Abstract
The total diffuse attenuation coefficient of Photosynthetically Available Radiation (Kd (PAR)) is derived by optimising the solution of a surface layer model to match temperature profiles measured with a precision thermistor chain; a non‐linear least‐squares Levenberg‐Marquardt scheme is applied to optimize Kd (PAR). The method was validated in Lake Kinneret (Israel) over 10 days in summer to early winter 2001, Valle de Bravo Reservoir (Mexico) on 1 July 2004 and Winam Gulf (Kenya) on 1 May and 12 August 2005. The method was applied for cases where the horizontal pressure gradient in the surface layer was negligible, the photic depth was deeper than the diurnal surface layer and the increment in temperature in photic region was considerably larger than the thermistor's precision. In all these tests, the errors of the estimated values of Kd (PAR) were mostly within 10% of the actual direct measurements. Investigations of the sensitivity of the model revealed that the accuracy of the method depended primarily on the precision and accuracy of the thermistors as well as the vertical resolution of the temperature measurements in the surface layer; all of these determine the rate of convergence of the optimisation scheme.
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