Scale dependence of gully investigations 319 Hungarian Geographical Bulletin 59 (3) (2010) pp. 319–330. Scale dependence of gully investigations1 Gergely Jakab2, Ádám Kertész1 and Zoltán Szalai1 Abstract Soil erosion causes considerable damage both for the agriculture by soil loss and for the environment by sedimentation via contribution to eutrophication. Disastrous soil erosion events due to extremely high intensity rainfalls have recently posed serious danger to roads, dams, and edifi ces in Hungary. Gully erosion plays a particular role in rapid and extreme erosion processes. Research at regional, catchment or gully scale may have diff erent objectives. The main aim of this paper is to compare methods from the local to regional scale. Three case studies are presented. The results suggest the following statements to be made. At local scale the processes are determined by gully morphology, material and energy transport as well as by soil properties. The topographical and lithological factors parallel with land use have growing importance with the decreasing scale. Keywords: gully erosion, scale dependence, GIS Introduction Gully erosion is the prevailing soil degradation process under arid and semi- arid climatic conditions. Due to the insuffi cient precipitation amount biomass production is very much restricted, almost inhibited and the low canopy cover cannot stop or slow down surface runoff (Kertész, Á. and Centeri, Cs. 2006; Kertész, Á. 2006; 2009). Rainfall events occur irregularly in time and the in- creasing probability of heavy rainstorms with extreme intensity generates more frequent gully development (Malzorff, I. and Poesen, J. 2009). As a consequence under arid and semiarid climatic conditions the contribution of gully erosion to total sediment loss can reach 50–80%, meanwhile in humid and subhumid regions it varies between 10–50% (Poesen, J. et al. 2006). Under subhumid climate the biomass production is rather high and the vegetation cover gives a certain protection against the direct impact of the rain- drops. In spite of the sheltering eff ect of the canopy rapidly developing gullies can be found even in forested areas (Kertész, Á. et al. 2000). The signifi cance of 1 The project was sponsored by National Scientifi c Research Fund (OTKA). Id. No: K76434 2 Geographical Research Institute Hungarian Academy of Sciences, H-1112 Budaörsi út 45. Budapest, Hungary. E-mail: jakabg@mtafk i.hu, kertesza@helka.iif.hu, szalaiz@mtafk i.hu 320 gully erosion under subhumid climate is shown by the results of an investigation carried out in Western Hungary pointing to 50 % of the total sediment amount in a catchment being delivered by gully erosion (Jakab, G. et al. (2009). Gully formation has always been an important geomorphic process in Europe. It is well known that gully development was launched and accelerated by various forms of anthropogenic activities over the last 2 to 3 thousand years (Gábris, Gy. et al. 2003; Lang, A. et al. 2003; Vanwalleghem, T. et al. 2003). Humans cleared the land in order to obtain new territories for agri- culture and land clearing completely changed the hydrological conditions of the fi eld. In many cases in line with land use modifi cations climatic condi- tions change, too with the involvement of possible interactions (Poesen, J. et al. 2006). A gully can be fi lled up because environmental conditions change; however, gully incision is generally faster, than the fi lling-up of gullies. Gullies aff ect their surroundings in a relatively long time span (Jakab, G. et al. 2005). Landforms of linear erosion collect surface waters, consequently they are deeply incised into the surface. Insolation into the gully is very limited and almost no wind can reach inside the gully. The consequence is lower evapora- tion intensity, wett er microclimate conditions and soil moisture surplus inside the gully (Mac Nally, R. et al. 2000). Because of the wett er conditions species composition, the number of individuals and even the fertility of the individu- als increase (Soderquist, T.R. and Mac Nally, R. 2000). Measurements performed by Palmer, G.C. and Bennett, A.F. (2006) in forests indicated that on wett er areas the fl ora composition changes and the diversity of bird species grows. In contrast to this statement Lindenmayer et al. (2009) did not fi nd more bird species in forest gullies than in the other parts of the forest. An explanation for these confl icting views can be the scale (size) dependence of various species. The size of an average gully is large enough to determine the micro- biological and non-arboreal composition but in case of vertebrates it is not so direct and evident (Naiman, R.J. et al. 1993). The environmental infl uence of gullies depends on their size, it is scale dependent. The local impact of gullies is manifested in the increase of the number of land mosaics. The role of gul- lies in soil erosion (Hegedűs, K. et al. 2008; Jakab, G. 2006; Kertész, Á. 2004) and in fertilizer and pesticide transport (Madarász, B. et al. 2003) is extremely signifi cant at catchment scale. The main objective of this paper is to analyze the distribution and morphological appearance regularity of gullies from the local to regional scale. The identifi cation of the most important characteristics of a gully is mainly aff ected by the scale. Research at diff erent scales requires diff erent methods. The case studies presented in this paper apply diff erent methods for gully investigation at diff erent scales. 