Geomorphology 149-150 (2012) 41–48
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Relationships between a new slope morphometric index and calanchi erosion in northern Sicily, Italy Marcello Buccolini a,⁎, Laura Coco a, Chiara Cappadonia b, Edoardo Rotigliano b a b
Dipartimento di Geotecnologie per l'Ambiente ed il Territorio, Università “G. d'Annunzio” Chieti-Pescara, Via dei Vestini 30, 66013 Chieti Scalo (CH), Italy Dipartimento di Scienze della Terra e del Mare, Università degli Studi di Palermo, Via Archirafi 20, 90123 Palermo, Italy
a r t i c l e
i n f o
Article history: Received 13 August 2011 Received in revised form 24 December 2011 Accepted 17 January 2012 Available online 24 January 2012 Keywords: Slope morphometry Badlands processes Drainage network
a b s t r a c t The Italian badlands, or “calanchi”, are common landforms in Mediterranean areas including central and southern Italy. Calanchi landforms may be compared to small hydrographic basins. These landforms are characterised by dense, hierarchical and rapidly evolving drainage systems carved into steep clayey slopes and by a sharply alternating pattern of furrows and narrow, generally sharp crests. This work presents a study of morphometric characteristics and a statistical analysis for two sites in northern Sicily (Italy), on outcrops of silty-clay deposits affected by active erosion processes, which give this area a typical calanchi landscape. In particular, factors closely linked to the characteristics of the hydrographic network and slope morphometry were considered and analysed. The initial geometry of the slopes was reconstructed and statistically compared with that of the current calanchi slopes including the drainage network. A new morphometric index (Morphometric Slope Index, MSI) was defined to represent the initial slope geometry as a whole. This index was found to be effective in defining the structure of hydrographic networks, summarising the characteristics and type of slope evolution, and quantifying the rate of soil erosion. The rate was determined based on both linear (gully erosion) and areal (landslides, sheet and rill erosion) morphogenetic processes, and our analysis based on MSI indicates the dominance of areal erosion. MSI could also be used for basins larger than calanchi to represent the characteristics of geomorphic processes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction cThe badlands in Italy are typically characterised by two types of landform: “biancane” and “calanchi”. Biancane (plural of biancana) are small clay domes that are found in groups, with the sides crossed by microrills and micro-pipes, and they are often covered by vegetation and surrounded by micro-pediment (Torri and Bryan, 1997; Farifteh and Soeters, 2006). Among the forms of accelerated erosion characterising the clay landscapes of Italy, calanchi (plural of calanco) are perhaps the most spectacular. Calanchi widely occur in hilly and mountain areas in and around the Apennines of central and southern Italy, namely Emilia, Marche, Abruzzo, Puglia, Basilicata, Tuscany, Lazio, Calabria and Sicily. Calanchi can be defined as very dense, hierarchised and rapidly evolving drainage systems carved into steep clayey slopes, characterised by a sharply alternating pattern of furrows and very narrow and generally sharp crests with a height between a few and several tens of metres (Castiglioni, 1933; Vittorini, 1977; Alexander, 1980; Dramis et al., 1982; Clarke and Rendell, 2000; Moretti and Rodolfi, 2000). The genesis of these landforms is controlled by the mechanical and chemical/mineralogical characteristics of the lithotypes involved, their particle size
⁎ Corresponding author. Tel.: + 39 0871 3556424; fax: + 39 0871 3556146. E-mail address:
[email protected] (M. Buccolini). 0169-555X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2012.01.012
composition and the structural setting of the geological substratum (Vittorini, 1977; Rodolfi and Frascati, 1979; Bryan and Campbell, 1986; de Lugt and Campbell, 1992; Torri and Bryan, 1997; Regüés et al., 2000; Farifteh and Soeters, 2006; Della Seta et al., 2007). The presence of steep vegetation-free clay slopes is a prerequisite for the development of calanchi (Castiglioni, 1933; Guasparri, 1978; Dramis et al., 1982). Also, climatic conditions influence the morphogenesis of the calanchi; it is favoured by strong seasonal contrasts typical of the Mediterranean climate (Csa of Köppen, 1936). The seasonal contrasts are related to the deep desiccation of soils in summer and intense hydration cycles in winter. The evolution of the drainage network can produce calanchi landforms, which may be compared to a small drainage basin or hydrographic unit. Erosion processes in calanchi include both linear erosion (gully erosion, rill erosion and piping) and areal erosion (slide or flow-type landslips and sheet erosion) (de Lugt and Campbell, 1992; Torri and Bryan, 1997; Moretti and Rodolfi, 2000). Linear erosion requires a sufficiently steep slope, the lack of which may lead to reticular piping and the emergence of biancane. The research described in this paper started from the assumption that the morphometric characteristics of the slope significantly affect the calanchi dynamics. There have been limited morphometric studies concerning the slope characteristics of calanchi, and they refer mainly to slope steepness (Castiglioni, 1933; Sfalanga and Rizzo, 1974; Rodolfi and Frascati, 1979). Other studies mainly analysed the
