LULC Default Parameter Basis
July 21, 2026 ยท View on GitHub
LULC_parameters.xlsx provides a consistent starting table for a new
HydroPol2D case. It does not provide universal physical constants. Terrain
resolution, vegetation structure, season, water depth, soil depth, and local
management all affect the effective parameters used by a distributed model.
Surface Routing, Rooting, and ET
| LULC class | Manning n [s m^-1/3] | Root depth [m] | Kc [-] | Basis for the generic value |
|---|---|---|---|---|
| Tree cover | 0.100 | 1.50 | 1.05 | Forest roughness within standard 2D floodplain guidance; deep woody root zone. |
| Shrubland | 0.080 | 1.00 | 0.70 | Shrub/scrub roughness range; intermediate woody rooting. |
| Grassland | 0.060 | 0.60 | 0.85 | Conservative shallow-flow roughness; shallow herbaceous rooting. |
| Cropland | 0.050 | 1.00 | 1.00 | Upper end of a generic crop roughness range; mid-season crop ET starting value. |
| Built-up | 0.030 | 0.00 | 0.00 | Smooth sealed-surface value. Use a separate urban class with larger n where buildings or obstructions must be represented. |
| Bare / sparse vegetation | 0.030 | 0.10 | 0.30 | Barren land roughness range; minimal vegetation water uptake. |
| Snow and ice | 0.020 | 0.00 | 0.00 | Initial smooth snow/ice runoff surface; revise for rough ice, debris, or crevassed terrain. |
| Permanent water bodies | 0.035 | 0.00 | 0.00 | Within the standard open-water range. Ponded cells evaporate at Ep, not through Kc. |
| Herbaceous wetland | 0.070 | 0.40 | 1.00 | Emergent-wetland roughness range and shallow, water-table-limited rooting. |
| Mangroves | 0.150 | 1.00 | 1.05 | High-end woody-wetland roughness; revise using local vegetation density and tidal/channel geometry. |
| Moss and lichen | 0.080 | 0.05 | 0.40 | Conservative shallow-flow resistance and near-surface water uptake. |
The roughness values follow the class ranges in the USACE HEC-RAS 2D land-cover
guidance. That guidance also notes that its tabulated values are intended for
appreciable flow depths; shallow overland flow can require larger effective
values. Root-depth values are simple representative depths consistent with
global land-model parameterizations, rather than estimates of the deepest roots
at a site. Kc is applied only to internally computed reference ET and is an
initial, time-invariant value. Seasonal crop curves, remotely sensed ET, or
locally calibrated values are preferable when available.
h0 and d0 both default to zero. h0 is used by the cellular-automata
routing option only; it is not a substitute for canopy storage or soil
infiltration. d0 represents the initial surface-water state and should be set
from a restart or an initial-depth raster when the event begins on a wet
surface.
Water Quality
The generic table sets C1 = C2 = C3 = 0 and C4 = 1. This deliberately
represents no initial pollutant buildup and no washoff. There is no defensible
universal set of buildup/washoff coefficients by land-cover class alone. In the
current mass-based HydroPol2D formulation,
B0 = C1 [1 - exp(-C2 ADD)] A / 10000
W = C3 Q^C4 B
where B0 is buildup mass, ADD is antecedent dry days, A is cell area in
m2, Q is runoff in m3 s^-1, and B is the available pollutant mass. C1
is in kg ha^-1, C2 is in d^-1, C4 is dimensionless, and the units of C3
depend on C4. Define these values for the pollutant, measurement units, and
case study, then calibrate them against concentration or load observations.
Snow
The snow columns use broadly plausible initial values: snow albedo of 0.55-0.80, emissivity of 0.98-0.99, a linear rain/snow transition of -1 to 2 degC, fresh-snow density of 100 kg m^-3, and maximum seasonal-snow density of 450 kg m^-3 (550 kg m^-3 for the snow-and-ice class). Degree-day melt, sublimation, and compaction coefficients are empirical rate parameters. They are retained as explicit inputs because they should be fitted or evaluated against local SWE, snow-depth, or runoff data; they should not be interpreted as transferable LULC constants.
Sources
- USACE HEC-RAS: land cover and Manning n guidance.
- Zeng (2001): Global Vegetation Root Distribution for Land Modeling.
- FAO-56: crop evapotranspiration and crop-coefficient method.
- Hock (2003): temperature-index melt modelling.
- Dai (2008): rain-snow phase transition.
- Vionnet et al. (2020): seasonal snow-density evolution.
- U.S. EPA SWMM: Reference Manual, Volume III: Water Quality.