Canopy evaluators

Interception

Partitions water between interception and throughfall, combining throughfall with drainage.

Based on CLM 4.5 and Lawrence et al 2007, interception is given by:

\[I = (P_{rain} + P_{snow}) * \alpha * (1 - exp(-.5(LAI)))\]

Throughfall is given by:

\[T = (P_{rain} + P_{snow}) - I\]

Drainage is provided as input here, as a total drainage from the canopy. The phase of this drainage is assumed to match the phase of the precipitation. So if it is raining, drainage is rain, while if it is 50/50 rain and snow, drainage is also 50/50 rain and snow. If total precipitation is 0, then drainage is partitioned by air temperature (above 0C –> all rain, otherwise all snow). This evaluator partitions the drainage and sums it with throughfall to compute the total source, in each phase, to the layer below the canopy (snow and/or ground surface).

interception-fraction-evaluator-spec

• “interception fraction parameters” [interception-fraction-model-spec]

MY KEYS: - “interception” DOMAIN-interception - “throughfall and drainage rain” DOMAIN-throughfall_drainage_rain - “throughfall and drainage snow” DOMAIN-throughfall_drainage_snow

KEYS: - “area index” DOMAIN-area_index - “precipitation rain” DOMAIN_SURFACE-precipitation_rain - “precipitation snow” DOMAIN_SNOW-precipitation - “drainage” DOMAIN-drainage - “air temperature” DOMAIN_SURFACE-air_temperature

Fraction of incoming water that is intercepted.

Based on CLM 4.5 and Lawrence et al 2007:

\[f_I = \alpha * (1 - exp(-.5 LAI)))\]

The interception fraction is everything here after the precip.

interception-fraction-model-spec

• “leaf area interception fraction [-]” [double] 0.25 The alpha

  term, this describes the fraction of leaf area that intercepts water.

Drainage

Drainage rate from the canopy to the lower layers.

A simple model based on relaxation from current water content to a saturated water content.

       |
       | source
       V
      /   \
   I /     \
    V       |
--Theta--    | T
    ^       |
    | D     |
    V       V
-- -- -- -- -- -- --

This is the model for drainage D.

Drainage is given by:

\[D = max(0, \frac{(\Theta - \Theta_sat)}{\tau})\]

drainage-evaluator-spec

• “drainage timescale [s]” [double] 864 Timescale over which drainage occurs.

• “saturated specific water content [m^3 H2O / m^2 leaf area]” [double] 1e-4

  The thickness of the wetting surface – determines maximum canopy water storage.

KEYS: - “area index” - “water equivalent”