Ground albedo

The ground albedo control defines the reflectance of the ground surface beneath and between module rows. It is found under Ground > Albedo within the system inputs. The control has three parts:

1. Selection of the ground optical surface: a preset material from the reflector library or a custom surface.
2. A scaling factor that adjusts the overall reflectance magnitude.
3. Effective integrated albedo values computed from the selected surface.

For details on how ground reflectance is handled during ray tracing, including the spectral treatment, scattering models and effective albedo equations, see System optics: Albedo.

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Material presets

The ground surface material is selected using two dropdown menus in the main view. The first selects the category of ground surface (e.g. Vegetation, Gravel, Snow/water) and the second selects the specific material within that category (e.g. Green grass, Limestone, Water). Below the dropdowns, → Show details opens a dialogue for inspecting or customising the underlying reflectance and scattering settings.

Selecting a material loads a pre-configured surface with a wavelength-dependent spectral reflectance profile from the reflector library, along with default scattering settings. Selecting Custom from the category dropdown bypasses the preset library and allows direct configuration of the reflectance and scattering settings, equivalent to clicking → Create custom and edit on a preset. See Using a custom fixed albedo for a step-by-step guide.

This is the same optical surface control used elsewhere in the system inputs; see Optical surfaces for a general description of the reflector models and scattering options.

Available categories

CategoryExample materialsConstruction material, Galvanised steel,, Concrete,, Aluminium alloysGravel, LimestoneSnow/water, Water,, SnowSoil, Brown gravelly loamVegetation, Green grass

These presets each carry a characteristic wavelength-dependent reflectance profile. For example, vegetation absorbs shorter-wavelength light, whereas snow has maximum reflectance at short wavelengths.

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Scaling factor

The Scaling factor input is visible when the selected material uses a wavelength-dependent reflectance profile. It applies a uniform multiplier to the selected material’s spectral reflectance at every wavelength. The default value is 1.0 (no scaling) and the allowable range is 0–5. Reflectance at any wavelength is clamped to a maximum of 100%, so scaling factors greater than 1.0 will not produce physically impossible values.

The primary use of the scaling factor is calibrating the library’s spectral profile against a site albedometer measurement. If a field albedometer reads a broadband albedo that differs from the library profile’s integrated value, adjust the scaling factor until the displayed effective albedo matches the measured value. This preserves the spectral shape of the library profile while aligning the overall magnitude to real-world observations.

The scaling factor is applied before the effective albedo values are computed.

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Effective albedo display

The control displays two computed effective albedo values:

• Albedo (Si detector): the effective albedo as it would be measured by a silicon-based pyranometer, weighted by the silicon spectral response (300–1200 nm)
• Albedo (thermopile): the effective albedo as it would be measured by a thermopile pyranometer, which has a flat spectral response across a broad wavelength range (300–4000 nm)

The Albedo (thermopile) value is more commonly used for comparison with on-site albedometer measurements. See System optics: Albedo for the equations used to compute these values.

When the reflectance model is set to constant (fixed), both values equal the entered reflectance.

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Surface details dialogue

Click → Show details to open the surface details dialogue. When a preset material is selected, the settings in this dialogue are read-only.

Once the dialogue is displayed the → Create custom and edit button unlocks all settings for manual editing.

Scattering

The ground surface uses Lambertian scattering with a fixed scattering fraction. The default fraction is 1.0, meaning all reflected light is scattered diffusely. See Scattering for more details on scattering models.

Reflectance model

The reflectance model dropdown has two options:

• Wavelength dependent: the surface reflectance varies with wavelength according to a spectral profile selected from the full reflector library. This is the default for all preset materials.
• Constant with wavelength: the surface has a single reflectance value R applied uniformly at all wavelengths. The remaining fraction ($1-R$) is absorbed. This mode is used when setting a custom fixed albedo.

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Using a custom fixed albedo

To set a custom fixed (constant) albedo value:

1. Select Custom from the first (category) dropdown in the main view. The second dropdown disappears.
2. Click → Show details to open the surface details dialogue. Fields are now editable.
3. In the reflectance model dropdown, select Constant with wavelength.
4. Set R (reflectance) to the desired value (e.g. 0.25). A (absorptance) adjusts automatically so that $R+A=1$.
5. Close the dialogue. Both effective albedo values display the entered value.

Scattering settings can also be modified in custom mode. Selecting a preset from the dropdowns discards any custom settings.

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Ground slope

Ground slope is displayed as a read-only value of 0°. The ground is assumed to be flat and horizontal.

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Spatially non-uniform albedo

Non-uniform ground albedo can be achieved by placing custom objects, such as reflective strips, on the ground surface and assigning them their own reflector settings. Where a custom object covers the ground, its reflectance is used instead of the ground albedo. See Simulating spatially varying albedo for a tutorial on combining ground albedo with reflective ground objects.

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Varying albedo with time

There is currently no input to define monthly or other time variation in albedo. To model this, run multiple simulations with different albedo settings, then combine the results manually.