C H A P T E R

8

Influence of Topographyon Soil Properties

O U T L I N E

8.1 Introduction 1458.2 Local Morphometric Variables and Soil 1468.3 Nonlocal Morphometric Variables and Soil 1488.4 Discussion 149

8.1 INTRODUCTION

It is well known that topography is one of the soil-forming factors(Dokuchaev, 1883, 1891; Sibirtsev, 1899; Vysotsky, 1906; Zakharov, 1911,1913; Neustruev, 1915, 1927, 1930; Jenny, 1941; Huggett, 1975; Fridland,1976; Gerrard, 1981; Schaetzl and Anderson, 2005). Topography influ-ences (micro)climatic and meteorological characteristics, which affectthe hydrological and temperature regimes of soils (Neustruev, 1927,1930; Geiger, 1927; Romanova, 1977; Kondratyev et al., 1978; Raupachand Finnigan, 1997; Bohner and Antonic, 2009; Emeis and Knoche,2009), the prerequisites of the gravity-driven lateral overland and intra-soil transport of water and other substances (Kirkby and Chorley, 1967;Young, 1972; Speight, 1980), as well as the spatial distribution of thevegetation cover (Yaroshenko, 1961; Franklin, 1995; Florinsky andKuryakova, 1996). Thus, it is natural that topography directly or indirectlycontrols the spatial distribution of physical, chemical, and biological soilproperties (Moore et al., 1991; Florinsky et al., 2002; Shary et al., 2002b;

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Schaetzl and Anderson, 2005, ch. 13). An in-depth understanding of thiscontrol is required for further modeling and mapping of soil propertiesbased on topographic data (Chapter 10).

Topography influences soil properties through two main “tools”:

1. The gravity-driven lateral migration and accumulation of water; and2. Spatial differentiation of the temperature regime of slopes.

Even in the early twentieth century, Vysotsky (1906, pp. 3�4) noted:

Water for soil is like blood for a living organism. It dissolves and transports var-ious more or less soluble and organic substances, which are created by the weather-ing of parent rocks or decomposition of organic residuals, or moved through theatmosphere as dust and precipitation. But apart from such an internal action, wateris an important external actor conducting (in known cases, together with windaction) a lot of work, such as erosion, denudation, and accumulation, as well asseparation of soil matter. . .. The direction of such actions depends, first and fore-most, on topography . . . because its forms influence not only overland water circu-lation but also intrasoil one.

Recently, Legates et al. (2011, p. 65) stressed the importance of soilmoisture:

Soil moisture is a critical component of the earth system and plays an integra-tive role among the various subfields of physical geography. . .. Soil moisture affectsatmospheric, geomorphic, hydrologic, and biologic processes . . . it lies at the inter-section of these areas of scientific inquiry. Soil moisture impacts earth surface pro-cesses in such a way that it creates an obvious synergistic relationship among thevarious subfields of physical geography. The dispersive and cohesive properties ofsoil moisture also make it an important variable in regional and microclimatic anal-yses, landscape denudation and change through weathering, runoff generation andpartitioning, mass wasting, and sediment transport.

Thus, it is reasonable to use the example of soil moisture to discussthe principal aspects of the topographic influence on soil properties.

The following quantitative topographic characteristics are responsiblefor the spatial distribution and redistribution of water in the landscape:slope gradient, slope aspect, horizontal, vertical, and mean curvature(local morphometric variables), as well as catchment area (the nonlocaltopographic attribute) (Table 2.1). Let us discuss their role in details.

8.2 LOCAL MORPHOMETRIC VARIABLES AND SOIL

Slope gradient controls soil moisture content as follows: as Gincreases, the slope area and velocity of water flow increase, so the pre-cipitation received per unit area and its infiltration decrease, the runoff

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and evaporation area increase, and hence soil moisture contentdecreases (Zakharov, 1913, 1940). This leads to the usual negative corre-lations between the soil moisture content and G (see results of correla-tion analyses in Chapters 9 and 11).

Slope aspect influences the soil water balance since A, in associationwith G, affects insolation (Kondratyev et al., 1978) and evapotranspira-tion (Romanova, 1977). It is well known that in the northern hemi-sphere, soil moisture content tends to be the highest on north slopes,intermediate on west and east slopes, and least on south slopes(Sibirtsev, 1899; Zakharov, 1913; Ponagaibo, 1915; Neustruev, 1927,1930). Also, A affects soil moisture content controlling the impact ofneighboring geographical objects (i.e., mountains, seas, and deserts),which determine the character and direction of atmospheric flows(Neustruev, 1915, 1930; Zakharov, 1940).

Slope gradient and aspect affect redistribution of snow over the landsurface. Thus, these morphometric variables influence the spatial differ-entiation and dynamics of freezing and melting of soils and, in turn, thespatial differentiation of soil water storage (Taychinov and Fayzullin,1958).