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M. Buccolini et al. / Geomorphology 149-150 (2012) 41–48
inclination and length of slopes within a gully (Clarke and Rendell, 2000; Clarke and Rendell, 2006; Farifteh and Soeters, 2006; Battaglia et al., 2011). Buccolini and Coco (2010) have shown that calanchi are also present on less steep slopes (9° − 30°) and the relationship between slope length and inclination is crucial in determining the characteristics of calanchi. The morphometric characteristics of hillsides in the calanco area seem to play a key role in characterising geomorphic processes. As calanchi are similar to miniature river basins, they can be studied using the quantitative geomorphic analysis typical of normal catchment areas. It is known that the morphodynamic processes and development of drainage systems are strongly influenced by the initial topography of hillslopes (Parker and Andres, 1976; Schumm et al., 1987; Oguchi, 1996; Pelletier, 2003; Lin and Oguchi, 2004, 2009; Buccolini and Coco, 2010). Slope geometry may be analysed by bringing together the size, shape, slope, and length and width of the slope into a single index to consider both areal (e.g., plan shape, 3D form, and extent) and nonareal (e.g., length, profile shape, and slope angle) features. Studies conducted so far have investigated these parameters individually, resulting in partial relationships with morphogenetic processes. For example, several authors have studied the relationship between drainage density and slope inclination of some river basins in different parts of the world (Oguchi, 1997; Tailing and Sowter, 1999; Lin and Oguchi, 2004). These studies showed that the correlation between drainage density and slope can be positive or negative depending on the dominant erosion processes (linear or areal), which are obviously influenced by other slope characteristics. The aim of this study is to analyse the influence of initial precalanchi slope geometry, the characteristics and evolution of calanchi hydrographic networks and the rate of soil erosion. To achieve this goal, a highly detailed morphometric analysis was carried out using a high-resolution (2 × 2 m) DTM (digital terrain model) for two
neighbouring calanchi sites in central–northern Sicily. The geometry of pre-calanchi slopes was reconstructed by correlating the topography inside the calanchi area to the heights of the watershed limits. Calanchi drainage networks were analysed using some geometric parameters (Horton, 1945; Miller, 1953; Strahler, 1964; Milton, 1967). Moreover, the volume of eroded material was estimated from the geometry of the pre-calanchi and present slopes. 2. General features of the study area The study area is located in the central segment of the Sicilian Apennines ridge (southern Italy) in the western sector of the Madonie mountain group (Fig. 1). The outcropping rocks in the Madonie mountains are mainly composed of terrigenous, carbonate and siliciclastic series forming imbricated tectonic units resulting from the compressive phase since the Oligocene that has given rise to the Sicilian ridge. The study area is located on a cuesta front with steep (>30°) scarp slopes, exposed bedrock of the fluvial-deltaic and marine deposits of the Terravecchia Formation (Upper Tortonian− Lower Messinian), and linear erosion features (rills and gullies) exhibiting a typical badlands landscape. The Terravecchia Formation consists of conglomeratic, arenitic, silty and clay facies forming a multi-cyclic sequence from alluvialclastic sediments to marine pelites. The sedimentation of the formation was generally controlled by synsedimentary tectonic movements and by the migration of the basement rock northwards (Abate et al., 1999). Two parts of this area were investigated in detail (Fig. 2). Notwithstanding the lithological homogeneity and spatial closeness of the two areas, different linear and areal erosion processes are recognised in terms of the prevalence of piping and gravitational processes and/ or rill and gully erosion processes. The outcropping deposits mainly consist of clayey silt. Mineralogical analyses of these deposits show that quartz and calcite are dominant in the silty and sandy fractions,
Fig. 1. Location of the study area and the Scillato (376 m a.s.l.), Collesano (460 m a.s.l.) and Cerda (274 m a.s.l.) weather stations.
