Overland Flow Rates Research Articles

Characteristics of the Sediment Transport Process in Vegetation Hillslopes under Different Flow Rates

Vegetation filter strips (VFSs) have always been an important measure to control agricultural soil erosion, especially in mountainous and hilly areas with more sloping farmland. To investigate the mechanism of the sediment-trapping process by VFSs, a series of tests were conducted with four gradients of flow rate, 7.5–45 L min−1 m−1, and two different sediment concentrations of 40 and 120 g L−1. The whole process of overland flow was monitored, and sediment and particle size samples from the inflow and outflow were collected and measured. The results showed that the changes in sediment concentration did not significantly affect the corresponding coefficients in the power function relationship between overland flow rate and velocity. Using the Reynolds number alone cannot effectively indicate the flow pattern of overland flow on vegetation hillslopes. The peak particle size and linear function were effective in describing the relationship between sediment particle composition and delivery rate during the sediment-trapping process by VFSs. During the sediment-trapping process, the sediment-trapping capacity of VFSs continued to decrease. The increase in sediment discharge was accompanied by a higher proportion of coarse particles. Under the same flow rate conditions, when the sediment concentration was higher, the coarse particles and their proportion also increased faster. Therefore, using only a certain particle size threshold to distinguish suspended and transported sediment may lead to inaccurate estimation of the sediment-trapping performance of VFSs. This study deepened the understanding of the mechanism of water–sediment processes on vegetation hillslopes and promoted the widespread and efficient application of VFSs management technology.

Effects of shrub-grass cover on the hillslope overland flow and soil erosion under simulated rainfall

Vegetation plays a vital role in regulating hydrological cycle and controlling soil erosion at multiple spatial and temporal scales. Establishing shrub-grass community is one of the widely adopted practices to increase rainfall infiltration and reduce soil erosion in water-limited and highland regions. To understand the effects of such vegetation communities on soil erosion and overland flow under different rainfall regimes at the hillslope scale, we conducted rainfall simulation experiments by setting up parallel plots at fixed slope of 15° including unvegetated (coverage 0%), shrub only (coverage 50%), grass only (coverage 50%), and shrub-grass covered (coverages 25%, 50%, 75%, and 100%) and constant rainfall intensities of 30, 60, and 90 mm h−1 rainfalls lasting 60 min each after the initiation of overland flow. Two native species Lespedeza bicolor and Carex giraldiana, distributed in the soil sampling region were planted on the plots to achieve designed coverages. We found that the overland flow and sediment load from vegetated slopes were reduced by 9%–58% and 27%–98%, respectively, compared with unvegetated slopes while the infiltration rate increased by over 45%. Shrub-grass community reduced the overland flow and sediment yield more significantly than shrub only and grass only treatments with the same coverage of 50% under three rainfall intensities. In addition, the overland flow rate linearly decreased while the mean sediment yield exponentially reduced against the increase in shrub-grass community coverage. Hydrodynamically, shrub-grass communities not only increased the critical hydrodynamic forces for the initiating soil erosion but also increased the resistance coefficient leading to reduce overland flow velocity, stream power, and thus soil erosion from the vegetative slope even under extreme rainfalls. Our research highlights the importance of developing the shrub-grass communities to reduce the quantity and energy of overland flow and control soil erosion on the hillslopes in water-limited and highland regions.

A modified wash-off function for stormwater suspended solids modelling

In urban areas, the classical exponential wash-off equation for modelling of stormwater suspended solids (SS) relates the washoff of solids to the overland flow rate, assuming that runoff is proportional to rainfall. However, in catchments where infiltration occurs through the pavement surface, this assumption may not be valid. Furthermore, many researchers have concluded that the effect of rain impaction, related to rain intensity, should be considered as a separate model parameter to runoff rate. To address this problem, a semi-deterministic stormwater quality model was developed which incorporated rain intensity directly in the exponential wash-off equation. The modified function was compared to the more classical wash-off equation using a dataset of 18 storm events sampled from a separate pipe and gully system in Dublin, Ireland. The modified wash-off equation showed a consistent improvement in the fit of observed SS fluxes at the outlet of the drainage system: the Nash-Sutcliffe index of modelled performance increased from 0.39 to 0.58 on average using the modified wash-off function. The form of the modified wash-off function allows for simple integration into many stormwater quality models currently in use, without the need for any additional model parameters.

