Land Surface Models (LSMs) are key components of the Earth System Models (ESMs) used in the studies of past and future global evolution. In order to increase the performance of LSMs, authors agree on the need to improve the representation of land surface processes, including hydrologic processes (Clark et al., 2015;Fan et al., 2019;Fisher & Koven, 2020). A joint effort from the hydrology and the LSMs modeling communities is needed to meet this challenge.The hydrology modeling community usually works at the local to regional basin scale (i.e., <10 3 km 2 ). The basin constitutes the basic unit for the study of hydrologic processes, and the physically based modeling approach is commonly used at this scale. Since its introduction by Freeze and Harlan (1969), this approach has been developed in the community with the appearance of several models, such as CATHY (

&lt;p&gt;Hydrological processes import across scales is known to constitute a key challenge to improve their representation in large-scale land surface models. Since these models describe continental hydrology with vertical one dimensional infiltration and evapotranspiration, the challenge mainly resides in the dimensionality reduction of the processes. Departing from the catchment three-dimensional scale, previous work has shown that an equivalent two-dimensional hillslope model is able to simulate long term watershed water balance with good accuracy. This work has been done on the Little Washita basin (Ok, USA) using the integrated code HydroGeoSphere. Following this framework, we show that hillslope hydrology can be described by using realistic simplifying assumptions, such as linear water table profile. These assumptions allow the writing of an analytical model relying on two hydrological variables: the seepage face extension, which describe the intersection length between the water table and the land surface, and the water table slope. The last step of the work will be to use these key variables and this simplified description of the driving processes for importing small-scale hydrological processes into large-scale models.&lt;/p&gt;

&lt;p&gt;Several authors in the literature, such as Khan (2014) and Loritz (2017), have previously suggested that 3D catchment hydrology can be predicted from 2D hillslope simulations. Following this idea, we propose an upscaling methodology for runoff and evapotranspiration fluxes. The first step consists of a geomorphic analysis of the studied watershed. The average mean slope and hillslope length are then used to build a 2D equivalent-hillslope model. The validity of the methodology is tested by comparing the resulting water balance with a 3D physically-based distributed model. 2D fluxes of the equivalent hillslope are converted into 3D by using the drainage density. This upscaling methodology is applied to the Little Washita (LW) watershed (Oklahoma, USA). Both the 3D reference model and the 2D equivalent model are built with the physically-based distributed code HydroGeoSphere, which is forced by LW reanalysis climatic data. Two decades are simulated. Regarding the evapotranspiration, the upscaling methodology with only one equivalent hillslope gives a good prediction of 3D fluxes. However, a combination of several hillslopes is needed for simulating the 3D flow rate at the basin&amp;#8217;s outlet. This work on the decrease of model dimensionality is a first step in the upscaling process from 3D physically-based models to 1D column models used in global Land Surface Models.&lt;/p&gt;