Soil Surface Microtopography Research Articles

Response of Two Semiarid Grasslands to a Second Fire Application

Prescribed fire was used in two semiarid grasslands to reduce shrub cover, promote grass production, and reduce erosional loss that represents a potential non-point-source of sediment to degrade water quality. This study measured transported soil sediment, dynamics in soil surface microtopography, cover of the woody shrub, grass, and bare ground cover classes, and soil fertility measured by nitrogen-mineralization potentials for the respective cover classes over a 9-year period during which 2 fires occurred. In general, the effects of two drought periods were equal to or greater than the effects of fire on the measured parameters. Following the second fire at the grama grass-dominated site, the number of live junipers was significantly lowered (P < 0.001). Fire consumed grass and shrub cover, which created the potential for greater transport of sediment and loss in soil surface elevation. Soil fertility under shrubs was significantly higher than soil fertility under grasses in both grassland sites before and after the first fire; however, that highly significant pattern was not present after the second fire for about 2 years at both sites. This suggests that the reestablishment of a natural fire frequency would likely dampen the well-documented pattern of greater soil fertility under shrub islands relative to grass islands. Repeat use of prescribed fire may keep shrub cover in check and promote fundamental changes in soil processes within semiarid grasslands, but its value must be weighed against the increased potential for erosion following fire.

Applying Improved Estimates of MODIS Productivity to Characterize Grassland Vegetation Dynamics

Prescribed fire was used in two semiarid grasslands to reduce shrub cover, promote grass production, and reduce erosional loss that represents a potential non-point-source of sediment to degrade water quality. This study measured transported soil sediment, dynamics in soil surface microtopography, cover of the woody shrub, grass, and bare ground cover classes, and soil fertility measured by nitrogen-mineralization potentials for the respective cover classes over a 9-year period during which 2 fires occurred. In general, the effects of two drought periods were equal to or greater than the effects of fire on the measured parameters. Following the second fire at the grama grass-dominated site, the number of live junipers was significantly lowered (P < 0.001). Fire consumed grass and shrub cover, which created the potential for greater transport of sediment and loss in soil surface elevation. Soil fertility under shrubs was significantly higher than soil fertility under grasses in both grassland sites before and after the first fire; however, that highly significant pattern was not present after the second fire for about 2 years at both sites. This suggests that the reestablishment of a natural fire frequency would likely dampen the well-documented pattern of greater soil fertility under shrub islands relative to grass islands. Repeat use of prescribed fire may keep shrub cover in check and promote fundamental changes in soil processes within semiarid grasslands, but its value must be weighed against the increased potential for erosion following fire.

Response of Two Semiarid Grasslands to a Second Fire Application

Prescribed fire was used in two semiarid grasslands to reduce shrub cover, promote grass production, and reduce erosional loss that represents a potential non‐point-source of sediment to degrade water quality. This study measured transported soil sediment, dynamics in soil surface microtopography, cover of the woody shrub, grass, and bare ground cover classes, and soil fertility measured by nitrogen-mineralization potentials for the respective cover classes over a 9-year period during which 2 fires occurred. In general, the effects of two drought periods were equal to or greater than the effects of fire on the measured parameters. Following the second fire at the grama grass-dominated site, the number of live junipers was significantly lowered (P , 0.001). Fire consumed grass and shrub cover, which created the potential for greater transport of sediment and loss in soil surface elevation. Soil fertility under shrubs was significantly higher than soil fertility under grasses in both grassland sites before and after the first fire; however, that highly significant pattern was not present after the second fire for about 2 years at both sites. This suggests that the reestablishment of a natural fire frequency would likely dampen the well-documented pattern of greater soil fertility under shrub islands relative to grass islands. Repeat use of prescribed fire may keep shrub cover in check and promote fundamental changes in soil processes within semiarid grasslands, but its value must be weighed against the increased potential for erosion following fire.

Landscape dysfunction and reduced spatial heterogeneity in soil resources and fertility in semi‐arid succulent thicket, South Africa

