Modelling solar irradiance on a slope under a leafless deciduous forest

This thesis investigates variations in solar irradiance incident upon sloping surfaces under deciduous forest in winter. A model is presented for prediction of solar irradiance at the surface which accounts for slope inclination and orientation, surrounding topography, isotropic absorption of solar radiation by the crown space, and shadows cast by the stem space.Field data from two sites of different slope and aspect attest to the validity of the model; errors, based on 20-minute averages of instantaneous values, are 15.5% (RMSE) and $-$1.9% (MBE). Error is partially due to reliance upon global radiation measurements above canopy at a different site (partially cloudy conditions) and sampling error (sunny sky conditions). The variability of solar irradiance at the surface, and in the error of predicted values, is found to vary with sky condition, solar zenith and incidence angles, and slope orientation. However, integration to hourly and/or daily time periods improves model performance significantly