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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?

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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

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Interception by forests

The amount of throughfall and stemflow for individual stormincreases 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.

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Number of Median canopyobservations interception (%)

Deciduous forest All data 10 13

Coniferous forest Rainfall only 11 22 Rain and snow 26 28 European data 9 35 North American data 27 27 Taiwan 1 8Dunne and Leopold (1978, Table 3-1)

Annual or seasonal total interception shows different pattern. Interception varies with tree types and geographical regions.

Why?

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.

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Interception by grasses and crops

Interception varies with plant height and cover density.

→ varies with growing season

Intercepation (% of rainfall)Growing season Low vegetation

Alfalfa 36 22Corn 16 3Soybean 15 9Oats 7 3Dunne 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?

radiation

evaporation

Root uptake

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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?

Interception still has significant roles. What are they?

time (hr)

rain

fall

inte

nsity

(m

m/h

r)

gross rainfall

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Snow interception

Snow is easily intercepted by coniferous trees.

What happens to intercepted snow?

Pomeroy JW et al. 1998. An evaluation of snow processes for land surface modelling. Hydrological Processes 12: 2339-2367.

PAMFJack pine

Seasonal pattern?

Tree types?

Buttle JM, Creed IF, Pomeroy JW. 2000. Advances in Canadian forest hydrology, 1995-1998. Hydrological Processes 14: 1551-1578.

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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:

λmax: Wavelength at the maximum intensity (µm)T: Temperature of the body (K)

TKµm2900

max =λ

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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).

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Radiation balance

Shortwave radiation

Direct and diffuse

Depends on the light angle and cloud cover

Reflection and albedoChristopherson (2000, Fig. 4-4)

Fig. 4-5

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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)

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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 basePhillips D.W. and Aston, D., 1980. Canadian solar radiation data.Library call number CA1 /EP 215/80R02

North American radiation model by NASAhttp://eosweb.larc.nasa.gov/sse/

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Pyranometer for incoming shortwave radiation.

Spectral characteristics of solar radiation and the pyranometer.Wavelength (µm)

Spec

tral i

rrad

ianc

e (W

m-2

µm-1

)

Net radiometer

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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?

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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.

Christopherson (2000, Fig. 4-17)Net radiation in W m-2

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Sensible heat in W m-2 Christopherson (2000, Fig. 4-19)

Latent heat in W m-2 Fig. 4-18