5

Silva Balcanica, 19(3)/2018 DOI: 10.6084/m9.figshare.8198255

EVALUATION OF WATER EROSION RISK IN DZHUBRENA RIVER WATERSHED

Eli Pavlova-Traykova, Ivan Ts. MarinovForest Research Institute - SofiaBulgarian Academy of Sciences

Abstract

The purpose of the investigation is to analyse the main erosion factors and to assess the potential and actual soil erosion risk in the watershed of Dzhubrena by applying a methodical approach for determine and mapping the territories of erosion risk by using GIS. The assessments were made according to accepted for implementation ‘Methodology for preparing the national long term programme for soil erosion and floods preventing’. The total assessment of Dzhubrena watershed is ‘low to moderate’ risk of potential soil erosion and a ‘very low to low’ risk of actual soil erosion.

Key words: soil erosion, risk assessment, Dzhubrena river, Struma river

INTRODUCTION

Soil erosion is one of the major threats of European soils with mean average annual soil loss of 3.13 t ha-¹ (Blinkov, 2015). This critical soil degradation hazard has been found on almost of 12% of European territory (Panagos et al., 2014). Climate changes also have significant influence on this hazard and in last years the changes of precipitation and temperature have led to significant damages.

Soil erosion is the most serious degradation problem on the territory of Bulgaria (Ruseva et al., 2012). One of the most affected areas in our country is the territory of Struma river watershed. The main reasons for soil erosion processes and flooding risk are mountain relief, climate conditions, hydrographical network density, changes in land use, unregulated forest activities and animal farming practice (Mandev, 1995, 1996, Velizarova et al., 2010, Zlatunova, Ziapkov, 2010). According to the indicator of torrent risk developed by Ziapkov (1988), the middle part and the upper of Struma watershed, where is our object of investigation, is with moderate torrential risk. It is assessed potential and actual soil erosion risk by using GIS for upper and middle part of Struma river (Ruseva et al., 2012a,b, Ruseva et al., 2010b), but tributaries in the upper part of Struma, are not studied in details. All the data above along with changes in wheatear conditions make it necessary additional investigation in this region.

For better future management and effectively mitigation strategies of great importance is to determinate areas in risk (Gobin et al., 2002). There are many models

6

for soil erosion assessment using GIS, but one of them suitable for our territories and well applied on watershed level is ‘Methodology for preparing the national long term programme for protecting from erosion and flooding in the forest lands’ (Marinov et al., 2009).

The purpose of the investigation is to analyse the main erosion factors and to assess the potential and actual soil erosion risk in the watershed of Dzhubrena by applying a methodical approach for determine and mapping the territories of erosion risk by using GIS.

MATERIALS AND METHODS

Object of investigation is Dzhubrena river watershed which is one of right tributaries of Dzherman river. The last significant overflow of the river was in 2010 when along with Bistritsa river caused significant material damages in the territory of Dupnitsa municipality. In 2012 and 2014 the river also had great levels of waters because of the heavy rains and appropriate environmental conditions. There is also data for torrential characteristics for one of Dzhubrena tributaries. In 1953, one of the tributaries - L02 caused damages on infrastructure and inhabitants of Saparevo village despite of conducted erosion control activities. In 1981 the torrent reminded itself again (Panov, 2000).

Assessment of potential and actual soil erosion risk is prepared only for forestlands. Adapted version of methodology of MERA project is used (Stoev et al., 1997), based on using of GIS opportunities for assessment and mapping the soil erosion factors (rainfalls, topography, soil and vegetation cover). Hydrographic system density is received from estimation D= L_

F, where L is river length in km and F is watershed area in km2.

For assessment of potential and actual soil erosion risk it is necessary to be assessed individual soil erosion factors. For rainfall erosivity index (R factor) it is used a map for area distribution according to R factor values (Rousseva, Stefanova 2006). All areas from forestland with altitude of 1000 m a.s.l. have annual index of erosion 1 (600 MJmm/hah), from 1000 to 1200 m – index 2 (601 – 1000 MJmm/hah), and over 1200 m are with rainfall index 3 (1001 – 2000 MJmm/hah).

