Environmental Conditions of Gully Erosion in Hungary 79 Hungarian Geographical Bulletin 2009. Vol. 58. No 2. pp. 79–89. Environmental conditions of gully erosion in Hungary Ádám Kertész1 Abstract Soil erosion research has become very important over the last decades. It is the subject matt er of several disciplines, geographers, geomorphologists, soil scientists, hydrologists, agronomists and other scientists publish on this topic and the authors of these publications are members of interdisciplinary and in most cases also international teams. Research on soil erosion by water concentrated, however, mainly on sheet erosion. The role of gully erosion has been recognized only lately. The presence and dynamics of various gully types (permanent, ephemeral and bank gullies) can be observed and their development can be followed under diff erent climatic conditions and various land use types and sheet and rill erosion measurements on runoff plots are not realistic indicators of total catchment erosion (Poesen, J. et al. 2003). Another weakness of plot measurements of sheet erosion is that they do not give information about the redistribution of eroded soil within a fi eld (Poesen, J. et al. 2003). Gully erosion plays a decisive role in the redistribution of eroded soil on a slope and in delivering it to watercourses (Evans, R. 1993, cited by Poesen, J. et al. 2003). All these statements point to the need of intensive research on gully erosion. Keywords: gully and rill erosion, soil loss, land use change Introduction The process of gully erosion generates 20–30 cm to 20 m deep gullies (Bergsma, E. 1996). Although there are contradictory views about the share of gully erosion in the total amount of soil loss our experiences show that gully erosion processes have a bigger share than those of sheet erosion (Jakab, G. et al. 2006). According to the Hungarian classifi cation gully erosion is one of the processes of linear erosion (see e.g. Jakab, G. 2008). Linear erosion is a logical scientifi c name for this group of processes but it is not used worldwide. Micro- rill, rill and gully erosion belong to the group of linear erosion processes. For the really big gullies, i.e. for those with signifi cant volume and especially deepness the expression of gorge (ravine) erosion is also used. Linearity is included in every defi nition. E.g. Poesen, J. et al. (2003) defi ne gully erosion “as the erosion process whereby runoff water accumulates and oft en recurs in 1 Geographical Research Institute, Hungarian Academy of Sciences. H-1112 Budapest, Budaörsi út 45. E-mail: kertesza@helka.iif.hu 1_Kertész_cikk.indd 791_Kertész_cikk.indd 79 2009.10.19. 10:32:552009.10.19. 10:32:55 80 narrow channels and, over short periods, removes the soil from this narrow area to considerable depths”. The defi nitions of various forms of linear erosion (rill and gully ero- sion) are given by Jakab, G. (2006) in Hungarian language. In this classifi cation the value of 50 cm (width and depth) separates rills from gullies and deep-cut tracks are defi ned as a special group of gullies. They were dealt with in detail by Kertész, Á. (1984). Kerényi, A. (1991) applied also the 50 cm value to diff erentiate between rills and gullies. Various Hungarian and foreign authors use diff erent threshold values and defi nitions. A detailed analysis and comparison of them will not be given here as it would not bring essential information on the topic. An important step in gully erosion research was the introduction of the term ephemeral gully erosion (Foster, G.R. 1986). The size of ephemeral gullies is between rills (Photo 1) and gullies (Photo 2), i.e. these gullies can still be removed by cultivation, while permanent gullies are too deep to ameliorate with tillage machines (Soil Science Society of America 2001). Bank gullies are defi ned as gullies developed on earth banks, i.e. where concentrated runoff crosses a bank (Poesen, J. et al. 2003). A very clear classifi cation and description of erosion processes is given by Laflen, J.M. (1985, see Table 1). Photo 1. Rills on arable land 1_Kertész_cikk.indd 801_Kertész_cikk.indd 80 2009.10.19. 10:32:562009.10.19. 