Modelling daily net radiation of open water surfaces using land-based meteorological data

Abstract

Accurate quantification of net irradiance of open water (R_{n water}) is of paramount importance for the estimation of open water evaporation, which is critical for the efficient management of water resources. Alternatively, model estimates of R_{n water} are often used when quality measurements of R_{n water} are not readily available for the water storage of interest. A Daily Penman, Monteith, Equilibrium Temperature Hargreaves-Samani (DPMETHS) model has been developed for the estimation of R_{n water} using land-based meteorological data. The DPMETHS model is a spreadsheet-based iterative procedure that computes R_{n water} using daily land-based meteorological measurements of solar irradiance (R_{s land}), minimum and maximum air temperatures (T_min and T_max), minimum and maximum relative humidity (RH_min and RH_max) and average wind speed (U_land). In this study, the DPMETHS model was evaluated using daily R_{n water} in-situ measurements acquired from 5 sites in both hemispheres, representing very different climatic conditions. Results showed reasonable model performance at all 5 sites, with the coefficient of determination (r²) values greater than 0.85 and root mean square error (RMSE) values ranging from 0.60 MJ∙m^-2 for Stratus Ocean (East Pacific Ocean) to 1.89 MJ∙m^-2 for Midmar Dam (South Africa). The results of this study suggested that the DPMETHS model can be reliably used to estimate R_{n water} for a wide range of climatic conditions. The performance of the DPMETHS model depends on the representativeness of the land-based meteorological data to the weather conditions above the open water surface. The DPMETHS model is user-friendly with minimal computational and data requirements that allows easy data handling and visual inspection.