Horizontal and vertical curvatures are the key topographic factorsdetermining overland and intrasoil water dynamics (Kirkby andChorley, 1967), being the measures of flow convergence/divergenceand flow deceleration/acceleration, respectively (Table 2.1). Lateralintrasoil flow of the saturation zone and soil moisture content increasewhen flows converge (kh takes negative values) and decrease whenflows diverge (kh takes positive values) (Kirkby and Chorley, 1967;Carson and Kirkby, 1972). Moreover, horizontal curvature influenceshydrological processes in unsaturated soils: infiltration flux divergeswhen kh.0 and converges when kh,0 (Zaslavsky and Rogowski, 1969).It was found experimentally that the dynamics of lateral flows of thesaturation zone and soil moisture content depend essentially on hori-zontal curvature (Anderson and Burt, 1978). This topographic attributecan play the key role in the formation of saturation zones: they are themost stable in areas of flow convergence (O’Loughlin, 1981). Soil mois-ture content also increases when flows decelerate (kv takes negativevalues) and decreases when flows accelerate (kv takes positive values)(Kirkby and Chorley, 1967; Kuryakova et al., 1992). These facts explainthe usual negative correlations of the soil moisture content with kh andkv (see results of correlation analyses in Chapters 9 and 11).

For the arid climatic conditions and relatively flat topography ofIsrael, Sinai et al. (1981) observed a high correlation (20.9) of soil mois-ture content of the root zone with mean curvature approximated by theLaplacian (Section A.3.4). Such a relation results from microtopographiccontrol of the lateral intrasoil water transport rather than from

1478.2 LOCAL MORPHOMETRIC VARIABLES AND SOIL

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redistribution of overland water flows (they are not typical for thatlandscape). A strong dependence of soil moisture on mean curvaturewas also found in Russia’s southern Moscow Region, which has a con-trast topography and is located in a moderate continental climate zone(Kuryakova et al., 1992) (Section 9.3).

Saturation zones are often correlated with landforms, for which bothhorizontal and vertical curvatures are negative (Feranec et al., 1991). Theseare relative accumulation zones, where both flow convergence andrelative deceleration of flows act together (for details, see Section 2.7.2).There are landsliding (Lanyon and Hall, 1983), soil gleying, maximumthickness of the A horizon, and maximum depth to calcium carbonate(Pennock et al., 1987) in these zones due to an increased water content insoils and grounds.

8.3 NONLOCAL MORPHOMETRICVARIABLES AND SOIL

Relationships between soil moisture content and a relative slopeposition (upslope, midslope, and downslope) were qualitatively under-standable even in the early twentieth century (Zakharov, 1913).Quantitatively, the dependence of soil moisture content on catchmentarea (which, in fact, describes the relative position of a point on thetopographic surface—Section 2.3) was probably first described byZakharov (1940, p. 384) as follows: “water amount per unit areaincreases from upslope to downslope due to additional water supply.”Thus, as CA increases, soil moisture content also increases. Thisexplains the usual positive correlations of the soil moisture content withCA (see results of correlation analyses in Chapters 9 and 11). Speight(1980) argued that catchment area rather than horizontal curvature is offirst importance for the soil moisture dynamics, because catchment areaconsiders a relative position of a given point in the landscape.

The topographic index (Section 2.6), combining the metrics of slopegradient with catchment area, can further improve the description ofmorphometric prerequisites for the spatial distribution of soil moisture(Moore et al., 1986). This is because TI takes into account both the localgeometry of a slope and the relative location of the given point in thelandscape. As CA increases and G decreases, TI and soil moisture con-tent increase. This results in higher absolute values of correlation coeffi-cients of soil moisture content with TI than with CA and G (Thompsonand Moore, 1996).

However, topographic index and horizontal curvature cannot sepa-rately offer the prospect of predicting soil moisture dynamics.Saturation zone depth may have higher correlations with some other

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empirically determined variables, such as a product of horizontal curva-ture and catchment area (Burt and Butcher, 1985). This generates a needfor the use of a representative set of morphometric attributes in soilstudies (Section 10.4).

8.4 DISCUSSION

It is obvious that soil moisture content depends not only on topogra-phy but also on some physical and hydraulic characteristics of soils,such as soil texture and soil water retention. However, spatial distribu-tion of these parameters also depends on morphometric variables(Moore et al., 1993; Pachepsky et al., 2001) because they are, in one wayor another, controlled by the intensity and direction of gravity-drivenoverland and intrasoil transport of substances.

Notice that the spatial distribution of moisture in a soil layer maysometimes depend on characteristics of the top surface of parent rocks.Among these are dense, water poor- or impermeable rocks (e.g., clays,granites). In such cases, topographic variables of the top surface of theC horizon may play similar roles as those of the land surface (Florinskyand Arlashina, 1998; Chaplot and Walter, 2003).

Results of the author’s studies of topographic influence on soil mois-ture can be found in Section 9.3 and Chapter 11.

Finally, we should note that the influence of topography on soilproperties depends on the management or tillage practice, for instance,zero tillage versus conventional tillage (Farenhorst et al., 2003;Senthilkumar et al., 2009). Most of the works dealing with relationshipsbetween topography and soil properties in agricultural landscapes havebeen conducted in Canada, the United States, and Australia on fieldstilled over a 50- to 150-year period without dramatic modifications ofthe land surface and soil cover. This may be one reason why high corre-lations have been systematically observed for the system “topogra-phy�soil” in agrolandscapes. A strong, long-term agricultural load canseriously reduce the topographic control of soil properties (Venteriset al., 2004; Samsonova et al., 2007).

1498.4 DISCUSSION

II. DIGITAL TERRAIN MODELING IN SOIL SCIENCE