## 2 Materials and Methods
### Hydrological Models
#### 2.4.1 LISFLOOD Model  
Brazil is vulnerable to extreme weather events, particularly floods and landslides caused by heavy rains and droughts, which can impact agricultural productivity. For this reason, a root zone model is important and applicable to different studies. LISFLOOD is a hydrological rainfall-runoff model capable to simulate the hydrological processes that occur in a catchment. LISFLOOD has been developed by the floods group of the Natural Hazards Project of the Joint Research Centre (JRC) of the European Commission [21].  
The model is made up of the following components: a 2-layer soil water balance sub-model, sub-models for the simulation of groundwater and subsurface flow (using 2 parallel interconnected linear reservoirs), a sub-model for the routing of surface runoff to the nearest river channel, and a sub-model for the routing of channel flow.  
The processes that are simulated by the model include snow melt, infiltration, interception of rainfall, leaf drainage, evaporation and water uptake by vegetation, surface runoff, preferential flow, exchange of soil moisture between the two soil layers and drainage to the groundwater, sub-surface and groundwater flow, and flow through river channels.  
Sample temporal series of the soil moisture (m\({}^{3}\)/m\({}^{3}\)) obtained from LISFLOOD model during the year 2015 for Brazilian Semiarid are illustrated in Figure 7. Of the 64 stations selected for the study of the LISFLOOD model, 25 stations were chosen in the State of Bahia (see Figure 7), since this state was the one who suffered the most severe drought in 2015, and because CEMADEM network became operational in 2015.  
Figure 7: Sample temporal series of SM obtained from CEMADEN stations (10 and 20 cm, gray lines; average value: red solid line), LISFLOOD model (50 cm depth, green line), SMOS L4 products (ascending and descending passes—red and blue line, respectively) during 2015–2016 for different sites located in Bahia (Brazilian Semiarid region). Sites selected to exhibit different correlation values with ascending-pass SMOS SM data.  
The 50 cm soil moisture data obtained was compared to CEMADEN stations (10 and 20 cm depth) [15], and SMOS L4 product (ascending and descending pass) for 2015-2016 (Figure 7) in the Brazilian semiarid region. It should be noted that the North East region of Brazil (NEB) is vulnerable to droughts with a long history of famine, migration and associated social problems. Currently this region presents a drought situation that began in 2012 and still persists in some areas with rainfall below the climatological average in the region. This drought can be considered the worst of the last 30 years. From 2012 to 2015, the drought hit the NEB, especially the state of Bahia [22; 23]. It is worth mentioning that SMOS L4 and LISFLOOD show good correlation with measurement data (Table 1). See [24] for a detailed study on the validation of SMOS products in different networks. For SMOS, 8 out of the 25 stations exhibit correlations smaller than 0.5, while for LISFLOOD the number grows up to 14, which is somehow expected as SMOS data are observations (despite at a different spatial resolution), and LISFLOOD models the results at the root zone, and the main difference is in the soil depth being modelled/sensed (CEMADEM stations = 10 cm, SMOS = 0-5 cm and LISFLOOD = 50 cm).