2.3.2. SMOS-CESBIO L4 Root Zone Soil Moisture

Regarding the current SMOS-derived RZSM products, the global SMOS L4 RZSM at 0–1 m depth over a 25-km EASE-2 grid was used. These data were obtained from the SMOS-CESBIO L3 SSM (using a 3-day average SSM) and other ancillary datasets, such as MODIS observations and climate data from the National Centers for Environmental Prediction (NCEP), among others. The RZSM is computed using a double bucket hydrological model, which is applied daily. The CESBIO model has two soil layers (0–40 cm and 40–100 cm). For the first layer, the selected model is the previously mentioned SWI [32,33]. For the second layer, a water balance model based on a linearized Richard's equation is employed [31].

Since the SMOS-CESBIO L4 RZSM is also distributed separately for daily ascending and descending orbits, the Iberian Peninsula area was clipped from these RZSM maps and a daily average of the ascending and descending orbits was then performed.
