Concentrated Overland Flow Research Articles

Study on Hydrological Calculation Model of Flood Control in the Middle and Lower Reaches of the Chuhe River under the Background of Urbanization

The traditional method of hydrology and water conservancy calculation is based on the assumption of natural runoff, while the applicability of method has weakened in the urbanization-affected river basins. The paper carried out research on hydrological calculation model of flood control in the middle and lower reaches of the Chuhe River under the background of urbanization. On the basis of the analysis of flood characteristics in river basins, the hydrological calculation model framework of flood control in the middle and lower reaches of the Chuhe River is constructed, which includes five modules: design rainstorm calculation, runoff calculation, regional flow concentration calculation, river-network confluence calculation and lake-reservoir regulation calculation. The parameters of runoff generation, overland flow concentration, hilly area flow concentration and river-network confluence are determined. Process simulation of the flood water levels and discharges in the middle and lower reaches of the Chuhe River in 1991 and 2008, which are the years with heavy rains, is conducted. The flood simulation of June 1991 shows that the error of water level is -1cm ~ + 8cm, and the error of discharge is +26 m3/s. The flood simulation of August 2008 shows that the error of water level is -8cm ~ + 2cm, and the error of discharge is -9 m3/s. The overall simulation results are relatively stable with all the calculation errors within the allowable range, and the results fit the facts well.

Rill erosion on an unpaved loess road surface: Effects and control

AbstractRill erosion on unpaved roads is widespread and highly destructive on the Loess Plateau of China, but the exploration of its evolution mechanism is scarce. In this study, a multiphase runoff scour experiment was performed on an abandoned field road in the Loess Plateau, China. Using a terrestrial laser scanner (TLS), scour experiments are used to evaluate the dynamic evolution of rill erosion and fill. The results show that soil erosion decreases significantly after the initial surge. During the first scouring, the depths of erosion and fill reached their maximum, which were 5.49 ± 7.62 mm and 5.72 ± 16.37 mm, respectively, and the fourth erosion and fill rates are only 24% and 35% of the first. Unlike the upstream and midstream, which are dominated by erosion, the downstream is mainly characterized by fill, and the downstream fill depth is 1.46 times the erosion depth; however, this ratio is 0.52 and 0.74 in the upstream and midstream, respectively. A comparison of the erosion raster with slope change revealed that rapid slope change due to erosion was mainly concentrated at 10–15°. In addition, a comparison of the fill raster with the slope found that slopes greater than 30° during the second scouring tended to flatten out most by filling, as the hydrodynamic forces of steep slopes were easily dissipated. The relationship between slope, erosion, and fill is complex and variable for each scouring. Orthophotos revealed that the U‐shaped bottom topography was conducive to concentrated overland flow, which significantly worsens the rill erosion on the road surface. This indicates that even if the slope is very gentle, the terrain easy to collect water will cause considerable erosion damage. Prevention of rill erosion on the unpaved loess road surface, therefore, requires focused drainage development. This study provides important further scientific insights into the evolution of loess road rill erosion and road erosion damage to inform erosion hazard management in the context of fragile loess landscapes.

Carbon concentrations in natural and restoration pools in blanket peatlands

AbstractOpen‐water perennial pools are common natural features of peatlands globally, and peatland restoration often results in new pool creation, yet the concentrations of different forms of aquatic carbon (C) in natural and artificial restoration pools are not well studied. We compared carbon concentrations in both natural pools and restoration pools (4–15 years old) on three blanket peatlands in northern Scotland. At all sites, restoration pools were more acidic and had mean dissolved organic carbon (DOC) concentrations in restoration pools of 23, 22, and 31 mg L−1 compared with natural pool means of 11, 11 and 15 mg L−1 respectively across the three sites. Restoration pools had a greater fulvic acid prevalence than the natural pools and their DOC was more aromatic. Restoration pools were supersaturated with dissolved CO2 at around 10 times atmospheric levels, whereas for natural pools, CO2 concentrations were just above atmospheric levels. Dissolved CH4 concentrations were not different between pool types, but were ~200 times higher than atmospheric levels. Regular sampling at one of the peatland sites over 2.5 years showed that particulate organic carbon (POC) concentrations were generally below 7 mg L−1 except during the warm, dry summer of 2013. At this regularly‐sampled site, natural pools were found to process DOC so that mean pool outflow concentrations in overland flow were significantly lower than mean inflow DOC concentrations. Such an effect was not found for the restoration pools. Soil solution and pool water chemistry, and relationships between DOC and CO2 concentrations suggest that different processes are controlling the transformation of C, and therefore the form and amount of C, in natural pools compared to restoration pools.