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Fig. 2. General geological sketch. 1 = calanchi fronts; 2 = contour lines; 3 = inclined bedding; 4 = fluvial deposits; 5 = clays and silty clays − Terravecchia Fm.; 6 = sandstones − Terravecchia Fm.; 7 = conglomerates −Terravecchia Fm. Modified after Catalano et al., 2010).
while the clayey fraction consists primarily of illite and kaolinite with smaller amounts of smectite (Agnesi et al., 2010). This mineralogy makes these deposits susceptible to cracking, as the expandable lattice clay minerals are prone to cycles of wetting and drying, leading to the swelling of soils. The climate is typically Mediterranean, with dry summers and moderately cold winters with light rainfall. The climatic data were obtained from three weather stations: Scillato (376 m a.s.l.), Collesano (460 m a.s.l.) and Cerda (274 m a.s.l.) (Fig. 1). The mean annual precipitation is 680 mm and is concentrated between the months of October and February. The mean annual temperature at the Cerda station is 16 °C.
3. Methodology The two calanchi fronts (Fig. 3), which are close (approximately 500 m) and characterised by the same environmental conditions (topographic, climate and litho-structural features), are coeval. These fronts were subdivided into 65 hydrographic units each of which corresponds to a single gully tributary, the surrounding calanco unit and upslope area, and are directly connected to the drainage network (Buccolini and Coco, 2010). The 65 units were inventoried with a reference index (Id) (Fig. 4), and their morphometric parameters were computed according to the following procedure.
Fig. 3. Photo of the studied calanchi sites viewed from the southwest. Two calanchi fronts are located in the central and left parts of the picture on the more inclined slopes of cuestas; between both, a small slope affected by badland erosion also occurs but it was not analysed in this paper.
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Fig. 4. Drainage units and example of their delimitation (lower left).
To investigate the relationships between pre-calanchi morphometry and calanchi characteristics, three types of data were acquired: attributes of calanchi, those of pre-calanchi slopes and eroded volume. Field surveys and orthophoto interpretation allowed the description of the current calanchi landforms, whose hydrographic and morphometric parameters were obtained by processing a laser scannerderived DTM, with a 2 m cell size, provided by the Sicilian Regional Assessorate for Territory and Environment. By processing the DTM with GIS (Geographic Information Systems) software, longitudinal topographic profiles were made parallel to the sloping line from the highest point to the base of the watershed, and the following morphometric parameters were computed for each profile: inclination (P), plan length (L) and their ratio (L/P) (Buccolini and Coco, 2010). For each watershed, the plan surface areas (A2D) and the circularity ratio (RC) (Miller, 1953) were also derived. Slope topography prior to the development of the calanchi hydrographic network was reconstructed for each watershed using straight contour lines connecting points with the same height on the watershed divide (Fig. 5). From the reconstructed contour lines, a DTM with a resolution of 2 m (the same as the lader DTM), which was assumed to represent the initial slope topography, was constructed through interpolation. An example of a reconstructed topography is presented in