10:32:56 81 Ta bl e 1. C la ss ifi ca ti on a nd d es cr ip ti on o f e ro si on p ro ce ss es b y La fl en , J .M . ( 19 85 ) Sh ee t a nd r ill e ro si on E p he m er al g u lly e ro si on G u lly e ro si on O cc u rs o n sm oo th s id e sl op es a bo ve d ra in ag el in e. O cc u rs a lo ng s ha llo w d ra in ag el in es u p - st re am fr om in ci se d c ha nn el s or g u lli es . G en er al ly o cc u r in w el l d efi n ed d ra in ag e- lin es . M ay b e of a ny s iz e bu t a re u su al ly sm al le r th an c on ce nt ra te d fl ow c ha nn el s. M ay b e of a ny s iz e bu t a re u su al ly la rg er th an r ill s an d s m al le r th an p er m an en t gu lli es . U su al ly la rg er th an c on ce nt ra te d fl ow ch an ne ls a nd r ill s. Fl ow p att e rn d ev el op s m an y sm al l d is - co nn ec te d p ar al le l c ha nn el s w hi ch e nd at c on ce nt ra te d fl ow c ha nn el s, te rr ac e ch an ne ls o r in d ep os it io na l a re as . U su al ly fo rm s a d en d ri ti c p att e rn a lo ng w at er c ou rs es b eg in ni ng w he re o ve rl an d fl ow , i nc lu d in g ri lls , c on ve rg e. F lo w p at - te rn s in fl u en ce d b y ti lla ge , r ow s, te rr ac es , m an m ad e fe at u re s. D en d ri ti c p att e rn a lo ng n at u ra l w at er co u rs es . M ay o cc u r in n on -d en d ri ti c p at - te rn s in r oa d d it ch es , t er ra ce o r d iv er si on ch an ne ls , e tc . R ill c ro ss -s ec ti on s u su al ly a re n ar ro w re la ti ve to d ep th . C ro ss -s ec ti on s u su al ly a re w id e re la ti ve to d ep th . S id ew al ls n ot w el l d efi n ed . H ea d cu ts n ot r ea d ily ; d o no t b ec om e p ro m in en t b ec au se o f t ill ag e. C ro ss -s ec ti on s u su al ly n ar ro w r el at iv e to d ep th . S id ew al ls a re s te ep . H ea d cu t p ro m in en t. E ro d in g ch an ne l a d va nc es u p st re am . R ill s no rm al ly r em ov ed b y ti lla ge , u su - al ly d o no t r eo cc u r in th e sa m e p la ce . Te m p or ar y fe at u re , u su al ly r em ov ed b y ti lla ge ; r eo cc u r in s am e p la ce . N ot r em ov ed b y ti lla ge . So il re m ov ed in th in la ye rs o r sh al lo w ch an ne ls . S oi l p ro fi le b ec om es th in ne r ov er e nt ir e sl op e. So il re m ov ed a lo ng n ar ro w fl ow p at h, to ti lla ge d ep th if u nt ill ed la ye r is r es is ta nt to e ro si on , o r d ee p er if u nt ill ed la ye r is le ss r es is ta nt . So il m ay e ro d e to d ep th o f p ro fi le , a nd ca n er od e in to s oft b ed ro ck . L ow e ro si on r at es n ot r ea d ily v is ib le . A re a m ay o r m ay n ot b e vi si bl y er od in g. E ro si on r ea d ily v is ib le D et ac hm en t a nd tr an sp or t b y ra in d ro p s an d fl ow in g w at er . D et ac hm en t a nd tr an sp or t b y fl ow in g w at er o nl y. D et ac hm en t b y fl ow in g w at er , s lu m p in g of u ns ta bl e ba nk s an d h ea d cu t r et re at ; tr an sp or t b y fl ow in g w at er . 1_Kertész_cikk.indd 811_Kertész_cikk.indd 81 2009.10.19. 10:32:562009.10.19. 10:32:56 82 In order to understand gully initiation and development usually the following questions are asked: (1) What is the importance of surface and near surface lithology? (2) What are the topographic threshold values leading to the formation of gullies? (3) What are the characteristics climatic conditions (fi rst of all rainfall amounts and intensities) to trigger gully development? (4) What is the role of land use and land use change? (5) What socio-economic factors infl uence gully initiation and extension in a given area? The present paper tries to answer these questions by examining the conditions of gully formation and development in Hungary. Photo 2. Gully in a forested area just below an arable fi eld 1_Kertész_cikk.indd 821_Kertész_cikk.indd 82 2009.10.19. 10:32:562009.10.19. 10:32:56 83 Soil erosion in Hungary Land degradation processes play an important role in relief formation and development in Hungary. Soil erosion is one of the most signifi cant land deg- radation processes on agricultural areas. Other land degradation processes, such as: mass movements, extreme soil reaction (including acidifi cation and salinization/alkalization), physical degradation and other chemical, physical and biological degradation processes (see Várallyay, Gy.–Leszták, M. 1990; Kertész, Á. 2001) are also important, but they are not as extended as soil ero- sion. Soil is one of the most important natural resources in Hungary, therefore soil erosion studies and soil erosion control are very important issues. 