Canopy interception

Overview

When precipitation falls on vegetated ground, some fraction is intercepted and stored as liquid water on leaves and stems. This intercepted water either evaporates directly back to the atmosphere or eventually reaches the ground. Leaf absorption is also possible but typically constitutes a fairly small fraction in intercepted water. The fraction of falling precipitation intercepted by the canopy directly depends on vegetation density and structural properties (leaf area index and stem area index).

subtypes(Terrarium.AbstractCanopyInterception)

2-element Vector{Any}:
 NoCanopyInterception
 PALADYNCanopyInterception

Canopy interception scheme from PALADYN

struct PALADYNCanopyInterception{NF} <: Terrarium.AbstractCanopyInterception{NF}

variables(PALADYNCanopyInterception(Float32))

Variables
├─ Prognostic: 
├── canopy_water [m] on XY{Center, Center}
├─ Auxiliary: 
├── canopy_water_interception [m s^-1] on XY{Center, Center}
├── canopy_water_removal [m s^-1] on XY{Center, Center}
├── saturation_canopy_water [-] on XY{Center, Center}
├── rainfall_ground [m s^-1] on XY{Center, Center}
├─ Inputs: 
├── leaf_area_index [-] on XY{Center, Center}
├── stem_area_index [-] on XY{Center, Center}
├─ Namespaces:

Interception fraction

Following PALADYN [11], the fraction of precipitation intercepted is

\[\begin{equation} I_{\text{can}} = \alpha_{\text{int}} P (1 - e^{-k_{\text{ext}}(\text{LAI} + \text{SAI})})\,, \end{equation}\]

where $\alpha_{\text{int}}$ is the interception factor (-), $P$ is incident precipitation (m/s), $k_{\text{ext}}$ is the radiation extinction coefficient (-), $\text{LAI}$ is the leaf area index (m²/m²), and $\text{SAI}$ is the stem area index (m²/m²).

The term $(1 - e^{-k_{\text{ext}}(\text{LAI} + \text{SAI})})$ represents the vegetation cover fraction following the Beer-Lambert law (see e.g. [21]) and increases from 0 (bare ground) toward 1 (dense forest).

Canopy water storage

The canopy water balance is expressed as

\[\begin{equation} \frac{\partial w_{\text{can}}}{\partial t} = I_{\text{can}} - E_{\text{can}} - R_{\text{can}}\,, \end{equation}\]

where $w_{\text{can}}$ is liquid water stored on the canopy (m), $I_{\text{can}}$ is the interception rate (m/s), $E_{\text{can}}$ is evaporation of intercepted water (m/s), and $R_{\text{can}}$ is the removal rate (m/s).

The canopy storage capacity $w_{\text{can, max}}$ (m) is diagnosed from the sum of the leaf and stem area indices,

\[\begin{equation} w_{\text{can, max}} = w_0 (\text{LAI} + \text{SAI})\,, \end{equation}\]

where $w_0$ (m) is the specific water capacity per unit leaf/stem area.

The removal rate of water from the canopy (e.g. due to gravity-induced drip) is computed as

\[\begin{equation} R_{\text{can}} = \frac{w_{\text{can}}}{\tau_w}\,, \end{equation}\]

where $\tau_w$ is the canopy water removal timescale (typically 1 day = 86400 s) (s).

The saturation fraction of the canopy is

\[\begin{equation} f_{\text{can}} = \frac{w_{\text{can}}}{w_{\text{can, max}}}\,, \end{equation}\]

which ranges from 0 (dry) to 1 (saturated) and controls the efficiency of canopy evaporation.

Precipitation

After accounting for interception and canopy , the precipitation reaching the ground is

\[\begin{equation} P_{\text{ground}} = P - I_{\text{can}} + R_{\text{can}} - E_{\text{can}}\,. \end{equation}\]

This represents the water available for soil infiltration and surface runoff.

Process interface

compute_auxiliary!(
    state,
    grid,
    canopy_interception::PALADYNCanopyInterception,
    atmos::Terrarium.AbstractAtmosphere
)

compute_tendencies!(
    state,
    grid,
    canopy_interception::PALADYNCanopyInterception,
    evapotranspiration::Terrarium.AbstractEvapotranspiration,
    args...
)

Methods

canopy_water(i, j, grid, fields, ::AbstractCanopyInterception)

saturation_canopy_water(i, j, grid, fields, ::AbstractCanopyInterception)

rainfall_ground(i, j, grid, fields, ::AbstractCanopyInterception)

Kernel functions

compute_canopy_interception(
    canopy_interception::PALADYNCanopyInterception{NF},
    precip,
    LAI,
    SAI
) -> Any

compute_canopy_saturation_fraction(
    canopy_interception::PALADYNCanopyInterception{NF},
    w_can,
    LAI,
    SAI
) -> Any

compute_canopy_water_removal(
    canopy_interception::PALADYNCanopyInterception{NF},
    w_can
) -> Any

compute_w_can_tendency(
    _::PALADYNCanopyInterception{NF},
    I_can,
    E_can,
    R_can
) -> Any

compute_precip_ground(_, precip, I_can, R_can)

References