UPDATED ROUTINE SCHEME IN ORCHIDEE

Meeting of the 18 / 02 / 2019

Present: Agnes Ducharne, Yann Meudesoif, Jan Polcher, Thomas Dubos, Catherine Ottle, Pascal Maugis, Matt Macgrath, Korato Hamada, Philippe Peylin

Excused: Josefine Ghattas, Xudong Zhou

A/ IMPROVE ROUTINE (Supermesh) TowardsParallelRouting.pdf​: Presentation by Jan P. of a new routing scheme based on the description of the Hydrological units (HTUs) within the grid of ORCHIDEE. Each grid cell of ORCHIDEE may contain several HTUs with the water flowing toward a common 'point'. HTUs may cover several grid cell of ORCHIDEE.

• This routing scheme has been implemented and used in ORCHIDEE since 2000.

• In the last few years it has been further developed and improved.

• Its parallelisation is not well implemented and requires work in order not to sow down ORCHIDEE too much.

B/ SIMPLE ROUTING orchidee.pdf: Presentation by Yann M. of the so-called "simple routing" scheme based on the current ORC routing scheme but externalised: the runoff and drainage fluxes calculated on the ORCHIDEE grid are map onto an independant grid representing the basins and their connections. The transport of the flow is done on this new "basin" grid and any return water flux should be interpolated on the ORC grid. This allows flexible parallel implementation.

From the discussion 3 majors directions of work emerge:

• Improving the "simple" scheme of Yann M.: Y.M needs some help to run some tests with a given precipitation field (or directly with a runoff+dranaige flux from an ORC run); This will allow to evaluate the potential of this simple external approach. (Agnes proposed to help).

• Define a common/optimal target for the routing scheme that the ORCHIDEE group would like to have. This could combine the advantages of both Yann and Jan approaches. For that a document should be written to resume the approach and iterate on the best solution (independent grid or embedded in the ORC grid; implementation of floods, ponds, etc...)

• Current works are using a relatively old "basin description" while Jan now has access to a new basin description at 90m (from Dai Yamazaki, based on the MERIT DEM). Jan need some help to work on this new hydrography database to extract the relevant information at a suitable resolution.

Note finally that the current ORCHIDEE routing scheme (used for CMIP6) should be corrected with respect to the transfer time of the water in the different reservoirs (bug that make this transfer time strongly dependent on the grid resolution).

Objectives for an optimal Routing Scheme

• ORCHIDEE needs to think for its future development if it wishes to keep the routing inside the LSM in order to optimise the description of the interactions of surface water and other processes or "externalise" the routing in order to simplify the model.

• How important is the lateral water transport to the land surface processes we wish to represent. The lateral water transport is the main cause of the complexity of the parallelisation of the routing. Thus the above discussion will also apply to groundwater fluxes as soon as they cross the grid cell boundary.