Abstract
Albedo is an important parameter in many environmental and renewable energy models. Satellite sensors can be used to derive broadband or narrowband albedos. However, the spatial resolution of such data can be insufficient in urban areas with complex morphology and land cover diversity. In this study, we propose the use of widely available aerial orthophotographs to derive visible band albedo in urban surfaces that can be effectively used in high-resolution applications. The solution is based on the estimation of the reflected irradiance captured by an RGB sensor and approximated by the brightness component in the hue-saturation-brightness (HSB) color model and incident solar irradiance modelled by the r.sun module in GRASS GIS. The visible band albedo values are calibrated by published reference values for selected land cover classes or, alternatively, by a spectroradiometer. The method is applied to the central part of Košice and compared to visible band albedo derived from the Landsat 8 OLI and Sentinel 2A sensors and previously published typical albedo values for various land cover classes, resulting in reasonable agreement. The proposed methodology is implemented using standard GIS tools that are easily applicable to any high-resolution urban data.
Highlights
The importance of albedo for determination of energy balance of land surfaces is well known
We present a methodology that is based on estimation of solar irradiance incident to a digital surface model (DSM) representing urban surfaces at the time of acquiring of orthophotographs as well as a conversion of the RGB images to the HSB color model to estimate the reflected radiance
The calibration of the VIS albedo values can be completed using appropriate in situ or published reference VIS albedo values
Summary
The importance of albedo for determination of energy balance of land surfaces is well known. Albedo determines the amount of solar radiation absorbed by the insolated material directly influencing land surface temperature (LST) especially in urban areas [1]. The reflected component of solar radiation directly influenced by albedo may be significant, in areas with complex surface geometry and high reflectivity (e.g., high mountains, urban areas). The surface albedo is generally defined as the ratio of reflected sunlight to the incident sunlight received by a surface. It depends on physical properties of the surface material as well as spectral and angular distribution of the incident sunlight. The broadband albedo refers to the entire spectrum of solar radiation, the spectral albedo uses a specific wavelength such as visible light (0.4–0.7 μm) or near-infrared (0.7–1.3 μm)
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