How to prevent drainage losses (w/o violating mass conservation)?

Author: A. Ducharne Last revision: 2020/06/01, A. Ducharne

Question by Daniel Goll on 2017/05/12

My aim is to prevent drainage completely in ORCHIDEE. However, I could not figure out how to achieve this due to the complex calculations of drainage w/o violating mass conservation.

Answer by Agnès Ducharne

It's simple, you only have to set

FREE_DRAIN_COEF = 0.0 0.0 0.0

in your run.def (one value for each soiltile in a gridcell).

This will preserve mass conservation, as FREE_DRAIN_COEF is accounted for when solving the Richards equation. This keyword defines F in Eq. 59, section 4.4 of the ​note on hydrol.f90 (version of April 25,2018).

What is not guaranteed, however, is numerical precision, if you have sharp mc gradients above the impermeable bottom and a loose vertical discretization (to improve this, it is advised to increase the number of soil layers close to the bootom, as explained in section 2.3 of the note).

Another reason for lack of numerical precision is if the soil mositure is close to saturation (prone to happen at the soil bottom if the latter is impermeable) because the form of Richards equation used in ORCHIDEE is not adapted to saturated or near-saturated conditions.