SOIL EROSION PROCESSES AND MODELLING IN THE UPPER BREGALNICA CATCHMENT Ivica Milevski, Ivan Blinkov, Aleksandar Trendafilov University “St. Cyril and Methodius” Skopje, Republic of Macedonia
[email protected]
Abstract Bregalnica is one of the largest rivers (225 km long, 4307 km2 catchment area) in the Republic of Macedonia, which drainage surface waters from most of the eastern part of the country. Because of suitable, sensitive natural factors (geology, soils, topography, climate, hidrography, vegetation) and human-related excess deforestation, huge area of Bregalnica catchment is characterized by strong erosion potential. This is especially evident upstream from Kalimanci dam, where most of the tributaries are torrential, and landscape represent real system of gullies, earth pyramids, badlands and landslides formed in soft Pliocene lacustrine sediment rocks. Aside of that are numerous deposition fans in the torrents bottom, as well as large flats of sedimentation. For that reason, in this upper part of Bregalnica catchment (1124.7 km2), are recorded sites with high annual erosion rates, among the highest in the Republic of Macedonia. In respond of such erosion risk, from the 1960-ties detailed studies with erosion modeling and mapping for the area are performed, dominantly based on traditional empiric model of prof. S. Gavrilovic. Having in mind fast transformation in the landscape, previous traditional approach is updated by implementation of suitable GIS tools. Keywords: Soil Erosion, Deposition, DTM (DEM), Satellite Imagery, GIS. 1
INTRODUCTION
Because of favourable natural factors and strong human impact over environment during centuries, area of the Republic of Macedonia is characterised with high soil erosion rate. According to “Erosion map of the Republic of Macedonia” (Djordjevic et al., 1993), average soil loss per year is near 700 m3/km2, which is among highest values in Europe. In last decades, faster economic growth of the country and lack of soil resources, fresh water, forests etc., reinforce the need for soil protection and soil conservation. That increases the interest for erosion research, especially in the areas highly affected by soil erosion. One of those areas is catchment of river Bregalnica - largest left tributary (225 km long) of Vardar, which is in turn a major river in the Republic of Macedonia. As a result of very suitable characteristics (soft rocks, sandy soils, steep slopes, climate, sparse vegetation, anthropogenic influence) upper part of its catchment has very high soil erosion rate. For that reason, in 1960-ties begins more detailed researches in this area, with primary goal to protect newly constructed Kalimanci reservoir from excess sedimentation. Later were published other studies in which fully or partly were treated soil erosion landforms in the area (Manakovic and Andonovski, 1979; Manakovic, 1980; An-
donovski, 1982; Milevski, 2004), soil erosion factors (Blinkov 2003), and soil erosion intensity (Rakicevic, 1975; Djordjevic et al., 1993; Blinkov, 1998). For soil erosion risk assessment in the upper part of Bregalnica catchment, previously mentioned “Erosion map of the Republic of Macedonia”, has great significance. This map is created according to the “Erosion Potential Model by prof. S. Gavrilovic, widely used and confirmed in former Yugoslavia. Recently, Gavrilovic model is updated by newly accessible DEM’s and remote sensing tools, offering even more valuable results. 2
STUDY AREA OVERWIEW
Bregalnica catchment is located in the east of the Republic of Macedonia, drainaging surface waters from area of 4307 sq. km or 16.7 % of the country. Upper part of the Bregalnica catchment, which is the subject of our interest, extending upstream of Kalimanci Dam (in the easternmost part of the Republic of Macedonia), from 22°27’44” to 23°02’03” East longitude, and from 41°35’09” to 42°09’16” North latitude.