Abstract Succulent thicket in the Eastern Cape of South Africa is one of many rangeland ecosystems in the world that displays evidence of unsustainable grazing pressure. Widespread transformation of succulent thicket has resulted in the replacement of the typical two‐phase perennial vegetation patches with a structurally simple field layer of ephemeral and weakly perennial grasses and forbs. We hypothesized that (i) transformation of succulent thicket leads to a switch from a spatially heterogeneous landscape to a homogenous and relatively infertile state; and (ii) that this loss of fertility is associated with a breakdown in the processes that conserve resources and promote water use efficiency. We tested these hypotheses at five fenceline contrast sites in Sundays River Thicket, an arid form of succulent thicket. We compared soil fertility (organic carbon, available nitrogen and phosphorus), texture, matric potential, and surface microtopography in the two main microhabitats on either side of the fencelines using a nested anova. Our results show that intact Sundays River thicket has a distinct spatial pattern of soil fertility where nutrients and organic carbon are concentrated under the patches of perennial shrubs, compared with under canopy trees. Transformation results in a significant homogenization of this pattern and an overall reduction in the fertility of the landscape. The proportion of the landscape surface that promotes infiltration due to a distinct litter layer decreases from 60% to 0.6%. Soil moisture retention (matric potential) also decreases with transformation. We interpret these patterns within the framework of semi‐arid landscape functionality.

Flow threads in surface run‐off: implications for the assessment of flow properties and friction coefficients in soil erosion and hydraulics investigations

AbstractSoil surface microtopography produces non‐uniform surface run‐off, in which narrow threads of relatively deep and fast flow move within broader, shallower, slower‐moving regions. This kind of flow is probably widespread, given that microtopography is itself common. Methods used to record the properties of surface run‐off include grid‐ or transect‐based depth observations, with a single mean flow speed derived by calculation from V = Q/WD, and the use of dye timing to estimate velocity, with an effective mean depth calculated from D = Q/WV.Because these methods allow only single, flow‐field mean values to be derived for V or D, neither is well suited to non‐uniform flows. The use of depth data to derive a flow‐field mean V furthermore implicitly applies area weighting to the depth data; likewise, the use of dye speeds for V inherently overestimates mean V because dye dominantly follows the faster flow threads. The associated errors in derived parameters such as friction coefficients are not readily quantified and appear not to have been addressed previously.New field experiments made on untilled soil surfaces in arid western NSW, Australia, explore these circumstances and the implications for deriving meaningful measures of flow properties, including friction coefficients. On surfaces deliberately chosen for their very subtle microtopography, average thread velocities are shown to be commonly 2·5 times greater than the flow‐field mean, and locally 6–7 times greater. On the other hand, non‐thread flow speeds lie below the flow‐field mean, on average reaching only 84 per cent of this value, and often considerably less. Flow‐field means conceal the existence of regions of the flow field whose properties are statistically distinct. Results confirm that a reliance on flow‐field average depths yields estimates of friction coefficients that are biased toward the shallower, high‐roughness parts of the flow, while if dye speeds are relied upon the results are biased toward the deeper, smoother threads of flow. A new approach to the evaluation of friction coefficients in non‐uniform flows is advanced, involving the determination of separate coefficients for threads and non‐thread zones of the flow field. In contrast, flow‐field friction coefficients as they are customarily derived in run‐off plot experiments subsume these distinct coefficients in proportions that are generally unknown. The value of such coefficients is therefore questionable. Copyright © 2004 John Wiley & Sons, Ltd.

Characterizing anisotropy and heterogeneity of soil surface microtopography using fractal models

Soil surface roughness and the complementary soil microrelief depression pattern determine water infiltration, overland flow and drainage network development. The characteristic soil surface roughness remains poorly defined, because of difficulties in quantifying soil microrelief by single indices. Moreover, self-affine fractal models have been found to adequately describe soil surface microtopographic features only in a small range of length scales. At least two parameters are required to assess fractal microrelief surfaces, fractal dimension, D, describing how roughness changes with scale, and the so-called crossover length, l, which specifies the variance at a reference scale. This study focuses on isotropy and heterogeneity analysis of soil surface point elevation data sets, measured with different resolutions, by pinmeter and laser scanner, under field and laboratory conditions. Selected examples of elevation data sets illustrate how real surfaces cannot be ideally fractal, even over limited length scales. Values estimated for the fractal parameters, D and l, by two independent methods, semivariogram (SMV) and local root mean square (RMS), presented some bias, although from a practical point of view both methods gave equivalent results. Irrespective of calculation method and after having removed both, slope and primary tillage effects, D SMV and D RMS values were always above 2.5, indicating anti-persistent behavior. The D SMV and D RMS values were highly correlated with each other, although the relationship was not one to one. Differences in l SMV and l RMS were only apparent for plots with very large structural units and random roughness. Fractal characterization of soil anisotropy and heterogeneity by the SMV and the RMS methods, respectively, may be very useful as a way to compare data sets obtained for different treatments or soil surface evolution conditions, when measured with the same resolution. Isotropic/anisotropic features of the soil microrelief were visualized by means of a Hurst rose plot, where D SMV values estimated for different directions were represented. Local spatially variable D RMS values provided a description of the heterogeneity/homogeneity of the soil surface.

