The effect of light extinction on thermal stratification in reservoirs andlakes

An examination is made of the effect of light attenuation in the water colu mn on the hydrodynamics, and in particular on temperature stratification in reservoirs. Numerical experiments are described which examine the differen ces in light profile in the water column when the spectral character of the incident radiation and light penetration is and is not taken into account. It was found that the exponential character of light penetration remains b ut the exponent may be increased by up to 20% in clear waterbodies and less in waterbodies with higher extinction values when I-0(lambda) and K-d(lamb da) are incorporated. The error may be higher when the spectral character o f the extinction is due to colored organics rather than phytoplankton. The numerical, one dimensional hydrodynamic model DYRESM is used to isolate and examine the influence of changing the light attenuation in the water colum n in a reservoir, and on its hydrodynamics, when keeping other conditions u nchanged. Not surprisingly, the simulations show that when the extinction r ate increases, mixing depth decreases markedly, and although the same energ y enters the water column, surface temperatures increase slightly while dee p temperatures decrease. Some other surprising effects were found and are d escribed. The results presented and comparison of the model results with re al-world data indicate that the pronounced dependence of mixing depth on li ght extinction is a general phenomenon which cannot be ignored.