DETERMINATION OF EROSION INTENSITY IN BRKA WATERSHED, BOSNIA AND HERZEGOVINA

The Bosnia and Herzegovina (BiH) erosion map was made in 1985, however, over a period of 35 years, there has been a substantial change in the values of most erosion factors, resulting in the change of the erosion intensity. Changes relate to demographics, urbanization and land use as well as climate. The increase in temperature and the occurrence of extremes caused significant environmental and economic consequences (May 2014 floods). This situation is more pronounced in the northern part of the country, especially in the lower parts of the larger basins. Risk assessment procedures using modern software and hardware solutions can help decision-makers to recognize sites where forest should not be cut down, certain crops should not be grown or soil conversation measures are necessary. Therefore, the aim of this research is to estimate the intensity of erosion processes in one such watershed in BiH the Brka watershed, taking into consideration current conditions and using modern hardware and software solutions. To calculate erosion intensity the Gavrilovic method supported with GIS techniques was used. The soil protection (x), soil erodibility (y) and type and extent of erosion (φ) coefficients were calculated using digital maps: CORINE 2018 (grid size 100 m x 100 m) land cover, soil map of BiH and open-source satellite images. The slope was calculated from the BiH digital elevation model (25 m x 25 m). The Brka watershed area (184.09 km 2 ) was divided into four basins: Maočka Rijeka (51.56 km 2 ), Rahička Rijeka (24.26 km 2 ), Zovičica (75.30 km 2 ) and direct basin of Brka (32.94 km 2 ). The highest average erosion intensity was determined for Zovičica basin, where Z=0.56. The calculated mean annual production of sediment per basin varies from 5,746 for Rahička Rijeka to 57,089 m 3 year -1 for Zovičica, with total Brka river watershed sediment yield of 120,754 m 3 year -1 .


INTRODUCTION
Soil erosion has been considered as the primary cause of soil degradation and loss. Lately, erosion has become a growing problem when it comes to environmental and biodiversity protection in the Balkans (Spalevic et al., 2015). In Bosnia and Herzegovina (BiH), soil erosion intensifies with the negative effects of man from the time of the ancient Illyrians, Romans, Slavs, etc. to this day. Logging and burning of forests and converting these areas to arable land resulted in the occurrence of excessive soil erosion (Šarić et al., 1999). Soil water erosion is one of the most important causes of soil degradation in BiH, this is especially true for agricultural land and smallholder farms that are often located in marginal areas, where the soil quality is poor and the topography is complexed (J. Žurovec et al., 2017a). With its complex relief, geological and pedological structure, hydrography, precipitation regime and land use, BiH is highly vulnerable to destructive processes of erosion and floods, especially in the northern part of the country (Čadro et al., 2019; O. Žurovec et al., 2017b). According to Lazarević (1985b), as much as 83% of the total area of BiH is threatened by water erosion.
When it comes to the analysis of erosion processes in BiH in addition to local surveys at the parcel level (J. Žurovec & Čadro, 2008; J. Žurovec et al., 2017a) the Gavrilovic method (Gavrilović, 1972) was used to map and analyze erosion at the larger-areas. An erosion map of the FR of Bosnia and Herzegovina was made in the period 1980-1985(Lazarević, 1985b. Recently, in 2012 an erosion map of the Entity Republika Srpska in scale 1:25,000 (Radislav Tošić et al., 2012a;Radislav Tošić et al., 2012b) was made as well as in 2018 the erosion map of the Vrbas River Basin at a scale of 1:25,000 (Lovrić & Tošić, 2018).
Latterly, there has been a substantial change in the values of most erosion factors, resulting in the change of erosion intensity. Changes relate to demographics, urbanization and land use as well as climate (Čadro et al., 2018;Čadro et al., 2019;Popov et al., 2018;Trbic et al., 2017;O. Žurovec et al., 2017b). The increase in temperature and the occurrence of extremes caused significant environmental and economic consequences (May 2014 floods). This situation is more pronounced in the northern part of the country, especially in the lower parts of the larger watersheds.
A map of the spatial distribution of the intensity of erosion processes should be the first step towards a better understanding of the situation in an area of a basin, as well as a more realistic view of the risks of natural disasters, especially erosion, floods and landslides. Such a map is essentially a measure for disaster risk reduction (DRR), a systematic approach to identifying, assessing and reducing the risks of disaster (Jamieson, 2016). Therefore, the main objective of this study was to analyze the basic soil erosion factors and estimate the intensity of erosion processes in the River Brka watershed, taking into consideration current conditions and using modern hardware and software solutions.