25% of the total area of Hungary (more than one-third of agricultural land) is aff ected by water erosion (on agricultural land 13.2% slightly, 13.6% moderately and 8.5% severely eroded) and 16% is aff ected by wind erosion (Stefanovits, P.–Várallyay, Gy. 1992, see Table 2). The signifi cance of soil erosion processes was recognized half a century ago and a soil erosion map was constructed by Stefanovits, P. and Duck, T. (1964) covering, however, only improved farmland (excluding non agricul- tural uses, e.g. forests, urban and industrial areas, roads, etc.). The mapping was based upon the analysis of soil profi les. As a consequence of the applied method only areas eff ected by sheet erosion are identifi ed on the map and the areas of gully erosion were not shown on it. Soil erosion research concentrated mainly on sheet erosion and the assessments were restricted to smaller areas, hillslopes or small catchments. a) Water erosion. Sheet erosion is an important problem on most of arable land. Before the change of the regime in 1989 large arable fi elds were created allowing for an even more extensive damage of sheet erosion. Most of the crop is harvested by the beginning of July leaving large surfaces without vegetation during the most sensitive period, i.e. between July and October. Sheet erosion processes are supported by micro-solifl uction and by splash erosion (Kerényi, A. 1991). Gully erosion will be dealt with below in detail. Table 2. Soil erosion in Hungary Indicator Thousand hectares % of the total area % of the agricultural land % of the eroded land Area of the country Agricultural land Arable land Total eroded land strongly moderately weakly 9,303 6,484 4,713 2,297 554 885 852 100.0 69.7 50.7 24.7 6.0 9.5 9.2 – 100.0 73.0 35.3 8.5 13.6 13.2 – – – 100.0 24.1 38.5 37.4 1_Kertész_cikk.indd 831_Kertész_cikk.indd 83 2009.10.19. 10:32:562009.10.19. 10:32:56 84 b) Wind erosion is highly extensive on the areas of wind blown sand, which occupy about 20% of the country’s territory. The thickness of the sand varies form a few centimeters to 25–30 meters. Damage is primarily caused on sandy soils, where crop yields may be reduced by up to 50%. Improperly cultivated peat soils with decomposed, powdery surfaces also have low resist- ance to wind erosion. Gully erosion research in Hungary As mentioned before, the role of gully erosion processes was not properly recognized until lately and it was believed that it is mainly sheet erosion which causes damage on agricultural land. This statement is also true in the case of Hungarian research. Gully classifi cation systems will be treated fi rst followed by a short review of scien- tifi c publications on gullying. A gully erosion survey was carried out to characterize gully erosion according to the length of gullies in a given area (Stefanovits, P.–Várallyay, Gy. 1992). Based on this survey the following categories were suggested. a) weakly gullied area: <200 m/km2 gullies; b) moderately gullied area: 200–500 m/km2; c) strongly gullied area: >500 m/km2. A classifi cation system based on soil loss values was suggested by Thyll, Sz. (1992). According to the method soil loss values will be identifi ed on a 40x40 m test area to give the rate of gully erosion. The categories are weak (<40 t/ha), medium (40–100 t/ha) and strong (>100 t/ha) gully erosion. The selection of the very small test area infl uences the obtained result very much and therefore this method cannot bring reliable results. Observations and descriptions of gully erosion date back to the last century (see. e.g. Pécsi M. 1955). The gullies of the Tokaj Hill were investigated by Pinczés Z. (1968, 1980). He used the number and extent of rills to identify the degree of soil erosion. Boros L. (1977) elaborated a simple method for mapping rills and gullies providing also some information on their morpho- metrical properties. Hilly areas with thick loess cover or with loose Pannonian sediments have unique geomorphological features and they are prone to rill and gully de- velopment (see. e.g. Kádár L. 1954; Ádám, L. 1969). Among them the Tolna and Szekszárd hilly countries were studied in detail by Ádám, L. (1969). Kerényi, A. and Kocsisné Hodosi, E. (1990) reported on the development of erosional forms in vineyards covered by loess. The role of piping was recognized by several authors (Kádár L. 1954; Ádám, L. 1969; Kerényi, A.–Kocsisné Hodosi, E. 1990). 