Lec 5: Interception and Throughfall

Question 1

Name some typical stores in the water cycle

Answer 1

• Surface water
  ‒ Rivers, lakes, reservoirs, wetlands
• Soil moisture
  ‒ Soils, wetlands
• Groundwater storage
  ‒ Below the water table
• Glaciers and snow
• Biomass

Question 2

Name some typical fluxes into and out of the water cycle

Answer 2

• Into
  ‒ Rainfall
  ‒ Melting of snow (depends on available energy)
  ‒ Groundwater inflow (from another system into the system under
  consideration)
  ‒ Irrigation water (when applicable)
• out of
  ‒ Evapotranspiration (loss to the atmosphere; depends on energy)
  ‒ Runoff from catchment (leaves catchment as discharge or stream flow)
  ‒ Groundwater outflow (leaves system under consideration)

Question 3

In what state is a system if the change in storage is = to 0, or not

Answer 3

steady state: = 0

transient state: not = 0

Question 4

give examples of a system in hydrology

Answer 4

System can be a watershed, or hillslope, plot, lake, wetland, stream, reservoir, etc

Question 5

Runoff = Total Precipitation – “Losses”

what are the losses? (5)

Answer 5

• Interception
• Wetland storage
• Evaporation and transpiration
• Infiltration and soil water/groundwater storage
• Lake and reservoir storage

Question 6

What is interception loss?

Answer 6

Proportion of precipitation that may be intercepted by a
vegetation canopy and lost to the atmosphere via evaporation.
without reaching the ground.

OR

Fraction of the gross precipitation input which:
* Does not reach the ground
* Wets and adheres to above ground objects until it is returned to the
atmosphere through evaporation

Question 7

Define the following terms:

• Gross precipitation
• Throughfall
• Stemflow
• Total Interception Loss
• Net precipitation

Answer 7

• Gross Precipitation (P):
  Measured above the canopy or in a clearing
• Throughfall (T):
  Precipitation reaching the ground directly or via canopy drip
• Stemflow (S):
  Water reaching the ground by flowing along tree trunks and plant stems
• Total Interception Loss (It):
  Sum of all canopy interception and losses
• Net Precipitation (N):
  Gross precipitation minus total interception loss

Question 8

Why is interception during rainfall events difficult to measure?

How is it approximated?

Answer 8

in sum: interception is spatially and temporally variable

‒ Spatially variable as a function of vegetation density and type, wind, etc.

‒ Temporally variable as well: interception increases exponentially during a
storm, until the interception capacity is achieved, and the weight of more
rain overcomes the surface tension holding the water on the plants

Estimated by approximating canopy storage during an event

Question 9

What are the units for canopy storage?

Rank vegetation types by canopy storage (from highest storage capacity to lowest)

Answer 9

mm/m2

~ 1 to 5 mm for tropical forests
~1 and 2 mm for conifers
~ 1-1.5 mm for grasses
~ ≤ 1 mm for deciduous trees

Question 10

Name 3 interception measurement types

Answer 10

• Above and below canopy
• Plastic sheets
• Cantilever effect
  measure the displacement of branches, which bend under the weight of precipitation

Question 11

Name 3 reasons for which interception is a critical hydrological process

Answer 11

‒ It can be a significant water
source for evapotranspiration
‒ It has a strong influence on
runoff
‒ Canopy drip gives rise to
larger drops, which can
increase local erosion

Question 12

Which factors influence interception? (external to the vegetation itself)

Answer 12

• precipitation intensity
  ‒ High-intensity rainfall events: water can be delivered too quickly for the plants
  to accommodate
  ‒ Low-intensity rain events: a larger proportion of precipitation will be
  intercepted
• precipitation duration
• precipitation frequency
  ‒ Long delays between consecutive events allow wet vegetation to dry out and
  canopy storage to become available before the next storm
• wind speed
• precipitation type

Question 13

How does wind increase interception loss?

Answer 13

• evaporation
• by blowing water into the interior of
  plants and plastering wet snow against trees and shrubs

Question 14

How does precipitation type affect interception loss?

Answer 14

‒ At temperatures around 0°C, rain can freeze to plants
‒ Snow is more easily blown off or away from plants
‒ Wet snow sticks more easily to plants, and some snowflakes can bridge gaps
between leaves: considerable interception

Question 15

Define free throughfall and release throughfall

Answer 15

free: never intercepted
release: intercepted but then released

Question 16

Define depression storage

Answer 16

‒ Gross precipitation or throughfall retained in puddles, stock ponds, ditches,
and other depressions on the ground surface.
‒ May be of considerable magnitude
‒ Can attenuate the impact of flooding
‒ Can either evaporate or contribute to soil moisture and/or subsurface flow
later on

Question 17

Compare retention and detention

Answer 17

‒ Retention means that storage is held for a long period of time and depleted by
evaporation.
‒ Detention rather refers to short-term storage depleted by flow away from the
storage location

Question 18

Define wetlands

Answer 18

Transitional systems between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water.

Question 19

What are the 3 criteria that wetlands need to meet at least one of?

Answer 19

‒ At least periodically, the land supports predominantly hydrophytes;

‒ The substrate is predominantly undrained hydric soil; and

‒ The substrate is saturated with water or covered by shallow water at some
time during the growing season of the year

Question 20

What is the major hydrologic benefit of wetlands?

Answer 20

They “trap” excess water in their depressions, leading to less runoff and less
intense flooding

Question 21

Define the following terms:
- hydrologic conditions
- hydric soils
- hydrophytic vegetation

Answer 21

Hydrologic conditions: Groundwater (water table or zone of saturation) is at the
surface or within the soil root zone during all or part of the growing season.

Hydric soils: soils that are saturated, flooded, or ponded long enough during the
growing season to develop oxygen-free conditions in the upper six inches

Hydrophytic vegetation: plants typically adapted to wetland and aquatic habitats;
plants which grow in water or on a substrate that is at least periodically deficient in oxygen due to excessive water content.

Question 22

Which characteristics of plants affect interception?

Which type of plants result in the most interception loss?

Answer 22

• bark
• canopy
• leaves

conifers

Question 23

compare soil water pressure in soaked and dry soils

Answer 23

soaked soil: positive water pressure
dry soil: strongly negative soil water pressure

Question 24

True or false: during low-intensity rainfall events, most of the incoming water can be intercepted by vegetation

Answer 24

true

Question 25

True or false: high winds lead to more interception of snow and rain by plants

Answer 25

it depends

Question 26

true or false: interception by plants is limited when a rain event occurs shortly after another one has ended

Answer 26

most likely true. there is a limit to canopy storage capacity