321 Methods At local scale morphometric measurements were carried out in Somogybabod, south of Lake Balaton. A detailed site description of the research area is given by Madarász, B. et al. 2003 and in Tóth, A. and Szalai, Z. 2007. In the winter of 2003 a fi eld survey was carried out using a laser re- fl ectance total station (Trimble 3305 DR). The survey was performed within the framework of the Unifi ed National Mapping System of Hungary. The aim of the survey was to determine morphometric properties of two gullies running next to each other. More than 1000 points were surveyed following a network of 20 by 20 meters grid, plus some representative points of the gullies (Jakab, G. 2009). For the estimation of the elevation of the surface between the surveyed points the kriging method was used. The digital elevation model (DEM) thus created is far from being perfect in case of banks, sharp relief forms and head- cuts. Using the kriging method these forms appear rounded without edges. If the number of surveyed points is suffi cient a bett er representation can be pro- vided via linear interpolation, namely by applying the Triangulated Irregular Network (TIN) method (Marzolff, I. et al. 2002). In this case the edges and breaks are bett er recognizable but the shapes of other parts of the surface are bett er approched by the kriging method. For a bett er representation of the ter- rain a more accurate DEM is needed and this can be achieved by the increase of the surveyed points via remote sensing (Ries, J.B. and Marzolff, I. 2003; Malzorff, I. and Poesen, J. 2009) or surface scanning methods. As these gullies formed under forest remote sensing is hardly applicable. Accuracy without additional measurements can be achieved by the combination of the above interpolation methods. Following this procedure the gullies are well presented and the DEM is useful for the morphometric comparison of the gullies. In 2009 the faster developing gully was surveyed again using the same method. The creation technique of the DEM was the same as well. With the comparison of the DEMs of various dates the dynamics of gully development can be estimated. Gully development investigations at fi eld scale were carried out in the Galga River valley next to the town of Galgagyörk to check the fi eld scale investigations at Somogybabod (Jakab, G. et al. 2010). Since the cross section of the Galga valley south of Galgagyörk has similar topographic and soil con- ditions (Marosi, S. and Somogyi, S. 1992) to those at Somogybabod it can be considered as an analogy of the Somogybabod site. Analyzing the available maps and remote sensing databases of the area the dynamics of gully devel- opment could be followed. At regional scale a gully database was created. 1:10,000 topographic maps from the 1980s were digitized. Land use categories were simplifi ed into three classes of forest, pasture and arable land. 322 Results and discussion Large scale investigations revealed the morphological diff erences between the two gullies. The diff erences are related to spatial and temporal changes of the geomorphic processes in the gullies (Jakab, G. 2008). The DEM shows that the northern gully is a rapidly developing one with quickly eroding vertical slopes and sharp edges (Figure 1), meanwhile the slopes of the southern gully are not so steep and covered with vegetation. The latt er is less active in spite of having similar topographic and land use characteristics as the northern gully and disposing of a twice larger drainage area. There are remarkable diff erences in the development of the two gullies. The topographic survey demonstrates that the southern gully is more or less inactive while the northern one retreats rather rapidly and deepens, too. The mor- phological changes during the investigated six years can be analyzed on the basis of selected cross sections (Figure 2). The heaviest material loss took place along cross section “A”, just below the headcut. Here the deepening was roughly 2.5 m and the retreat of the northern slope accounted for 1–1.5 m. Downwards along the gully the sides are less and less eroded while gully incision seems to be constant (ca 2 m). The same tendency can be seen on the soil loss map (Figure 3). Since slope steepness, land use and vegetation cover are the same in both gullies (Jakab, G. 2009) the morphological diff erences can probably be explained by the variation in lithological properties. Investigating gully development at large scale the interactions of the environmental conditions aff ecting gully formation and development have to be identifi ed. Fig. 1. Digital Elevation Model of the investigated gullies at Somogybabod. The lett ers refer to the cross section locations in Fig. 2. 