Effect of Napier grass shoots characteristics on runoff and sediment outflow under concentrated overland flow conditions

Vegetation cover is very effective measure to control environmental conditions. In this study, the potential of Napier grass to reduce soil erosion was evaluated. Parameters include number of stems, shoot length, number of leaves, leaf area index and experiments were performed using a hydraulic tilting flume and controlled water circulation system. Extensive experiments were carried out using (i) control (ii) whole plant plot and (iii) roots plot at 2% slope with 10.7 l/s/m discharge rate. Results summarized that Napier grass produced significantly lesser soil erosion and runoff as compared to the control. Further, it was observed that the sediment outflow and runoff generation significantly decreased with the growth stage of Napier grass. Roots were found more effective in controlling sediment rates, while shoots were more effective in reducing runoff rate. The correlation analysis indicated that runoff and sediment outflow was highly negatively correlated with the leaf area index and the number of stems, respectively.

Long‐term evidence for fire as an ecohydrologic threshold‐reversal mechanism on woodland‐encroached sagebrush shrublands

AbstractEncroachment of sagebrush (Artemisia spp.) shrublands by pinyon (Pinus spp.) and juniper (Juniperus spp.) conifers (woodland encroachment) induces a shift from biotic‐controlled resource retention to abiotic‐driven loss of soil resources. This shift is driven by a coarsening of the vegetation structure with increasing dominance of site resources by trees. Competition between the encroaching trees and understory vegetation for limited soil and water resources facilitates extensive bare intercanopy area between trees and concomitant increases in run‐off and erosion that, over time, propagate persistence of the shrubland‐to‐woodland conversion. We evaluated whether tree removal by burning can decrease late‐succession woodland ecohydrologic resilience by increasing vegetation and ground cover over a 9‐year period after fire and whether the soil erosion feedback on late‐succession woodlands is reversible by burning. To address these questions, we employed a suite of vegetation and soil measurements and rainfall simulation and concentrated overland flow experiments across multiple plot scales on unburned and burned areas at two sagebrush sites in the later stages of woodland succession. Prior to burning, tree cover was approximately 28% at the sites, and more than 70% of the area at the sites was intercanopy with depauperate understory vegetation and extensive bare ground (52–60% bare soil and rock). Burning initially increased bare ground across fine (<1 m2) to patch (tens of metres) scales, resulting in enhanced sediment availability at the fine scale, sustained high run‐off and erosion within degraded intercanopies, amplified run‐off and erosion from tree canopy areas, and amplified sediment delivery across fine to patch scales. However, fire‐induced increases in grass cover over nine growing seasons improved infiltration, limited run‐off and sediment delivery from the fine scale, and reduced intercanopy run‐off and erosion at the patch scale. These changes reflect a switch in vegetation structure, triggered by burning and subsequent vegetation re‐establishment, and a shift to biotic control on run‐off and erosion across spatial scales. The responses and persistence over the 9‐year period postfire at the two sites demonstrate that fire can decrease woodland ecohydrologic resilience by altering plant community physiognomy and thereby can reverse the soil erosion feedback on sagebrush shrublands in the later stages of woodland encroachment.

Soil loss: Effect of plant litter incorporation rate under simulated rainfall conditions

AbstractPlant litter can control land degradation by increasing soil organic matter and decreasing soil erosion by covering the soil surface or being incorporated into topsoil. However, the effect of litter incorporation on soil erosion is not clear. This study aimed to investigate the effect of litter incorporation into topsoil on interrill erosion. The poorly decomposed litter was collected from the field and incorporated into topsoil (5 cm) at six rates (ranging from 0 to 0.5 kg m−2). Laboratory rainfall (80 mm hr−1) experiments were conducted under five slope gradients (ranging from 9% to 47%). The results showed that the soil loss rates declined exponentially with increase in litter incorporation rate on slope gradients between 9% and 36%. However, on slope gradient of 47%, litter incorporation caused an increase in the soil loss rate. The efficiency of litter incorporation in controlling soil loss varied from −21.7% to 40.5% with a mean of 10.8%. It decreased linearly with the slope gradient (R2 = 0.34). The decrease can be explained by two simultaneous but opposite effects: the effect of preventing erosion and that of promoting erosion. The former was induced by decreases in the effective erosive force of raindrop and overland flow exerted on soil, and soil erodibility, along with increases in sediment interception. The latter was caused by rapid saturation of the surface soil and concentrated overland flow. The results demonstrate the complexity of the effect of litter incorporation on soil erosion and can improve our understanding of litter in soil loss control.

