table and improve agricultural productivity. However, it accelerates the movement of water through the soil profile, increasing the volume and speed of throughflow reaching the channel, resulting in a shorter lag time and higher peak discharge.

Common Pitfalls: Confusing Overland Flow Types

Two distinct mechanisms generate overland flow, and confusing them is a frequent error. Hortonian overland flow occurs when rainfall intensity exceeds the infiltration capacity of the soil, generating surface runoff regardless of how much water the soil can hold in total. This is common in arid and semi-arid environments, urban areas, and compacted soils. Saturation overland flow (also called the Dunne mechanism) occurs when the soil profile becomes saturated from below (e.g., by rising groundwater or throughflow convergence at the base of slopes), and any additional rainfall cannot infiltrate and flows over the surface. This is common in humid environments with shallow soils. In many temperate environments, saturation overland flow is the dominant mechanism, not Hortonian overland flow. Always specify which mechanism you are describing.

Case Study: The River Severn, UK

The River Severn (approximately 354 km) is the longest river in the UK, draining a basin of approximately 11 400 km$^2$ in mid-Wales and western England. The hydrology of the Severn illustrates the interaction of physical factors across a heterogeneous basin.

Upper Severn (Plynlimon). The headwaters rise on the Ordovician and Silurian shales of the Cambrian Mountains at elevations above 600 m. The steep slopes, impermeable geology, and high rainfall (approximately 2500 mm per year) produce a flashy hydrological response. The Institute of Hydrology operated experimental catchments at Plynlimon from the 1960s to the 1980s, comparing the hydrology of the afforested Severn catchment (conifer plantation) with the adjacent grassland Wye catchment. Key findings included: the forested catchment had higher interception losses (approximately 30% of gross rainfall, compared to approximately 10% for grassland), lower annual runoff, and more moderated peak flows during moderate storms. However, during extreme storms, the forested catchment produced similar peak discharges because the canopy became saturated and interception capacity was exhausted.

Middle and Lower Severn. As the river crosses the lowlands of the Severn Valley, the geology becomes more varied (Permo-Triassic sandstones, which are highly permeable and store large volumes of groundwater). Baseflow contribution increases, and the hydrograph becomes less flashy. The Severn is prone to flooding in its middle and lower reaches, particularly at Shrewsbury, Worcester, and Gloucester. The autumn and winter floods of 2019--2020 produced the highest recorded flows on the Severn at several gauging stations, attributed to a succession of storms (Storm Ciara, Storm Dennis) falling on already-saturated ground.

Case Study: The Ganges-Brahmaputra-Meghna Basin

The Ganges-Brahmaputra-Meghna (GBM) basin, draining approximately 1.7 million km$^2$ across India, Nepal, Bangladesh, Bhutan, and China, illustrates how basin-scale hydrology is shaped by extreme climate variability and large-scale physical processes.

Seasonal hydrology. The GBM basin has a strongly seasonal hydrological regime driven by the South Asian monsoon. Approximately 80% of annual rainfall occurs during the monsoon season (June--September). Discharge on the Ganges at Farakka (the downstream gauging station before it enters Bangladesh) varies from approximately 500 m$^3$/s in the dry season (March--May) to over 70 000 m$^3$/s during the monsoon peak. The Brahmaputra at Bahadurabad shows even greater variation, from approximately 3000 m$^3$/s to over 100 000 m$^3$/s.

Snow and glacial melt. The upper reaches of both the Ganges and Brahmaputra receive significant contributions from snow and glacial melt from the Himalayas and the Tibetan Plateau. Meltwater contributes approximately 10% of annual flow of the Ganges, but this proportion can reach 30--40% during the pre-monsoon period (April--June), when meltwater is critical for irrigation in the Gangetic Plain.

Flooding. The confluence of the Ganges and Brahmaputra in Bangladesh, combined with monsoonal rainfall and Himalayan snowmelt, produces catastrophic flooding almost annually. The 1998 flood inundated approximately 100 000 km$^2$ of Bangladesh (approximately 70% of the country), affecting over 30 million people, destroying approximately 500 000 homes, and causing estimated damage of approximately USD 2 billion. The 2022 floods affected approximately 7.2 million people in Bangladesh's northeastern Sylhet region.

Hydrological Fieldwork and Skills

Measuring Discharge

River discharge ($Q$) is calculated as:

$Q = w \times \bar{d} \times \bar{v}$

where $w$ is channel width, $\bar{d}$ is mean depth, and $\bar{v}$ is mean velocity. Velocity is measured using a flow meter (impeller or electromagnetic) at multiple points across the channel (typically at 0.6 of the depth from the surface, or as the average of measurements at 0.2 and 0.8 of the depth for deeper channels). The velocity-area method integrates velocity measurements across the channel cross-section.

Hydrograph Analysis

Key analytical skills for IB Geography include:

• Calculating lag time from a hydrograph by measuring the time interval between peak rainfall and peak discharge.
• Identifying the proportion of baseflow vs stormflow by separating the hydrograph into its components (a straight line drawn from the point where the rising limb begins to the point where the recession curve returns to baseflow level).
• Comparing hydrographs for different storm events or different drainage basins and explaining the differences in terms of physical and human factors.
• Relating hydrograph characteristics to flood risk, recognising that flashy hydrographs with short lag times and high peak discharges pose a greater flood risk than damped hydrographs.

Common Pitfalls: Assuming a Single Factor Controls Hydrograph Shape

Examination questions frequently ask students to explain the shape of a given hydrograph. A common error is to attribute the hydrograph shape to a single factor (e.g., "the basin is urban, so the hydrograph is flashy"). In reality, the hydrograph shape results from the interaction of multiple factors. A strong answer will identify the dominant factor and then explain how other factors modify its effect. For example, "The short lag time is primarily due to the impermeable clay geology, which limits infiltration. However, the mature forest cover partially offsets this by intercepting rainfall and increasing infiltration through root action, which explains why the peak discharge is lower than would be expected for bare clay."

For an overview of freshwater issues and management, see ./water-scarcity-and-management and ./flood-management. The parent topic page is at ../freshwater-issues.