P2

Question 1

The drainage basin water cycle

Answer 1

• 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.

Question 2

Diagram of The drainage basin cycle

Answer 2

Question 3

Catchment:

Answer 3

The area of land drained by a river and its tributaries.

Question 4

Watershed:

Answer 4

The highland which divides and separates waters flowing to different rivers.

Question 5

Precipitation
For precipitation (rain, snow, hail) to form, certain conditions are needed:

Answer 5

• 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).

Question 6

As far as the impacts on the drainage basin hydrological system are concerned, there are six key influencing factors:

Answer 6

amount of precipitation,

type of precipitation

Seasonality.

Intensity of precipitation

Variability

The distribution of precipitation within a basin.

Question 7

The amount of precipitation,

Answer 7

which can have a direct impact on drainage discharge: as a general rule, the higher the amount the less variability in its pattern.

Question 8

The type of precipitation

Answer 8

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.

Question 9

Seasonality.

Answer 9

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.

Question 10

Intensity of precipitation

Answer 10

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.

Question 11

Variability can be seen in three ways:

Answer 11

• 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.

Question 12

The distribution of precipitation within a basin.

Answer 12

• 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.

Question 13

Convectional rainfall

Answer 13

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

Question 14

Cyclonic rainfall

Answer 14

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.

Question 15

Orographic rainfall

Answer 15

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.

Question 16

Precipitation data

Answer 16

• 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.

Question 17

Rain shadow

Answer 17

• 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.

Question 18

Interception

Answer 18

• 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.

Question 19

factors impacting interception

Answer 19

• 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.

Question 20

Interception loss:

Answer 20

• 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).

Question 21

Throughfall:

Answer 21

This is when the rainfall persists or is relatively intense, and the water drops from the leaves, twigs, needles, etc.

Question 22

Stem flow:

Answer 22

This is when water trickles along twigs and branches and then down the trunk.

Question 23

Infiltration

Answer 23

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

Question 24

Duration of rainfall

Answer 24

• Infiltration capacity decreases with time through a period of rainfall until a more or less constant low value is reached.

Question 25

Antecedent soil moisture

Answer 25

• The rate of infiltration also depends on the amount of water already in the soil (antecedent soil moisture) as surface or overland flow will take place when the soil is saturated.

Question 26

Soil porosity

Answer 26

• The rate of infiltration also depends on the amount of water already in the soil (antecedent soil moisture) as surface or overland flow will take place when the soil is saturated.

Question 27

Vegetation cover

Answer 27

• The type, amount and seasonal changes in vegetation cover are a key factor, with infiltration far more significant in land covered by forests (50 mm/hour) or moorland (42 mm/hour), hence the recent drive to vegetate upland catchments that flow into areas liable to flooding. Permanent pasture has infiltration rates of 13-23 mm/hour depending on grazing density and soil type.

Question 28

raindrop size

Answer 28

• The nature of the soil surface and structure is also important. Compacted surfaces inhibit infiltration (around 10 mm/hour), especially when rain splash impact occurs.

Question 29

Slope angle

Answer 29

• Slope angle can also be significant: very steep slopes tend to encourage overland run-off, with shallower slopes promoting infiltration.

Question 30

Infiltration capacity:

Answer 30

The maximum rate at which rain can be absorbed by a soil.

Question 31

Surface run-off:

Answer 31

The movement of water that is unconfined by a channel across the surface of the ground. Also known as overland flow.

Question 32

Flows and transfers

Answer 32

Overland flow (variously known as surface run-off or direct overland flow

Throughflow

Percolation

Saturated overland flow

Groundwater flow (also known as base flow or interflow)

Channel flow

Evapotranspiration (EVT)

Potential evapotranspiration (PEVT)

Question 33

Overland flow

Answer 33

• (variously known as surface run-off or direct overland flow on account of its rapidity in reaching the channel) is a concept developed by Horton.
• For this type of flow to occur, precipitation intensity must exceed the infiltration rate.
• Circumstances include an intense torrential storm, persistently high levels of precipitation over a longer period, or the release of very large quantities of melt water from the rapid melting of snow.
• Alternatively, bare, ‘baked’ unvegetated surfaces, which commonly occur in arid or semi-arid regions, also lend themselves to overland flow as this type of ground has very limited infiltration capacity.
• This type of flow is the primary agent of soil erosion as sediment is removed by a range of erosive processes: rain splash, sheet, rill and gully erosion.
• Direct overland flow occurs once depression storage capacity in puddles has been exceeded.
• Overland flow is also a feature of many urban areas (see page 18), especially when the capacity of storm drains and sewers has been exceeded.

