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Albedo

Albedo represents the fraction of incoming shortwave radiation that is reflected by the surface.

In HydroPol2D, albedo is used to characterize surface radiative properties and their influence on energy-related processes, particularly evapotranspiration and snow dynamics.

Role in HydroPol2D​

Albedo is used to represent:

  • Surface reflectivity to incoming solar radiation
  • Energy available for evapotranspiration
  • Surface energy balance conditions
  • Snowmelt behavior (when snow processes are activated)

Lower albedo surfaces absorb more energy, while higher albedo surfaces reflect a larger fraction of incoming radiation.

Representation​

Albedo is provided as a continuous raster map, where each grid cell contains a value between 0 and 1.

Typical values include:

  • 0.05 – 0.15 → water bodies, dark surfaces
  • 0.15 – 0.25 → vegetation
  • 0.25 – 0.40 → dry soils
  • 0.60 – 0.90 → snow-covered surfaces

Values must be physically consistent with land cover and environmental conditions.

Temporal Representation​

In the current implementation of HydroPol2D, albedo is treated as a static input.

Users should select values representative of the simulation conditions:

  • Long-term simulations → use average or median albedo
  • Seasonal simulations → use values consistent with the dominant surface condition

Dynamic albedo variations (e.g., due to snow cover or vegetation changes) are not explicitly resolved unless provided externally.

Data Sources​

Several datasets provide global albedo estimates:

MODIS Albedo (MCD43A3)​

Copernicus Global Land Service (Albedo)​

Google Earth Engine​

MODIS and Copernicus albedo datasets are available in Google Earth Engine and can be processed to extract representative values for the simulation domain.

Consistency with Other Inputs​

Albedo must be consistent with:

  • LULC → defines surface type
  • LAI → reflects vegetation density
  • Snow conditions → strongly influence reflectivity

Inconsistent albedo values may lead to unrealistic energy partitioning.

Practical Considerations​

When preparing albedo inputs:

  • Avoid extreme or non-physical values
  • Use temporally aggregated values when necessary
  • Ensure spatial alignment with all raster inputs
  • Verify consistency with land cover and climate conditions

If no albedo dataset is available, values may be assigned based on LULC classes, with appropriate justification.

Summary​

Albedo defines surface radiative properties in HydroPol2D and influences energy-driven processes such as evapotranspiration and snowmelt.

Although simple in structure, it plays an important role in ensuring physically consistent representation of surface–atmosphere interactions.