Solar Radiation. Radiant energy of light may be transformed to another form of energy Potential energy (through photosynthesis) Heat energy

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1 Light

2 Solar Radiation Radiant energy of light may be transformed to another form of energy Potential energy (through photosynthesis) Heat energy Source of energy for biota

3 Solar Radiation Influences physical properties of lakes Thermal structure Density stratification Water movement

4 Solar Radiation Influences water chemistry of lakes Thermal influence on reaction rates Photodegradation Influence on biotic activity related to nutrient cycling

5 Electromagnetic Spectrum Spectrum of electromagnetic energy is defined in terms of frequency and wavelength In lakes, solar radiation ranges from infrared radiation (greater than 3000 nm) to ultraviolet radiation (100 nm)

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8 Plants and Light Plants absorb light energy in the range from 400 nm to 700 nm This range is known as Photosynthetically Active Radiation (PAR)

9 Nature of light Light is really waves and particles Particle nature is packets of energy - quanta or photons Light is a transverse wave of energy Photon carries energy in a wave conformation

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11 Wavelength and Frequency Wavelength (λ)( ) is the distance between crests of waves λ = c/v x cm sec -1 λ =

12 Energy of a Photon Energy of photon is proportional to frequency and inversely proportional to wavelength e = hv e = energy of photon in ergs h = Planck's constant (6.63 x erg- seconds) v = frequency of the radiation in cycles per second

13 Total Incoming Energy Maximum energy of solar irradiance is found in infrared portion of spectrum

14 Light Impinging on Lakes Amount of light that reaches the surface of a lake is determined by many factors Amount of direct solar radiation per unit of time incident on a perpendicular surface outside the earth's atmosphere is a constant - solar constant (1.97 cal cm -2 min -1 ) Direct solar radiation impinging on the earth's surface is a function of latitude, season, time of day, altitude, and meteorological conditions

15 Light Impinging on Lakes Angle of rays of light is a major factor Time of day Latitude Light is absorbed by O 2, O 3, H 2 O, CO 2 Distance traveled is critical Moisture increases both absorption and scattering Contaminants increase both absorption and scattering

16 Light Impinging on Lakes Indirect solar radiation is a function of scattering Scattering is a function of the fourth power of frequency (UV is reduced by 25% as a result of scattering) Scattering is affected by the same factors as described by absorption but solar height and atmospheric distance are of major importance

17 Reflection of light from surface All radiation does not penetrate Reflection is a function of several factors Angle of sun Surface characteristics Topography Meteorological conditions

18 Reflection of light from surface Lower the angle of incidence, the greater the reflection Reflection of 6.5% of incoming light is common Reflection is 5-6% in summer and 10% in winter

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20 Reflection of light from surface Surface roughness increases reflection Reflection may decrease if waves are very large and angles approach perpendicular

21 Reflection of light from surface Percentage of reflected light is termed the albedo Sun's angle is less of a factor with increasing cloud cover because of increased diffuse radiation Snow cover greatly increases reflection (approximately 75% of incident light is reflected)

22 Reflection of light from surface Algae and submerged macrophytes on the lake surface reflect green light and near-infrared light Longer wavelengths are reflected to a slightly greater degree Light leaving the lake by reflection generally equals light leaving lake by scattering

23 Attenuation of Light Decrease in light intensity (photon flux) as light passes through water Light is attenuated with depth by scattering and absorption Scattering is the deflection of the direction of the photon Absorption is the transfer of energy of a photon to vibrational energy of a molecule (heat)

24 Scattering Scattering is the deflection of light energy by molecular components, solutes, and particulates Shorter wavelengths are scattered more than longer wavelengths by water

25 Scattering Scattering is not uniform throughout the water column because particulates are not evenly distributed Particulates scatter longer wavelengths preferentially Light is scattered by the lake bottom Sand and CaCO 3 scatter more Organics scatter less

26 Absorption Absorption of light by pure water is greatest for long wavelengths DOC absorbs short wavelengths to a greater degree Low concentrations of particulates do not absorb selectively

27 Measures of Attenuation Birgean Percentile Absorption 100 (I o - I z ) Percentile Absorption = I o

28 Measures of Attenuation Light extinction coefficient (η)( ηz z = ln l - ln o l z or l = z l o e-ηz

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32 Attenuation of Light High at IR Minimal at blue Increases in UV 53% of light is generally transformed to heat within one meter

33 Selective Transmission of Light Longer wavelengths are absorbed by water to a greater degree than shorter wavelengths

34 Selective Transmission of Light Shorter wavelengths are scattered to a greater degree by water than longer wavelengths

35 Visible Spectrum - ROYGBIV

36 Color and Sight

37 Selective Transmission of Light Dissolved organic matter absorb UV to a greater degree than longer wavelengths

38 Selective Transmission of Light In pure water, blue light penetrates the deepest Backscatter is predominantly blue in clear water Commonly green penetrates deepest

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41 Transparency An Italian, Secchi,, developed method of measuring transparency in cm disc or larger is lowered till it disappears Secchi depth is the mean depth of disappearance and reappearance Results are erratic near dawn and near dusk

42 Transparency Secchi depths range from centimeters to greater than 40 meters Roughly represents 1-15% 1 15% transmission

43 Secchi Depth