Soil index (Is) is defined depending on erosion degree and erosion type, mentioned in forest management plans. The values obtained by multiplying the class according to the degree of erosion and class according to the type of erosion are divided into three soil indexes– 1 (with value 1), 2 (with value 2 or 3) and 3 (with value 4 and 6).

For topography factor assessment (slopes gradient), a digital elevation model (DEM) is used, obtained from topography maps in scale 1:25 000. Slopes gradient is classified in four indexes: 1 – to 10о, 2 - 11- 20о, 3 - 21 – 30о and 4 – above 30о.

Potential soil erosion risk is determined by multiplication of R factor, slope index and soil index. For vegetation cover influence assessment, data was used from forest management plans. Vegetation index 1 have plantation and forestation with density above 0,6, these with density 0.3-0.6 - index 2, and open stands not suitable for forest area, barrens, gullies, landslides and landslips - index 3.

7

Actual soil erosion risk is determined by multiplication of potential soil erosion risk index and vegetation cover index in six grade scale. The methodology is presented detailed in Marinov et al. (2009).

RESULTS AND DISCUSSION

Watershed characteristic

Total area of Dzhubrena river watershed is 11 215.44 ha of them 4153.05 are forestland (Fig. 1).The river is right tributary of Dzherman river, which is one of main tributaries of Struma river. The main river length of Dzhubrena is 25.6 km. The area of river watershed is 112.2 km2 with average altitude of 1016 m and average slope gradient 11° (Table 1). This data are received from DEM of the watershed. There are more slopes with sunny exposure then shady one. Hydrographic system density is 1.4 km/km2.

The data from Forest Management Plan (FMP 2007-2017) showed that afforestation in watershed is significant part of forestlands. They are 1555.2 ha with main tree species Pinus sylvestris L. The main soil types are cambisols and with same participation are chromic cambisols and mollic cambisols (this classification of soils is according FAO).

Potential soil erosion risk

For assessment of soil erosion risk, first it is necessary to conduct some of main erosion factors. Heavy rains and their erosive force are one of main erosion factors which led to erosion processes.

To characterized heavy rains and rainfall erosivity it is usually used indexes calculated on different ways (Wishmeier, Smith, 1958, Malinov, 2003, Marinov, 2004, Rousseva, Stefanova 2006; Marinov et al., 2009, Ruseva et al., 2010a,b, Panagos et al., 2014, 2015). In Methodology rainfall erosivity considers the rainfall amount and intensity and it is expressed as R-factor.

The obtained results for R factor (Table 2) showed that with index 1 erosion rainfalls are 8967.15 ha (79.95%) and about 14.5 % are the territories with index 2 erosion rainfalls. The territories with index 3 are 621.9 ha, which is about 6% from the total watershed territory.

The area of distribution of individual soil erosion factors

Another key parameter for modeling soil erosion is soil index (Is). Soil index was obtained from forest management plans. The results are as follow: the territories without erosion processes – 3707.55 ha (89.27%). There are no territories with low erosion processes and the territories with strong erosion are 445.5 ha (10.72%).

Slopes are divided in 4 indexes. With index 1 (<10°) are about 53%, with index 2 (11°-20°) are 32% from the territory. With very steep slopes index 3 (21°-30°) are about 13% from the territory and with index 4 (>30°) are only 2% from the territories.

Potential soil erosion risk assessment is presented in Table 3. Predominant degree for potential soil erosion is ‘moderate’ with 46.61% from the watershed. With ‘low’

8

degree of erosion are 52.01% of the territory and with ‘strong’ degree of erosion are only 1.39 % of the territory. This distribution of degrees point that the total assessment of Dzhubrena watershed as with ‘low to moderate’ potential soil erosion risk.

For tributaries L02, L05, R10 that we used as representative the distribution of degree of erosion are with same trend. For all chosen tributaries predominant degree of erosion is ‘moderate’ potential soil erosion. It varies between 52% and 64%. The next predominant degree is ‘low’, and also there are some territories with ‘strong’ degrees. R12 tributary is with predominant degree of ‘low’ and without presence of ‘strong’ degree of soil erosion.

