TechDis Accessibility Essentials 3: Creating Accessible Presentations

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TechDis Accessibility Essentials 3 Creating
Accessible Presentations

• This presentation on Measuring Infiltration
  depicts how a staggered reveal of complex images
  can aid understanding and retention in a learner.
  
• Navigate through the slide sequentially,
  revealing the images and associated text.

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Measuring Infiltration

• During first half minute...
• The water rapidly leaks away into the pore
  spaces in the soil. Rate of infiltration will
  depend on previous weather. If the soil is
  already wet less infiltration will take place.
  Sandy or well structured soils (e.g. crumb
  structures) will encourage rapid infiltration.

• Place a sawn off tube in the ground and fill to a
  marked line with water.
• After 30 seconds measure new level

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Measuring Infiltration

• End of first half minute
• The new water level is recorded and the water
  level rapidly refilled to the original marker
  line. The timer continues.
• The first point can be plotted on the graph.

• Record new water level and top up again

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Measuring Infiltration

• After first full minute
• In the second half minute the water level
  dropped less rapidly because there are fewer
  unfilled soil pores near the infiltration ring.
  
• Capillary action in dry soil is effective at
  sucking in water. Once the soil is wet,
  capillary forces become less important and
  infiltration slows.

• Record new water level and top up again

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Measuring Infiltration

• Second minute
• Depending on the difference between the first
  and second readings you may wish to either (i)
  continue recording at 30 second intervals or (ii)
  record every minute only.
• In this example we will next measure at the end
  of the second minute as shown below.

• Record new water level and top up again

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Measuring Infiltration

• Subsequent Minutes
• As the soil beneath the infiltration ring
  becomes saturated the main control on rate of
  infiltration is gravity pulling water through the
  matrix of pore spaces. This represents the true
  infiltration rate of the soil.
• This is normally given in mm/minute and can be
  read off the graph as the final horizontal line.

• Record new water level and top up again

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Measuring Infiltration

• Factors Affecting Infiltration Rates 1
• Infiltration curves can give information about
• Differing weather conditions for the same soil.
• Differing soil types for the same weather.
• In the top graph, infiltration rates are plotted
  for the same soil in different weather conditions
  - yellow drought red normal blue wet.
  Steepness of the curve reflects how long it takes
  to fill soil pore spaces - this depends on amount
  of water already there. The final flat part of
  the curve represents the rate of flow through
  full pore spaces - i.e. an indication of soil
  porosity.
• In the lower graph, infiltration rates have been
  plotted for different soils in similar weather
  conditions. Steepness reflects how long it takes
  to fill the soil pore spaces - dependent on soil
  porosity (because weather was the same for each).
  The final level represents rate of flow through
  full pore spaces - also dependent on soil
  porosity. In these examples green is the soil
  with highest infiltration rate and yellow the
  lowest.

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Measuring Infiltration

• Factors Affecting Infiltration Rates 2
• Management of soil influences infiltration rates.
  Management can influence positively or
  negatively.
• The diagram show a soil ped - a collection of
  textural particles (clay, sand etc) glued
  together by organic material such as humus (shown
  brown) and capillary forces of water beads
  between soil grains (shown blue).
• After heavy rainfall the soil pores fill with
  water and instead of capillary action holding
  grains together, excess water pressure pushes
  them apart (positive pore water pressure). This
  makes the peds very fragile.
• If the soil is stressed in these conditions - by
  heavy machinery (e.g. tractors) or trampling by
  people or livestock - the peds break down
  entirely, resulting in a structureless mass
  described as a poached soil. The destruction of
  peds means that the large pores between peds
  disappear, vastly reducing infiltration rates.

Tractors on wet soil destroy soil structure
Good infiltration Poor infiltration
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Managing Infiltration

• Managing Infiltration Structure Positively
• Good infiltration is a key to a good soil,
  allowing drainage (to avoid waterlogging) and
  aeration (to allow micro-organisms to breed and
  breathe!).
• Good infiltration depends to a large extent on
  good soil structure.
• Soil structure is currently declining in many
  parts of the developed world because artificial
  fertilizer has been used instead of organic
  manure. Whilst artificial fertilizer puts the
  right nutrients in the ground it does not provide
  the raw organic matter for glueing textural
  particles into peds.
• Structure can be improved by
• adding organic fertilizers to the soil
• maintaining an effective crop rotation
• ploughing the soil to break up poached layers
• avoid cultivating the soil during wet conditions

Cross section of sloping field
Higher infiltration rates result in lower erosion
rates as water infiltrates underground, leaving
little for surface erosion.