-
[show abstract]
[hide abstract]
ABSTRACT: The notion that the desert areas of the world possess a distinct geomorphology has a long history and, in many ways, is informed
by the popular concept of deserts as places that are different. Not surprisingly, early explorers in deserts, particularly
Europeans travelling in the Sahara from the late 18th century onwards, were impressed by, and reported on, the unusual features
of these areas. Rock pedestals, sand dunes, and bare-rock hills rising almost vertically from near-horizontal, gravel-covered
plains all contributed to the impression of a unique landscape. This spirit of exploration in a totally alien landscape continued
into the 20th century, so that as late as 1935 R.A. Bagnold wrote of his travels in North Africa during the preceding decade
under the title Libyan sands: travels in a dead world (Bagnold 1935). Emphasis on the unusual and remarkable landforms of desert areas and a coincident emphasis on the hot tropical
deserts had a profound impact on attempts to explain the geomorphology of deserts.
12/2008: pages 3-7;
-
[show abstract]
[hide abstract]
ABSTRACT: Desert hillslopes below the angle of repose are dominated by the weathering characteristics of the underlying lithology, and
specifically by the rate of production of fine material compared to the rate of removal. The previous chapter considered hillslopes
underlain by massive rocks, or those in layered rocks dominated by outcropping resistant layers. These lithologies are weathering
limited, and give rise to hillslopes where a surficial layer of weathered material is thin or absent. On more readily weathered
lithologies a more-or-less continuous layer of debris is found. This layer of debris is subject to pedogenic processes. This
chapter deals with such hillslopes.
12/2008: pages 233-263;
-
[show abstract]
[hide abstract]
ABSTRACT: A simple model of raindrop erosion—the combined effects of the detachment of sediment by raindrops and its transport by splash or by overland flow—is developed to examine the role of this process in the formation of desert pavements. Application of the model to soils in areas of existing pavement initially simulates the formation of pavements, but the changing sediment size distributions lead to the subsequent destruction of these modelled surfaces. An improved model that accounts for the feedback effects of the changing size distributions on infiltration and microtopography is then developed. Incorporating these effects allows simulated pavements to be maintained over longer periods. The model yields desert pavements whose particle size compositions differ in response to differences in initial soil characteristics, slope and rainfall intensity. This model is tested against empirical data from a site where there is intershrub pavement and associated mounds of fines beneath desert shrubs. The results successfully predict the accumulation of fines under shrubs but underestimate the development of the pavement between shrubs. These findings suggest that the raindrop erosion mechanism on its own cannot account for the development of the pavement and that some other mechanism leading to the surface concentration of coarse particles must also be operating.
Earth Surface Processes and Landforms 07/2006; 20(3):277 - 291. · 2.43 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Experiments were undertaken to determine the feasibility of tracing sediment movement in interrill overland flow. Crushed magnetite was introduced as a source-line 10 cm wide by 8 m long on a runoff plot 18 m wide by 29 m long located in southern Arizona. Initial magnetic susceptibilities along this source line, and along three transects located 0·25, 2·95 and 5 m downslope of the source-line, were measured. Movement of the magnetite in response to three rainfall simulation experiments was monitored. During the first two experiments, overland flow discharge was sampled at miniature flumes located along two cross sections on the plot downslope of the source-line, and at a supercritical flume at the plot outlet. Magnetic susceptibilities along the source-line and transects were measured after all three experiments. Results show that the magnetite moves very early in the experiments and that it reaches one of the flumes 2 m downslope of the source-line in 3 min. Most of the tracer moves a very short distance: 29·7 per cent is deposited within 25 cm of the source-line and only 2·2 per cent is deposited 2·95 m away. The deposition rate appears to decrease exponentially away from the source-line. Very little magnetite is recorded in the flow through the miniature flumes: in general it makes up less than 1 per cent of the total sediment load. No temporal pattern in these percentages is observed. Magnetite appears to be an effective tracer of sediment movement in interrill overland flow, though its higher density than natural soil may affect its detachment and transport.
