Solar Heating of Water Bodies as Influenced by Their Inherent Optical Properties

A simple one-dimensional model has the world [Jerlov, 1976; Kirk, 1983]. It seems been developed for calculating the development of thermal structure in water bodies under specified meteorological conditions, as a function of their inherent optical properties (spectral absorption coefficients, scattering coefficient). Penetration of radiant solar energy is modeled with a previously derived equation relating attenuation of irradiance in narrow wavebands to inherently likely that the extreme variability of penetration of solar radiation in water bodies is accompanied by comparable variability in thermal behavior, but the relationship is not well characterized. This variability, it should be noted, applies mainly to the shortwave (visible) part of the solar flux. The infrared (A > 700 nm) band, constituting about half the total, is absorption coefficient, scattering coefficient and absorbed so strongly by water itself that most of solar angle. Evaporative and other heat exchange it is converted to heat in the upper half-meter processes at the surface are taken into account. layer, regardless of the optical character of the Vertical heat transfer is calculated by making use water. of a recently published parameterization of the Klein [1980] carried out numerical modeling eddy diffusion coefficient. The calculations show of thermocline development for two optical types marked changes in thermal structure as absorption and scattering are varied over a range of optical water types from coastal seawater to highly colored and turbid inland water. In general terms, increasing the color and/or turbidity shifts the zone of shortwave energy absorption more toward the surface and leads to warmer but shallower mixed layers. Introduction of oceanic water and predicted a higher sea surface temperature and more stable stratification in the more turbid water. Zaneveld et al. [1981] carried out calculations of the rate of heating in the surface mixed layer of marine waters, ranging in optical character from oceanic type I to coastal type 9 in the Jerlov [1976] classification. For a constant mixed layer depth, the heating rate (in degrees Celsius per day) increased progressively as the transparency of the water decreased. In a 10-m-deep mixed layer the An understanding of the annual cycle of solar heating rate increased by 56 to 78% (depending on heating of water bodies is essential for an the assumed diffusivity) in proceeding from understanding of the physical, chemical, and oceanic water type I to coastal water type 9. biological processes going on within them. Solar Harleman [1982], in his modeling of thermocline heating influences these processes not only by development in lakes and reservoirs, found the determining the temperature at which they go on, calculated depth of the upper, wind-mixed, layer but also by bringing about thermal stratification, to decrease markedly as the assumed extinction with its associated chemical and biological coefficient for shortwave radiation increased. stratification of the water column. Because of The Kraus and Turner [1967] model of oceanic its great importance, this phenomenon has been thermocline formation predicted a decrease in extensively studied in marine and inland waters, mixed layer depth with increasing attenuation by both by field measurement and by numerical the water, as did the model of Simpson and Dickey modeling, but no general review of this field will [1981]. Calculations by Woods et al. (1984) for be attempted here. The intention rather is to oceanic waters types I to III (Jerlov consider in detail one particular aspect of the classification) showed that the seasonal rate of solar heating process, namely, the way in which it solar heating below the mixed layer increased as is affected by the inherent optical properties of the aquatic medium. Natural waters vary enormously in their optical character. Some oligotrophic oceanic waters are optically equivalent to distilled water, whereas some inland waters in eroding catchments can present the appearance of liquid mud. The values of inherent optical properties, such as the absorption coefficient (a) and the scattering coefficient (b), or apparent optical properties, such as the vertical attenuation coefficient (Kd) for downward irradiance of the photosynthetic waveband (400-700 nm), vary by orders of magnitude amongst the water bodies of Copyright 1988 by the American Geophysical Union Paper number 8D0283. 0148-0227/88/008D-0283505. O0 the attenuation by the water decreased. In addition to these modeling studies, there are some field observations relating thermal behavior of water bodies to their optical properties. Idso and Foster [1974] found, in a highly eutrophic pond, that when an algal bloom developed, with a consequent increased attenuation of solar radiation, the upper layer of the water showed a much greater rise in temperature during the day than it did before the bloom appeared. Zimmerman et al. [1981] compared two nearby lakes in Georgia, one highly colored and the other only moderately colored. The highly colored lake stratified in the summer: the other did not. Schreiner (1984) observed, in an eutrophic pond, intense heating in the near-surface waters during periods of high surface turbidity and less intense heating to a greater depth when surface turbidity was lower. Various workers have reported enhanced heating within patches of algal blooms in inland