1_Kertész_cikk.indd 841_Kertész_cikk.indd 84 2009.10.19. 10:32:572009.10.19. 10:32:57 85 Investigations in Lake Balaton catchment by various authors in- cluded also some aspects of gully development. The Department of Physical Geography of the Hungarian Academy of Sciences carried out several research projects on soil erosion forms and processes in the catchment. Tóth A. (2004) analysed the ratio of sheet and gully erosion in the Tetves catchment. Jakab, G. et al. (2005) made a very detailed morphometrical survey of gullies in the same catchment. Kertész, Á. (2004a) studied geomorphic processes on collapsible and dispersive soils. Rill initiation and development was part of various rain- fall simulation experiments (Csepinszky B. et al. 1998; Csepinszky B.–Jakab G. 1999; Sisák, I. et al. 2002; Centeri, Cs. 2002; Centeri, Cs.–Pataki, R. 2003, 2005; 2005, Szűcs, P. et al. 2006; Jakab, G.–Szalai, Z. 2005; Balogh J. et al. 2008). There is also historical evidence (see e.g. Gábris, Gy. et al. 2003) that a very intensive gully erosion activity took place in the nineteenth century when large areas covered by loose sediments were deforested and opened for arable farming. Conditions of gully development a) Slope gradient Stefanovits, P.–Várallyay, Gy. (1992) investigated the eff ect of relief on water erosion (including both sheet and gully erosion) in Hungary according to slope gradient categories. On slopes <5% erosion hazard is negligible. As slopes >25% are generally forested they do not imply a high erosion risk. The 17–25% slopes are either under forest or were deforested in the recent past. Most of the 5–17% slopes are used for agriculture and deteriorated by soil erosion to a certain extent (Krisztián J. 1992). There are no studies car- ried out on the threshold value of slope gradient for gully initiation. It would be interesting to investigate the relationship between critical slope gradient versus upslope drainage area for (ephemeral) gully initiation (see Vandaele, K. et al. 1996). b) Soil parent material About two thirds of the total area of Hungary are covered by loose sediments, mainly by loess and loess like deposits, susceptible to soil erosion and mass movement processes in the hilly regions of the country. Soil erosion is the great- est environmental hazard on hillslopes under cultivation. The thickness of slope loess varies between 5 and 25 m. Recent processes acting on loose sediments were mainly dealt with as part of geomorphological mapping activities and geomor- 1_Kertész_cikk.indd 851_Kertész_cikk.indd 85 2009.10.19. 10:32:572009.10.19. 10:32:57 86 phological surveys (Kertész, Á. 2004b). The best conditions for gully erosion are provided in the areas of thick loess cover (e.g. Szekszárd hilly country). Other loose sediments like Pannonian sands are also susceptible to gully erosion. c) Soil properties Soils of the loess covered areas are generally highly erodible because the par- ent material of the soil is a loose sediment. The initiation and development of gullies is in some cases promoted by subsurface erosion, i.e. by piping (called also suff osion in Hungarian literature, see Jakab, G. et al. 2005). Physical and chemical properties of loess and loess-like sediments off er favourable conditions for the development of pipes. Collapsibility is primarily connected with calcium carbonate content (including lime concretions in older loess deposits), with the very high porosity (volume of pores is 40–60%). The most important processes on collapsible/dispersive rocks and soils include sheet erosion, rill erosion, gully erosion, piping (tunnel erosion, suberosion), wind erosion and mass movements. d) Climatic conditions Gully erosion is more frequent under arid conditions and less frequent under humid climatic conditions (Poesen J. et al. 1996). Recent research concentrates on the occurrence of erosive rainfall events. In most cases the role of rainfall characteristics in SL Gully % (the percentage of soil loss caused by gully ero- sion in the total soil loss of the catchment. Evidently the amplitude and fre- quency of rainfall events are the most important rainfall characteristics. It is also evident that any change in rainfall regime (e.g. because of climate change) will lead to the change of the value of SL Gully %. For the development of sheet and gully erosion, „erosion-sensitive days” characterized by >30 mm daily rainfall are of crucial importance. (Stefanovits, P.