Table 1. Characteristics of Dzhubrena River watershed

Characteristics MeasureArea,

calculated with GIS

Wat

ersh

ed

Area km2 112.2 (11 220 ha)

Length km 25.6 – main stream length

Average altitude m 1016

Average slope o 11

Slope distribution:< 10 o

11–20 o

21-30 o

>30 o

ha

5940.83594.51452.2227.9

Slope exposure:sunny (S, SE, SW, W)shady (N, NW, NE, E)

flat

%7246.13961.5

7.8

Hydrographic system density km/km2 1.4

Table 2. Distribution of individual soil erosion factors

Rainfall erosivity index (R factor)

Area, ha Soil index (Is) Area, ha Slope index Area, ha

1 8967.15 1 3707.55 1 5940.81

2 1626.39 2 0 2 3594.51

3 621.9 3 445.5 3 1452.24

        4 227.88

Total area 11 215.44   4153.05   11 215.44

9

It is good to mention that for all chosen tributaries, most forest territories are with good density and they are well managed. But there are presence of territories with ‘strong’ potential erosion risk which showed the necessity of additional erosion control activities and research work.

In L02 tributary along with forestation they are also conducted erosion control activities on the river bed. Despite all this this tributary caused damages several times and presence of territories in ‘strong’ degree is a signal for future risk of floods and damages.

As we mentioned above forest territories in watershed of Dzhubrena river are well managed and with good density. This is proved with data for vegetation index (Table 4). With index 1 (full protection) are 2934.1 ha (70.6%). The territories with

Fig. 1. Location of the Dzherman river watershed and its tributaries

Fig. 2. Forestland distribution by actual soil erosion risk

Table 3. Forestland area distribution by potential soil erosion risk

Potential soil erosion risk

Forestland area distribution by tributaries

Dzhubrena river watershed

L02 L05 R10 R12

Index Degree ha % ha % ha % ha % ha %

1 Low 2159.82 52.01 179.82 29.40 169.20 46.65 296.82 47.73 266.31 54.04

2 Moderate 1935.63 46.61 391.05 63.93 189.36 52.21 321.48 51.69 226.49 45.96

3 Strong 57.60 1.39 40.86 6.68 4.14 1.14 3.60 0.58 0 0

Total area 4153.05 100 611.73 100 362.70 100 621.90 100 492.8 100

10

moderate protection (index 2) are 448.9 ha (10.8%) and 770.2 ha (18.5%) are with poor protection.

The results for forestland distribution by actual soil erosion risk are presented in Table 5. Total assessment for the watershed is ‘very low to low’ actual soil erosion risk. There is significant existence of territories assessed with ‘moderate’ degree which are mainly on the upper part of the watershed. The territories with ‘strong’ degree of soil erosion are mainly around Saparevo settlement where L02 tributary is (Fig. 2).

This is also seen from distribution by tributaries. For L02 and R12 tributaries have territories with ‘strong’ degree of actual erosion risk. For R12 tributary this assessment is mainly because territories with open stands. For L02 the data for actual erosion risk along with assessment for potential soil erosion risk shows that all soil erosion factors are in terms for high risk of soil erosion, floods and future damages.

CONCLUSION

From single soil erosion factors database for potential and actual soil erosion risk it was identified that the watershed of Dzhubrena river is characterized with high erosion sustainability.

