The drainage basin water cycle
- On a smaller scale (variable from regional to local, depending on the size of the drainage basin) the drainage basin is a subsystem within the global hydrological cycle.
- It is an open system as it has external inputs and outputs that cause the amount of water in the basin to vary over time.
- These variations can occur at different temporal scales, from short-term hourly through to daily, seasonal and annual
- A drainage basin can be defined as the area of land drained by a river and its tributaries, and is frequently referred to as a river catchment.
- The boundary of a drainage basin is defined by the watershed, which is usually a ridge of high land which divides and separates waters flowing to different rivers.
- Drainage basins can be of any size, from that of a small stream possibly without tributaries up to a major international river flowing across borders of several countries.
Diagram of The drainage basin cycle
Catchment:
The area of land drained by a river and its tributaries.
Watershed:
The highland which divides and separates waters flowing to different rivers.
Precipitation
For precipitation (rain, snow, hail) to form, certain conditions are needed:
• air cooled to saturation point with a relative humidity of 100 per cent
• condensation nuclei, such as dust particles, to facilitate the growth of droplets in clouds
• a temperature below dew point.
There are three main triggers for the development of rainfall, all of which involve uplift and cooling and condensation (Figure 1.5).
As far as the impacts on the drainage basin hydrological system are concerned, there are six key influencing factors:
amount of precipitation,
type of precipitation
Seasonality.
Intensity of precipitation
Variability
The distribution of precipitation within a basin.
The amount of precipitation,
which can have a direct impact on drainage discharge: as a general rule, the higher the amount the less variability in its pattern.
The type of precipitation
rain, snow or hail): the formation of snow, for example, can act as a temporary store and large fluxes (flows) of water can be released into the system after a period of rapid melting resulting from a thaw.
Seasonality.
In some climates, such as monsoon, Mediterranean or continental climates, strong seasonal patterns of rainfall or snowfall will have a major impact on the physical processes operating in the drainage basin system.
Intensity of precipitation
has a major impact on flows on or below the surface. It is difficult for rainfall to infiltrate if it is very intense, as the soil capacity is exceeded.
Variability can be seen in three ways:
• Secular variability happens long term, for example as a result of climate change trends.
• Periodic variability happens in an annual, seasonal, monthly or diurnal context.
• Stochastic variability results from random factors, for example in the localisation of a thunderstorm within a basin.
The distribution of precipitation within a basin.
- The impact is particularly noticeable in very large basins such as the Rhone or the Nile, where tributaries start in different climatic zones.
- At a local scale and shorter time scale the location of a thunderstorm within a small river basin can have a major impact temporarily as inputs will vary, with contrasting
storm hydrographs for different stream tributaries.
Convectional rainfall
This type of rainfall is common in tropical areas, and in the UK during the summer. When the land becomes hot, the air above it becomes warmer, expands and rises. As it rises, the air cools and its ability to hold water vapour decreases. Condensation occurs and clouds develop. If the air continues to rise, rain will fall.
1 The Earth’s hot surface heats the air above it
2 Rain - The heated air rises, expands and cools; condensation takes place
3 Rising warm air - Further ascent causes more expansion and more cooling: rain takes place
4 Cool air descends and replaces the warm air
Cyclonic rainfall
This happens when warm air, which is lighter and less dense, is forced to rise over cold, denser air. As it rises, the air cools and its ability to hold water vapour decreases. Condensation occurs and clouds and rain form.
Orographic rainfall
When air is forced to rise over a barrier, such as a mountain, it cools and condensation takes place forming rain. The leeward (downwind) slope receives relatively little rain, which is known as the rain shadow effect.
Precipitation data
- It is important to recognise that data on precipitation may not always be reliable.
- In the UK, 200 automated weather stations spaced about 40 km apart continuously collect precipitation data.
- In the semi-arid Sahel countries of Mali, Chad and Burkina Faso roughly 35 weather stations collect data across an area of 2.8 million square kilometres (more than 10 times the area of the UK).
- Major storms can easily fall between these weather stations because rainfall is geographically patchy, especially when it is non-frontal.
- Understanding rainfall patterns and trends is critical in semi-arid areas but data reliability in these regions is often low.
Rain shadow
- A rain shadow is a dry area on the leeward (downwind) side of the mountain.
- It receives little rainfall as the mountains shelter it from rain-producing weather systems.
- As the moist air is forced to rise on the windward side of the mountain, rainfall occurs as a result of adiabatic cooling (when the volume or air increases but there is no addition of heat), and condensation to dew point.
- The air, without much water left in it, is then drawn over the mountains where it descends and is adiabatically warmed by compression.
- This leads to a very dry ‘shadow’ area, for example, the Owens Valley is in the rain shadow of the Sierra Nevada range in California.
Interception
- Interception is the process by which water is stored in the vegetation.
- It has three main components: interception loss, throughfall and stem flow.
- Interception loss from the vegetation is usually greatest at the start of a storm, especially when it follows a dry period.
factors impacting interception
- The interception capacity of the vegetation cover varies considerably with the type of tree, with the dense needles of coniferous forests allowing greater accumulation of water.
- There are also contrasts between deciduous forests in summer and in winter - interception losses are around 40 per cent in summer for certain Chiltern beech forests, but under 20 per cent in winter.
- Meteorological conditions also have a major impact.
- Interception varies by vegetation cover.
- Coniferous forest intercepts 25-35 per cent of annual rainfall, whereas deciduous forest only 15-25 per cent and arable crops
10-15 per cent. - Wind speeds can decrease interception loss as intercepted rain is dislodged, and they can also increase evaporation rates.
- The intensity and duration of rainfall is a key factor too.
- As the amount of rainfall increases, the relative importance of interception losses will decrease: as the tree canopies become saturated, so more excess water will reach the ground.
- There are also variations for agricultural crops, with interception rates increasing with crop density.
Interception loss:
- This is water that is retained by plant surfaces and later evaporated or absorbed by the vegetation and transpired.
- When the rain is light, for example drizzle, or of short duration, much of the water will never reach the ground and will be recycled by this process (it’s the reason you can stand under trees when it is raining and not get wet).
Throughfall:
This is when the rainfall persists or is relatively intense, and the water drops from the leaves, twigs, needles, etc.
Stem flow:
This is when water trickles along twigs and branches and then down the trunk.
Infiltration
Infiltration is the process by which water soaks into (or is absorbed by) the soil. The infiltration capacity is the maximum rate at which rain can be absorbed by a soil in a ‘given condition’ and is expressed in mm/hr. The rate of infiltration depends on a number of factors, as shown in Figure 1.6.
Such as
Duration of rainfall
Antecedent soil moisture
Soil porosity
Vegetation cover
Raindrop size
Slope angle
Duration of rainfall
- Infiltration capacity decreases with time through a period of rainfall until a more or less constant low value is reached.
