Improvement in the Estimation of Daily Net Surface Radiation in China

Abstract

Earth’s surface net radiation (Rn) governs sensitive and latent heat fluxes, plant photosynthesis, and the terrestrial carbon cycle. However, Rn records are scarce in China. The accurate quantification of Rn is therefore of great importance. The Rn is the difference between net shortwave (incoming) and net longwave radiation (outgoing), and the Food and Agricultural Organization of the United Nations (FAO) 56 Penman-Monteith (FAO-56 PM) method has been widely used to estimate Rn. However, site-specific calibrations of parameters are required for the FAO-56 PM method. Thereby, an improved Rn estimation in China was done by (1) quantifying the spatial and temporal variations of surface albedo to better estimate net shortwave radiation and (2) recalibrating the parameters of FAO-56 PM method based on in situ observations to better estimate net longwave radiation. Results indicate that the monthly value of surface albedo over the course of a year is U-shaped, and the minimum value appears in the boreal growing season (April to September). The monthly surface albedo during the nongrowing season was best fit by a latitude-dependent quadratic function. By better accuracy in the estimation of net shortwave and longwave radiation, results at 31 validation stations indicated that the improved FAO-56 PM method enhanced the accuracy of estimated Rn by 33% (the median Nash-Sutcliffe coefficient increased from 0.6 to 0.8) when compared to the default FAO-56 PM method. The improved method developed in this study has universal applicability in data-scarce regions in China and elsewhere, which is valuable for various practical applications including evapotranspiration simulation, hydrological modeling, and agricultural crop planning and management.