Calculating solar and longwave radiation for a Mediterranean lake under various weather conditions

Two years of hourly measurements of global solar radiation and downward longwave radiation were used to estimate the radiative balance of Lake Banyoles, a small karstic Mediterranean lake. The performance of several empirical estimation equations on an hourly and a daily basis were evaluated taking into account various meteorological conditions. The new coefficients in the formulae for the effective emittance, which have been derived from meteorological data, demonstrate that these coefficients strongly depend on the radiation climate of the respective area. In the Mediterranean climate of Lake Banyoles, changes in wind direction and relative humidity after the vertical distribution of vapour pressure beneath the cloud layer. Different median experimental errors were found depending on the time scale and cloud cover. Comparison of global solar radiation measured at Banyoles with estimated values shows good agreement for clear skies, especially for daily means. On a daily scale, the root mean squared (r.m.s.) error was found to be 8% for solar radiation and 12% for longwave radiation. On an hourly scale, different r.m.s. errors were obtained depending on the degree of cloud cover.