Zero plane displacement heights in forests

[SnowHydrology]

PET option 5, the Penman-Monteith method is optimized for relatively even grass/crop surfaces. In these areas, the zero plane displacement height, d, is typically less than the wind speed and/or relative humidity measurement height. This is important because estimating surface resistance, rs, requires taking the natural log of the measurement height, z, minus d. The natural log of a negative number (i.e., d > z) is NaN.

Equation from FAO.

Current behavior

When PET is run with option 5 in forested locations (d > z), it returns NaN. Part of this is because we are using AORC forcing data, which assumes z for wind is 10 m and z for humidity is 2 m. If running option 5 with above-canopy met towers, this would not be a problem.

Expected behavior

We should be able to estimate PET in forested locations using P-M as other researchers have done. We need to either modify the z estimate to be less than the measurement height or substitute empirically derived resistance values. E.g., 43–67 s/m from Amatya et al. (2016).

[WanruWu-NOAA]

Thanks, Keith. I'd like to add that the equation ra was the aerodynamic resistance; the surface resistance rs was assigned a constant value in /src/pet.c as model->pet_forcing.canopy_resistance_sec_per_m = 50.0

[SnowHydrology]

Closed with PR #13, #15, and #16