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Leaf Area Index (LAI)

Leaf Area Index (LAI) represents the total leaf area per unit ground area and is used in HydroPol2D to characterize vegetation density and its interaction with rainfall and atmospheric processes.

In the current implementation of the model, LAI is used to represent canopy effects on interception and evapotranspiration.

Role in HydroPol2D​

LAI is used to represent:

  • Rainfall interception by the vegetation canopy
  • Partitioning between throughfall and canopy storage
  • Vegetation influence on evapotranspiration

Higher LAI values correspond to denser vegetation and increased canopy storage capacity.

Representation​

LAI is provided as a continuous raster map, where each grid cell contains a value representing vegetation density.

Typical values range from:

  • ~0 → bare soil or impervious surfaces
  • 1–3 → sparse to moderate vegetation
  • 4–7+ → dense vegetation (e.g., forests)

Values must be consistent with the land cover conditions in the domain.

Temporal Representation​

Dynamic LAI is not currently implemented in HydroPol2D.

As a result, LAI must be provided as a static input.

The recommended approaches are:

  • Median LAI → for long-term or climatological simulations
  • Wet-season LAI → for simulations focused on high vegetation activity
  • Dry-season LAI → for simulations focused on reduced vegetation conditions

The choice should be consistent with the simulation objective and time period.

Data Sources​

Several datasets provide global LAI estimates:

MODIS LAI (MOD15A2H)​

Copernicus Global Land Service (LAI)​

VIIRS LAI​

  • Region: Global
  • Moderate resolution
  • Continuation of MODIS-era products

Google Earth Engine​

LAI datasets from MODIS, Copernicus, and VIIRS are available in Google Earth Engine and can be processed to extract median or seasonal values.

Consistency with LULC​

LAI must be consistent with the LULC classification.

Typical correspondence includes:

  • Forest → high LAI
  • Grassland → moderate LAI
  • Urban → low or near-zero LAI

Inconsistencies between LULC and LAI will result in unrealistic canopy behavior.

Practical Considerations​

When preparing LAI inputs:

  • Use representative values for the simulation period
  • Avoid extreme or noisy values from raw satellite products
  • Apply temporal aggregation (median or seasonal averages) when needed
  • Ensure spatial consistency with other raster inputs

If no LAI dataset is available, values can be assigned based on LULC classes, although this introduces additional assumptions.

Summary​

LAI is used in HydroPol2D to represent vegetation effects on interception and evapotranspiration.

The model currently assumes a static LAI field, and users must select representative values based on the intended simulation period.

Careful selection of LAI is required to ensure consistency with land cover and realistic representation of vegetation processes.