Gully headcut susceptibility modeling using functional trees, naïve Bayes tree, and random forest models

Gully headcuts are due to erosion generated by concentrated overland flow, non-uniform infiltration, the presence of impermeable sub-surface soil layers, and a hydraulic gradient. It has been seen in an extensive range of continuous and categorical conditioning factors in several countries. Researchers fail to determine the factor that is the most important cause of GHs occurrence. This study determines the morphological and physio-chemical soil features, the locations with significant GHs, and the soil and geomorphic parameters in the loess area of Golestan Province, Iran. The spatial distribution of GHs was determined by field surveys and analysis of 1 m-resolution aerial photography collected with an UAV. Physical and chemical features of soil samples were determined through lab analysis. The relationships between features and GHs were ranked by statistical significance using chi-square tests. Functional trees - (FT), naïve Bayes tree (NBTree), and random forest (RF) models were employed to generate GHs sensitivity maps. One-hundred and twenty-seven GHs were identified and randomly divided into a training dataset comprised of 70% (89) of the GHs, and a validation dataset encompassing 30% (38) of the GHs. Twenty-two GHs conditioning factors were input into the models. The results of the chi-square analysis confirmed land use (90.8%) had the greatest influence on GHs occurrence. The results of training indicated that the AUC value for the FT, NBTree, and RF models were 83.4%, 94.8%, and 96.5%, respectively. The confidence interval (CI) for these methods was 0.771–0.885, 0.905–0.975, and 0.926–0.987, respectively. The highest standard error (SE) was related to the FT model (0.0314), followed by NBTree (0.0151), and RF (0.0119). The validation results indicated an AUC of 0.888, 0.947, and 0.981 for these methods, respectively. Therefore, RF was found to be the most effective model for predicting and mapping GHs in the future.

Hydrogeomorphic processes affecting dryland gully erosion: Implications for modelling

Gullies contribute high sediment loads to receiving waters and significantly degrade landscapes. In drylands, low annual rainfall and resultant poor ground cover, coupled with high-intensity storms and dispersive soils, predispose these landscapes to gully erosion. Land management, such as grazing, exacerbates gully-forming processes by degrading ground cover and compacting soils, thereby increasing and concentrating overland flow. Current surface erosion models do not adequately represent sediment export from gullied terrain due to lack of distributed data and complex hydrogeomorphic processes, such as overland flow concentration, waterfall erosion, soil pipe collapse, and mass wasting. Here, we outline the strengths and weaknesses of past modelling approaches in erodible terrain and focus on how gully erosion processes can be better simulated at appropriate scales using newly available remote-sensing techniques and databases, coupled with improved understanding of relevant hydrogeomorphic processes. We also discuss and present examples of challenges related to assessing land management practices in drylands that affect gully erosion.

Vegetation, Hydrologic, and Erosion Responses of Sagebrush Steppe 9 Yr Following Mechanical Tree Removal

