3-1
GEOG415
Lecture 3A: Interception
What is interception?
Canopy interception (C)
Litter interception (L)
Interception ( I = C + L )
Precipitation (P)
Throughfall (T)
Stemflow (S)
Net precipitation (R)
Dunne and Leopold (1978, Fig. 3-1)
Interception “storage” - expressed as mm of water.
What does it mean?
What happens to the intercepted water?
3-2
Canopy interception is defined by:
C = P - (T + S)
What control the amount of canopy interception?
(1) vegetation
(2) storm characteristics
(3) ??
Measurement of interception
Above canopy precipitation (gross precipitation)
Throughfall
Stemflow
Litter interception
3-3
Interception by forests
The amount of throughfall and stemflow for individual storm
increases with gross precipitation. Deciduous and coniferous
trees have similar canopy interception. This pattern is similar
regardless of geographical regions.
Dunne and Leopold (1978, Fig. 3-2)
Problem 3-1 from DL
Calculate net rainfall under hardwoods and conifers for:
(1) sequence of 4 storms applying 50 mm each.
(2) sequence of 20 storms applying 10 mm each.
Stemflow is a minor component of gross rainfall in most
forests (less than a few percent), but could be significant for
certain types of trees and crops.
3-4
Annual or seasonal total interception shows different pattern.
Interception varies with tree types and geographical regions.
Why?
Number of Median canopy
observations interception (%)
Deciduous forest
All data
10
13
Coniferous forest
Rainfall only
Rain and snow
European data
North American data
Taiwan
11
26
9
27
1
22
28
35
27
8
Dunne and Leopold (1978, Table 3-1)
Recent study in Prince Albert Model Forest, SK
Type of tree
Growth stage
Canopy cover
Buttle JM, Creed IF, Pomeroy JW. 2000. Advances in Canadian
forest hydrology, 1995-1998. Hydrological Processes 14: 1551-1578.
3-5
Interception by grasses and crops
Interception varies with plant height and cover density.
→ varies with growing season
Alfalfa
Corn
Soybean
Oats
Intercepation (% of rainfall)
Growing season Low vegetation
36
22
16
3
15
9
7
3
Dunne and Leopold (1978, Table 3-2)
Interception and water balance
In agriculture and forestry, interception is viewed as a “loss”
of moisture. Is this really true?
Interception vs transpiration?
evaporation
radiation
Root
uptake
3-6
Condensation of fog
How does the morning dew collect on leaves?
‘Negative’ interception
Potential source of groundwater in arid regions (e.g. Kenya).
Ingraham NL, Matthews RA. 1988. Fog drip as a source of groundwater recharge in Northern Kenya.
Water Resources Research 24: 1406-1410.
Interception during heavy storm
During heavy storms, the amount of interception is relatively
insignificant compared to the total amount of precipitation.
Why?
rainfall intensity
(mm/hr)
Interception still has significant roles. What are they?
gross rainfall
time (hr)
3-7
Snow interception
Snow is easily intercepted by coniferous trees.
What happens to intercepted snow?
PAMF
Jack pine
Pomeroy JW et al. 1998. An evaluation of snow processes for land surface modelling. Hydrological
Processes 12: 2339-2367.
Seasonal pattern?
Tree types?
Buttle JM, Creed IF, Pomeroy JW. 2000. Advances in Canadian
forest hydrology, 1995-1998. Hydrological Processes 14: 1551-1578.
3-8
GEOG415
Lecture 3B: Energy Balance
Radiation and wave length
Radiation can be considered as electromagnetic wave. Solar
radiation has relatively short wavelengths, while the
radiation from the earth has long wavelengths.
Wien’s law:
λ max =
2900 µm K
T
λmax: Wavelength at the maximum intensity (µm)
T: Temperature of the body (K)
3-9
The solar-energy input depends on the angle of the surface
to the sun’s rays.
→ Four seasons
→ Climatic regions
→ Microclimate affected by the slope angle and aspect
Christopherson (2000, Fig. 2-9)
The unit of radiation is W m-2 or J s-1 m-2. In climate databases,
they are commonly reported as daily radiation (MJ m-2 day-1).
The average insolation at the top of the atmosphere is called
solar constant (= 1372 W m-2).
3-10
Radiation balance
Shortwave radiation
Direct and diffuse
Depends on the light angle and cloud cover
Reflection and albedo
Christopherson (2000, Fig. 4-4)
Fig. 4-5
3-11
Long wave radiation
Radiation by ground surface
Radiation by atmosphere
Stefan-Boltzman law
Radiation (E) emitted by a body (e.g. soil, water, plants) is a
function of the surface temperature (T)
E = εσT4
σ = 5.67 × 10-8 W m-2 K-4
ε: emissivity (soil 0.9-0.98, water 0.92-0.97, snow 0.82-0.99)
Net radiation = incoming - outgoing radiation
Christopherson (2000, Fig. 4-1)
3-12
Clear-sky insolation is essentially a function of the latitude
only (why?), and its values are found in DL, p.107.
Actual incoming and outgoing radiation depends on many
factors →
Measurement of radiation
Different instruments are used for different purposes.
- incoming, outgoing, or net radiation
- wavelength
Sources of radiation data
Canadian radiation data base
Phillips D.W. and Aston, D., 1980. Canadian solar radiation data.
Library call number CA1 /EP 215/80R02
North American radiation model by NASA
http://eosweb.larc.nasa.gov/sse/
3-13
Spectral irradiance (W m-2 µm-1)
Pyranometer for incoming shortwave radiation.
Wavelength (µm)
Spectral characteristics of solar radiation and the pyranometer.
Net radiometer
3-14
Energy balance
Heat storage = net radiation - conduction - convection
- latent heat
Heat storage ∝
Incoming radiation = constant
Outgoing radiation ∝
Conduction and convection (sensible heat) ∝
Latent heat ∝
Christopherson (2000, Fig. 4-9)
Implications in hydrology?
3-15
Energy balance equation of the earth surface
G = Rnet - H - LE
G: Ground (or water) heating
H: Sensible heat transfer to the atmosphere
LE: Latent heat transfer to the atmosphere
Over a short period, G may be significant (e.g. seasonal
temperature fluctuation). Over a longer period, G is negligible.
Net radiation in W m-2
Christopherson (2000, Fig. 4-17)
3-16
Sensible heat in W m-2
Latent heat in W m-2
Christopherson (2000, Fig. 4-19)
Fig. 4-18