Study area and data collection
The Brka River Basin is located in the northeast of BiH, it covers the northern slopes of mountain Majevica and part of the Bosnian Posavina ( Figure  1). The total watershed area is about 184.09 km 2 . The highest point is the Okresanica peak, 815 meters above sea level, while the lowest point is the delta of the Brka River at 84 meters above sea level. Most of the watershed area is located within the Brcko District, and only a small part to the south is in the Federation of Bosnia and Herzegovina (FBiH), the municipalities of Srebrenik and Čelić. The Brka watershed belongs to the temperate continental climate zone. The characteristics of this climate are quite cold winters and warm summers. The average air temperature is 11.12 o C and the average precipitation is 780 mm (Table 1). In the south, due to the increase in altitude, average temperatures are decreasing and precipitation is increasing (Majstorović, 2000). This is an area of great potential for the development of the economy, due to favorable population density, significant areas of arable land, developed road infrastructure and favorable position towards the three major regional centers, Belgrade, Zagreb and Sarajevo (Čardaklija, 2015;Smajlović, 2014 56 67 76 95 73 70 55 47 70 69 780 1 T max -Maximum average air temperature; 2 T min -Minimum average air temperature; 3 T mean -Average air temperature; 4 RH mean -Average relative humidity in %; 5 u (2) -Average wind speed in m s -1 ; 6 PRCP -Average sum of precipitation in mm.

Erosion intensity calculation method
In this research, the Gavrilovic method (Gavrilović, 1972) also known as the Erosion potential method (EPM) modified according to Lazarević (1985a) and adapted for use in the geographical information system environment -GIS (N. Dragičević et al., 2013;Mustafić, 2012;Radislav Tošić & Dragićević, 2012) was used to create maps and calculate erosion intensity (Z), mean annual production of sediment (Wyear) and basin sediment yield (Gyear).
The soil erosion coefficient, or erosion intensity (Z) was calculated using the analytical method with following equation: (1)

Where:
Y -Coefficient of the resistance of the land to erosion (soil erodibility) X-Coefficient of the protection of the land from the atmospheric impact, vegetation protection coefficient -Coefficient of the type of erosion √Jsr -Average slope (inclination) in % The quantitative values of the erosion coefficient (Z) have been used to separate erosion intensity to 5 classes: Excessive erosion (I), Z > 1.00; Intensive erosion, Z=0.71-1.00; Medium erosion (III), Z=0.41-0.70; Slight erosion (IV), Z=0.21-0.40; Very slight erosion (V). Z=0.01-0.20 (Lazarević, 1985a). To calculate mean annual production of sediment per basin -W year (m 3 year -1 ) the following equations ware used: Mean annual air temperature (°C) H year Mean annual sum of precipitation (mm) F Area of the basin (km 2 ) Multiplying the mean annual production of sediment per basin (W year ) with Coefficient of the retention of sediment (R u ) we calculated the mean annual volume of suspended and transported sediment per basin, or the basin sediment yield -G year (m -3 year -1 ). To do so the following equations were applied: The boundary of the basin area was determined using Digital terrain model (DEM: 25 m x 25 m) and Hydrographic network map of BiH; the soil protection coefficient (X) from CORINE 2018 (grid size 100 m x 100 m) land cover map based on the X values proposed by Globevnik et al. (2003). Soil erodibility (Y) was determined on the basis of the BiH soil map (scale 1: 50,000), while for the determination of type and extent of erosion (ϕ) coefficients open-source satellite images were used.
Esri® ArGIS 10.2.1 software was used to determine all required elements of the basin (√Jsr, F, O, D, I p , D d , L and I a ). The raster calculator tool was used to create Z and W year maps.
Also, climate data from the Brčko weather station (period 1961 -1990) was used to analyze the climatic conditions as well as the calculation of certain parameters within the EMP methods (T and H year ). The largest area is occupied by the Zovičica river basin, with about 41% of the total area, while the Rahička river basin occupies the smallest area or about 13% of the total watershed.

