Modelling of solar radiation influenced by topographic shading - Evaluation and application for precision farming

Solar radiation is the major energy source on earth. Until now, little attention has been spent on how the spatially differencing solar radiation affects crop production and causes spatial crop yield differences in agricultural fields. For homogeneous soil conditions one would expect a similar crop yield all over the field. However yield variability is often observed even though management and soil conditions are relatively homogeneous. The objective of this study was to investigate, whether the distribution of solar radiation influenced by surface topography contributes to crop yield variability. For a hummocky region in Luettewitz (State of Saxony, Germany) a Digital Elevation Model (DEM) was obtained by Laser- Altimetry with 1m spatial resolution. In such a terrain, the amount of incoming solar radiation is affected by the topography. For a given point the amount of shading differs throughout a day and a year causing spatial differences of radiation and soil temperature. Using the GIS Arc/Info and the SRAD-Module from the TapesG-package the short wave incoming radiation was calculated based on a 6m by 6m grid cell size for a field site of 20ha. The crop growth and nitrogen model "HERMES" was adapted to take into account site specific solar radiation in the calculation of biomass production and nitrogen and water dynamics. Model runs with and without topographic shading were performed for 225 points and compared against combine harvested grain yield for 1998 and 1999. Good agreements were observed between simulated and measured shortwave solar radiation. A spatially related pattern (north or south exposed positions) could be observed for HERMES model runs including site specific solar radiation. Other factors like previous landuse or moisture distribution in flowlines could be identified additionally as influencing crop grain yield development.