2.The Drainage Basin as an ‘Open System’

Using the diagram of the drainage basin hydrological cycle to help you; complete the flow diagram by filling in the missing boxes

The Drainage Basin Cycle (ppt, slide 8)

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Why is a Drainage Basin an ‘Open System’?

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Notes:

Key processes within a drainage basin and factors that impact upon it

• Type of precipitation – Orographic, frontal and convectional

• Basin size and shape – look at this briefly but again in a few lessons

• Drainage Density – amount of river tributaries/channels

• Relief - steepness of the land

• Temperature and Climate – colder vs warmer/hotter

• Land Use – rural vs urban

• Rock Type - including permeable vs impermeable rocks and aquifers

• Soil Type - including the structure of it

• Tides and Storms – what influence this has on the drainage basin

Precipitation

For precipitation to form it requires the following conditions: Air cooled to saturation point with a relative humidity of 100%

Condensation(the conversion of a vapour or gas to a liquid) nuclei, such as dust particles, to facilitate the growth of droplets in clouds

A temperature below dew point (Dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water. When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface)

6 key points: (See PPT slide 16)

1. The amount of rainfall can have a direct impact on drainage discharge2. The type of precipitation e.g. the formation of snow, can act as a temporary store and large

fluxes of water can be released into the system after a period of rapid melting resulting from a thaw.

3. Seasonality. In some climates such as monsoon, Mediterranean or continental seasonal patterns of rainfall or snowfall can have a major impact on the physical processes within the drainage basin

4. Intensity of precipitation has a major impact on flows on or below the surface. It is difficult for rainfall to infiltrate if rain is intense as soil is saturated and has no capacity.

5. Variability of rainfall1. Secular(persisting) variability happens long term e.g. result of climate change trends2. Periodic(intervals) variability happens in an annual, seasonal or daily basis3. Stochastic(random) variability results from random factors e.g. localised thunderstorms

6. The distribution of precipitation within a basin. This is more noticeable in large river basins e.g. Rhone or Nile which have tributaries that start in different climatic zones.

Types of precipitation (numbers as per Hodder page 7)

1. = Cyclonic 2. = Convectional3. = Rain Shadow

Further notes to supplement these images, p7 Hodder

1.

2.3.

Heads and tails: Influxes(inflow of water)

Convectional rainfall

A period of sustained, moderately intensive rain; it is associated with the passage of depressions

Cyclonic rainfall Concentrated on the windward slopes and summits of mountains

Orographic rainfall

Water retained by plant surfaces and later evaporated or absorbed by the vegetation and transpired

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

Throughfall Often associated with intense thunderstorms, which occur widely in areas with ground heating such as the Tropics and continental interiors.

Stem flow When the rainfall persists or is relatively intense and the water drops from the leaves, twigs, needles etc.

Rain shadow A dry area on the leeward side of a mountainous area (away from the wind). The mountains block the passage of rain-producing weather systems and cast a "shadow" of dryness behind them.

Fluxes in a Drainage Basin

Infiltration Variations - Match up the diagrams with the statements to show how different factors can affect infiltration rates

Heads and tails: Flows and Transfers

Flows and Transfers - Match up the flow terms with their definitions(next page) and then use them to annotate your version of the diagram below.

Drainage basin system outputs (PPT slide 25)

Physical factors affect the drainage basin cycle

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

Climate Determines the amount of infiltration and throughflow and, indirectly, the type of vegetation

Vegetation Can impact on the subsurface processes such as percolation and groundwater flow (and, therefore, on aquifers). Indirectly, it can alter soil formation

Geology Can impact on the amount of precipitation. Slopes can affect the amount of runoff

Soils The presence or absence of this can have a major impact on the amount of interception, infiltration and occurrence of overland flow, as well as transpiration rates.

BASIN SIZE, SHAPE and DRAINAGE DENSITY

RELIEF

TEMPERATURE AND CLIMATE

Temperature and Climate

LAND USE

ROCK TYPES

SOIL TYPE

TIDES AND STORMS

• Water level fluctuations include astronomical tides, storm surges, and long-term sea level rise or fall. Water level is important in coastal processes and engineering in part because it controls the location of wave influence on shorelines and structures.

• Tides- The tide is the slow rise and fall of the ocean waters in response to the gravitational pull of the moon and the sun

• Storms Surges - The rise of water level above the astronomical tide as a result of meteorological forcing. This forcing is primarily wind but also includes the barometric pressure and, for some coast allocations, local rainfall runoff.

• Sea level rise/fall - The level of the oceans of the world has been gradually increasing for thousands of years. The important change is the relative sea level change, the combined effect of the ocean water elevation and the land-mass elevation change.

• All 3 can have dramatic effects on water levels – surplus which can cause flooding or deficit/shortage which can result in droughts

Human Impacts on the Hydrological Cycle