–Várallyay, Gy. 1992), which may occur 4–12-times per year in Hungary. Concerning rainfall characteristics the most informative value is the rainfall threshold leading to the development of gullies in various environ- ments. According to Poesen J. et al. (2003) there is not much diff erence in threshold rains of rills and gullies. There are no data available on threshold rains in Hungary. Global climate change is very likely to increase gully erosion risk. Extreme events are going to be more frequent. In summer long periods of draught will alternate with storms (high intensity rainfalls). In winter freezing, melting and intensive rainfalls will alternate. 1_Kertész_cikk.indd 861_Kertész_cikk.indd 86 2009.10.19. 10:32:572009.10.19. 10:32:57 87 e) Land use change Land use plays a key role in the development of land degradation processes. Recent studies indicate that (1) gully erosion represents an important sediment source in a range of environments and (2) gullies are eff ective links for transfer- ring runoff and sediment from uplands to valley bott oms and permanent chan- nels where they aggravate off site eff ects of water erosion. In other words, once gullies develop, they increase the connectivity in the landscape. Many cases of damage (sediment and chemical) to watercourses and properties by runoff from agricultural land relate to (ephemeral) gullying. There is a huge number of studies on the eff ect of land use on gully development. Gábris, Gy. et al. (2003) reported on a very intensive gully erosion activity in the nineteenth century when large areas deforested. Deforestation and starting agricultural activity on former forested areas increases gully erosion risk also in Hungary. Conclusions The hilly countries of Hungary are mainly covered by unconsolidated sedi- ments, with a prevalence of loess and loess like sediments among them. Loess covered areas are prone to erosion and mass movements. The paper provided an analysis of the physico-geographical conditions of gully development in Hungary. Gully erosion risk is present on various landscapes because of the environmental conditions. a) Great Hungarian Plain. Even lowlands covered by a thick layer of loess and other loose sediments are prone to gullying. Along the banks of riv- ers (e.g. some sections of the Danube valley) various forms of erosion includ- ing gullies are present. Rills and gullies will be formed on sand accumulation areas. These forms are very dynamic, change rapidly and disappear on moving sand (see Boros L.–Boros L.-né. 1980) and on sandy soils. b) Hilly countries and mountains. Being covered by loess and other loose sediments hilly countries all are prone to gully erosion. Big elevation diff erences in a small area, i.e. high relative relief values point to a high risk of gullying (e.g. Somogy, Tolna, Szekszárd hilly countries). Deforested areas used by agriculture, especially arable lands and vineyards have an enhanced risk. Mountains are mostly forested with spots of clearings. Even in the moun- tain forest there is a risk of gullying. Antropogenous activities (e.g. timber trans- port tracks, unpaved forest roads etc.) contribute to the risk of gully erosion. Medium and long term land use planning should ensure a minimum risk of gully erosion with special emphasis on aff orestation. Acknowledgements: The present study was supported by the Hungarian Scientifi c Research Fund (OTKA), project number: T 76434. The support is gratefully acknowledged. 1_Kertész_cikk.indd 871_Kertész_cikk.indd 87 2009.10.19. 10:32:572009.10.19. 10:32:57 88 REFERENCES Ádám, L. 1969. A Tolnai-dombság kialakulása és felszínalaktana. – Akadémiai Kiadó, Budapest. 83 p. Balogh J.–Baloghné Di Gléria M.–Jakab G.–Szalai Z. 2008. Talajeróziós vizsgálatok esőszimulátorral. – In: Schweitzer F.–Bérci K.–Balogh J. szerk.: A Bátaapátiban épülő nemzeti radioaktívhulladék-tároló környezetföldrajzi vizsgálata. – MTA Földrajztudományi Kutatóintézet, Budapest, pp. 90–104. Bergsma, E. 1996. Terminology for soil erosion and conservation. – International Society of Soil Science (ISSS). 313 p. Boros L.–Boros L.-né. 1980. Hóolvadékvíz által előidézett talajpusztulás a Nyírség északnyugati részén. – Földrajzi Értesítő 29. (2–3.) pp. 217–232. Boros L. 1977. 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