Question 34

Throughflow

Answer 34

refers to the lateral transfer of water down slope through the soil via natural pipes and percolines (lines of concentrated water flow between soil horizons to the river channel). While slower than direct overland flow, this shallow transfer can occur quite rapidly in porous, sandy soils.

Question 35

Percolation

Answer 35

can be regarded as a continuation of the infiltration process; it is the deep transfer of water into permeable rocks - those with joints (pervious rocks such as carboniferous limestone) or those with pores (porous rocks such as chalk and sandstone).
The throughflow percolation route is much more likely to be associated with humid climates with vegetated slopes.

Question 36

Saturated overland flow

Answer 36

is a much slower transfer process as it results from the upward movement of the water table into the evaporation zone. After a succession of winter storms (for example, in the UK during winter 2015) the water table rises to the surface in depressions and at the base of hill sides.
This leads to saturated overland flow making a major contribution to channel flow.

Question 37

Groundwater flow (also known as base flow or interflow)

Answer 37

is the very slow transfer of percolated water through pervious or porous rocks. It is a vital regulatory component in maintaining a steady level of channel flow in varying weather conditions.

Question 38

Channel flow

Answer 38

takes place in the river once water from the three transfer processes - overland flow, throughflow or groundwater flow - reaches it. Direct channel precipitation is added to channel storage. depend on the time of year, the type and amount of vegetation cover, the degree of availability of moisture in the atmosphere and the length of growing season.

Question 39

Evapotranspiration (EVT)

Answer 39

is the combined effect of evaporation and transpiration. EVT represents the most important aspect of water loss to the atmosphere, accounting for the removal of nearly 100 per cent of the annual precipitation in arid and semi-arid areas, and around 75 per cent in humid areas. Obviously over ice/snow fields, bare rock slopes and soils, desert areas and the majority of water surfaces, the losses are purely evaporative.

Question 40

Potential evapotranspiration (PEVT)

Answer 40

is the water loss that would occur if there was an unlimited supply of water in the soil for use by vegetation. Therefore, the difference between PEVT and EVT is much greater in arid areas than in humid areas.

Question 41

Figure 1.8 How the various flows operate within the drainage basin system

Answer 41

Question 42

Physical factors that influence the drainage basin cycle

Answer 42

Climate

Soils

Geology

Relief

Vegetation

Question 43

Climate

Answer 43

Climate has a role in influencing the type and amount of precipitation overall and the amount of evaporation, i.e. the major inputs and outputs. Climate also has an impact on the vegetation type.

Question 44

Soils

Answer 44

Soils determine the amount of infiltration and throughflow and, indirectly, the type of vegetation.

Question 45

Geology

Answer 45

Geology can impact on subsurface processes such as percolation and groundwater flow (and, therefore, on aquifers). Indirectly, geology alters soil formation.

Question 46

Relief

Answer 46

Altitude can impact on precipitation totals.
Slopes can affect the amount of run-off.

Question 47

Vegetation

Answer 47

The presence or absence of vegetation has a major impact on the amount of interception, infiltration and occurrence of overland flow, as well as on transpiration rates.

Question 48

Figure 1.9a and Figure 1.9b show contrasting hydrological cycles in two different areas with completely different physical factors. This leads to contrasting inputs, stores, flows and outputs.

Answer 48

• Area A is a semi-arid area, for example on the fringe of the Atacama Desert in northern Chile. It has a low level of water security as there is very little storage potential, and outputs exceed inputs. Other sources are not accessible (fossil water, melt water from the cryosphere).
• Area B is a temperate rainforest area in southern Chile with a high level of water security; inputs of precipitation exceed outputs and there is also abundant groundwater storage.