Runoff reaction from extreme rainfall events on natural hillslopes: a data set from 132 large-scale sprinkling experiments in south-western Germany

Abstract. Pluvial or flash floods generated by heavy precipitation events cause large economic damage and loss of life worldwide. As discharge observations from such extreme occurrences are rare, especially on the scale of small catchments or even hillslopes, data from artificial sprinkling experiments offer valuable information on runoff generation processes, overland and subsurface flow rates, and response times. We conducted 132 large-scale sprinkling experiments on natural hillslopes at 23 sites with different soil types and geology on pastures and arable land within the federal state of Baden-Württemberg in south-western Germany. The experiments were realized between 2016 and 2017. Simulated rainfall events of varying durations were based on (a) the site-specific 100-year return periods of rainfall with different durations and (b) the maximum rainfall intensity observed locally. The 100 m2 experimental area was divided into three individual plots, and overland and subsurface flow, soil moisture, and water level dynamics in the temporarily saturated soil zone were measured at 1 min resolution. Furthermore, soil characteristics were described in detail for each site. The data were carefully processed and corrected for measurement errors and combined into a consistent and easy-to-use database. The experiments revealed large variability in possible runoff responses to similar rainfall characteristics. In general, agricultural fields produced more overland flow than grassland. The latter generated hardly any runoff during the first simulated 100-year event on initially dry soils. The data set provides valuable information on runoff generation variability from natural hillslopes and may be used for the development and evaluation of hydrological models, especially those considering physical processes governing runoff generation during extreme precipitation events. The data set presented in this paper is freely available from the FreiDok plus data repository at https://doi.org/10.6094/UNIFR/151460 (Ries et al., 2019).

Overland runoff erosion dynamics on steep slopes with forages under field simulated rainfall and inflow

AbstractAlthough numerous studies have acknowledged that vegetation can reduce erosion, few process‐based studies have examined how vegetation cover affect runoff hydraulics and erosion processes. We present field observations of overland flow hydraulics using rainfall simulations in a typical semiarid area in China. Field plots (5 × 2 m2) were constructed on a loess hillslope (25°), including bare soil plot as control and three plots with planted forage species as treatments—Astragalus adsurgens, Medicago sativa and Cosmos bipinnatus. Both simulated rainfall and simulated rainfall + inflow were applied. Forages reduced soil loss by 55–85% and decreased overland flow rate by 12–37%. Forages significantly increased flow hydraulic resistance expressed by Darcy–Weisbach friction factor by 188–202% and expressed by Manning's friction factor by 66–75%; and decreased overland flow velocity by 28–30%. The upslope inflow significantly increased overland flow velocity by 67% and stream power by 449%, resulting in increased sediment yield rate by 108%. Erosion rate exhibited a significant linear relationship with stream power. M. sativa exhibited the best in reducing soil loss which probably resulted from its role in reducing stream power. Forages on the downslope performed better at reducing sediment yield than upslope due to decreased rill formation and stream power. The findings contribute to an improved understanding of using vegetation to control water and soil loss and land degradation in semiarid environments.

Effects of Needle Ice on Peat Erosion Processes During Overland Flow Events

AbstractFreeze‐thaw processes play a role in increasing erosion potential in upland areas, but their impact on overland flow hydraulics and fluvial erosion processes are not clearly established. We provide the first quantitative analysis demonstrating that needle ice production is a primary process contributing to upland peat erosion by enhancing peat erodibility during runoff events following thaw. To quantify the effects of needle ice on peat physical properties, overland flow hydraulics, and erosion processes, physical overland flow simulation experiments were conducted on highly frost‐susceptible blanket peat with and without needle ice processes. For each treatment, overland flow rates of 0.5, 1.0, and 2.0 L/min and slopes of 2.5° and 7.5° were applied. Peat erodibility, sediment concentration, and sediment yield were significantly increased in treatments subjected to needle ice processes. Median peat losses were nearly 6 times higher in peat blocks subject to needle ice processes than in peat blocks not subject to needle ice processes. Needle ice processes decreased mean overland flow velocities by 32–44% via increased hydraulic roughness and changes to surface microtopographic features, with microrills and headcut development. Needle ice processes increased the hydrodynamic force of shear stress by 55–85%. Erosion rates under needle ice processes exhibited a significant linear relationship with stream power. Our findings indicate that models of overland flow‐induced peat erosion would benefit from a winter component that properly accounts for the effects of needle ice processes on peat erodibility and erosion.

