Spatial Davidson, 1986) or induced as a result

Spatial heterogeneity of the watershed characteristics such as soils, vegetation, surface roughness and topography can have a remarkable influence on the soil erosion and sediment. The aim of this study is to understand the general concepts and quantification methods of spatial heterogeneity. Due to soil erosion and sediment issues and its natural complexity and variability, it is necessary to quantify spatial heterogeneity. It was used data types for quantifying the spatial heterogeneity that include non-spatial, spatial patterns, quantitative lattice and qualitative lattice. Each of them was classified to several parts. The results of this review showed that the study and quantifying of conceptual frameworks of spatial heterogeneity and addressing these complicated problems are indispensable. Because quantifying of heterogeneity contribute to facilitate understanding of nature complexities and variability. Therefore, this commentary explores a number of related new avenues for research in spatial heterogeneity, including the use of classification, defining, understanding watershed function and effect of spatial heterogeneity of soil erosion and sediment on watershed or sub watershed.Key words: Spatial heterogeneity, Quantification methods, Soil erosion and sediment, Natural complexity1. IntroductionSoil erosion has been an important dynamic land surface and land degradation process in all over earth history (Dearing 1994; Ananda and Herath, 2003; Marques et al. 2008). It has exacerbated human civilization and the quest for better live by man. It is either caused by natural agents including rainfall, flowing water and ice and wind which cause the soil or rock materials loosened and removed from one place and deposited to new place (Morgan and Davidson, 1986) or induced as a result socioeconomic development (Burkes 1979, Evans 1996; Souchere et al., 2003; Mathieu and Joannon, 2003; Boardman et al, 2003) over the years. Soil erosion causes both on-site loss of topsoil and also has serious offsite environmental effects which are damaging to both flora and fauna (Roberts 1994; Morgan 1995; Lal 2001; Dearing et al. 2003). Due to aforementioned paragraph, soil erosion is a main problem in areas with expanding population and water and food security, agricultural production, construction, urbanization and human activities (Ding et al., 2015; Leh et al., 2011; Wu and Xie, 2011; Wen, 1993; Brown and Halweil, 1998; He et al., 2003) flood, drought and famine(the Soil and Water Conservation Society, 2003; Lal 2003) as well as depleting soil fertility (Dotterweich, 2013; Morgan, 2005). Among the factors poor land management is the most important factors that cause soil erosion which causes damage to the soil and results in runoff across outlook instead of sufficient infiltration (Liu, 2016; Montgomery et al, 2014; Nadeu et al, 2012; Niu et al., 2015). The effects of soil erosion and sediment could be deteriorate by inter and intra reactions within the ecosystem. Inter and intra reactions caused to recognize likely variations across scales in watershed. For example, at local scales, trait variability in general is generated by local processes like disturbances, heterogeneity in resource availability and species interaction (Moreira et al., 2012) and caused to influence the population dynamics and the community structure which, is much dependent on this local trait variability (Jung et al., 2010; Bolnick et al., 2011). These variations divided to multiple classifications. They are including variability, pattern, distribution and heterogeneity. All of above classification are divided spatial and temporal. Due to tittle article, we explain more about the differences among spatial variability, spatial pattern, spatial distribution and spatial heterogeneity. These descriptions caused to recognize the differences among them.Spatial variability occurs when a quantity that is measured at different spatial locations displays values that differ across the locations. Spatial variability can be assessed using spatial descriptive statistics such as the range (the largest and smallest values) (Isaaks and Srivastava, 1989; Fortin and Dale, 2005; Kristensen et al., 2015).Temporal variability is a variant that is a function of time. It is including daily, monthly and annual (Olden and Poff (2003)).For example, several researcher have presented that dominant climatic and landscape controls on hydrologic behavior are time-scale dependent (Sivapalan et al., Atkinson et al. 2003; Farmer et al. 2003; Son and Sivapalan 2007). A spatial pattern is defned as a perceptual structure, placement, or arrangement of objects on Earth. It also includes the space in between those objects. Patterns may be recognized because of their arrangement; maybe in a line or by a clustering of points. It is including