Fig. 6. The reconstructed surface area (Ar) of each watershed was calculated using this DTM, as the extent of the watershed before the presentday calanchi development. To combine the main morphometric aspects of the slope (surface area, plan area, form, inclination, length and width) in a single index, the morphometric slope index (MSI) was calculated for the reconstructed slope of each calanco: MSI ¼
Ar A2D
L
RC
ð1Þ
This index reflects the slope size Ar, width in terms of L×RC (perimeter p is a function of L and width), and overall inclination which affects the ratio Ar/A2D (Fig. 6). MSI can be simplified as: MSI ¼
Ar 2D A2D
L
ð4π
A2D p2
¼ 4π
Ar p2
L
ð2Þ
MSI was used for the following reasons: i) availability of a high-resolution DTM, essential for a detailed analysis of small hydrographic basins; ii) characteristics of calanchi processes, which are similar to miniature hydrographic basins, where different morphogenetic
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To highlight the dependence of drainage network organisation on the initial topography, the relationships between slope parameters, calanchi parameters and eroded volumes were investigated using an exploratory statistical analysis tool (SPSS 10.0) and the Pearson correlation analysis. A bivariate linear regression was carried out for the variables with a significant correlation to investigate the relation between independent and dependent variables (Davis, 1990). 4. Results and discussion
Fig. 5. Example of hillside reconstruction for hydrographic unit #14 in Fig. 4. Numbers indicate the height of points on the divide; contour interval is 2 m.
processes (water erosion, landslides, and piping) occur; and iii) the possibility to evaluate and compare data derived from coeval hydrographic units. The comparison between the three-dimensional models of the slope's current and reconstructed geometries allows the estimation of the total eroded volume (V). The eroded volume was in turn normalised to A2D (V/A2D) to obtain an average magnitude of erosion for the unit area of the supply basin. For the calanchi surfaces in each individual hydrographic unit, orthophotos were analysed to map the furrows of the calanchi drainage network, and to perform detailed quantitative geomorphic analyses. The following parameters were calculated: drainage density (D), drainage frequency (F) (Horton, 1945), the number of furrows (N), their total length (l) and the ratio between the two (l/N). Also the hierarchical order (U) and the direct bifurcation ratio (Rbd) between the first and second orders were calculated (Strahler, 1964; Milton, 1967).