Land managers across the western United States are faced with selecting and applying tree-removal treatments on pinyon (Pinus spp.) and juniper (Juniperus spp.) woodland-encroached sagebrush (Artemisia spp.) rangelands, but current understanding of long-term vegetation and hydrological responses of sagebrush sites to tree removal is inadequate for guiding management. This study applied a suite of vegetation and soil measures (0.5−990 m2), small-plot rainfall simulations (0.5 m2), and overland flow experiments (9 m2) to quantify the effects of mechanical tree removal (tree cutting and mastication) on vegetation, runoff, and erosion at two mid- to late-succession woodland-encroached sagebrush sites in the Great Basin, United States, 9 yr after treatment. Low amounts of hillslope-scale shrub (3−15%) and grass (7−12%) canopy cover and extensive intercanopy (area between tree canopies) bare ground (69−88% bare, 75% of area) in untreated areas at both sites facilitated high levels of runoff and sediment from high-intensity (102 mm • h−1, 45 min) rainfall simulations in interspaces (~45 mm runoff, 59−381 g • m−2 sediment) between trees and shrubs and from concentrated overland flow experiments (15, 30, and 45 L • min−1, 8 min each) in the intercanopy (371−501 L runoff, 2 342−3 015 g sediment). Tree cutting increased hillslope-scale density of sagebrush by 5% and perennial grass cover by twofold at one site while tree cutting and mastication increased hillslope-scale sagebrush density by 36% and 16%, respectively, and perennial grass cover by threefold at a second more-degraded (initially more sparsely vegetated) site over nine growing seasons. Cover of cheatgrass (Bromus tectorum L.) was < 1% at the sites pretreatment and 1−7% 9 yr after treatment. Bare ground remained high across both sites 9 yr after tree removal and was reduced by treatments solely at the more degraded site. Increases in hillslope-scale vegetation following tree removal had limited impact on runoff and erosion for rainfall simulations and concentrated flow experiments at both sites due to persistent high bare ground. The one exception was reduced runoff and erosion within the cut treatments for intercanopy plots with cut-downed-trees. The cut-downed-trees provided ample litter cover and tree debris at the ground surface to reduce the amount and erosive energy of concentrated overland flow. Trends in hillslope-scale vegetation responses to tree removal in this study demonstrate the effectiveness of mechanical treatments to reestablish sagebrush steppe vegetation without increasing cheatgrass for mid- to late-succession woodland-encroached sites along the warm-dry to cool-moist soil temperature−moisture threshold in the Great Basin. Our results indicate improved hydrologic function through sagebrush steppe vegetation recruitment after mechanical tree removal on mid- to late-succession woodlands can require more than 9 yr. We anticipate intercanopy runoff and erosion rates will decrease over time at both sites as shrub and grass cover continue to increase, but follow-up tree removal will be needed to prevent pinyon and juniper recolonization. The low intercanopy runoff and erosion measured underneath isolated cut-downed-trees in this study clearly demonstrate that tree debris following mechanical treatments can effectively limit microsite-scale runoff and erosion over time where tree debris settles in good contact with the soil surface.

Identification of karst sinkholes in a forested karst landscape using airborne laser scanning data and water flow analysis

Karst sinkholes (dolines) play an important role in a karst landscape by controlling infiltration of surficial water, air flow or spatial distribution of solar energy. These landforms also present a limiting factor for human activities in agriculture or construction. Therefore, mapping such geomorphological forms is vital for appropriate landscape management and planning. There are several mapping techniques available; however, their applicability can be reduced in densely forested areas with poor accessibility and visibility of the landforms. In such conditions, airborne laser scanning (ALS) provides means for efficient and accurate mapping of both land and landscape canopy surfaces. Taking the benefits of ALS into account, we present an innovative method for identification and evaluation of karst sinkholes based on numerical water flow modelling. The suggested method was compared to traditional techniques for sinkhole mapping which use topographic maps and digital terrain modelling. The approach based on simulation of a rainfall event very closely matched the reference datasets derived by manual inspection of the ALS digital elevation model and field surveys. However, our process-based approach provides advantage of assessing the magnitude how sinkholes influence concentration of overland water flow during extreme rainfall events. This was performed by calculating the volume of water accumulated in sinkholes during the simulated rainfall. In this way, the influence of particular sinkholes on underground geomorphological systems can be assessed. The method was demonstrated in a case study of Slovak Karst in the West Carpathians where extreme rainfalls or snow-thaw events occur annually. We identified three spatially contiguous groups of sinkholes with a different effect on overland flow concentration. These results are discussed in relation to the known underground hydrological systems.

