## 3 Results and Discussion  
The annual average precipitation observed by the rain gauges and estimated by the IMERG are shown in Figure 2. The precipitation in the SFB presents a characteristic distribution with higher annual average accumulations above 1500 mm in the South (Upper Sao Francisco). In the Middle Sao Francisco, the observed precipitation was around 600 and 1050 mm, decreasing to values below 500 mm in the northern portion of the basin (Lower Sao Francisco). This gradual decrease in precipitation from South to North observed in the rain gauges (Figure 2a) is represented consistently by the algorithm, but IMERG underestimates precipitation in the Lower Sao Francisco. The analysis of precision and accuracy is shown in Figure 3. The underestimation in the Lower Sao Francisco (Figure 3a) is confined to the northernmost of the SFB, located in the coastal region. In the rest of the SFB, the algorithm presents biases between \(-0.5\) and \(0.5\) mm. The RMSE exceeds 10 mm in the southernmost portion of the SFB, decreasing towards the central region (Figure 3b). The lower the RMSE values, the better the capacity of the IMERG product to represent the precipitation values observed by rain gauges. The correlation is below 0.4 in the Lower Sao Francisco (Figure 3c) and it is in this region where the lowest POD (Figure 3d) and highest FAR (Figure 3e) are found. Similar values were found by [18] who observed in inland and coast northeastern regions, regioes que abrangem o SFB, that 3B42 product of TRMM demonstrated a better performance, as demonstrated in the metrics for inland and coast northeastern regions, bias = 2.82 mm/day and \(-\) 2.94 mm/day; r = 0.18 and 0.30; std = 8.53 mm/day and 6.97 mm/day; rmse = 14.75 mm/day and 7.03 mm/day, respectively. Figure 3: Spatial distributions of (**a**) BIAS, (**b**) RMSE, (**c**) correlation coefficient (r), (**d**) POD and (**e**) FAR. Figure 2: Spatial distributions of mean annual precipitation (2000 to 2019): (**a**) Rain gauges, (**b**) IMERG. The precipitation in the SFB shows an evident spatial and temporal variability (Figure 4). The positive aspect regarding the IMERG data is the ability to adequately capture the seasonality of precipitation, observed over most of the basin. In this figure, it is clear that the rain regime is different among the sites. The majority presents its rainy period in the austral summer. On the other hand, the dry period prevails in the austral winter. For these sites, its clearly observed that the IMERG product is well related to the in-situ data. However, for the sites which presented their rainy period in the austral winter and the dry period during the summer, the results are not the same. in the municipalities of Agua Branca (Figure 4a), Pao de Acucar (Figure 4n) and Propria (Figure 4r), the satellite did not consistently reproduce the distribution of precipitation between the months of May and August, underestimating both the average, median and quartiles measured using rain gauges. This could be explained by the occurrence of warm cloud in these regions [5; 50], which in turn, generate worse precipitation estimates by sattelites. The density curves (gauge and IMERG) referring to the monthly data distributions are similar in a large part of the basin (Figure 5). As indicated by the boxplot (Figure 4), the exception is in the municipalities of Agua Branca (Figure 5a), Pao de Acucar (Figure 5n) and Propria (Figure 5r), where the distributions of the satellite data show a higher con  
Figure 4: Boxplot of monthly precipitation during 2000 to 2019 at each gauge (**a**) Agua Branca, (**b**) Barra, (**c**) Barreiras, (**d**) Belo Horizonte, (**e**) Bom Jesus da Lapa, (**f**) Cabrobó, (**g**) Carinhanha, (**h**) Curvelo, (**i**) Formosa, (**j**) Ibipetuba, (**l**) Junaria, (**m**) Montes Claros, (**n**) Pao de acücar, (**o**) Paracatu, (**p**) Petrolina, (**q**) Pirapora, (**r**) Propriá, (**s**) Remanso. centration on the left compared to the data from the rain gauges.