323 Fig. 2. Four cross sections from 2003 and 2009 of the rapidly developing gully at Somogybabod Fig. 3. Soil loss map of the rapidly developing gully at Somogybabod prepared using the TIN method. Decrease in surface height refers to soil loss between 2003 and 2009 BA C D 324 The next dimension of the investigations is the catchment scale. At this scale gully development should be investigated in the context of catchment processes. Jakab, G. et al. (2010) carried out investigations in a hilly country (Somogybabod, Tolna County) covered by forest at the end of the eighteenth century. The appearance of gullies is due to the forest clearance at the turn of the 18–19th centuries. In those areas where agricultural activities had been carried out prior to 1780 when the fi rst Military Survey of Hungary com- menced, gully initiation and development started earlier. The evidence of this can also be detected on the fi rst military maps of Galgagyörk, Northern Hungary (Figure 4). In the surroundings of Galgagyörk the valley bott om was occupied by arable fi elds in the 1780s. On both sides of the river there are roads and gully-like forms running up on the hill slopes towards the divides. On the eastern slope there is a thin forest strip extending to the river bank. This strip used to be probably an abandoned part of the arable fi eld because its cultivation was hindered by the presence of a gully there. At the end of the 18th century huge areas were covered by vineyards mainly on the slopes with western and south-western exposures. The analysis of the Galgagyörk map drawn during the 2nd Military Survey in the 1840s (B) showed that the arable land/forest ratio of the areas has not changed signifi cantly since the end of the 18th century. The opposite of this is true in Somogy county. Due to the bett er illustration of topographic forms on the second military survey maps gullies and ditches indicated by the gully bott om line can be well identifi ed. In the investigated section of the Galga valley there are more than ten gullies shown on the map. They must be either ephemeral gullies or deep cut tracks. Two inscriptions refer to gullies developed long before the surveying. In the north-eastern part of the valley a gully name suggests a deep and wide landform. In the other valley of the sample site there are also several gullies in arable fi elds and vineyards. On the map of the third military survey (C) gullies are represented by the same symbols as today. Gullies are well recognizable from the contour lines and from the indication of the gully bott om line. Looking at the map prepared 100 years aft er the third military survey (D) no major changes can be identifi ed. Gullies are represented more pre- cisely and even the gully bott oms are recognizable on the map. Some of the ephemeral gullies became permanent ones, some of them had transformed into valleys. This development is typical for ephemeral gullies formed on arable fi elds pointing to the dominant contribution of ephemeral gullies in the total soil loss at catchment scale (Poesen, J. et al. 2003; Jakab, G. 2009). The most remarkable land use change is the decrease of vineyards as a consequence of the Viteus vitifolii infection between 1875 and 1890. On the aerial image (E) the recent status dominated by an extended ephemeral gully system can be seen. Some parts of former ephemeral gullies 325 Fig. 4. Changes in topography and land use at Galgagyörk during the last 200 years. A = 1st military survey (1780s), B = 2nd military survey (1850s), C = 3rd military survey (1870s), D = 1:10.000 map (1960s), E = aerial image (2006, Google Earth). Arrows and circles indicate areas of intensive gullying and those of land use change A B C D E on certain spots became permanent. In these cases the deepening had been so eff ective and rapid, that tillage was not possible any more. Accordingly this gully cannot be classifi ed into a defi nite group as a whole but it has to be divided into parts. Looking at the Galgagyörk area one could think to deal with a similar situation as in Somogy county, namely that gully initiation started aft er defor- estation when woodland was converted into arable land (Jakab, G. et al. 2010). The Galga valley gullies are much older and they are probably active since the arable fi elds nearby were tilled. Judging from the age of the gullies a huge amount of fertile soil must have been eroded since their appearance. 326 Land use types are of primary importance from the point of view of erosion (Centeri, Cs. et al. 2009). This statement is confi rmed by the results obtained at the sites of Somogybabod and Galgagyörk where the main fac- tor of gully initiation has been land use change. If there had not been land use change gully development would have been negligible and new gullies would not be formed. Under such circumstances gully erosion is manifested in a relatively slow development of the existing gullies. The next dimension is the regional scale and the question arises how the above statements can be generalized at this scale. A national survey of gul- lies was launched in 2009 in order to build up a gully database. In the 1960s the whole country was surveyed in order to compile an up-to-date 1: 10.000 topographical map series of Hungary. Gullies and land use were also represented on