From shifting cultivation to teak plantation: effect on overland flow and sediment yield in a montane tropical catchment

Soil erosion supplies large quantities of sediments to rivers of Southeastern Asia. It reduces soil fertility of agro-ecosystems located on hillslopes, and it degrades, downstream, water resource quality and leads to the siltation of reservoirs. An increase in the surface area covered with commercial perennial monocultures such as teak plantations is currently observed at the expanse of traditional slash-and-burn cultivation systems in steep montane environments of these regions. The impacts of land-use change on the hydrological response and sediment yields have been investigated in a representative catchment of Laos monitored for 13 years. After the gradual conversion of rice-based shifting cultivation to teak plantation-based systems, overland flow contribution to stream flow increased from 16 to 31% and sediment yield raised from 98 to 609 Mg km−2. This result is explained by the higher kinetic energy of raindrops falling from the canopy, the virtual absence of understorey vegetation cover to dissipate drop energy and the formation of an impermeable surface crust accelerating the formation and concentration of overland flow. The 25-to-50% lower 137Cs activities measured in soils collected under mature teak plantations compared to soils under other land uses illustrate the severity of soil erosion processes occurring in teak plantations.

Ecohydrologic response and recovery of a semi-arid shrubland over a five year period following burning

Increasing trends in wildfire activity on semi-arid rangelands necessitate advancement in understanding of fire impacts on vegetation, soils, and runoff and erosion processes. This study used artificially applied rainfall and concentrated overland flow experiments to evaluate the ecohydrologic response and recovery of a semi-arid shrubland in the Great Basin Region, USA, following fire. Rainfall experiments were conducted at the 0.5m2 plot scale to assess fire impacts on rainsplash and sheetflow processes. Concentrated flow experiments were applied on 9m2 plots to evaluate fire impacts on concentrated overland flow processes. Vegetation, soil, hydrologic, and erosion variables were assessed at each scale pre-fire and 1, 2, and 5yr post-fire. Infiltration and runoff on rainfall simulation plots were affected more by measured background soil water repellency than fire effects on vegetation and soils. Runoff from rainfall on shrub-dominated plots was unchanged 1yr post-fire, but runoff from interspace plots between shrubs declined 1yr post-fire. Runoff increased on shrub and interspace rainfall plots 2yr post-fire and then declined in the 5yr post-fire. Bare ground generally declined across study years, implicating the temporal variability in soil water repellency as the causal factor for infiltration and runoff trends. Erosion on rainfall plots increased by factors of 8 to more than 10 following fire removal of vegetation and ground cover and declined with vegetation recovery through five growing seasons. Concentrated overland flow plots generated slightly more total runoff and 26-fold more total sediment 1yr following burning relative to pre-fire measures. Erosion from concentrated overland flow remained greater on burned than unburned plots after five growing seasons even though ground cover returned to approximately 85%. The relative recovery of vegetation and total ground cover were typical for the shrubland community assessed, but elevated erosion with 85% ground cover 5yr post-fire was unexpected. The persistent high sediment delivery from concentrated plots is attributed to the fine textured soils and thin litter accumulation. The importance of considering erodibility in context with sediment supply and vegetative recovery is discussed. The results demonstrate the complexity of post-fire ecohydrologic interactions, advance process understanding of post-fire ecohydrologic responses for semi-arid rangelands, and underscore the need for additional studies on post-fire recovery over time.

Impact of urbanization on rainfall-runoff processes: case study in the Liangshui River Basin in Beijing, China

Abstract. China is undergoing rapid urbanization during the past decades. For example, the proportion of urban population in Beijing has increased from 57.6 % in 1980 to 86.3 % in 2013. Rapid urbanization has an adverse impact on the urban rainfall-runoff processes, which may result in the increase of urban flood risk. In the present study, the major purpose is to investigate the impact of land use/cover change on hydrological processes. The intensive human activities, such as the increase of impervious area, changes of river network morphology, construction of drainage system and water transfer, were considered in this study. Landsat TM images were adopted to monitor urbanization process based on Urban Land-use Index (ULI). The SWMM model considering different urbanized scenarios and anthropogenic disturbance was developed. The measured streamflow data was used for model calibration and validation. Precipitation with different return periods was taken as model input to analyse the changes of flood characteristics under different urbanized scenarios. The results indicated that SWMM provided a good estimation for storms under different urbanized scenarios. The volume of surface runoff after urbanization was 3.5 times greater than that before urbanization; the coefficient of runoff changed from 0.12 to 0.41, and the ratio of infiltration decreased from 88 to 60 %. After urbanization, the time of overland flow concentration increased while the time of river concentration decreased; the peak time did not show much difference in this study. It was found that the peak flow of 20-year return-period after urbanization is greater than that of 100-year return-period before urbanization. The amplification effect of urbanization on flood is significant, resulting in an increase of the flooding risk. These effects are especially noticeable for extreme precipitation. The results in this study will provide technical support for the planning and management of urban storm water and the evaluation on Low Impact Development (LID) measures.