Table 4. Forestland territory distribution by vegetation index

Vegetation index

Type of subsection Area, ha

1 Plantation and forestation with density above 0.6 2934.1

2 Plantation and forestation with density 0.3-0.6 448.9

3Open stands, not suitable for forest lands, barren, guillies, burn out area,

sole, landslides, screes, landslips, embarkment, marshes, cutting area770.2

Table 5. Forestland distribution by actual soil erosion index

Actual soil erosion risk

Forestland distribution by tributaries

Dzhubrena river watershed

L02 L05 R10 R12

Degree Index ha % ha % ha % ha % ha %

Very low 1 1095.57 26.38 133.47 21.82 68.67 18.93 258.84 41.62 169.92 34.48

Low 2 1735.56 41.79 317.16 51.85 173.07 47.72 264.60 42.55 142.74 28.97

Low to moderate

3 338.94 8.16 58.14 9.50 20.52 5.66 33.93 5.46 37.80 7.67

Moderate 4 771.57 18.58 30.96 5.06 90.99 25.09 26.10 4.20 109.89 22.30

Moderate to strong

5 204.57 4.93 70.74 11.56 9.45 2.61 38.43 6.18 31.59 6.41

Strong 6 6.84 0.16 1.26 0.21 0.00 0.00 0.00 0.00 0.81 0.16

Total area 4153.05 100 611.73 100 362.7 100 621.9 100 492.8 100

11

Without soil erosion processes is major part of the watershed, mainly because slight slopes, low erosion power of the rains and significant vegetation cover of forest fund majority. The application of the Methodology in forest fund territory and assessment indicates the presence of high risk of potential and actual soil erosion risk in certain parts of the watershed which matched with the territories with previous floods and risky events.

The total assessment of the watershed is ‘low to moderate’ potential soil erosion risk and ‘very low to low’ actual soil erosion risk. The existence of territories with ‘strong’ risk indicates the need for further research, especially in L02, where despite of erosion control activities the potential and actual soil erosion risk remain high.

REFERENCES

Blinkov, I. 2015. The Balkans - the most erosive part of Europe? Glasnik Shumarskog Faculteta. 111, 9-20. DOI: 10.2298/GSF1511009B.

Gobin, A, G. Govers, R. Jones, M. Kirkby, C. Kosmas. 2002. Assessment and reporting on soil erosion. European Environment Agency, 103 ISBN: 92-9167-519-9.

Malinov, I. 2003. Using N. Onchev’s rainfall erosivity factor to estimate R-factor by W. Wischmeier. Forest science Sofia, 1, 83-91 (In Bulgarian, English summary).

Mandev, A. 1996. Evaluation of the soil protective effectiveness of some forest ecosystems in Southwestern Bulgaria. – In: Proceedings of the Second Balkan Scientific Conference: Study, Conservation and Utilization of Forest Resources, II, 43-47.

Mandev, A. 1995. Regularities in the variation of the intensity of sheet water erosion in upland watershed areas managed in a number of ways. – In: Scientific Conference with Participation of International Specialists ‘90 Years of Soil Erosion Control in Bulgaria’ (Marinov, I. Ts., Ed). Lotus Publishers, Sofia, 37-42.

Marinov, I. Ts. 2004. Determination of rainfall erosivity on the territory of Ihtimanska Sredna Gora. Forest science Sofia, 4, 55-67.

Marinov, I. Ts., Р. Rajkov R., Petrov I. 2009. Method for determination of the erosion control activities in forest fund. Forest science Sofia, 1, 29-40 (In Bulgarian, English summary).

Panagos, P, Ch. Karydas, C. Ballabio, Io. Gitas. 2014. Seasonal monitoring of soil erosion at regional scale: An application of the G2 model in Crete focusing on agricultural land uses, International Journal of Applied Earth Observation and Geoinformation, 27, Part B, Pages 147-155, ISSN 0303-2434,https://doi.org/10.1016/j.jag.2013.09.012.

Panagos, P., C. Ballabio, P. Borrelli, K. Meusburger, A. Klik, Andreas, Sv. Rousseva, P. Tadić, Melita S. Michaelides, M. Hrabalikova, P. Olsen, J. Aalto, M. Lakatos, A. Rymszewicz, A. Dumitrescu, S. Beguería, Ch. Alewell, Christine. 2015. Rainfall erosivity in Europe. Sci. Total Environ. Science of Total Environment. 511. 801-814.

Panov, P. 2000. The Torrents under Control in Bulgaria. Monograph. Apricom., Sofia, 292, ISBN: 954-90748-1-1 (In Bulgarian).