Impacts of changing hydrology on permanent gully growth: experimental results

Abstract. Permanent gullies grow through head cut propagation in response to overland flow coupled with incision and widening in the channel bottom leading to hillslope failures. Altered hydrology can impact the rate at which permanent gullies grow by changing head cut propagation, channel incision, and channel widening rates. Using a set of small physical experiments, we tested how changing overland flow rates and flow volumes alter the total volume of erosion and resulting gully morphology. Permanent gullies were modeled as both detachment-limited and transport-limited systems, using two different substrates with varying cohesion. In both cases, the erosion rate varied linearly with water discharge, such that the volume of sediment eroded was a function not of flow rate, but of total water volume. This implies that efforts to reduce peak flow rates alone without addressing flow volumes entering gully systems may not reduce erosion. The documented response in these experiments is not typical when compared to larger preexisting channels where higher flow rates result in greater erosion through nonlinear relationships between water discharge and sediment discharge. Permanent gullies do not respond like preexisting channels because channel slope remains a free parameter and can adjust relatively quickly in response to changing flows.

A simulation of rill bed incision processes in upland concentrated flows

Quantifying rill bed incision provides fundamental information for process-based erosion modeling; while the morphodynamic and hydrodynamic mechanism in bed incision processes are still unclear. Thus, experiments were conducted to examine rill bed incision processes in upland concentrated flows. DEMs (2 mm × 2 mm resolution) obtained by photogrammetry were used for rill bed morphology analysis. Rill channel (2.0 m-long, 0.08 m-wide and 0.15 m-deep) with two slope gradients (15° and 20°) were subjected to four overland flow rates (1.0, 2.0, 3.0 and 4.0 L min−1). The results showed that sediment delivery, rill bed incision rate and average rill depth increased with inflow rate and bed slope. Sediment delivery increased from 0.060 to 0.226 kg min−1 per 1 L min−1 inflow increment and from 0.043 to 0.207 kg min−1 when bed slope increased from 15° to 20°. In a well-developed rill channel, rill bed incision could be divided into three phases: pre-headcut formation (dominated by rill flow shear stress), headcut incision (dominated by headcut advancing) and post-headcut incision (dominated by rill flow shear stress). Headcut incision phase, which only accounted for <15% of total experimental time, produced >65% of rill bed sediment. In the pre-headcut formation phase, rill flow velocity, shear stress and stream power increased with increases of inflow rate and slope gradient. Conversely, flow velocity showed no evident trend with increased inflow rate and bed slope during headcut incision phase. Initial headcut advancing rate could be predicted by a non-linear function based upon soil characteristics, rill flow shear stress and headcut height. Sediment delivery showed a power function with the product of inflow rate and squared bed slope. Because rill bed incision is dominated by headcut advancement and incision, practices for controlling headcut initiation should be implemented to decrease hillslope soil loss.

The relative importance of different grass components in controlling runoff and erosion on a hillslope under simulated rainfall

The effects of vegetation cover on overland flow and erosion processes on hillslopes vary with vegetation type and spatial distribution and the different vegetation components, including the above- and below-ground biomass. However, few attempts have been made to quantify how these factors affect erosion processes. Field experimental plots (5 m × 2 m) with a slope of approximately 25° were constructed and simulated rainfall (60 mm hr−1) (Rainfall) and simulated rainfall combined with upslope overland flow (20 L min−1) (Rainfall + Flow) were applied. Three grass species were planted, specifically Astragalus adsurgens (A. adsurgens), Medicago sativa (M. sativa) and Cosmos bipinnatus (C. bipinnatus). To isolate and quantify the relative contributions of the above-ground grass parts (stems, litter cover and leaves) and the roots to reducing surface runoff and erosion, each of the three grass species was subjected to three treatments: intact grass control (IG), no litter or leaves (only the grass stems and roots were reserved) (NLL), and only roots remaining (OR). The results showed that planting grass significantly reduced overland flow rate and velocity and sediment yield, and the mean reductions were 21.8%, 29.1% and 67.1%, respectively. M. sativa performed the best in controlling water and soil losses due to its thick canopy and dense, fine roots. Grasses reduced soil erosion mainly during the early stage of overland flow generation. The above-ground grass parts primarily contributed to reducing overland flow rate and velocity, with mean relative contributions of 64% and 86%, respectively. The roots played a predominant role in reducing soil erosion, with mean contribution of 84%. Due to the impact of upslope inflow, overland flow rate and velocity and sediment yield increased under the Rainfall + Flow conditions. The results suggest that grass species on downslope parts of semi-arid hillslopes performed better in reducing water and soil losses. This study is beneficial for forage selection, allocation and management practices, such as forage harvesting, when implementing restoration strategies to control soil and water losses.