Evolution of overland flow connectivity in bare agricultural plots

AbstractSoil surface roughness not only delays overland flow generation but also strongly affects the spatial distribution and concentration of overland flow. Previous studies generally aimed at predicting the delay in overland flow generation by means of a single parameter characterizing soil roughness. However, little work has been done to find a link between soil roughness and overland flow dynamics. This is made difficult because soil roughness and hence overland flow characteristics evolve differently depending on whether diffuse or concentrated erosion dominates. The present study examined whether the concept of connectivity can be used to link roughness characteristics to overland flow dynamics. For this purpose, soil roughness of three 30‐m2 tilled plots exposed to natural rainfall was monitored for two years. Soil micro‐topography was characterized by means of photogrammetry on a monthly basis. Soil roughness was characterized by the variogram, the surface stream network was characterized by network‐based indices and overland flow connectivity was characterized by Relative Surface Connection function (RSCf) functional connectivity indicator. Overland flow hydrographs were generated by means of a physically‐based overland flow model based on 1‐cm resolution digital elevation models. The development of eroded flow paths at the soil surface not only reduced the delay in overland flow generation but also resulted in a higher continuity of high flow velocity paths, an increase in erosive energy and a higher rate of increase of the overland flow hydrograph. Overland flow dynamics were found to be highly correlated to the RSCf characteristic points. By providing information regarding overland flow dynamics, the RSCf may thus serve as a quantitative link between soil roughness and overland flow generation in order to improve the overland flow hydrograph prediction. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Short-Term Impacts of Tree Removal on Runoff and Erosion From Pinyon- and Juniper-Dominated Sagebrush Hillslopes

Tree removal is often applied to woodland-encroached rangelands to restore vegetation and improve hydrologic function, but knowledge is limited regarding effects of tree removal on hydrologic response. This study used artificial rainfall and overland flow experiments (9–13 m2) and measures of vegetation and ground cover to investigate short-term (1–2 yr) responses to tree removal at two woodland-encroached sites. Plots were located under trees (tree zone) and in the intercanopy (shrub-interspace zone, 75% of area). Before tree removal, vegetation and ground cover were degraded and intercanopy runoff and erosion rates were high. Cutting and placing trees into the intercanopy did not significantly affect vegetation, ground cover, runoff, or erosion 1 yr posttreatment. Whole-tree mastication as applied in this study did not redistribute tree mulch within the intercanopy, but the treatment did result in enhanced herbaceous cover and hydrologic function in the intercanopy. Fire removal of litter and herbaceous cover increased tree-zone runoff and erosion under high-intensity rainfall by 4- and 30-fold at one site but had minimal impact at the other site. Site response differences were attributed to variability in burn conditions and site-specific erodibility. Burning had minimal impact on shrub-interspace runoff and erosion from applied high-intensity rainfall. However, 1 yr postfire, erosion from concentrated overland flow experiments was 2- to 13-fold greater on burned than unburned tree-zone and shrub-interspace plots and erosion for burned tree zones was 3-fold greater for the more erodible site. Two yr postfire, overland flow erosion remained higher for burned versus unburned tree zones, but enhanced intercanopy herbaceous cover reduced erosion from shrub-interspace zones. The net impact of burning included an initial increase in erosion risk, particularly for tree zones, followed by enhanced herbaceous cover and improved hydrologic function within the intercanopy. The overall results suggest that erosion from late-succession woodlands is reduced primarily through recruitment of intercanopy herbaceous vegetation and ground cover.

