Lecture 2: interception and infiltration

Question 1

What re the three components of interception?

Answer 1

Interception loss
Throughfall
Stemflow

Question 2

How does radiant energy balance affect precipitation?

Answer 2

• Affects temperature of leaves, intercepted water and air around the canopy. Tends to be higher in forests as lower albedo.

Question 3

How does Interception storage affect interception?

Answer 3

Dry vegetation has max storage, whilst for wet leaves, leaf drip and evaporation renew interception storage.

Question 4

How do vegetation characteristics affect interception?

Answer 4

Controls maximum water storage in the canopy by:

• Leaf shape, size, roughness, flexibility, orientation
• The total biomass present
• Deciduous or evergreen habitat

Question 5

How does rainfall intensity affect interception?

Answer 5

• Controls rate of reaching storage capacity
• low intensity = evaporation may occur before canopy saturated
• relative importance of interception decrease with time.

Question 6

How do windspeed / turbulence affect interception?

Answer 6

• Wind speed controls evaporation from leaves

- Affect the supply of unsaturated air and heat transfer to the canopy zone

Question 7

How does the phase of precipitation interact with interception?

Answer 7

• Snow more susceptible to interception, however evaporation for snow is much lower.
• Trees can reduce the amount of snowfall reaching ground by 80%

Question 8

For fir trees, in both summer and winter, what are the main components of interception?

Answer 8

1. ) Throughfall

2. ) interception loss

Question 9

For beech trees, what are the relative proportions of throughfall, stemflow and interception loss?

Answer 9

Throughfall = 70%

Stemflow and interception loss = 15% each (with canopy interception loss slightly smaller in winter)

Question 10

How is interception loss calculated?

Answer 10

Precipitation above canopy - net precipitation below

Question 11

How can throughfall be measured?

Answer 11

Using gauges placed below the forest canopy.

Question 12

How can stemflow be measured?

Answer 12

Using guttering around tree

Question 13

How might net rainfall (encompassing through and stem flow) be calculated?

Answer 13

Large sheet gauges

Question 14

What is the vertical process of water movement called in the soil zone compared to the intermediate zone?

Answer 14

Soil zone - infiltration

Intemediate zone - percolation

Question 15

What is the difference between infiltration rate and infiltration capacity?

Answer 15

Rate - volumetric rate of seepage (mm/hr) into the top few cms of soil
Capacity - the maximum rate at which water can be absorbed by the soil at a steady rate.

Question 16

How does the rate of infiltration vary across a rainstorm event?

Answer 16

Initially rapid (more so for dry than wet soil) eventually levelling off at the infiltration capacity.

Question 17

Under ponding, what physical characteristic affects infiltration rate?

Answer 17

Soil with open cracks with have higher infiltration rate that more rapidly declines than soil without.

Question 18

What factors affect infiltration capacity?

Answer 18

• Soil properties
• Land management (ploughing aerates soil so there is more infiltration, but machinery compacts it so less)
• Soil faunal activity - increased pore space so more infiltration
• Vegetation
• Slope
• Geomorphic location (convex/concave/uniform slope)

Question 19

Describe the relative permeability of massive, prismatic/blocky, platy and crumby/granular soil structures?

Answer 19

Massive - poor permeability
Prismatic / blocky - good vertical permeability
Crumby / granular - Good general permeability
Platy - good horizontal permeability

Question 20

What are the three infiltration conditions that have different characteristics in the soil profile?

Answer 20

No ponding - supply controlled
Ponding condition from above - soil controlled
Ponding condition from below - infiltration controlled (no infiltration)

Question 21

Which soil types accumulate infiltration most rapidly?

Answer 21

Sandy loam (gravely>pasture>mulch>wheat) then silt loam then finally (most slowly) clay.

Question 22

What is characteristic of unimproved pasture?

Answer 22

No interception, very high runoff

Question 23

What is different about forests without litter / humus in terms of their infiltration?

Answer 23

Have a much higher HOF and less infiltration

Question 24

Describe and name the different point methods of measuring infiltration capacity?

Answer 24

Point measurements can by collected by :

• Single ring infiltrometers (allow vertical and lateral flow into soil)
• Double ring infiltrometers (allow vertical flow only from inner tube, lateral also from outer)
• Twin ring infiltrometers
• Tension infiltrometer (air entered via syringe through porous plate - water flows into smaller pores only)

Question 25

How else might infiltration be measured? Give some advantages and disadvantages of this method.

Answer 25

Rainfall simulation
Used to control / standardize experiment conditions - can create and replicate identical rainfall events whilst varying another parameter such as foliage. Variety of durations / scales / apparatus / methods.
Disadvantages - lack of correspondence between actual and simulated rainfall, focuses too much on drop size and kinetic energy at the expense of intensity /duration properties, with the influence of drop arrival rate unexplored.