Relation between underwater irradiance and quantum irradiance in dependence on water transparency at different depths in the water bodies

Abstract

Abstract. The ratio between underwater quantum irradiance (q) and irradiance (E) for derlov's [ 1976] oceanic and coastal water types and for 11 Estonian and Finnish lakes was studied. This ratio was found to depend on the depth in the water body and the transparency of the water. The ratio q/E for the photosynthetically active radiation (PAR) region of the spectrum may differ from its value in air by up to 24%. The results of the present paper can be used to convert the underwater radiation data from units of W m -2 to gmol s '• m '2 (or vice versa) and to estimate the errors caused by applying the "air" value of q/E instead of its real value in the water. derlov [1976] presents data on the ratio (E/q)350.700 for his water types L II, III, 1, and 3. The biggest difference between E/q at the surface and in the deeper layers of ocean was found for type I, where, beginning from a depth of 40 m, this difference is about 16%. We computed the vertical profiles of (q/E)4oo-7oo for Jerlov's water types I, II, III, 1, 3, 5, 7, and 9. The spectral distribution of the solar radiation just below the water surface was taken analogical to that presented by Dera [1992] for the conditions of medium air turbidity and a solar zenith angle of 40 ø. Our results (Figure 1) correspond well with Jerlov's data (taking into account that they are the reciprocal of our q/E and for a slightly different spectral range). Table 1 presents the values of (q/E)4oo-7oo for a depth of 20 m, computed by us for Jerlov's water types. These values are close to the asymptotic ones for each type. In Table 1 also, the wavelengths (Amax) are shown, for which the spectral irradiance at this depth has the maximum value, and the relative difference (in percent) between q/E at 20 7749

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“…Quantum PAR irradiance Q PAR is given in PAR quanta per unit area and unit time; transformation to PAR irradiance in units of power (E PAR ) depends on the spectral distribution of irradiance. For white light, which can here be assumed for the atmosphere, we have Q PAR [µmol m -2 s -1 ] = 4.60 × E PAR [W m -2 ]; in water (and ice) the spectrum of light is different and the factor 4.60 is replaced by a factor in the range 4.8-5.5, large values for more turbid waters (Reinart et al 1998). The correction factor of 5.0 can thus be taken as a representative one in the present study.…”

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“…where k is the wavelength of light, h is the Planck's constant 6.625 9 10 -34 J s, and c 0 is the speed of light 3 9 10 8 m s -1 (Reinart & Arst, 1998). K d(k) was defined as the slope of the least-square regression of ln [E d(z,k) /E s(0-,k) ] with respect to depth over the depth interval just below the water surface to Z 1% , where most of the attenuation of solar energy takes place (Kirk, 2003).…”

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“…The short-wave surface irradiance flux Q S is obtained generally from an atmospheric radiative transfer model and is converted from W m −2 to the units of µE m −2 s −1 with the constant factor 1/0.215 (Reinart et al, 1998). ε PAR is the coefficient determining the portion of PAR in Q S .…”