Figure 1: Geographic location of the upper Bregalnica catchment. The catchment cover Malesh and Pijanec tectonic depressions, and it is bounded by: Vlaina Mountain (1932 m) from the East, Maleshevo Mountain (1745 m) from the South, Plackovica Mountain (1754 m) and Golak with Obozna Mountain (1536 m) from the West and Osogovo Mountain from the North. In that extent, upper part of Bregalnica
catchment has area of 1124.7 sq. km which is 26.1% of total area, or 4.4% of country area. It has southeast-northwest direction in length of 55 km, with altitude from 2203 m (Mal Ruen peak on the Osogovo Mountain) to 510 m (Kalimanci Lake) which means altitude difference of 1693 m. There are tree mid-to small size towns in the area: Delcevo, Berovo and Makedonska Kamenica, as well as numerous villages. 3
FACTORS OF SOIL EROSION
There are many natural and anthropogenic factors influencing soil erosion in upper part of Bregalnica catchment: lithology and soil structure, topography, climate, hydrography, vegetation (land cover), and human impact in the environment. However, aside that almost all natural factors forcing soil erosion process, human impact has leading significance trough the influence to most of other factors, especially of land cover. Lithology of the research area is represented by various metamorphic (gneiss, micaschist’s, schist’s), magmatic (granitites, gabro) and clastic sediment rocks, with variable erodibility. It is important that huge areas in the central part of the catchment (up to 1200 m of altitude), are composed by easily erodible Pliocene sands and sandstones - deposits from the Neogene lacoustrine phase in the Malesh and Pijanec basins. Exactly in that areas are sites with highest erosion rate and greatest production of deposits. Green schist’s and other schist’s, who cover significant area, are also highly erodible, exposed to intensive weathering and decomposition. Except this direct effect on soil erosion, lithology influencing soil structure as well. So most of the area is cover by eroded sandy soils and clay soils, excluding lowest parts of the basins and valleys, and dense forests. In latest, soils have better structure which is more resistant to soil erosion. Topography is another significant factor of soil erosion expressed by hypsometry, slopes, slope inclination (aspects), terrain relief etc. Hypsometry of upper Bregalnica catchment has generally indirect influence on soil erosion processes, thru the climatic and biogenic (vegetation) zonality, as well as intensity of human impact. The catchment lay between 510 m (Kalimanci Lake), and 2203 m (Mal Ruen peak on Osogovo Mountain), with mean altitude of 1000.5 m. Lower parts of the catchment, from 510 m to 1000 m, cover greatest area (56.6%, from which nearly 40% is between 750-1000 m). Exactly on these lower altitudes, there are low precipitations but with frequent stormy rain occurrences, with high daily and seasonal temperature amplitudes, sparse (human changed) vegetation, and significant human activity. Thus here it can be expecting elevated erosion potential, which is shown in further analysis. Higher elevations (1000-1500 m cover 38.4%, 1500-2000 m cover 5.3%, and above 2000 m is only 0.05% of the total area), are better protected from raindrop impact because of the better vegetation cover. Slopes like first hypsometry derivatives, has strong effect on soil erosion processes, especially by slope angle. Moreover, some earlier soil erosion models and equations were based only on slope parameters (Zingg, 1940). Thus, slopes in upper Bregalnica catchment are generally moderate with average value of 12°. Largest areas have slopes of 510° (31.7%), and 10-15° (28.3%), or both of them have 60%. Those are moderate slopes, here usually characterized with surface sheet to rill erosion. Higher slopes of 1520° and 20-25° cover 15.9% and 7.2% respectively, typically related with gully erosion, badlands (“mel”), landslides and rock falls. Lower slopes of 0-5° cover 12.4%, and in some cases are related to deposition areas (on the bottom of wider river valleys).
