Radiation and energy balance on a hillslope forest: horizontal versus slope-parallel installation of radiometer

Radiation is a major driver to the carbon, water, and energy exchanges of an ecosystem. For local radiation balance measurements, one essential question is whether the measurement systems should be installed horizontally or parallel to inclined slope surface. With a case study over a temperate deciduous forest on a moderate inclined (9°) northwest-facing slope, we quantified the slope effect on net radiation (R_n) and its components and the energy balance closure measured by an eddy covariance (EC) system. Compared with the slope-parallel radiometer, the horizontal sensor overestimated the incident solar radiation (SR) by 7%, the incoming photosynthetically active radiation (PAR) by 1.5%, and the incoming near-infrared radiation (NIR) by 10%; while underestimated the reflected shortwave radiation (SR) by 4% and NIR by 5%. The influence of radiometer-orientation on incoming longwave radiation (LR) was about 3%, while that on outgoing LR was negligible. Summing all these components, horizontal sensor overestimated the R_n by 9%. Converting the horizontally-measured incident radiation to slope-surface reduced a half of the biases on incoming SR and R_n. Measuring the R_n with slope-parallel radiometer and correcting the slope-effect on horizontally-measured incident SR improved the energy balance ratio (EBR) by 8% and 5%, respectively. A mini-review indicated that, the horizontal sensor underestimated (overestimated) the EBR on north-facing (south-facing) slopes in temperate zone in the Northern Hemisphere, with an inclination angular sensitivity of EBR as high as 1.17% per degree of inclination angle. We recommend measuring radiations on inclined terrains with slope-parallel radiometers, or correcting at least for the incident SR in energy balance studies.