RESULTS AND DISCUSSION
The direction of fall of the Brka River basin is southwest-northeast, which is the result of higher altitudes in the south (814 m.a.s.l.) that is, in the area of the Majevica Mountain and on the other side, low altitudes (84 m.a.s.l.) of the Posavina valleys in the north (Figure 2). The average basin elevation is 276 m, with an almost equal proportion of lowlands with elevations up to 150 m (33%) and elevations ranging from 330 to 570 m (31%). Less than 5% of the watershed area is located at an altitude of more than 570 m (Table 2). An overview of the basic spatial and hydrological characteristics required for the EPM method calculation for the 4 defined sub-basins of the Brka River is given in Table 3.  The individual sub-basins are quite different, this is especially true for the Zovičica river basin, which occupies the largest surface area. The main watercourse, the river Brka is 26.57 km long. The Zovičica River is similar in length (24.39 km), however, the total length of its tributaries is more than 3 times greater. Also, the difference between the lowest and highest points of the Brka River is only 185 m, unlike the Maočka River where this difference is 661 m or Zovičica where it is 611 m. This situation results in high river network density (D d ) as well as a significant retention coefficient (R u ), which is especially true of the Zovičica River basin area.When it comes to soil type, the Dystric Kambisol occupies the largest area of the Brka watershed (Table 4). In most cases, this soil is covered with forest, but due to its favorable properties it is often used as agricultural land (Miljković, 2005;Resulović et al., 2008). Most of the areas under this type of soil are located in the southern part of the basin, respectively within the sub-basins of the Maočka and Rahiča rivers. Pseudogley and Luvisol are in second and third place, respectively. These are heavy soils, with poor permeability and high erodibility (Dugalić & Gajić, 2012;Resulović et al., 2008). These soils are very susceptible to erosion, especially if located on slopes greater than 12% (J. Žurovec, 2012). It is very important to note that these soils occupy 82 km 2 or 45% of the Brka watershed. They are mainly located in the north part of the basin, at altitudes less than 330 m.Based on the land use, the watershed can be divided into three zones, an urban zone in the far north that includes the city of Brčko itself, then an agricultural zone located in the middle part of the watershed, ie along the river Brka itself and within the Zovičica river basin. The third zone, the forest zone, is located in the south of the basin, that is, on the slopes of mountain Majevica, or sub-basins Maočka and Rahička Rijeka (Table 5). Nearly half (49%) of the watershed area is covered by forest vegetation, dominated by the broad-leaved forests. Agricultural production takes place at 82 km 2 .Based on mentioned soil erosion factors in the Brka watershed, first, the individual land use and soil type categories were assigned with the values of the coefficients X and Y, then their spatial distribution was created in Esri® ArGIS 10.2.1 software (Figure 3). In this process, using the DEM and open-source satiate images, a slope map, as well as an ϕ map, were created ( Figure 3). Based on the erosion categories 16.68% of the territory is affected by excessive erosion, 7.24% by intensive erosion, 7.31% by medium erosion, 12.66% by slight erosion, 48.85% by very slight erosion, and 7.72% has no erosion (Table 6). In this way, all the maps necessary for the calculation (Equation 1) and spatial representation of the erosion intensity (Z) were obtained. The next step was use of the Raster calculator tool to calculate and create erosion intensity (Z) map of Brka watershed as shown in Figure 4.
The spatial distribution of erosion intensity (Figure 4) shows the highest intensity of erosion in the central part of the watershed. Although the upper part of the watershed has a higher slope, most of these areas are covered with forests, which very well protects the soil from erosion. This is not the case in the central and lower parts of the watershed, which are characterized by smaller slopes, but where intensive agricultural production is carried out on soils with poor waterphysical characteristics. This means that soil characteristics and land use have a dominant influence on the intensity of erosion processes in the Brka watershed. According to the results, the intensity of the erosion process in Brka watershed has a medium erosion character, with an average erosion coefficient of Z=0.46 (Table 7). In comparison, the average value of Z for the Vrbas basin is much smaller Z=0.18 (Lovrić & Tošić, 2018), as well as most of the other watersheds in BiH entity Republic of Srpska: Bosna Z=0.20;Drina=0.45;Sana=0.15 (Radislav Tošić et al., 2012a). This indicates the pronounced erosion processes in the Brka watershed. This is especially true for the Zovičica River sub-basin (Z=0.56) and the Brka River direct basin (Z=0.46). This situation is probably the result of the high prevalence of high erodibility soils (Pseudogley and Luvisol) that are mostly used for agricultural production. The mean annual production of sediment per km 2 (W year ) varied between 672 and 1639 m 3 year -1 km -2 . The calculated mean annual sediment yield (G year ) varies from 5,746 for Rahička Rijeka to 57,089 m 3 year -1 for Zovičica, with total Brka river watershed sediment yield of 120,754 m 3 year -1 .

CONCLUSIONS
The average Z value of 0.46 (medium erosion intensity), 43.89% of the territory threatened by water erosion, and 16.68% affected by excessive erosion indicates that at the Brka watershed certain soil conservation measures are more than necessary.The upper part of the watershed is covered with forest vegetation and therefore well protected from erosion processes. This is especially true for the sub-basins of the Maočka and Rahička rivers. Most of the agricultural production in this watershed takes place in the central part of the basin. However, this production takes place on soils with poor water-physical characteristics (Pseudogley and Luvisol). Since land use is an erosion factor that humans can control, it is necessary to act in this direction and prevent erosion conducting agro-technical and biological soil conservation measures.
In these circumstances, the cultivated soil should not be left bare -not sown at any cost, especially when it is plowed in the direction of the slope. Additionally, special attention should be paid to the length of parcels located on higher slopes. Contour soil cultivation and contour sowing/planting are recommended whenever the size and shape of the plot allow it.