point, line and area.A spatial distribution is defned as the arrangement of a phenomenon across the Earth’s surface and a graphical display of such an arrangement is an important tool in geographical and environmental statistics. A graphical display of a spatial distribution may summarize raw data directly or may reflect the outcome of more sophisticated analysis. Several studies carried out on spatial distribution of sediment (Faran Ali and De Boer, 2010; Schrott et al., 2003; Me´ar et al., 2006 ). Spatial heterogeneity is defined either as the variation in space in distribution of a point pattern, or variation of a qualitative or quantitative value of a surface pattern (Dutilleul & Legendre 1993). It can be caused by habitat factors (Tscharntke et al. 2002) and their temporal variations (Leyequien et al. 2007), individual traits (Tilman & Kareiva 1997), and neutral processes (Rosindell, et al., 2008). Therefore, spatial heterogeneity is of great importance in the study of populations, communities, ecosystems, and landscapes (Shaver 2005).A major challenge is to find appropriate conceptual frameworks to address these complicated problems. Understanding spatial heterogeneity is now recognized as one of the most significant aspects of this challenge. However, because spatial heterogeneity have been ignored for so long in watershed systems, there is a pressing need to integrate it into their studies, theories, and models. With new frameworks and tools, ecology is now poised to make important strides forward in the focused study of heterogeneity from an ecosystem and landscape perspective. Ecology has accepted the challenge of understanding these complicated systems overall, and is making good progress toward doing so. Such knowledge is vital to guide conservation initiatives, sustainable management, mitigation of environmental impacts, and future breakthroughs in understanding (Likens, 2003).1.1. Motivation and importance of spatial heterogeneityThe earth is not really flat or homogeneous, so the shortest effective distance between two points is different for soil erosion which moving about on the same landscape. In some sense, different notions of epidemic space are motivated by the different ways a soil erosion factor moves within watershed and sub watersheds. For example, regional- and global-scale management issues have progressively compelled ecologists to work on large, heterogeneous study areas (e.g., Possingham et al. 2006).At the same time, the rapid rise of landscape ecology (Turner et al. 2001) has provided the intellectual motivation to understand large, heterogeneous landscapes. Indeed, the move by ecologists and engineering to understand regional and global problems has probably been one of the important motivations for bringing the subject of the functional consequences of heterogeneity to the fore. Also, these problems exist in watersheds and sub watersheds such as spatial heterogeneity of soil erosion and sediment, on-site and off-site threats and so on that needs to be more discussed. According to the occurrence and effects of soil erosion vary considerably over multiple spatial and temporal scales. At a regional scale, variation in watershed characteristics results in spatial heterogeneity of soil erosion. Thereby, the dynamics or spatial variation at the lower level are so high-frequency that the average value is experienced at the focal level. Furthermore, Turner et al. (2001) point out that a shift in the relative importance of variables influence a process, or even a change in the direction of the relationship; often happen when scales are changed. Variation gradually changes and makes spatial patterns in different watersheds. This causes to emerge spatial heterogeneity. Therefore, analysis and quantifying the spatial heterogeneity play an essential role in simplifying the perplexing complexity of natural systems.1.2. Factors effecting on sediment spatial heterogeneityHeterogeneity exists at multiple spatial scales and hence the effects of that heterogeneity also obvious over a wide range of scales. One approach to dealing with heterogeneity without their precise characterization is to focus on those properties that become manifest with increasing scales, and their resulting hydrological effects. It is possible that as we look more closely and find a way to filter out unimportant details, we might begin to see emergent features or properties that play as a natural skeleton to connect descriptions of hydrological responses across scales (Sivapalan, 2003).1.2.1. Vegetation and sediment spatial heterogeneityIn general, vegetation cover is able to prevent water from flowing through the increasingly rough surface. The sediment-carrying capacity of fluid flow compared with bare land would decrease with increased vegetation on the surface. As a result, soil erosion would be hindered and sediment