## 3 Earth Surface Modelling Advances and Links with EO Datasets
### Land-Surface Reservoirs
#### 3.1.1 Soil  
Soil is a porous medium that can store water, energy and carbon and these can be exchanged with the atmosphere and the oceans via transport mechanisms. The observations within the soil layer are typically in situ measurements, while remote sensing data can only penetrate the top few centimeters; therefore, a combination of satellite based and in situ based information is essential in constraining soil models. The amount of water in the soil and its vertical distribution in the column are important for the regulation of heat and water vapor fluxes towards the atmosphere and involves a range of time scales from minutes to years in the coupled land-atmosphere system. For example, at ECMWF, the Hydrology--Tiled ECMWF Surface Scheme for Exchanges over Land (H-TESSEL, [89])--has included a revised soil hydrology developed and tested for the global scale. These model developments were a response to known weaknesses of the TESSEL hydrology as used in the ERA-Interim reanalysis: specifically the choice of a single global soil texture, which does not characterize different soil moisture regimes, and a Hortonian runoff scheme which produces hardly any surface runoff. Therefore, a revised formulation of the soil hydrological conductivity and diffusivity (spatially variable according to a global soil texture map) and surface runoff (based on the variable infiltration capacity approach) were introduced in the Integrated Forecasting System (IFS) in November 2007, and is used in weather and sub-seasonal-to-seasonal forecasting applications and climate reanalysis products at ECMWF. The soil hydrology revisions involved the use of selected field sites as well as global atmospheric coupled simulations and data assimilation experiments [89]. Figure 6 illustrates the impact of these hydrology changes on the water budget of a number of European river catchments. H-TESSEL increases the seasonal amplitude of the Terrestrial Water Storage (TWS, Figure 6) change due the increased water holding capacity of the soil resulting from the new hydrological parameters and soil texture. The H-TESSEL scheme compared better than TESSEL with the terrestrial water storage dataset [90], which was derived as the residual of reanalysed atmospheric moisture convergence and observed river catchment runoff and referred to as the Basin Scale Water Budget (BSWB) dataset. Bare ground evaporation was improved by adopting a lower soil water content threshold for evaporation than the one used for vegetation. The most right panel of Figure 7 shows the difference in Root Mean Square Error (RMSE) of soil moisture between the revised model and the old model. RMSE is computed with respect to in situ measurements of soil moisture over the United States of America [54]. Figure 6: Monthly Terrestrial Water Storage (TWS) changes (left panel) for the Central European catchments Wisla, Oder, Elbe, Weser, Rhine, Seine, Rhone, Po, North-Danube (the coverage is shown in the right panel). The curves are for TESSEL (GSWP-2-driven, green line), H-TESSEL (GSWP-2-driven, blue line), TESSEL in ERA-40 (black dashed line). The red diamonds are the monthly values derived from atmospheric moisture convergence and runoff for the years 1986–1995 in the BSWB dataset [90]. Figure 7: Mean soil moisture in the ECMWF model (August 2010), before (BEVAP OLD, **upper left**) and after (BEVAP NEW, **upper right**) the bare soil evaporation revision, evaluated using the US in situ soil network (**lower left**) to calculate the difference in soil moisture Root Mean Square Error when using the revised bare soil evaporation threshold instead of the old model version (curve with dots, **lower right**). The solid curve indicates the number of in situ observations in each of the bare soil fraction classes. For further details, see Figures 1, 2, 4 and 5 in [54]. The new bare ground H-TESSEL evaporation formulation resulted in more realistic soil moisture values when compare to in situ data, particularly over dry areas.