Characterizing anisotropy and heterogeneity of soil surface microtopography using fractal models

Characterizing anisotropy and heterogeneity of soil surface microtopography using fractal models

Effects of intense, short-duration grazing on microtopography in a Chihuahuan Desert grassland

We studied the effect of three consecutive years of short duration (<48 h per year), and intense grazing (20–40 yearling cows per hectare) on soil surface microtopography in a Chihuhuan Desert grassland. We also studied the effects of shrub removal plus grazing on microtopography. Microtopography was measured in 18 plots (treatments). Treatments were a combination of two factors: (1) three levels of grazing (winter-grazed, summer-grazed, and not grazed), and (2) two levels of habitat structure (shrubs-removed and shrubs-intact). Mesquite ( Prosopis glandulosa) shrubs were removed from half of the plots (nine out of 18 plots). The average height of the micromounds, the average depths of intermound depressions, and the number of micromounds were significantly reduced on the grazed plots. Shrub removal had no significant effect on the height of the micromounds or the depth of the intermound depressions of ungrazed plots. There were significant differences in average micromound heights and intermound microdepression depths attributable to the season of grazing. Microtopography was significantly reduced on grazed plots from which shrubs were removed, compared to ungrazed plots, and grazed plots with shrubs present. Grass canopy reduction, and destruction of the micromound structure in a short duration, plus intense grazing results in erosion of micromounds and in-filling of intermound depressions. The loss of microtopography coupled with reduction in vegetation height and cover resulting from short-duration intense grazing by cattle exposed soils to an increased risk of soil erosion Destruction of the micromound/microdepression topography by cattle changes the spatial patterns of water infiltration, and may homogenize nutrients in desert grasslands.

An Instantaneous‐Profile Laser Scanner to Measure Soil Surface Microtopography

Soil surface roughness affects overland flow and soil erosion processes, yet studies on roughness effects are hampered by the difficulty of acquiring microtopographic data. The purpose of this study is to develop an instantaneous surface‐profile laser scanner that has significantly higher data acquisition rate and smaller overall size than previous laser scanners. This laser scanner consists of two diode lasers and a digital camera mounted on a single rail. The lasers project a bright line on the surface and the shape of this line, digitized by the camera from an oblique angle, changes depending on the surface microtopography. From the geometry of the laser‐camera assembly, the line image is converted to surface heights using a calibration procedure based on a triangulation principle. A computer drives the translation of the laser‐camera assembly along the rail, processes the images from the camera, and records the surface profile data during the scan. This ensures the reconstruction of the surface morphology by juxtaposing successive profiles. The current system can measure the microtopography of a 50 cm by 4 m surface with a positional and elevational accuracy of 0.5 mm. The scanner can digitize six surface profiles per second. This translates to a time of 7.4 min to scan a 4‐m long section with profiles taken every 1.5 mm apart. The instantaneous‐profile laser scanner is significantly faster than previous scanner technologies and allows new research opportunities in quantifying surface boundary processes, such as soil erosion.

Automated Digital Photogrammetry: A Valuable Tool for Small‐scale Geomorphological Research for the Non‐photogrammetrist?

Digital photogrammetry provides a tool with which to automatically generate digital elevation models (DEMs). The necessary equipment is now both readily available and affordable: thus there is considerable potential for this technique to be widely adopted in geomorphological studies. But is it possible for geomorphologists without a background in photogrammety to use it successfully? As part of a larger study into rill initiation by overland flow, a non‐metric digital camera and ERDAS IMAGINE OrthoMAX software were used to generate small‐scale DEMs of soil surface microtopography. This paper reports on the procedure used, highlights potential pitfalls, and comments on the quality of the resultant DEMs. Whilst acquisition of high‐quality images using a digital camera is relatively straightforward, problems were subsequently encountered due to the small size of the internal imager and the need for camera calibration. Potential stumbling blocks in the use of the software lay in the setting‐up of ground control points and the use of tie‐points and check‐points, as well as several software glitches not identified in the current manual. Nonetheless, once these problems were overcome the technique proved to be a simple, effective and fast tool for generating high quality microtopographical DEMs. This methodology shows great promise for future geomorphological studies that require these kinds of surface data.

