Hydrological modelling on atmospheric grids: using graphs ofsub-grid elements to transport energy and water

Land surface models (LSMs) use the atmospheric grid as their basic spatial decomposition because their main objective is to provide the lower boundary conditions to the atmosphere. Lateral water flows at the surface on the other hand require a much higher spatial discretization as they areclosely linked to topographic details. We propose here a methodology to automatically tile the atmospheric grid into hydrological coherent unitswhich are connected through a graph. As water is transported on sub-grids of the LSM, land variables can easily be transferred to the routingnetwork and advected if needed. This is demonstrated here for temperature. The quality of the river networks generated, as represented by theconnected hydrological transfer units, are compared to the original data in order to quantify the degradation introduced by the discretizationmethod. The conditions the sub-grid elements impose on the time step of the water transport scheme are evaluated, and a methodology is proposed tofind an optimal value. Finally the scheme is applied in an off-line version of the ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems) LSM over Europe to show that realistic river dischargeand temperatures are predicted over the major catchments of the region. The simulated solutions are largely independent of the atmospheric grid usedthanks to the proposed sub-grid approach.