Inclination or terrain aspect is another valuable parameter related to soil erosion potential. In study area (according to latitude), south aspects are dryer, hotter, barer, and more eroded from north aspects. Analyses show that in general, south side terrains has greater fraction (53.7%) than north sided (46.3%). According to 4 main inclinations, west (30.5%) and south (24.8%) aspects prevails, then east (24.0%), and last north aspects (20.7%). It is clear that greater domination of south slopes trigger higher erosion risk, especially evident on south-east Osogovo and Vlaina Mountain slopes. Climate is very important factor of soil erosion in the region, especially in regard to temperatures and rains. Because of the geographic position, closeness of the region and high altitude, local climate has continental characteristics with high annual, seasonal and daily amplitude of the temperature. From meteorological station records in Berovo and Delcevo it is evident that minimal winter temperatures may drop bellow -30°C, while maximal summer temperatures can reach 38°C (Lazarevski, 1993). Daily temperatures can have high amplitude of more than 20°C. It is clear that such differences provoke strong weathering and decomposition of the rocks and deterioration of soil structure, making them more vulnerable to erosion. High summer temperatures for prolonged period, lead to faster drying of the soil which is then very susceptible to stormy rains, because then vegetation is weak as well. Because of dominance of water erosion in the area, most important climate factor are precipitations, especially the rains. According to local meteorological station records, mean annual sum of precipitation is from 562 mm (gs.Delcevo - 630 masl), 595 mm (gs Berovo - 824 masl) up to 802 mm (gs Sasa - 920 masl). Seasonality is not so much expressed like in other parts of the Republic of Macedonia, so the maximum of precipitations are in months of May and November (70-80 mm), and minimum is in August or September (40 mm). Stormy rains affect this area from May to November sometime with intensity of more than 50 mm/day causing very high erosion rate. As already mentioned, main hydrographic object is river Bregalnica, which has overall length of 225 km, while upstream of Kalimanci Dam (in the analyzed part) its length is 93.3 km. Altitudinal difference from the river spring on the Malesevska Mountain (1670 masl) to the Kalimanci reservoir (519 masl) is 1170 m, which is in average 79,7 %o. Almost all tributaries of Bregalnica, has torrential character with high amplitude in water discharge. The most significant tributaries are: Kamenicka (21.5 km), Zelevica (19.2 km), Ribnicka (10.3 km), Lukovicka River (10.5 km) etc. Vegetation is another significant factor of soil erosion in the area, which is highly influenced in past by excess human impact. According to historical sources, in last few centuries, natural vegetation represented generally by forests is almost totally destroyed or degraded in lower part of the cathcment, especially up to 1200 masl. For defining of the present vegetation cover, Landsat ETM+ imagery (acquired in 2000) was used in the study. Vegetation indices are derived from the spectral bands 4 (near infrared) and 3 (red) in widely used NDVI-Normalized Difference Vegetation Index, where: NDVI=(b4b3)/(b4+b3). NDVI for upper Bregalnica catchment is in range of -0.96 to 0.82, and for better calculation (without negative numbers), these values are transformed according to equation: tNDVI =(NDVI+1)*100. Resulting tNDVI generally range from 60 (water surfaces) to 160 (dense forests). Areas with vegetation index from 70-100, cover significant 34.4% (387.2 sq. km), representing bare soils, sparse grasslands and croplands (lower altitudes of Malesh and Pijanets depression). On the other side, values of tNDVI in range 140-180 represent dense or medium forest, and these areas occupy about 24.9%
(higher altitude on mountain areas on: Osogovo, Vlaina, Malesevska). Average vegetation index for the catchment is 121.5, and it significantly rise with altitude. These data’s tell about weak overall vegetation, with slight protection effect against raindrop impacts. If other elements are appropriate (slopes, aspects, precipitation), which is case along the rim of depression-mountain areas (altitude of 600-1000 m), frequently sites with severe erosion occur. 4
SOIL EROSION LANDFORMS IN UPPER BREGALNICA CATCHMENT