Characterization of complex pebble movement patterns in channel flow – a laboratory study

For a long time, studies concerning erosion caused by concentrated overland flow mainly dealt with the erosion and the transport of fine material. More recent studies have shown that rock fragments reduce the intensity of soil erosion processes on the one hand, but on the other hand rock fragment movements also have been observed both in the rill- and interrill erosion processes. However, there is little knowledge about the movement process of rock fragments in shallow channel flow. Are certain movement patterns typical for different shapes? Are there relationships between movement patterns and slope and flow velocity? Are all these patterns and relationships reproducible? To answer these questions, we performed laboratory channel experiments. With these experiments, we could obtain information about movement patterns of pebbles, by varying the following parameters: shape (flat, ellipsoidal, nearly spherical), size (diameter between 1.97 and 4.0 cm) and channel slope (5°, 10°). During the experiments, a high-speed camera was used to capture the motion of eight specially painted pebbles. The resulting image sequences were processed using both automatic image processing and manual visual inspection. Besides the movement patterns, the pebbles velocity, the water velocity and the water depth were estimated. We could show that there were different movement patterns depending on the shape and the slope. For the 5° experiments, the big, flat pebbles lie at the beginning of the tests. After the following yawing, the pebbles mainly showed the movement form rolling around the longest axis. For the 10° experiments the big, flat pebbles showed the same movement pattern firstly, but later in the sequence, they started to roll around their shortest axis and in the end this movement form was combined with saltation. These patterns are described using a simple symbolic language: sequences of pictograms describe the consecutive movement forms. Furthermore, we detected five different velocity groups of the pebbles for each slope: different cross-section shapes of the pebbles result in different acceleration behavior. The methodology is limited to clear water in laboratory use. Even a larger water depth restricts the image processing. Thus, in the future the experiments will be combined with a small sensor that is implanted in the pebbles and measures forces (acceleration), compass (magnetic flux density) and rotations (gyroscope).

Linking fine root and understory vegetation to channel erosion in forested hillslopes of southwestern China

The forestland understory vegetation reduces concentrated overland flow through infiltration improvement by roots and raindrop interception by surface cover. However, little has been done to quantify the linkages between understory vegetation cover, roots, and channel erosion, and such information can help assessing the role of the reforestation in soil erosion control. In this study, we evaluated the relationships between channel density, root density, and vegetation cover in forested hillslopes of southwestern China. Twelve locations (four slopes and three positions) of forested hillslopes with a wide range of understory degradation due to litterfall extraction and livestock grazing were selected for the study. Channel density as a measure of rill and (small) gully erosion, root density of different diameter classes, and vegetation cover of all types were determined using field measuring, soil coring and the line transect method, respectively. Soil loss rates were estimated using the caesium-137 (137Cs) technique. Rills (depth 0.3 m) with a width 0.5–1.0 m were found at the locations of hillslopes with high understory-degradation. Channel density and soil loss rate increased with the increase in understory-degradation in the forested hillslopes. Simple correlation analysis indicated that channel density was negatively correlated with fine root density (diameter < 1 mm and 1–2 mm) and grass and shrub covers, but not with coarse roots (diam. 2–5 mm and 5–10 mm) and mulch and tree covers. The principal component regression revealed fine root density (diam. <1 mm), shrub and grass covers were the most important predictors for channel density in the forested hillslopes. Tree cover, mulch cover and coarse root density were found to have much less influence on channel density. For the model established from this study using principle component regression, vegetation variables could explain 82 % variance of channel density. We conclude that fine root density and grass and shrub covers are the most important factors in controlling soil erosions in forested hillslopes. These parameters should be taken into consideration in assessing reforestation for soil erosion control in hilly areas such as that in southwestern China or other similar regions.

Biosolids and distillery effluent amendments to Irish Miscanthus plantations: Impacts on overland flow and surface water quality

Abstract There are plans to use Irish energy crop plantations as repositories of organic by-products (OBs). It is first necessary to determine the risk of surface-water (SW) pollution from amendment of OBs to energy crops under typical Irish conditions. Therefore, the impact of application of two OBs on the quality of overland flow (OLF) emanating from plantations of Miscanthus × giganteus was assessed. Municipal biosolids (BS) and distillery effluent (DE) were spread annually (for four years) on six 0.1174 ha plots at “100%”, “50%” and “0%” treatment rates. The 100% rate was the maximum P-load of 15 t ha−1 allowed by regulation. Surface flows were sampled over 25 months and tested for pH, electrical conductivity, NO3−, P, K, Cu, Cd, Cr, Pb, Ni, and Zn. Results were compared to drinking-water (DW) and SW quality limits; total exports were used to assess risk to SW. The concentrations of nitrate and heavy metals were within DW limits. Concentrations of PO43− were above limits in all cases; K in OLF from M-DEx plots was above DW limits (in some cases). Despite high concentrations, all exports were relatively small, as OLF events were rare and involved small volumes. There was correlation between OB treatment and increases in OLF concentration in some cases, implying higher applications could result in larger exports. Results imply very low SW quality risks exist in areas that are not prone to OLF (at treatment rates and plot scales used). It is recommended further work is conducted to assess risks at catchment scales, particularly for P-loss.