Earth Surface Processes and Landforms 07/2006; 18(8):721 - 732. · 2.43 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This study examines the size characteristics of sediment removed from a semiarid hillslope by interrill overland flow. Rainfall simulation experiments were conducted on a runoff plot 18 m wide and 35 m long established on a piedmont hillslope in southern Arizona. The top of the plot coincided with the hillslope divide, and its outlet was located within a shallow rill. Samples of runoff were obtained from two cross-sections located in the interrill portion of the plot upslope of the rill and from a calibrated flume through which was directed interrill overland flow reaching the bottom of the plot. Analyses of sediment contained in these samples showed that sediment in interrill flow is finer than the matrix soil. The fineness of the interrill sediment compared to the matrix soil appears to be due to the inability of interrill overland flow to transport the coarser fraction of the sediment supplied to it by raindrop detachment. This finding implies that the rate of soil erosion in interrill areas is not. as is commonly supposed, limited by the rate at which raindrops can detach sediment but by the rate at which they detach sediment of a size that the overland flow is competent to transport. The relative fineness of sediment eroded from this hillslope is consistent with other evidence for the recent evolution of shrub-covered hillslopes in southern Arizona.
Earth Surface Processes and Landforms 07/2006; 16(2):143 - 152. · 2.43 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Previous research has shown that either hydraulic action or creep may be the dominant process transporting coarse debris down hillslopes in the American Southwest. This study analyses the movement over 16 years of painted stones on two hillslopes in the central Mojave Desert to ascertain which of these two processes dominate in this region. The distance moved (M) is found to be directly related to length of overland flow (X) and hillslope gradient (S), and inversely related to particle size (D). The fact that M is more highly correlated with X than with S suggests that hydraulic action rather than creep is the dominant process. It is concluded that this is probably the case over most of the Mojave and Sonoran Deserts on slopes up to at least 24°, and that it is only at higher elevations where winters are more severe that creep may become dominant.
Earth Surface Processes and Landforms 07/2006; 9(4):365 - 370. · 2.43 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Although the Shields relation was developed for rivers, it has been applied to sediment transport by overland flow. According to the Shields relation, where the critical boundary Reynolds number Re*c exceeds 40, the critical Shields number F*c is independent of both Re*c and the ratio of the critical flow depth to particle diameter dc/D. Analyses of data collected from runoff plots in southern Arizona reveal that F*c is positively correlated with both Re*c and dc/D. Thus the Shields relation does not apply to overland flow on debris-covered desert hillslopes.Multiple regression analysis is employed to develop alternative threshold relations in which critical boundary shear stress τc is related to D and dc/D (R2 = 0.782) and to D and Sc (critical gradient) (R2 = 0.625). The computed R2 values derive in large part from the spurious correlations of dc/D and Sc with τc. Nevertheless, the relations may be utilized to predict τc. In this regard, the latter relation is likely to prove more useful than the former because Sc is generally known, whereas dc is not.An investigation of the functional relation between τc and D reveals that τc varies approximately with D2 for overland flow on the desert hillslopes under study, whereas the Shields relation predicts a linear relation (i.e. a D exponent of 1). This result is consistent with Cheng's data which show that F*c varies with (dc/D)−1 where 0.4 < dc/D < 2 and may be explained in terms of increased energy dissipation both in separation zones downslope of particles and in distortion of the water surface as dc/D decreases. Consequently, larger values of τc, and hence F*c, are required to initiate the transport of particles of a given size D as dc decreases.
Earth Surface Processes and Landforms 07/2006; 13(5):407 - 419. · 2.43 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A new method of computing the mean velocity of overland flow using dye tracing is proposed in which a specified cross-section is divided into zones of relatively uniform flow characteristics, termed partial sections. The mean surface velocity for each partial section is determined by timing the passage of injected dye, and this figure is multiplied by 0.67, 0.70, or 0.80, depending on whether the flow is laminar, transitional, or turbulent, to give mean velocity. The mean velocity for the entire cross-section is calculated by multiplying the mean velocity of each partial section by its cross-sectional area, summing the products for all the partial sections, and dividing by the total area of the cross-section. A field test shows that mean velocity derived in this manner differs from mean velocity derived by the discharge method (i.e. by dividing discharge measured volumetrically by cross-sectional area) by an average of only 7.07 per cent. Thus the partial-section technique appears to provide a reliable method of estimating mean velocity of overland flow.