Strong erosion processes in the upper Bregalnica catchment, resulted in waste variety of erosion and deposition landforms. There are several types of erosion landforms: pluvial forms (rill, gully erosion, badlands, earth pyramids (earth pillars), landslides, landfalls, rockfalls etc. Rills frequently occur on cultivated land areas with processes of irrigation, or on erodible hilly terrain with overland flow. They are narrow and shallow ephemeral V-form incisions in the soil, and usually produce significant amount of deposits. Depending of soil erodibility and slope, rills often develop in more permanent gullies. Because of suitable environment, large areas in the upper Bregalnica catchment, as well as in their tributaries (Kamenicka River, Zelevica and others) have extremely developed gully erosion. The gullies are in variable shape (V, U, W-shaped) and evolution stage, characterized with excess sediment production. Larger gullies, like those in Kamenicka River watershed, formed alluvial fans in the bottoms with continual deposition of sediments. Landslides are also very common in this area, especially in the lower parts of the catchment, below 1200 m altitude. Their formation is enabled with characteristic lithology resembled by soft sediment rocks (sands, sandstones) on top which lie above inclined Pliocene clay layer. Another reason is high human impact on the environment. One of greatest landslide is recorded in the valley of Crnicka River near Crnik village. The landslide is located on the left (south) valley side, and it is 300 m long, 60-100 m wide, and 15-20 m deep. According to estimations, Pliocene sands of 650.000-700.000 m3 volume are under sliding. This “stratigenic” type of landslide, slowly moving toward Crnicka River, which in turn permanently detaches material from their frontal part, especially during the torrential event (Milevski, 2004). Numerous landslides are recorded in the valley of Kamenicka River downstream from the village Sasa. This part of the valley is composed by soft erodible schist’s and Pliocene sediments, mostly uncover, eroded and under steep slope. Because of that, soft weathered rocks on the valley sides almost entirely slide-down toward the valley bottom, making and enormous system of landslides with total estimated volume of 5 millions m3. That is particularly evident near the village Kosevica where the village itself has in danger. Aside of that, several landslides are registered on the coastal region around the Kalimanci Lake, then in the catchments of Lukovicka and Ribnicka River etc. All of them highly influence the soil loss rate and deposits intake in the Bregalnica River. Interesting forms of soil erosion in this region are so called “mels”. Mels are particular form of badlands with amphitheatric, semi-rounded shape, deeply incised in soft clastic sands and sandstones. Usually they have huge dimensions: 150-300 m width and 30100 m height, representing forms of excessive deep erosion with extreme deposit pro-
duction. Most remarkable mels in the area appear near villages Crnik, Smojmirovo and Istevnik.
Figure 2: Large “mel” near Crnik village In the upper Bregalnica catchment, there are several occurrences of earth pyramids (pillars), especially near v. Trabotiviste. Here, on the side of one large “mel”, numerous individual pillars are shaped in forms of animals, human being etc. Local people call them “Kukulje”. Similar but in smaller extent are earth pyramids near v. Kosevica which are in initial phase. Both occurrences are result of pluvial erosion in soft clastic sediment rocks which are partly protected on top with more resistant patches of rocks and soils. Landforms of rock weathering are present where terrain is composed by rocks with heterogeneous erodibility. Weathering processes in mica-schist, gneiss, granite, result in many stone pillars, rock falls, talus-cones etc. located mostly in the Kamenicka River watershed, then on Golak, Bejaz Tepe and Vlaina Mountain. Deposition landforms appear in the areas where erosion and transport capacity of the waters decrease, usually with decreasing of slope. These landforms strictly correlate with erosion intensity, so largest deposition landforms was created in the regions of severe erosion, and most of them are along the valley bottom of Kamenicka River. Downstream from Sasa village, valley bottom represent large alluvial plain with huge amount of fresh deposits from the near torrents. Along the left side of the bottom, these torrents formed 6 large alluvial fans (200x250 m). Most of the upper Bregalnica torrential tributaries have smaller alluvial fans and alluvial plains.
