Precipitation categories and the descriptive statistical variables of different

989 rainfall events have been separated based on the records of the tipping bucket rain gauge

“hhm” during the hydrological years 2000-2010 and 1123 rainfall events have been observed according to the “c1” rain gauge during the hydrological years 2003-2010. Deviations have also been obtained between the rainfall variables, due to the previously described failures of the instruments. However, this work primarily focuses on the impact of some selected rainfall

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events on the sediment transport, thus the ten-years-long precipitation database has not been corrected in the frame of the dissertation.

Description of the rainfall conditions are based on five precipitation categories according to Kucsara (1996), where the classes are: 0.0-2.0 mm, 2.1-5.0 mm, 5.1-10.0 mm, 10.1-20.0 mm and > 20.0 mm. Precipitation events with 0.5 mm or lower rainfall depth (P) are considered as trace of precipitation, but they are errors in several cases. Therefore, it would be necessary to neglect these data in the future, to make the evaluation more plausible. Precipitation categories in the different time scales give a comprehensive view of rainfall depth, intensity and erosivity distribution and also reflect the relation of the rainfall variables.

Figure 4.2. Precipitation categories and number of precipitation events according to the records of

“hhm” rain gauge (left) and “c1” rain gauge (right)

According to the Figure 4.2 the frequency of precipitation events increases in the higher categories. Disregarded the 0.0-2.0 mm category, the number of observations are comparable based on the two rain gauges:

 As the histogram of “hhm” shows, 35% of the rainfall events belong to the 2.1-5.0 mm category, 27% to the 5.1-10.0 mm category, 22% to the 10.1-20.0 mm category and 16% to the > 20.0 mm category.

 Rainfall distributions are similar according to the “c1”, as for the 36% proportion of the 2.1-5.0 mm precipitation class, 27% proportion of the 5.1-10.0 mm category, 21%

proportion of the 10.1-20.0 mm category and 16 % proportion of the > 20.0 mm category.

Since the contribution of precipitations lower than 2.0 mm to the total annual erosivity index (EI) are not higher than 0.8% at “c1” and 0.9% at “hhm”, the lowest category could be omitted in the sediment transport examinations.

Considering the seasonal distribution of precipitation events according to the database of

“hhm” (Figure 4.3), the rate of heavy rainfalls are dominant in summer (September-November): 31% all of the precipitation events are observed in summer (45% of the total rainfall amount in the ten-years-long study period) and 29% of the summer rainfalls belong to the >10.1 mm precipitation category. The ratio of the >10.1 mm category is 17% in autumn (September-November), 14% in winter (December-February) and 16% in spring

(March-61

May). As expected, the descriptive statistical variables also prove the high frequency of summer storms and the major role of the summer season in soil erosion (Annex IV.I.2), as well the records by the “c1” rain gauge.

Figure 4.3. Seasonal distribution of the precipitation categories according to the records of “hhm” rain gauge

Average and maximal values of maximal 30-min rainfall intensity (I_max30) and EI are the highest in summer and the lowest in winter. Despite of the bit higher ratio of the upper two precipitation category in autumn (17% vs. 16%), the late spring storms indicate higher erosion risk regarding the maximum rainfall depth (66.0 mm vs. 74.0 mm), Imax30 (58.0 mmh^-1 vs. 74.0 mmh^-1) and EI (47.2 kJm^-2mmh^-1 vs. 92.0 kJm^-2mmh^-1). The sum of EI in summer represents 71.5% of the total EI in the entire study period, while the proportion of the sum of EI to the total EI is 16.5% in spring, 9.5% in autumn and 2.5% in winter. Knowledge of these ratios is especially useful in the mirror of the vegetation cover. Since heavy rainfall events are the most frequent in late spring and summer, forestry activities should be avoided in these periods, to ensure the soil protection role of the vegetation.

Suspended sediment dynamics have also been evaluated at annual scale. To describe the rainfall conditions for these examinations, this section summarizes the precipitation categories for each hydrological year from 2000 to 2010 according to the records of “hhm” rain gauge (Annex IV.I.3). A detailed precipitation analysis of the hydrological year 2008-2009 has been

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represented according to the “hhm” rain gauge, because the sediment yield (SY) calculations and the assessment of an outwashing sediment deposit have been accomplished for this period.

Considering the Annex IV.I.3, the average annual precipitation is 746 mm in the study period based on the “hhm” rain gauge and omitted the inaccurate rainfall depth in 2002-2003. This value coincides well with the average annual precipitation of Sopron Hills according to Dövényi (2010). The average annual EI is 187.2 kJ·m^-2·mm·h^-1; furthermore, the sum of EI remarkably exceeds the average value in the hydrological years 2007-2008 and 2009-2010.

The extremely heavy rainfall events have a major role for providing high EI and triggering high rate of soil erosion. Rainfall events with rainfall depth higher than 20.0 mm have an 89.8% contribution to the annual sum of EI in 2008 and 88.1% in 2010.

Table 4.1. Rainfall events with high EI from the years 2008-2010 based on the “hhm” rain gauge (List of symbols: P – rainfall depth; Imax30 – maximal 30-min rainfall intensity; EI – erosivity index)

Start End Duration

As an example, Table 4.1 shows some selected events from the years 2008 and 2010 which compose a large proportion to the annual sum of EI. The two rainfall events from 2009 in the marked by italic numbers generated the sampled flood events which are analysed in detail (examination of suspended sediment dynamics and SY calculation at event-scale).

The hydrological year 2008-2009 can be considered as an average year as for the annual sum of P (723.5 mm) and EI (133.9 kJ·m^-2·mm·h^-1), thus it is ideal for the SY calculation. In this period, 96 single rainfall events have been observed, from which 38.5% of the precipitations belong to the <2.0 mm category, 20.8% to the 2.1-5.0 mm category, 19.8% to the 5.1-10.0 mm category, while the proportion of the upper two categories are similarly 10.4%. Rainfalls with rainfall depth higher than 10.0 mm have 66.6% ratio to the annual precipitation and 80.1%

ratio to the annual EI pointing at the importance of heavy rainfalls in soil destruction.

Considering the seasonal distribution of P, I_max30 and EI (Table 4.2), trends are mostly similar to the seasonal fluctuation of rainfall variables based on the entire study period. Although in the hydrological year 2008-2009 the autumn is the driest season, intensity and erosivity

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parameters follow the “usual” order: winter < autumn < spring < summer. Sum of EI of the summer rainfalls compose the 82.7% proportion of the annual EI.

Table 4.2. Seasonal fluctuation of the descriptive statistical variables of the rainfall parameters in the hydrological year 2008-2009 based on the “hhm” rain gauge (List of symbols: P – rainfall depth; Imax30

– maximal 30-min rainfall intensity; EI – erosivity index)

Valid N Average Sum Maximum Std.Dev.