## 3 Model Development
### Updated Soil Parametrization  
With higher spatial resolution and more accurate grid parameters characterisation, in addition to broader applications and the need to improve the water budget in the model and coupled hydrological components, the importance of the soil depth and vertical discretisation within land surface models has increased. It affects the way land surfaces store and regulate water, energy and also carbon fluxes. The amount of water in the soil and its vertical distribution in the column are important for the regulation of heat and water vapour fluxes towards the atmosphere spanning a range of time scales from minutes to months in the coupled land-atmosphere system. This also depend on the soil parameters and is further modulated by the vertical roots distribution, and soil moisture stress function, which control evapotranspiration under soil moisture stress conditions. Currently in the ECMWF land Surface Scheme the soil column is represented by a fixed 4-layer configuration with a total of 2.89 m depth (7 cm, 21 cm, 72 cm, 189 cm). In the ECLand system we introduce new configurations with increased soil depth (up to 8 m with fixed layers depths) and higher vertical discretisation (up to 10-layers; 1 cm, 2 cm,  
Figure 5: Schematic of the multi-layer snow scheme (**a**) for a 1-metre thick snowpack with its vertical discretization (\(d_{1},\ldots,d_{5}\), in m) for flat terrains. (**b**) Idealized time-series of snow depth accumulation and ablation (continuous line) over flat terrain, with the vertical discretization used in the multi-layer scheme. The list of symbols used in (**a,b**) is also reported.  
4 cm, 9 cm, 12 cm, 30 cm, 42 cm, 100 cm, 200 cm, 400 cm), including a dissociation between the treatment of water and heat fluxes (Figure 6) following [47]. In these configurations, the upper layers which have a thinner depth, allows a better match with satellite microwave soil moisture observations.  
An optional formulation for the distribution of roots density has been also developed in ECLand. It is based on a uniform distribution, which is appealing due to its simplicity and straightforward first-order assumption. This uniform distribution replaces the current exponential formulation and depends only on the maximum rooting depth (\(R_{DMAX}\)). This root distribution formulation was investigated by [PERSON] et al. [48] and showed potential for improvement of the water and energy fluxes. The transformation of root zone soil moisture to water stress is also an open question, and there are different approaches to relate root zone soil moisture and the water stress factor [49; 50]. In ECLand it is also possible to change the linear transformation between root zone soil moisture and water stress by a non-linear relation, which showed a large sensitivity in offline and coupled forecasts [51]. Such approaches, combined with the rooting depth distribution, are likely to further enhance the model capability to represent root water uptake, but require further evaluation and calibration. The quality of the soil ancillary data and its related parameters is also essential for the regulation of water, energy and carbon fluxes. The coupling of ECLand with the multiscale parameter regionalization system (MPR [52]) would facilitate the integration and calibration of such parameters related to new soil maps.