# Rainfall Erosivity Main Features and Their Associated Synoptic Conditions in North-Eastern Romania
## 3 Results and Discussions  
### Spatial Distribution of Rainfall Aggressiveness in Northeastern Romania  
In the eastern part of Romania, the annual distribution of heavy rains is recorded between May and September, with the highest frequency occurring in June, followed by July, August, September, and May [12]. For larger periods of time, it appears that 56.8% of the heavy rainfall was recorded between 15 May and 20 June, an interval that overlaps with the critical erosion season for the Moldavian Plateau [50]. Thus, we calculated the Fournier Index for the period 1979-2018 using ERA-5 reanalysis dataset. The Fournier Index is based on the amount of rainfall, and therefore, the minimum values obtained over the analyzed region are between 13 and 16 (very low erosion class with soil losses between 0-5 t/ha/year), located in the southeastern part of the extra-Carpathian Moldova where the annual precipitation amount is the lowest over the entire analyzed region (Figure 2a). Conversely, the maximum values reach 36 (low erosion class with soil losses of 5-12 t/ha/year) along the boundaries with the mountainous terrain. \begin{table}
\begin{tabular}{c c} \hline \hline
**Erosion Class** & **Fournier Index** \\ \hline Very Low & 0–60 \\ Low & 60–90 \\ Moderate & 90–120 \\ High & 120–160 \\ Very high & \textgreater{}160 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Erosion classes based on Modified Fournier Index values [43]. Although this estimated value of erosion does not seem high, it signifies an important loss, significantly exceeding the natural soil regeneration capacity. Furthermore, the topography of the region, as indicated in Figure 1b, has been demonstrated to exert a substantial influence on erosion rates, with significantly higher values observed in hilly regions compared to plains, particularly in the central and southeastern regions of the study area [51,52]. Our results indicate similar values to those reported for other regions in Romania. Thus, Fournier Index values range from 10 to 16 over the Bucharest area [40], while in the Secasul Mare Basin, values show very low aggressiveness in the lower area of the basin and low to moderate aggressiveness in the higher area of the basin [9]. During the same time interval, the Modified Fournier Index registered values between 42.3 (very low erosion class) and 103.7 (moderate erosion class). The maximum values were concentrated mainly in the area of the Suceava Plateau in the northwest and the Moldavian Subcarpathians in the west, while the minimum values were found in the southeastern part (Figure 2b). However, when lithological and land cover factors are considered, the soil erosion is highest in this south-eastern part, with annual soil erosion loss peaking up to 25 t/h/year [51], being expected to increase at the highest rate across the arable region of Romania in both moderate and extreme climate change scenarios [52]. Moreover, in situ measurements of soil erosion in this critical region shown that peak years reach 60 t/ha/year, in direct relation with rainfall aggressiveness index [53]. At the European scale, values like these are among the highest values for lowland regions besides important areas of southern Spain or Italy [3]. Figure 2: The spatial distribution of Fournier Index Erodibility classes (**a**) and Modified Fournier Index Erodibility classes (**b**) in northeastern Romania for 1979–2018. Other studies have shown that the Modified Fournier Index ranges from 53.6 to 66.6 in Bucharest [40]. According to the aggressiveness classes of the Modified Fournier Index, in the Secasul Mare Basin, for altitudes below 200 m, the rainfall aggressiveness is very low, increasing at altitudes over 800 m to moderate [9]. Moreover, the annual cycle reveals substantial differences from one month to another: from very low aggressiveness between December and March to very high aggressiveness in June and July [12].