Modelling incident solar radiation in equatorial rainforest in Indonesia by integrating remote sensing data and in situ measurements

Solar radiation is a very important limiting factor for photosynthesis activity of vegetation and is one of the key input variables to the modelling of the terrestrial carbon cycle, vegetation productivity and yield assessments. Last three decades have been signified by great efforts in investigation of the Earth’s cycle and growing interest of researches in modelling different parameters of solar radiation at all scales from global to local. This paper presents an algorithm for obtaining solar radiation balance in equatorial rainforest in Sulawesi, Indonesia. The algorithm developed in this study is based on the use of the remotely sensed derived data and quantitative information from in situ measurements in the study region. The algorithm enabled us to calculate total radiation as well as its separate components above and under the canopy of equatorial forest at the regional scale with the spatial resolution of 250 m at 10-day time-step throughout the period of 2002-2005.
In this algorithm, the amount of incoming solar radiation above the canopy was derived from the combination of the extraterrestrial irradiance and the local atmospheric conditions such as cloudness information and the optical air mass. The amount of the extraterrestrial irradiance was computed using common equations integrating the variables of Earth-Sun distance, solar inclination, solar elevation angle, and geographical position. The calculations were made at a daily time-step. After that, the results were summed to 10-day values. The product of this calculation was corrected for relief slope and relief aspect using a digital elevation map. The proportion of diffuse radiation in the total radiation on the top of the canopy was determined by a simple model of atmospheric attenuation of radiation under a cloudless sky.
The amount of direct and diffuse radiation under the canopy was modelled from the combination of the canopy structure variables such as Leaf Area Index (LAI), leaf-sun angle a, canopy light extinction coefficient k, and leaf transmission coefficient t, with the results of the radiation calculations above the canopy. The LAI was obtained by extrapolation of the in situ measurements of LAI over the study region using the Normalized Difference Vegetation Index (NDVI) resulting from the data of MODIS satellite. The value of a is 60° for a canopy with spherical leaf angle distribution but a is considered to vary with the change of the solar zenith angle and was calculated for each 10-day period individually. The value of k was determined empirically from in situ measurements of light intensity outside the canopy and inside the canopy. The transmission of light through leaves was considered to be a constant for a certain forest type and was borrowed from literature.
The developed model yields detailed information on a region-wide solar radiation balance both above and below the forest canopy and is proposed for the further use in a flux model.