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 11 22 Rain and snow 26 28 European data 9 35 North American data 27 27 Taiwan 1 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 Intercepation (% of rainfall) Growing season Low vegetation Alfalfa 36 22 Corn 16 3 Soybean 15 9 Oats 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? Interception still has significant roles. What are they? rainfall intensity (mm/hr) 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.
GEOG415 Lecture 3B: Energy Balance 3-8 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 λ max : 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 ).
Radiation balance Shortwave radiation Direct and diffuse Depends on the light angle and cloud cover 3-10 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 = εσt 4 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 Pyranometer for incoming shortwave radiation. Spectral irradiance (W m -2 µm -1 ) 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 = R net - 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 Christopherson (2000, Fig. 4-19) Latent heat in W m -2 Fig. 4-18