The data derived from the morphometric analysis of the 65 hydrographic units can be divided into three categories: those related to the calanchi network (D, F, N, l, l/N, Rbd, and U), those for the tributary area (L, P, L/P, and MSI), and those related to the eroded volume (V and V/A2D). These data are summarised in Table 1. The parameters Rbd and U were not used in the analysis because they have narrow value ranges. Table 2 shows the Pearson correlation matrix for all parameters. MSI obviously depends on the parameters that constitute the formula (Ar, A2D, L, and RC); moreover, it depends on the other slope parameters (P and L/P). Of particular relevance is the correlation between F and MSI (r = −0.561, p b 0.01) and D and MSI (r = −0.577, p b 0.01). V/A2D is directly related to MSI (r = 0.910, p b 0.01). V/A2D is inversely proportional to P (r = −0.719, p b 0.01), and directly proportional to L/P (r = 0.856, p b 0.01). V/A2D is also correlated to F (r = −0.440, p b 0.01) and D (r = −0.455, p b 0.01). The regression analysis revealed the dependence of the calanchi network parameters on the tributary slope parameters. The calanchi parameters are related to MSI: D decreases with MSI (r 2 = 0.333), and F decreases with MSI (r 2 = 0.314) (Fig. 7). The regression between V/A2D and the slope parameters reveal the dependence of the erosion amount on the slope characteristics prior to the deepening of the drainage network. The dependence of V/A2D on MSI is crucial: V/A2D increases with increasing MSI (r 2 = 0.829) (Fig. 8). Moreover, V/A2D is also related to the characteristics of the calanchi network. In particular, V/A2D decreases with increasing F (r 2 = 0.194) and D (r 2 = 0.207) (Fig. 9). In calanchi water erosion processes such as sheet, rill and gully erosion are active. Piping and gravitational processes including landslides are also present. The predominance of a single phenomenon in each of the study areas with similar litho-structural characteristics and climatic conditions is likely to ascribable to the morphometric characteristics. We focused on gully erosion, because it was impossible to quantify the extent of sheet and rill erosion processes due to their ephemeral nature, and it was difficult to quantify the distribution of gravitational processes which may be quickly remodelled and modified. V/A2D is the product of several erosion processes, and D can be directly attributed to the effect of deep linear erosion (gully). The low values of r 2 relative to the regression between D and V/A2D, although statistically significant, confirm the important contribution of the other erosional processes. D is inversely proportional to L/P, with a correlation value higher than the case of MSI. However, MSI, representing both linear and areal features of the slope, is also related to morphogenetic processes such as areal erosion, as shown by correlations with V/A2D (between V/A2D and L, r = 0.762, p b 0.01; between V/A2D and P, r = − 0.610, p b 0.01; between V/A2D and L/P, r = 0.849, p b 0.01; and between V/A2D and MSI, r = 0.888, p b 0.01). 5. Conclusions
Fig. 6. Parameters used for the determination of MSI: an example of hydrographic unit #2 in Fig. 4. A2D = plane area, Ar = reconstructed surface area, L = plane length, p = perimeter.
The validity of MSI over common parameters such as P and L/P is reinforced not only by the values of the regression, but also by the fact that slopes with very different planimetric and dimensional characteristics can have similar values of P and L/P. It is fair to say that in a morphoclimatic and litho-structural context applicable to the examined calanchi area, the contribution of areal erosion (landslides, sheet erosion and rill erosion) probably provides the greatest amount of eroded