5
ESTIMATION OF EROSION RATES
There are numerous GIS-based methods of soil erosion rate estimation, depending of the purpose of that estimation, available tools and expected results. Usually those are empiric equations, frequently adapted to GIS-aided computations. Worldwide there are many such methods: USLE, WEPP, EUROSEM etc., providing better or fever outputs. In the Republic of Macedonia, estimation of average soil erosion potential and soil erosion risk is generally achieved with equation of prof. S. Gavrilovic (1972). The equation is in form: G=T*H*3.14*sqrtZ3*f, where: G is average annual soil erosion in m3, T is temperature coefficient (0.1*t+0.1), H is annual precipitation in mm, Z is erosion factor and f is catchment area in km2. Among these factors, Z has special importance combining: soil erodibility (Y), land cover index (Xa), index of visual erosion processes (φ), and mean slope (J) in relation: Z=Y*Xa*(φ+sqrtJ). Values of Z usually ranged between 0 (no erosion) and 1.5 (excess erosion). Because of proven accuracy, there are several recent GIS adaptation of this equation (Milevski, 2001; Milevski, 2005; Petras et al., 2007). Instead of traditional cartographic tools, in our approach most of the equation parameters are derived from 3”SRTM DEM and thematic layers of Landsat ETM+ satellite imagery. Thus, from preprocessed DEM (60m), with evaluated vertical gradient interpolation, climate indices T and H were obtained, as well as slope value J (in %) for every cell on the grid. Land cover index Xa is prepared from CORINE Land cover map or from transformed NDVI with values from 0.1 (dense forests) to 1 (bare soils). Index φ is prepared from Landsat ETM+ band 3, according to relation: φ=sqrt(b3/255). In GIS procedure factor Y (soil and rock erodibility) is taking to be optional, unless sufficient parameters are available. Finally with successive calculations of all grid layers, average erosion rate G is produced. In such way finished, GIS-based soil erosion map (Fig. 3A) is pretty close with the traditional Gavrilovic-based, catchment oriented soil erosion map (Fig. 3B) prepared by Djordjevic et al., (1993). Traditional map show average soil erosion potential of the upper Bregalnica catchment of 977 m3/km2/y, and our GIS-based map show in average 925 m3/km2/y, which is in line with Kalimanci basin sedimentation measurement (Blinkov, 1998).
Figure 3: A. Map of GIS-based (according to Gavrilovic model) estimation; B. Map of soil erosion potential (according to Djordjevic et al., 1993). 30.0
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Figure 4: Chart of specific deposits production according to traditional and GIS based method in the upper Bregalnica catchment. Coefficient of erosion Z itself, is almost the same on both maps, or about 0.55. However, there are big differences in the area of the sites with weak erosion (less than 500 m3/km2/y), which in GIS model account about 307.5 km2 (27.2%) and on traditional map only 31 km2 (2.8%). It is interesting that sites with high erosion risk (specific erosion rate more than 1500 m3/km2/y) on classical map cover 129.6 km2 (11.5%), on GIS-based map cover slightly more or 190.0 km2 (16.8%). Previous show that unlike traditional map which has smaller value amplitude, GIS-based map has large differences in coefficient Z and overall erosion production. Further research will confirm which model has to be cor-
rected. Never less, in regard to both maps as well as to several sedimentation measurement of Kalimanci reservoir, average annual sedimentation is pretty high; almost 420.000 m3/y. That is significant erosion rate, permanently occurring last decades. 6 CONCLUSION Because of favorable natural factors and significant human impact on the environment during long time, upper Bregalnica catchment is one of the most erosive areas in the Republic of Macedonia. Strong arguments for that are numerous soil erosion landforms (especially gully erosion, landslides, badlands) as well as estimated soil erosion. Soil loss assessment of the area is performed thru the traditional empiric model of Gavrilovic, and newly available GIS-based adaptation of this model. Both show high erosion rate (near 1000 m3/km2/y, or 1 mm topsoil/y), especially for some areas. Comparison of the models with sedimentation measurement, confirm close fitting, but further