deposition would be promoted (Li 2011; Xu 2006), accumulating the organic matter and the moderation of soilmicroclimate (Kittredge 1948), development of water stable soil aggregates (La, 1987), increasing infiltration rates (Blackbum 1975; Wood 1981; Knight 1984; Thurow, 1986; Amin, 2005), increasing cover of ground-storey plants, particularly grasses, on reducing runoff and erosion (Pressland, 1982; Eldridge, 1993),  storing water and on controlling sediment yield through Interception (Ghimire et al. 2012; Wang et al. 2012) and absorption (Kurothe et al. 2014; Park et al. 2010).Therefore, the different vegetation could be have different behavior because of heterogeneity environments.                                                                                                                                                         It is considered that heterogeneous environments lead to heterogeneous vegetation and tend to exhibit a high vegetation diversity (Begon et al., 1996).However, the measurement of spatial heterogeneity in community/vegetation is often difficult for at least the following three reasons: (1) The spatial heterogeneity of each vegetation and vegetation diversity are highly variable and may depend on the climate, geographical conditions and human agricultural activities, etc. (Patil et al., 1974; Pacala and Crawley, 1992; McNaughton, 1994 ). (2) Although many methods or indices can be used for estimation of heterogeneity, no single index is superior (Greig, 1983; Hurlbert, 1990; Downing, 1991). (3) Measurements of such indices are often difficult, tedious and tend to disturb the vegetation measured. Therefore, comparison of different communities/landscapes is usually difficult.If we look carefully this issue a few vegetation species do not enable less the spatial heterogeneity sediment. For example, the growth of Vallisneria natans and Prosopis glandulosa did not differ significantly between various spatial heterogeneity sediments (Maestre and Reynolds, 2006; Xie et al., 2007). Therefore, the influence of sediment heterogeneity on plant growth is species-specific, and may be connected to differences in nutrient-obtaining ability between species.1.2.2. Rainfall and sediment spatial heterogeneityDuring the process of water erosion, the transformation of surface morphology affects the pro¬duction of surface runoff, water flows, and convergence, and accordingly it affects the evolution of divers’ soil erosion types and erosion sediment yield (Burwell et al. 1968; Johnson et al. 1979; Onstad et al. 1984). This usually separate the soil and dispersing the materials. A typical rainwater runoff will impact lighter materials like organic matter, silt, and finer sand particles, but in most heavy rainfalls larger material components are also affected as well. With rainfall duration, peak runoff discharge was presented, resulting from soil sealing (Duiker et al., 2001; Gómez and Nearing, 2005; Ran et al., 2012).In fact high intensity caused that soil erosion occurs easily (Brunton and Bryan, 2000; Woodward, 1999; Di Stefano et al., 2013).Liang et al. (1996) developed a statistical dynamic approach to include the spatial heterogeneity of rainfall. These authors evaluated the approach in terms of its ability to predict spatially averaged surface fluxes, runoff, and soil moisture produced by an explicit model. Sivapalan et al. (1997) highlighted the profound effect of the subgrid scale variability of rainfall on land; surface fluxes and on the long; term water balance. They demonstrated that spatially heterogeneous rainfall produces variability in the constitutive relationships for evapotranspiration and runoff and noted that the variability is particularly important for runoff predictions. Rainfall spatially heterogeneity has been caused to create a spatial heterogeneity of sediment. Therefore, all the essential variables that indicate sediment dynamics are to be taken into consideration, especially sediment concentrations, sediment yield and transportability of soil particles (Defersha, and Melesse, 2012).1.2.3. Topography and sediment spatial heterogeneityTopography is one of the main factors controlling sediment transport at the catchment scale through spatial patterns of upslope contributing area and local slope.  