M. Buccolini et al. / Geomorphology 149-150 (2012) 41–48
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Table 1 Calanco and hillside parameters. Id = calanco identification number; F = drainage frequency; D = drainage density; l = total drainage network length; N = total number of furrows in the drainage unit; Rbd = direct bifurcation ratio; U = hierarchical order; L = plane length; P = inclination; RC = circularity ratio; MSI = morphometric slope parameter; V = eroded volume; A2D = plane area. Id
F (m− 2)
D (m− 1)
l (m)
N
l/N (m)
Rbd
U
L (m)
P (°)
L/P (m/°)
RC
MSI (m)
V (m3)
V/A2D (m)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
0.00581 0.00587 0.00294 0.00302 0.00197 0.00241 0.00559 0.00814 0.00598 0.00228 0.00353 0.00488 0.00651 0.00581 0.00941 0.00655 0.01569 0.00829 0.01974 0.00777 0.00802 0.01075 0.00629 0.00569 0.00911 0.00974 0.01549 0.03912 0.00142 0.00475 0.00397 0.01399 0.01961 0.01573 0.01679 0.01060 0.01408 0.01782 0.01054 0.03213 0.01473 0.01496 0.02282 0.03612 0.02370 0.01840 0.01665 0.01967 0.03792 0.02034 0.03634 0.03144 0.01176 0.01073 0.00738 0.00964 0.00434 0.01877 0.00910 0.00736 0.02245 0.02462 0.01818 0.03408 0.01096
0.1201 0.1469 0.0822 0.0955 0.0792 0.0680 0.0980 0.1401 0.1111 0.0536 0.0771 0.0993 0.1343 0.1160 0.1492 0.1117 0.1607 0.1459 0.1659 0.1367 0.1261 0.1506 0.1105 0.1203 0.1722 0.1544 0.1696 0.4177 0.0669 0.1048 0.0901 0.1417 0.2086 0.1684 0.1880 0.1445 0.3265 0.2074 0.1947 0.3568 0.2072 0.3590 0.2681 0.2111 0.2753 0.3532 0.1977 0.2152 0.2605 0.2519 0.3004 0.2898 0.2739 0.0692 0.2433 0.2996 0.1168 0.2037 0.1863 0.1424 0.3123 0.1966 0.2363 0.2200 0.1460
1384.13 875.51 251.65 505.32 1123.13 564.09 157.70 120.52 130.05 188.03 174.44 569.50 824.76 498.80 459.76 768.29 92.17 387.37 33.61 211.17 62.87 112.15 52.71 148.16 188.95 95.09 43.79 42.71 47.08 66.12 68.06 30.38 31.90 53.52 33.60 54.56 23.19 34.93 55.39 33.32 42.20 24.00 11.75 17.53 34.84 19.20 118.74 76.57 20.60 49.53 24.79 9.22 23.30 6.45 32.97 31.10 26.91 32.55 20.46 19.36 13.91 23.96 13.00 6.45 13.33
67 35 9 16 28 20 9 7 7 8 8 28 40 25 29 45 9 22 4 12 4 8 3 7 10 6 4 4 1 3 3 3 3 5 3 4 1 3 3 3 3 1 1 3 3 1 10 7 3 4 3 1 1 1 1 1 1 3 1 1 1 3 1 1 1
20.66 25.01 27.96 31.58 40.11 28.20 17.52 17.22 18.58 23.50 21.81 20.34 20.62 19.95 15.85 17.07 10.24 17.61 8.40 17.60 15.72 14.02 17.57 21.17 18.90 15.85 10.95 10.68 47.08 22.04 22.69 10.13 10.63 10.70 11.20 13.64 23.19 11.64 18.46 11.11 14.07 24.00 11.75 5.84 11.61 19.20 11.87 10.94 6.87 12.38 8.26 9.22 23.30 6.45 32.97 31.10 26.91 10.85 20.46 19.36 13.91 7.99 13.00 6.45 13.33
3.55 3.14 3.00 4.00 3.80 4.67 2.50 6.00 6.00 7.00 7.00 3.00 3.88 6.33 4.00 2.75 8.00 6.00 3.00 11.00 3.00 2.50 2.00 2.00 3.00 5.00 3.00 3.00
4 3 3 3 3 3 3 2 2 2 2 4 3 3 3 4 2 3 2 2 2 3 2 3 3 2 2 2 1 2 2 2 2 2 2 2 1 2 2 2 2 1 1 2 2 1 3 3 2 2 2 1 1 1 1 1 1 2 1 1 1 2 1 1 1
191.93 148.05 141.43 138.21 223.45 125.16 73.59 62.53 59.88 110.79 127.58 121.84 113.62 118.25 125.13 120.73 45.34 127.55 35.33 94.57 58.8 47.38 52.54 79.52 76.26 57.41 55.53 25.87 55.83 68.73 73.49 31.81 26.69 45.1 34.46 47.54 29.02 45.28 43.16 27.08 43.5 30.26 18.77 24.65 34.44 22.87 44.13 39.91 29.89 36.42 20.29 13.75 30.01 35.53 37.76 37.98 37.02 34.7 32.04 29.91 19.87 21.46 18.93 13.73 26.42