analysis and research must be done especially on the GIS-based model, which will allow better accuracy. Aside of that, because of high erosion rate, complex measures must be taken for soil conservation and environmental protection in the upper Bregalnica catchment (Milevski, 2006). That is key requirement for better socio-economic growth in the future. References Andonovski T. (1982): Erosion sites in SR Macedonia. XI Congress of geographers in SFRY, Budva Blinkov I. (1998): Influence of the rains on the intensity of soil erosion in the Bregalnica catchment up to the profile “Kalimanci Dam”, doctoral dissertation-manuscript, Faculty of Forestry, Skopje Blinkov I., (2003): Water and sediment discharge as a result of the forests and forest activities, scientific project, MES of RM, Skopje 2003, final report. Blinkov I., Trendafilov A. (2006): Effects of erosion control works in some torrents in the RM, Conference on Water Observation and Information System for Decision Support, BALWOIS, Ohrid, 2004 Djordjevic M., Trendafilov A., Jelic D., Georgievski S., Popovski A. (1993): Erosion map of the Republic of Macedonia, Skopje-textual part Gavrilovic S. (1972): Engineering of torrents flows and erosion. Special edition, Belgrade Jarvis A., Reuter H.I., Nelson A., Guevara E. (2006): Hole-filled SRTM for the globe, Version 3, available from the CGIAR-CSI SRTM 90m Database: http://srtm.csi.cgiar.org Lazarevski A. (1993): Climate of Macedonia. Skopje Liberti M., Simoniello T., Carone M.T., Coppola R., D’Emilio M., Lanfredi M., Macciato M. (2006): Badnd lands area mapping from Landsat-ETM data. Proceedings of the 2 Workshop of the EARSeL SIG on Land Use and Land Cover, 434-440 Manakovic D., Andonovski T. (1979): Relief characteristics of East Macedonia. Geographical review No. 17, Skopje 5-32 Manakovic D. (1980): Geomorphology of Males and Pijanets. Natural and socio-geographic characteristics of malesh and Pijanets - scientific project, Skopje Markoski B., Milevski I. (2005): Digital elevation model (DEM) of the Republic of Macedonia. Proceedings: Global automatization and energy optimization of technical processes. Skopje Milevski I. (2001): Modeling of soil erosion intensity with software tools, in the example of Kumanovo Basin. Proceedings of II Congress of Macedonia Geographic Society, Ohrid, 49-57 Milevski I. (2002): Recent soil erosion in Kumanovo Basin. Proceedings of scientific conference in the honor of D. Jaranoff. Varna, Bulgaria, 344-352 Milevski I. (2004): Forms of soil erosion in the Vinichka (Crnichka) valley. Geographic Review No. 39, Skopje, 5-20
Milevski I. (2004): Soil erosion in the Zelevica catchment. Bulletin for Physical geography, Skopje, 59-75 Milevski I. (2005): Possibilities for analysis of soil erosion in Republic of Macedonia by using of satellite imagery. III Congress of Macedonia Geographic Society, Skopje pp. 74-80 Milevski I. (2006): Erosion processes and development of rural areas in the Republic of Macedonia. Rural areas in the modern conditions of development. Ohrid, 539-556 Milevski I. (2007): Morphometric elements of terrain morphology in the Republic of Macedonia and their influence on soil erosion. International Conference Erosion and Torrent Control as a Factor in Sustainable River Basin Management 25-28 September 2007, Belgrade – Serbia Milevski I., Dragicevic S., Kostadinov S. (2007): Digital elevation model and satellite images in assessment of soil erosion potential in the Pcinja catchment. Annual of Serbian Geographic Association, Book LXXXVII-No 2, Belgrade, 11-20 Moore, I.D., R.B. Grayson, A.R. Ladson. (1991): Digital terrain modeling: A review of hydrological, geomorphological, and biological applications. Hydrol. Processes, 5 3–30 Zingg A.W. (1940): Degree and length of land slope as it affects soil loss in runoff. Agricultural Engineering, 21, 59-64 Olaya V. (2001): A gentle introduction to SAGA GIS. http://www.saga-gis.uni-goettingen.de Petras J., Holjevic D., Kunstek D. (2007): Implementation of GIS-technology in Gavrilovi’s method for estimation soil erosion production and sediment transport. International Conference Erosion and Torrent Control as a Factor in Sustainable River Basin Management 25-28 September 2007, Belgrade – Serbia Rakicevic T. (1975): Muddy of rivers in the Vardar catchment. Annual of Geographical Institute of FNSM, Belgrade, 21-33