Earth Surface Processes and Landforms 07/2006; 11(6):653 - 657. · 2.43 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In desert shrubland ecosystems water and nutrients are concentrated beneath shrub canopies in ‘resource islands’. Rain falling on to these islands reaches the ground as either stemflow or throughfall and then either infiltrates into the soil or runs off as overland flow. This study investigates the partitioning of rainwater between stemflow and throughfall in the first instance and between infiltration and runoff in the second.Two series of 40 rainfall simulation experiments were performed on 16 creosotebush shrubs in the Jornada Basin, New Mexico. The first series of experiments was designed to measure the surface runoff and was performed with each shrub in its growth position. The second series was designed to measure stemflow reaching the shrub base and was conducted with the shrub suspended above the ground. The experimental data show that once equilibrium is achieved, 16% of the rainfall intercepted by the canopy or 6·7% of the rain falling inside the shrub area (i.e. the area inside the shrub's circumscribing ellipse) is funnelled to the shrub base as stemflow. This redistribution of the rainfall by stemflow is a function of the ratio of canopy area (i.e. the area covered by the shrub canopy) to collar area (i.e. a circular area centred on the shrub base), with stemflow rate being positively correlated and throughfall rate being negatively correlated with this ratio.The surface runoff rate expressed as a proportion of the rate at which rainwater arrives at a point (i.e. stemflow rate plus throughfall rate) is the runoff coefficient. A multiple regression reveals that 75% of the variance in the runoff coefficient can be explained by three independent variables: the rainfall rate, the ratio of the canopy area to the collar area, and the presence or absence of subcanopy vegetation. Although the last variable is a dummy variable, it accounts for 66·4% of the variance in the runoff coefficient. This suggests that the density and extent of the subcanopy vegetation is the single most important control of the partitioning of rainwater between runoff and infiltration beneath creosotebush. Although these findings pertain to creosotebush, similar findings might be expected for other desert shrubs that generate significant stemflow and have subcanopy vegetation. Copyright © 2003 John Wiley & Sons, Ltd.
Hydrological Processes 08/2003; 17(13):2555 - 2566. · 2.49 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Rainfall-simulation experiments have been carried out on a series of plots ranging in size from 1 m2 to c 500 m2 in order to observe process and flux-rate changes resulting from the replacement of the dominant vegetation type from grassland to shrubland in the American South-west. Results have demonstrated variations in infiltration rates, flow hydraulics, splash and interrill erosion rates and nutrient transport rates. Furthermore, the shrubland areas develop rills, which are responsible for significant increases in overall erosion rates. The small-plot experiments allow the definition of controlling factors on the processes, and highlight the importance of vegetation controls. Although the small-plot approach has a number of significant advantages, it also has a number of disadvantages, which are discussed in detail. Some of these problems can be overcome with a careful consideration of experimental design. It is argued that plot-scale studies play an important part in improving our understanding of complex, open systems, but need to be integrated with other approaches such as the monitoring of natural events and computer modelling so that mutually consistent understandings of complex ecohydrological systems can be achieved. Copyright © 2000 John Wiley & Sons, Ltd.
Hydrological Processes 01/2000; 14(16-17):2921-2943. · 2.49 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Transmission losses through the beds of hillslope rills were studied at two sites in the semi-arid south-western United States. At one site the rills were sand-bedded and at the other they were gravel-bedded. Transmission losses in the sand-bedded rills are about 66% higher than those in the gravel-bedded rills. The former show evidence of increasing transmission loss with increasing depth of flow, whereas the latter do not. In both cases, transmission losses in the rills are about an order of magnitude greater than infiltration losses in adjacent interrill areas. Copyright © 1999 John Wiley & Sons, Ltd.
Hydrological Processes 12/1999; 13(17):2897 - 2905. · 2.49 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A series of rainfall simulation experiments was carried out at the Walnut Gulch Experimental Watershed, Tombstone, Arizona (31° 43′N, 110° 41′W), to observe the speed at which desert pavement surfaces could be re-established following disturbance. The results of these experiments, which consisted of repeated, 5 min rainfall events, demonstrate that pavements can reform within 10 events, which is compatible with observations of the recovery of surfaces under natural rainfall on an annual cycle. A model for the development of pavements by raindrop erosion processes had previously shown the importance of these processes. The rainfall simulation experiments were used to test the general applicability of this model. The model was able to reproduce the general characteristics of the regenerated surfaces and the timing of their development. However, details of the particle size fractions produced were less well simulated by the model. Testing of the sensitivity of the model to the sediment transport parameters suggests that this problem is not related to the soil characteristics, but is more likely to be an indication of a poor understanding of all the feedbacks operating in the raindrop erosion processes. Copyright © 1999 John Wiley & Sons, Ltd.