31.37 34.30 33.33 32.49 22.16 25.98 34.19 36.93 35.68 28.73 26.28 29.88 33.43 32.04 31.27 25.27 34.36 35.21 30.72 37.67 36.18 35.66 35.88 37.03 36.77 39.31 39.64 40.38 29.04 36.58 34.23 41.36 41.96 42.27 41.97 42.13 42.94 43.52 43.53 43.83 43.31 46.60 38.63 44.24 47.06 46.38 42.19 42.05 46.05 44.66 49.78 41.10 44.99 43.74 42.00 41.84 40.84 40.84 41.15 43.11 46.58 48.20 50.55 47.54 44.54
6.12 4.32 4.24 4.25 10.08 4.82 2.15 1.69 1.68 3.86 4.85 4.08 3.40 3.69 4.00 4.78 1.32 3.62 1.15 2.51 1.63 1.33 1.46 2.15 2.07 1.46 1.40 0.64 1.92 1.88 2.15 0.77 0.64 1.07 0.82 1.13 0.68 1.04 0.99 0.62 1.00 0.65 0.49 0.56 0.73 0.49 1.05 0.95 0.65 0.82 0.41 0.33 0.67 0.81 0.90 0.91 0.91 0.85 0.78 0.69 0.43 0.45 0.37 0.29 0.59
0.64 0.49 0.31 0.61 0.57 0.75 0.52 0.45 0.59 0.48 0.41 0.73 0.70 0.65 0.50 0.77 0.63 0.41 0.42 0.43 0.35 0.70 0.40 0.49 0.47 0.49 0.24 0.42 0.55 0.35 0.35 0.53 0.55 0.42 0.40 0.45 0.24 0.24 0.37 0.36 0.27 0.21 0.36 0.36 0.31 0.30 0.54 0.48 0.19 0.41 0.49 0.45 0.27 0.20 0.27 0.20 0.35 0.36 0.26 0.40 0.32 0.52 0.40 0.41 0.33
143.22 88.85 53.35 103.10 142.06 106.03 46.06 35.26 44.16 61.45 59.87 106.91 101.75 97.53 76.92 106.26 33.25 63.77 17.71 51.59 25.69 41.59 26.03 48.27 45.55 37.21 17.68 14.79 36.29 28.57 31.58 22.21 19.44 25.51 18.63 28.79 9.33 14.50 21.69 13.52 16.31 9.04 8.89 12.61 15.39 9.68 31.84 26.64 7.94 21.41 14.98 7.61 11.40 9.51 13.78 10.05 16.78 16.52 11.03 16.33 9.20 16.26 11.02 7.45 11.99
66416.91 27899.46 7061.58 13717.41 141088.02 64703.88 4293.48 1164.92 2813.25 10452.73 3253.00 25442.17 52422.63 18131.02 16835.26 58159.44 930.48 6141.07 56.10 1682.59 145.57 177.91 110.65 918.69 237.79 487.33 40.55 5.19 134.08 85.29 349.35 82.39 69.47 59.20 18.95 35.84 10.27 18.93 31.73 18.37 24.00 2.95 4.73 6.55 6.56 1.49 252.46 131.55 8.78 10.12 4.96 6.68 5.58 1.29 4.21 2.55 20.94 9.82 1.08 17.31 11.24 14.32 5.33 8.56 3.01
5.35 4.32 2.20 2.52 8.91 6.93 2.53 1.33 2.40 2.33 1.08 4.28 8.17 3.89 4.73 7.07 1.62 2.31 0.28 1.09 0.29 0.24 0.23 0.75 0.22 0.79 0.16 0.05 0.19 0.14 0.46 0.38 0.45 0.19 0.11 0.09 0.14 0.11 0.11 0.20 0.12 0.04 0.11 0.08 0.05 0.03 0.42 0.37 0.15 0.05 0.06 0.21 0.07 0.01 0.03 0.02 0.09 0.06 0.01 0.13 0.25 0.12 0.10 0.29 0.03
2.00 2.00 2.00 2.00 4.00 2.00 3.00 2.00 2.00 2.00 2.00
2.00 2.00 3.50 2.00 2.00 3.00 0.90
2.00
2.00
volume in a long run (Descroix et al., 2008). The highest rates of erosion, which are comparable to higher V/A2D values because the calanchi are coeval, occur on less steep but larger slopes with a higher value of MSI. The value of MSI can be considered peculiar to each singular slope. In future studies MSI could also be used for larger catchment areas and
could be correlated more closely with morphogenetic processes. Further use of the index in a predictive way could be implemented by replacing Ar with A3D (actual surface of the tributary area). The data derived from the comparison between the MSI–Ar relation and reference morphological processes could be projected onto the MSI–A3D
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Table 2 Pearson's correlation matrix. F = drainage frequency; D = drainage density; l = total drainage network length; N = total number of furrows in the drainage unit; L = plane length; P = inclination; RC = circularity ratio; MSI = morphometric slope parameter; V = eroded volume; A2D = plane area.
