Abstract
This article presents a methodology for estimating total incoming solar radiation from Triangular Irregular Network (TIN) topographic meshes. The algorithm also computes terrain slope degree and aspect (slope orientation) and accounts for self shading and cast shadows, sky view fractions for diffuse radiation, remote albedo and atmospheric backscattering, by using a vectorial approach within a topocentric coordinate system establishing geometric relations between groups of TIN elements and the sun position. A normal vector to the surface of each TIN element describes its slope and aspect while spherical trigonometry allows computing a unit vector defining the position of the sun at each hour and day of the year. Sky view fraction, useful to determine diffuse and backscattered radiation, is computed for each TIN element at prescribed azimuth intervals targeting the steepest elevation gradient. A comparison between the sun zenith angle and the steepest gradient allows deciding whether or not the pivot element is shaded. Finally, remote albedo is computed from the sky view fraction complementary functions for observed albedo values of the surrounding terrain. The sensitivity of the different radiative components to seasonal changes in atmospheric transmissivitties and surrounding albedo is tested in a mountainous watershed in Wyoming. This methodology represents an improvement on the current algorithms to compute terrain and radiation values on unstructured-mesh terrain models. All terrain-related features (e.g., slope, aspect, sky view fraction) can be pre-computed and stored for easy access into a subsequent, progressive-in-time, numerical simulation.
Highlights
IntroductionTerrain properties such as slope gradient and aspect are fundamental in controlling a number of point-surface, and shallow subsurface, hydrologic processes including runoff rates and generation mechanisms [1,2], infiltration and wetting front development [3,4], weathering and erosion rates [5,6]
Terrain properties such as slope gradient and aspect are fundamental in controlling a number of point-surface, and shallow subsurface, hydrologic processes including runoff rates and generation mechanisms [1,2], infiltration and wetting front development [3,4], weathering and erosion rates [5,6]and incident solar radiation [7], among other ecohydrologic processes
We describe the development and testing of a Triangular Irregular Network (TIN)-based algorithm to compute terrain parameters and clear-sky incident solar radiation for implementation in continuous and distributed hydrologic models that employ unstructured TIN geometry
Summary
Terrain properties such as slope gradient and aspect are fundamental in controlling a number of point-surface, and shallow subsurface, hydrologic processes including runoff rates and generation mechanisms [1,2], infiltration and wetting front development [3,4], weathering and erosion rates [5,6]. Incident solar radiation [7], among other ecohydrologic processes. The solar radiation flux striking an inclined surface consists of direct, diffuse, backscattered and landscape reflected (albedo) radiation [3,8]. The total incident solar radiation and its variability in topographically complex areas is responsible for controlling magnitude and spatial variability of turbulent heat fluxes, snow ablation rates, soil moisture losses and skin temperature, among other physical variables. The uneven distribution of snow and vegetation on contrasting mountain flanks is a clear evidence of this topographically-induced variability [11,12,13]
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