Biosolids and distillery effluent amendments to Irish short rotation coppiced willow plantations: Impacts on overland flow and surface water quality

It's necessary to determine risks of pollution arising from amendment of organic by-products (OBs) to energy crops. The impact of applying two OBs on the quality of overland flow (OLF) from plantations of short rotation coppiced willow was assessed. Municipal biosolids (BS) and distillery effluent (DE) were spread annually on six 0.1174ha treatment plots at rates of 100% (W-BS100, W-DE100), 50% (W-BS50, W-DE50) and 0% (W-BS0, W-DE0). The 100% rate was the maximum load of 15t Pha−1 as per the current Irish regulation based on crop uptake of P. Surface flows were sampled over 18 months and tested for pH, electrical conductivity, NO3−, PO43−, total P, K, Cu, Cd, Cr, Pb, Ni, and Zn. Results showed NO3−, Cu, Cd, Cr, Pb, Ni, and Zn concentrations in OLF were well below quality standard limits set out in European Union (EU) drinking water (DW) and surface water (SW) regulation. For P–PO43− and K, concentrations in OLF exceeded these limits. There were treatment responses for all OLF species, except P and Ni; in some cases there were large increases in OLF concentration as OB treatment rate increased. There was build-up in soil-P, -K and in heavy metals (HMs) following application of BS, but not distillery effluent (DE). Results suggest increased OB application could result in increased export from plots treated with OBs. Despite high PO43− and K OLF concentrations, the occurrence of OLF was rare and volumes lost from plots were low; actual exports of nutrient and HMs by this pathway were small. It was unlikely OLF streams transported particulate matter far from plots, and no preferential flow routes were available to the OLF streams. For these reasons, the OLF encountered during experiments did not pose any serious threat to the quality of nearby surface waters. CapsuleOrganic by-products (OBs), biosolid and distillery effluent, amended to short rotation coppice willow. Impact on quality of surface (overland) flows (OLF) assessed by analysis of nutrients and heavy metals (HMs). Evidence of export of all three major agricultural nutrients and some heavy metals. Concentrations of P and K exceeded DW regulation limits in some instances. However, small volumes and rare occurrence of OLF indicated no risk to nearby surface waters. The practice of OB amendment resulted in little risk to surface waters at these scales. If scaled up to plantation scale, in areas vulnerable to OLF, careful consideration must be made about P-loss (on a catchment scale) before amendments carried out.

Seasonal Surface Drainage of Sloping Farmland: A Review of Its Hydrogeomorphic Impacts

AbstractThe combination of runoff‐generating areas (saturated soils) and overland flow concentration in features such as drainage ditches makes sloping farmland vulnerable to soil erosion. The establishment of drainage ditches aims at draining the excess of water from the farmland, particularly in areas where soils are saturated in the rainy season. The hydrogeomorphic impacts on the farmland itself and on downstream areas need however also to be studied. Off site, downstream problems comprise higher peak discharges, leading to gully initiation, an increase in sediment load, and flooding problems. On‐site problems such as the development of the drainage ditches into (ephemeral) gullies are less documented, although they may be important, as illustrated in the Lake Tana Basin (Ethiopia). The similarities and interactions between ephemeral gully channels and drainage ditches have to be considered to better understand all effects of drainage. Drainage ditches are a potential source of conflict between farmers with different interests and power, as well as between upstream and downstream users. A case study on drainage ditches on sloping farmlands in the Lake Tana Basin showed that nine out of ten catchments had drainage densities by ditches ranging from 53 to 510 m ha−1. Drainage ditches were constructed with an average top width of 27 (±9) cm. A significant correlation was found between stone bund density (physical conservation structures) and ditch drainage density (R = −0·72), in line with the Ethiopian government's ban on drainage ditches in farmlands where stone bunds have been constructed. Copyright © 2014 John Wiley &amp; Sons, Ltd.