Pavlova-Traykova, E., I. Marinov, P. Dimov. 2017. Evaluation of water erosion risk of Badinska river catchment, Southwest Bulgaria. Glasnik Sumarskog fakulteta, 115, 89-98. ISSN:0353-4537, DOI:10.2298/GSF1715089P.

Rousseva, S., L. Lozanova, D. Nekova, V. Stefanova, H. Dzhodzov, E. Tzvetkova, I. Malinov, V. Krumov, S. Chehlarova-Simeonova. 2010a Soil erosion risk in Bulgaria and recommendations for soil protective use of agricultural land. Part I. Northern Bulgaria. 304. ISBN 978-954-749-086-4 (In Bulgarian).

Rousseva, S., L. Lozanova, D. Nekova, V. Stefanova, H. Dzhodzov, E. Tzvetkova, I. Malinov, V. Krumov, S. Chehlarova-Simeonova. 2010b Soil erosion risk in Bulgaria and recommendations for soil protective use of agricultural land. Part II. Southern Bulgaria, 319, ISBN 978-954-749-087-1 (In Bulgarian).

Rousseva, S., L. Lozanova, D. Nekova, V. Stefanova, I. Nikolov. 2012a Evaluations of the Factors and

12

the Risk of Water Erosion of Soil for the Catchment of Struma River – Upper Reaches, Bulgaria. BALWOIS 28 May – 2 June 2012, Ohrid, Republic of Macedonia; M. Morell (Ed.) Available at: https://www.researchgate.net/publication/267264393_Evaluations_of_the_Factors_and_the_Risk_of_Water_Erosion_of_Soil_for_the_Catchment_of_Struma_River_-_Middle_Reaches.

Rousseva, S., L. Lozanova, V. Stefanova, I. Nikolov. 2012b Evaluations of the Factors and the Risk of Water Erosion of Soil for the Catchment of Struma River – Middle Reaches Bulgaria. 28 May-2 June 2012, Ohrid, Republic of Macedonia; M. Morell (Ed). Available at: https://www.researchgate.net/publication/267264393_Evaluations_of_the_Factors_and_the_Risk_of_Water_Erosion_of_Soil_for_the_Catchment_of_Struma_River_-_Middle_Reaches.

Rousseva, S., V. Stefanova. 2006. Assessment and mapping of soil erodibility and rain-fall erosivity in Bulgaria. Proceedings of the Conference on Water Observation and Information System for Decision Support BALWOIS 2006, Ohrid, Republic of Macedonia. Available at: https://www.researchgate.net/publication/251813968_Assessment_and_Mapping_of_Soil_Erodibility_and_Rainfall_Erosivity_in_Bulgaria.

Stoev, D., I. Malinov, V. Dimitrov, S. Rashkov, V. Stefanova, I. Nikolov. 1997. PHARE-MERA project BULGARIA - Land degradation mapping, Service contract 95 - 0021.00 . PHARE: ZZ9211/0502, Final report JRC ISPRA. Ministry of Environment, Sofia, 51.

Velizarova, E., I. Ts. Marinov, T. Lubenov. 2010. Changes of Some Soil Characteristics in Result of Degradation Processes in Maleshevska Mountain, Bulgaria. BALWOIS 25 – 29 May, Ohrid, Republic of Macedonia; M. Morell (Ed.). Available at: https://www.researchgate.net/publication/268197939_Changes_of_Some_Soil_Characteristics_in_Result_of_Degradation_Processes_in_Maleshevska_Mountain_Bulgaria.

Wishmeier, W. H., D.Smith. 1958. Rainfall energy and its relationship to soil losses. Transactions American Geographycal Union, 39, 2.

Zlatunova D., L. Ziapkov. 2010. Flood processes in Struma river. Annuaire de L’universite de Sofia St. Kliment Ohridski Faculte de Geologie et Geographie livre 2 – Geographie, 102, 13-29.

Zyapkov, L. 1988. Torrent degrees of Bulgarian rivers. - In: Problems of Geography. BAS, 3, 35-42.

E-mail: pavlova.eli77@gmail.com