The microtopography can be divided to three types of terrains: plane, slope, and uneven terrain (Zhang et al. 2011). The microto¬pography in tilled loess slopes generated by human management is not only the direct result of slope erosion but also the principal factor leading to the further development of slope erosion. Based on Kirkby (2001) microtopography can be examined to be a lively place which can reflect various elements of slope erosion kinetics as well as their interaction. Furthermore, the microtopography affects the amount of surface depression storage, which is the part of the soil surface cov¬ered by water. Topography influence not only surface hydrological and hydraulic characteristics (Yang and Chu, 2013; Peñuela et al., 2015), but also many abiotic processes and biotic interactions(Burke et al., 1999; Frouz and Kindlmann, 2001) surface depression storage (Planchon and Darboux, 2002), infiltration rate (Morbidelli et al., 2015), surface runoff (Rai et al., 2010; Vermang et al., 2015) and transition and deposition of soil particles during water erosion process (Shi et al., 2012; Wang et al., 2014) and relationship between microtopology and soil erosion, hydrological processes on sloped surfaces (Abell et al. 2008; Yvonne et al. 2008; Zhao et al. 2010) .It is necessary to study the spatial heterogeneity in linking microtopography and soil erosion, hydrological processes on sloped surfaces.The evolution of spatial heterogeneity variability in the soil erosion process on a microtopography scale, has drastically hindered our under¬standing of the role that microtopography plays in the soil erosion process (Huang et al. 1992, 2001, 2003).Since slope and length of slope are the main factor in topography. Hence, it is important that these factors were studied in soil erosion and sediment process. Several studies presented that soil loss increased with increasing slope gradient (e.g. Kinnell, 2000; Assouline and Ben-Hur, 2006; Berger et al., 2010), while some studies determined that there is no correlations between soil loss and slope gradient (Chaplot and Le Bissonnais, 2003). As steep land eroded sediments reach the stream network, river capacity reduces, and flood risk increases (Morgan, 2005). Sedimentation also can reduce the capacity of reservoirs, decreasing water storage and shortening the lifespan of hydroelectric power plants (Verstraeten et al., 2003). Hence, most of soil properties display remarkable spatial variations in erosion and sediment dynamics, which may lead to spatial heterogeneity of soil detachment processes. Spatial heterogeneity stem from long-term temporal–spatial interaction between basic ecological processes and physical processes (Li and Wu, 1992). Soil properties (physical, chemical, and biological) differ in space; this difference is termed spatial heterogeneity of soil. It means that spatial pattern of slopes in different environment produce different sediment spatial heterogeneity. 1.2.4. Soil moisture and sediment spatial heterogeneitySoil resources are spatially heterogeneous at various scales (Kelly and Canham 1992; Jackson and Caldwell 1993; Maestre and Cortina 2002). Among soil resources properties, antecedent soil moisture is one of the most important soil properties which affects on soil erosion because it affects the structure and hydraulic response of the soil (Cresswell et al. 1992; Luk, 1985).As an important research subject in hydrological research, pedology and environmental studies (Lin et al., 2006), soil moisture is influenced by many environmental factors, such as rainfall, topography (Qiu et al., 2001; Kim, 2012), solar radiation (Lu et al., 2002; Western and Blöschl, 1999), soil texture (Baroni et al., 2013; Jawson and Niemann, 2007) and land use (Fu et al., 2003; Venkatesh et al, 2011). The spatial distribution of soil moisture is complex, and the factors controlling the pattern’s formation are disputable, due to the scale dependence of the spatial variability of soil moisture and the increase in soil moisture heterogeneity as scale expansion (Mark et al., 2007; Famiglietti et al., 2008). Therefore, the strong spatial heterogeneity of soil moisture is controlled by many environmental factors, including topography and land use. Further, the spatial patterns and soil hydrological processes belong to the scale of the site being investigated, which engenders a challenge for soil moisture spatially heterogeneity.Based on factors affecting on spatial heterogeneity on soil erosion and sediment, it is understand that every watershed is unique and the way every watershed has evolved in response to unique climatic and geological features and the history and initial conditions is different. And so the exact pattern and process that outcome will be different, even under similar conditions. However, the more we understand the general, the more we accept and understand the anomaly or the outlier from the mainstream (Bloschl, 2006).It can be stated that watersheds/sub watersheds affect each together under similar conditions. Therefore, understanding the effects of spatial heterogeneity on subclass watersheds is necessity to quantify interactions among factors of affecting on spatial heterogeneity on soil erosion and sediment. The key objectives of this review is to provide concepts of spatial heterogeneity, major issues facing spatial heterogeneity of soil erosion and sediment, understanding the natural complexity and its quantification.