Earth Surface Processes and Landforms 09/1999; 24(11):1025 - 1037. · 2.43 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Rainfall simulation experiments were performed in areas of semiarid grassland (Bouteloua eriopoda) and arid shrubland (Larrea tridentata) in the Chihuahuan desert of New Mexico. The objective was to compare the runoff of nitrogen (N) and phosphorus (P) from these habitats to assess whether losses of soil nutrients are associated with the invasion of grasslands by shrubs. Runoff losses from grass- and shrub-dominated plots were similar, and much less than from bare plots located in the shrubland. Weighted average concentrations of total dissolved N compounds in runoff were greatest in the grassland (1.72 mg/1) and lowest in bare plots in the shrubland (0.55 mg/1). More than half of the N transported in runoff was carried in dissolved organic compounds. In grassland and shrub plots, the total N loss was highly correlated to the total volume of discharge. We estimate that the total annual loss of N in runoff is 0.25 kg/ha/yr in grasslands and 0.43 kg/ha/yr in shrublands — consistent with the depletion of soil N during desertification of these habitats. Losses of P from both habitats were very small.
Biogeochemistry 01/1999; 45(1):21-34. · 3.07 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The velocity of overland flow has been conventionally measured using tracers, but it is difficult to measure the mean flow velocity directly because the centroid of the tracer plume is not easily identified. Consequently, previous investigators have measured the velocity of the leading edge of the plume and multiplied it by a correction factor α to obtain an estimate of mean velocity. An alternative method is to measure the velocity of the peak concentration in the tracer plume and multiply this velocity by another correction factor β to estimate mean velocity. To investigate the controls of α and β and develop predictive models for these correction factors, 40 experiments were performed in a flume with a mobile sand bed. Multiple regression analyses reveal that both α and β vary inversely with slope and directly with Reynolds number. The derived regression equations may be used to calculate the mean velocity of other shallow overland flows, at least within the range of slope and Reynolds number for which the equations were developed. In the experiments, slope ranged from 2.7;° to 10° and Reynolds number from 1900 to 12 600.
Earth Surface Processes and Landforms 12/1998; 21(6):509 - 515. · 2.43 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A distributed, dynamic, process-based model for interrill overland flow that has previously been shown to predict accurately both total runoff and runoff hydraulics for a site on semi-arid shrubland is assessed in terms of (i) its portability, (ii) its sensitivity to the quality of data inputs, and (iii) its sensitivity to the size of cell used in the model. It is found that the model can be used at another site, but only after modifications to take account of the local controls of runoff routing. The model is portable, but not readily so. The model is sensitive to both the quality of data input and the size of cell. Data input cannot be reduced by use of stochastic distribution of model parameters without significant loss of accuracy in model predictions, particularly of runoff hydraulics. Larger cells produce poorer predictions of the runoff hydrograph. It is concluded that process-based modelling of interrill runoff may not be a realistic tool for predicting soil erosion, but is one that may be useful for identification of our present poor understanding of erosion processes. Such models help to define the research agenda for soil erosion studies. © 1997 John Wiley & Sons, Ltd.
Hydrological Processes 12/1998; 11(14):1833 - 1859. · 2.49 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Seventy field experiments were conducted in seven rills located on a semiarid rangeland hillslope underlain by gravelly soils at Walnut Gulch, Arizona. The rills, which are characterized by wide, shallow cross-sections and gravel-covered beds, have mean at-a-station hydraulic geometry exponents of b = 0·33, f = 0·34 and m = 0·33. Although the differences between these values and typical values of b = 0·30, f = 0·40 and m = 0·30 for cropland rills are not statistically significant, they are thought to be real, as cropland rills often have more rectangular cross-sections and steeper sides than the rangeland rills under study. For rills formed in silty loamy soils, Govers developed an empirical relation between mean flow velocity and discharge. Emphasizing the generality of this relation, he suggested that it may be used as a simple means of routing runoff through rills. He also noted that this relation appeared to be unaffected by either slope or soil materials. The present data represent rills underlain by coarser and somewhat more varied gravel-rich soils. These data do not conform to Govers' relation, and a multiple regression analysis reveals that slope and soil materials, either directly or indirectly through bed roughness, exert almost as much influence on flow velocity as does discharge. Three alternative methods are developed for predicting flow velocity in the rills under study. All three methods give good results with the largest root mean square deviation being 3·115 cm s−1.
Earth Surface Processes and Landforms 12/1995; 21(1):35 - 47. · 2.43 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Summary In this study we investigate the relation between soil loss and slope length in an interrill portion of a gentle, plan-planar, semiarid hillslope in southern Arizona. Simulated rainfall experiments were conducted on an 18-m-wide by 35-m-long runoff plot, the upper end of which coincided with the hillslope divide. Sediment loads were measured at two cross sections, S1 and S2, located 12.5 and 21 m, respectively, downslope from the divide. From these data, section soil losses under equilibrium runoff conditions were computed. Average section soil loss was found to increase from the divide to S1 and to decrease from S1 to S2. To explain this downslope pattern of soil loss, a simulation model was developed for soil detachment by raindrop impact and sediment removal by overland flow at a cross section. Input data for the model consist of overland flow depths and velocities measured at closely spaced points along a cross section. The model was applied to three cross sections located at 3.3, 12.5, and 21 m downslope from the divide and predicted a downslope pattern of soil loss similar to that observed. The success of the model suggests that in the interrill portions of a wide range of gentle arid and semiarid hillslopes, downslope patterns of soil loss are controlled in large part by downslope changes in across-slope distributions of overland flow depth and velocity.