Dynamics of fluvial and slope processes in the changing geomorphic environment of singapore

AbstractThe nature and rates of fluvial and slope processes change over time and space as urbanized areas replace forested land in Singapore. Storm‐based and time‐based data, from undisturbed rainforests, heavily disturbed construction sites, urban grass‐covered slopes and an experimental plot, are collected to observe the impact of rainwater on the soil moisture conditions, surface microtopography, runoff generation, sediment movement, and ground lowering in the three different categories of land use.The undisturbed forested environment is characterized by high throughfall (58% of total rainfall) and frequent negative soil moisture suctions. The slow and unconcentrated overland flow during heavy storms is restricted by the forest floor microtopography. No rills develop. Ground lowering is recorded as 3·2–3·4 mm a−1. But sediment movement is episodic and suspended sediment concentrations in overland flow are 172–222 mg l−1. During urban construction, gully development is rapid on the bare slopes, runoff generation, voluminous, and sediment‐laden discharges (5200–75498 mg l−1) lead to sediment plumes at channel mouths. Ground lowering rates are measured at 132·4 mm a−1. Once grass‐covered, runoff carries less suspended sediment (800 mg l−1) and ground lowering rates are reduced, but depend on the condition of the cover, ranging from 0·2 to 8·2 mm a−1.As urban development continues, environments are altered both in time as well as spatially.

Effects of Soil Surface Microtopography and Litter Cover on Germination, Growth and Biomass Production of Chicory (Cichorium intybus L.)

-A greenhouse experiment was conducted to determine the effect of soil surface and litter cover on germination, survival and biomass production of chicory (Cichorium intybus L.). Surface treatments which had more microtopographical relief (soil covered with clods or rocks) were better for germination and growth compared to treatments in which surfaces were smooth or covered with fine or coarse litter. Seeds germinated in crevices formed where clods or rocks interfaced with the soil surface creating safe sites which retained moisture and provided seedling roots an opportunity to become established. Optimal experimental treatments were similar to chicory's roadside habitat which appears to be hot, dry and inhospitable, but provides the topographical heterogeneity necessary for successful germination and seedling establishment.

Recent developments regarding the influence of soil surface characteristics on overland flow and erosion

Recent developments in modelling overland flow and soil erosion, which permit the consilidation of soil surface characteristics, are reviewed. The models of overland flow are found to be sensitive to soil surface micro-topography and surface pore structure. The former affects depressional storage and hydraulic routing and the latter soil hydraulic properties, notably infiltration rate. The models of soil erosion by water are found to be sensitive to surface roughness, strength and the aggregate size distribution, and the deposition characteristics, of the eroded sediment. Methods for describing the roughness of a soil surface are described and the links to the above processes discussed. The implications of new techniques of roughness measurement for this area of research are briefly examined. It is concluded that the new models of erosion and overland flow provide a more realistic description of the influence of soil surface characteristics than previous semi-empirical models. The review identified the case of large form roughness and shallow flow as a poorly described regime. It was also concluded that the relationship between structural degradation and surface roughness warranted investigation.

Spatial and temporal heterogeneity in soil nutrient supply measured usingin situ ion-exchange resin bags

Data are presented which illustrate the range of ion values obtained from soil solutions eluted fromin situ ion exchange resin bags in grazed and ungrazed grassland soils sampled in the summer and early autumn. Overall, higher levels of cations were being supplied in both the grazed and ungrazed plots in the autumn compared with during the summer. Variation in ion levels reflected spatial heterogeneity in ion supply in these soils. This variation was correlated with the distribution and abundance of the dominant plants and soil surface microtopography. The use ofin situ ion exchange resin bags allow an understanding of short-term temporal and spatial heterogeneity in ion supply.

Interference in Populations of Three Weed Species

species. The life-span and the timing of the life-cycle of the three species are similar. They flower and germinate throughout the year and large numbers of their seedlings often emerge in mixed clusters. The three species occur together and are likely to compete for limited available resources. Pattern analysis has shown that the distribution of the three weeds is related to soil surface microtopography (M. A. Pemadasa, unpublished); Bidens chinensis is relatively more abundant in 'hollows' than on 'hummocks', while the converse is true for the other two species. Soil surface heterogeneity, therefore, appears to play an important role in controlling the size of the populations of the three species. Harper, Williams & Sagar (1965) argued that the relative abundance and cohabitation of different species in nature may be determined by the availability of 'welcoming microsites' on soil surfaces, but information regarding the effect of soil microtopography on interference between species is still scanty. Two aspects of the ecology of the three weed species were studied in the present investigation. Firstly, the reaction of the three species to density at various stages of the life-cycle was studied, and secondly the influence of soil surface heterogeneity on interference between species was examined.