these maps. The map off ers a possibility to analyze the spatial distribution of gullies. A simple ratio, i.e. total gully length/km2, suggested by Stefanovits, P. and Várallyay, Gy. (1992) can be calculated. The classifi cation system proposed by the authors is applied in this study, i.e. (a) weakly gullied area: < 200 m/km2; (b) moderately gullied area: 200–500 m/km2; (c) strongly gullied area: > 500 m/km2. Further data layers of the national gully database include topographic parameters (slope gradient, slope exposure, derived from the Shutt le Radar Topography Mission /SRTM/ database), recent land use given in the CORINE database and soil characteristics (soil type, soil texture, parent material etc.) from the AGROTOPO database of Hungary (Várallyay, Gy. et al. 1998). The database allows for the identifi cation and analysis of the areas endangered by gully erosion. Sample areas will be selected and temporal change will be revealed based on the database and on remote sensing materials. The detailed investigation of the sample areas allows for the identifi cation of present-day trends in gully development and for the estimation of its rate in the future. Up to now gullies and land use types of fi ve map sheets of the 1960s were digitized (Figure 5). Preliminary results of the analysis of the database are as follows: 1. Ephemeral gullies are diffi cult to identify because they are oft en shown as valleys on the map and the distinction of gullies from valleys is diffi cult. 2. It is hardly possible to give the exact length of a gully because it is diffi cult to defi ne independent gullies from those which belong together. The general statement about gully length is that the average gully on arable land is longer than on pasture and in most cases also in forest (Table 1). 3. Most of the gullies (85–90%) can be found in forests. The percentage of gullies outside the forest has increased only on less dissected areas. 327 Fig. 5. Digitized map sheets with the indication of the gullies 4. The areas presented in Figure 5 are highly dissected (500 m km-2 <) with the exception of the area shown on map sheet 88 which is partly lowland, i.e. part of the Great Hungarian Plain (Table 2). 5. In this study the classification system of Stefanovits, P. and Várallyay, Gy. (1992, see above) was applied. Values within the strongly gullied category vary widely therefore the classifi cation system has to be changed. Conclusions Gullies determine the development and material fl ux of the landscape. Gully formation is triggered by the given environmental conditions, but an already existing gully will remain in the landscape for a long time. Our investigations carried out at distinct scales had diff erent objectives and the applied methods were diff erent, too. At local scale the role of gully mor- phology, parent material, soil properties and energy transport determine the processes. Along with the decreasing scale the importance of topographical and lithological factors and of land use increases. The gully classifi cation system has to be improved by adding a new category of “extremely gullied” (1,000 m km-2 <). 328 Ta bl e 1 . M ai n st at ist ic al p ar am et er s o f t he g ul ly le ng th cl as sifi ed b y la nd u se ty pe s i n th e l at e 1 96 0s St at is tic al pa ra m et er s of g ul ly le ng th (m ) N um be r o f m ap s he et 23 76 86 87 88 23 76 86 87 88 23 76 86 87 88 A ra bl e la nd Pa st ur e Fo re st M ea n M ed ia n SD M ax . M in . 19 8 16 4 13 9 66 5 2 25 8 18 0 21 4 1, 01 5 34 25 6 23 4 15 1 63 1 70 17 0 14 9 11 9 63 2 28 22 0 18 0 24 0 1, 77 3 27 15 3 12 0 12 2 77 8 7 16 9 10 5 18 4 1, 44 0 17 12 7 76 21 1 2, 99 4 8 11 6 80 12 1 1, 74 6 1 15 4 10 6 15 5 1, 18 0 1 29 2 15 4 43 1 5, 97 0 3 22 9 10 0 48 5 9, 74 0 10 18 6 83 38 6 7, 71 3 7 22 2 10 4 40 2 8, 63 6 1 22 5 12 1 41 3 7, 14 1 1 Ta bl e 2 . D ist rib ut io n of g ul lie s a cc or di ng to la nd u se ty pe s N um be r of g ul lie s G ul lie s La nd u se (l at e 19 60 s) To ta l le ng th M ea n le ng th G ul ly d en si ty A ra bl e la nd Pa st ur e Fo re st m m k m -2 ca te go ry N o. To ta l le ng th , m % N o. To ta l le ng th , m % N o. To ta l le ng th , m % 3, 36 9 4, 53 6 13 ,3 85 13 ,3 22 1, 48 9 93 0, 93 0 1, 02 3, 81 6 2, 47 1, 45 5 2, 70 2, 77 0 30 4, 78 8 27 6 22 6 18 5 20 3 20 5 60 6 66 7 1, 60 9 1, 76 0 19 8 st ro ng ly g ul lie d st ro ng ly g ul lie d st ro ng ly g ul lie d st ro ng ly g ul lie d w ea kl y gu lli ed 12 3 86 23 61 70 24 ,3 40 22 ,2 03 5, 88 5 10 ,3 82 15 ,4 16 2. 6 2. 2 0. 2 0. 4 5. 1 28 4 29 2 34 4 23 92 42 0 43 ,3 35 49 ,2 84 43 ,7 43 27 8, 47 6 43 ,3 35 4. 7 4. 8 1. 8 10 .3 14 .2 2, 96 2 4, 15 8 13 ,0 18 10 ,8 69 99 9 86 3, 25 5 95 2, 32 9 2, 42 1, 82 7 2, 41 3, 91 2 24 6, 03 7 92 .7 93 .0 98 .0 89 .3 80 .7 329 REFERENCES Centeri, Cs., Herczeg, E., Vona, M., Balázs, K. and Penksza, K. 2009. The eff ects of land- use change on plant-soil-erosion relations, Nyereg Hill, Hungary. Journal of Plant Nutrition and Soil Science 172. (4): 586–592. Gábris, Gy., Kertész, Á. and Zámbó, L. 2003. Land use change and gully formation over the last 200 years in a hilly catchment. Catena 50. 151–164. 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