CATENA. 01/1991; 18(3-4):255-270.
-
[show abstract]
[hide abstract]
ABSTRACT: Simultaneous measurements of rainsplash (as a surrogate for total raindrop detachment) and erosion were made during two rainfall simulation experiments on an 18 by 29 m runoff plot located in an interrill area on a semi-arid, grassland hillslope in southern Arizona. The temporal variation in splash was found to be more complex than hitherto reported. In the first experiment maximum splash occurred 15–20 minutes after the onset of rain and well after runoff began, whereas in the second experiment splash declined throughout the period of observation. The erosion rate, both within and between storms, was not closely related to the splash rate. Higher and increasing erosion rates were identified at times of lower and decreasing splash rates. Whereas splash (raindrop detachment) is controlled by surface-soil moisture and the availability of loose, detachable sediment, erosion is controlled not only by raindrop detachment but also by the areal extent of overland flow. Prediction of interrill soil erosion should not be based upon the assumption that the rate of sediment detachment by raindrops determines the rate of erosion, but upon the interactions among raindrop detachment, overland-flow distribution and ground-surface characteristics.
CATENA.
-
[show abstract]
[hide abstract]
ABSTRACT: Simulated rainfall experiments were conducted on six runoff plots ranging in gradient up to 33° in Walnut Gulch Experimental Watershed, southern Arizona. On the steeper, coarser-textured plots infiltration rates are so high that runoff fails to achieve equilibrium during the c.40-minute experiments. On the gentler, finer-textured plots the rising limb of the hydrograph is stepped. It is speculated that the step is due to a decrease in permeability at the top of the B horizon. Discharge levels off at the step as the surface infiltration rate approaches the saturated hydraulic conductivity of the A horizon, but then increases again to a higher equilibrium discharge as infiltrated water penetrates and eventually saturates the upper B horizon. On five plots, sediment concentration tends to decline with the passage of time since the start of rainfall and in most cases is negatively correlated with discharge. These results are attributed to the progressive exhaustion of fine sediments loosened by weathering and raindrop impact prior to runoff. On the remaining plot, sediment concentration tends to increase with the passage of time and is positively correlated with discharge. Sediment concentration on this plot appears to be determined by flow detachment. Particle size, stone cover, and surface roughness all increase with gradient and together promote infiltration and delay the onset of runoff and the attainment of equilibrium discharge. Sediment yield is curvilinearly related to gradient with a maximum at about 12°. On slopes less than 12°, runoff changes little with gradient, permitting sediment yield to increase with gradient as the effect of gravity increases. On steeper slopes, runoff decreases rapidly as gradient increases, causing sediment yield to decrease despite the increase in the effect of gravity.
CATENA. 15(2):103-117.
-
[show abstract]
[hide abstract]
ABSTRACT: Rainfall-simulation experiments were conducted within the Jornada Experimental Range, southern New Mexico, on small plots located on mesquite nabkha and interdune spaces. Data from these experiments were used to determine runoff, sediment and nitrogen losses, and to measure the effect of soil crusts in interdune spaces on these quantities. Results show that runoff from the nabkha plots amounted to only 7·8% of the applied rainfall, whereas from the interdune plots it was 75·7%, rising to 90·8% from plots where the soil crust was protected from raindrop impact. Sediment yield from the nabkha plots averaged 0·24gm−2s−1 and from the interdune plots it averaged 0·70gm−2s−1, with no difference between the protected and unprotected plots. Nitrogen losses in runoff averaged 1·45mgl−1 from the nabkha plots and 0·61mgl−1 from the interdune plots. However, because of the much greater runoff from the interdune areas total nitrogen loss from interdune plots was three times that from the nabkha plots. Rates of annual sediment loss by runoff are estimated to be similar to rates of aeolian erosion, but rates of dissolved nitrogen loss are estimated at only between 2% and 7% of total atmospheric deposition and less than 1% of nitrogen fixation by mesquite.
Journal of Arid Environments.