F (m− 2) D (m− 1) l (m) N l/N (m) L (m) P (°) L/P (m/°) RC V (m3) V/A2D (m) MSI (m)
F (m− 2)
D (m− 1)
l (m)
N
1 0.752⁎⁎ − 0.430⁎⁎ − 0.369⁎⁎ − 0.704⁎⁎ − 0.626⁎* 0.655⁎⁎ − 0.583⁎⁎ − 0.290⁎ − 0.561⁎⁎ − 0.338⁎⁎ − 0.440⁎⁎
1 −0.411⁎⁎ −0.377⁎⁎ −0.340⁎⁎ − 0.613⁎⁎ 0.696⁎⁎ − 0.587⁎⁎ −0.465⁎⁎ −0.577⁎⁎ −0.341⁎⁎ −0.455⁎⁎
1 0.953⁎⁎ 0.391⁎⁎ 0.888⁎⁎ − 0.657⁎⁎ 0.875⁎⁎ 0.594⁎⁎ 0.941⁎⁎ 0.863⁎⁎ 0.902⁎⁎
1 0.23 0.808⁎⁎ − 0.608⁎⁎ 0.764⁎⁎ 0.630⁎⁎ 0.893⁎⁎ 0.727⁎⁎ 0.851⁎⁎
l/N (m)
1 0.532⁎⁎ − 0.529⁎⁎ 0.549⁎⁎ 0.131 0.458⁎⁎ 0.421⁎⁎ 0.383⁎⁎
L (m)
P (°)
L/P (m/°)
RC
V (m3)
V/A2D (m)
MSI (m)
1 − 0.810⁎⁎ 0.976⁎⁎ 0.505⁎⁎ 0.941⁎⁎ 0.762⁎⁎ 0.830⁎⁎
1 −0.832⁎⁎ −0.602⁎⁎ −0.796⁎⁎ −0.610⁎⁎ − 0.719⁎⁎
1 0.516⁎⁎ 0.928⁎⁎ 0.849⁎⁎ 0.856⁎⁎
1 0.719⁎⁎ 0.501⁎⁎ 0.663⁎⁎
1 0.812⁎⁎ 0.910⁎⁎
1 0.888⁎⁎
1
⁎⁎ Significant correlation at the 0.01 level ⁎ Significant correlation at the 0.05 level.
relation. The latter depends on the erosional processes that shaped the slope, but this term subsequently conditions the future evolution of the processes themselves.
(San Casciano in Val di Pesa, Firenze), Prof. Jan Kalvoda (Charles University in Prague) and Prof. Takashi Oguchi (University of Tokyo) for their critical review and useful comments and suggestions.
This study was financially supported by the project MIUR-PRIN 2007, “Development of an integrated model for a preventive assessment of soil degradation processes in the Mediterranean environment” (coordinator: M. Märker), the sub-project “Contribution of slope anthropization analysis to the development of an integrated model for the quantitative assessment of hydric soil erosion processes in the Mediterranean environment” (local coordinator: G. Pambianchi), and University “G. d'Annuzio”. The authors thank Dr. David Alexander
V/A2D(m)
Acknowledgements
10 9 8 7 6 5 4 3 2 1 0
R² = 0.829
0
50
100
150
200
MSI (m)
a
Fig. 8. Linear regression for V/A2D versus MSI.
0.05
a 0.05
0.04 0.03
R2 = 0.194
0.04
0.02
F (m- -2)
F (m-2 )
R2 = 0.314
0.01
0.03 0.02 0.01
0 0
50
100
150
200 0
MSI (m)
0
2
4
b
6
8
10
V/A2D(m)
0.5
b
R2 = 0.332
0.5
0.4
R2 = 0.207
0.3
D(m--1)
D (m-1)
0.4
0.2 0.1
0.3 0.2 0.1
0
0 0
50
100
150
MSI (m) Fig. 7. Linear regressions for F (a) and D (b) versus MSI.
200
0
2
4
6
8
V/A2D (m) Fig. 9. Linear regressions for F (a) and D (b) versus V/A2D.
10
48
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