Slope–velocity equilibrium and evolution of surface roughness on a stony hillslope

Abstract. Slope–velocity equilibrium is hypothesized as a state that evolves naturally over time due to the interaction between overland flow and surface morphology, wherein steeper areas develop a relative increase in physical and hydraulic roughness such that flow velocity is a unique function of overland flow rate independent of slope gradient. This study tests this hypothesis under controlled conditions. Artificial rainfall was applied to 2 m by 6 m plots at 5, 12, and 20 % slope gradients. A series of simulations were made with two replications for each treatment with measurements of runoff rate, velocity, rock cover, and surface roughness. Velocities measured at the end of each experiment were a unique function of discharge rates, independent of slope gradient or rainfall intensity. Physical surface roughness was greater at steeper slopes. The data clearly showed that there was no unique hydraulic coefficient for a given slope, surface condition, or rainfall rate, with hydraulic roughness greater at steeper slopes and lower intensities. This study supports the hypothesis of slope–velocity equilibrium, implying that use of hydraulic equations, such as Chezy and Manning, in hillslope-scale runoff models is problematic because the coefficients vary with both slope and rainfall intensity.

Flow-driven soil erosion processes and the size selectivity of eroded sediment on steep slopes using colluvial deposits in a permanent gully

Colluvial deposits with loose, coarse material are easily erodible in permanent gullies, but the mechanisms of erosion and sedimentation during overland flow remain obscure. Hence, the processes and mechanisms of the transportation of soil particles by overland flow were investigated in this study. Experiments were carried out in a 5.0m long by 1.0m wide flume using colluvial deposits. The slope gradient varied from 36 to 84%, and the flow rate ranged from 0.72Lm−2min−1 to 2.88Lm−2min−1. The runoff rate and sediment yield rapidly increased with increasing overland duration. Runoff and sediment were highly variable when the flume was treated with a high flow rate compared with a low flow rate, with the fluctuation of sediment concentration under the high flow rate usually reaching 500gL−1. The slope gradient and overland flow rate have strong impacts on sediment transport capacity. The mean flow velocity and the unit stream power can be an optimal composite force predictor for estimating sediment transport capacity. Experimental results also revealed that the percentage of gravel-sized particles increased with increasing flow rate and slope gradient, but silt and clay fractions observed opposite trend. The average enrichment ratio (ER) of gravel was usually <1. However, the ER of silt and clay fractions decreased following the increase of the flow rate and slope gradient, and the values stabilized in the range of 1.1 to 1.4. The predicted values of runoff, sediment yield, and sediment concentrations were simulated by an empirical equation with measured values, and the regression coefficients were 0.983, 0.996 and 0.877, respectively. When the flow rate was >2.16Lm−2min−1, the bed load transport became an important mechanism; however, the simulation model overestimated these values.

Understanding soil erosion processes in Mediterranean sloping vineyards (Montes de Málaga, Spain)

Sloping vineyards in the Mediterranean cultivated on bare soils show several types of evidence of soil erosion processes. However, little is known about the key factors that condition and enhance these processes at the intra-plot scale. There is a need to assess soil conservation methods to reach sustainability of vineyards and high grape quality, and for this it is necessary to investigate the factors and rates of soil erosion processes under natural conditions. The main goal of this research, conducted in traditional Mediterranean vineyards in Los Montes de Málaga (South Spain), was to carry out a precision analysis of the patterns of soil erosion and the soil surface components at the intra-plot scale. The analysis was performed after monitoring soil erosion processes during 25 natural rainfall events. Soil loss, overland flow, and runoff threshold were calculated using six Gerlach troughs. Fine soil particles and rock fragments were also assessed after each natural rainfall event and tillage practice. The research showed an example of sloping vineyards in a Mediterranean environment with bare soils that are associated with high soil losses and an uneven spatiotemporal distribution of hydrological and geomorphological processes. Key factors enhancing soil erosion processes are: i) extreme rainfall events and ii) management practices (pruning, ploughing and trampling). The runoff threshold reached very low values (between 4.55mm and 8.5mm) and the highest soil loss and overland flow rates were obtained from a few rainfall events during the rainiest period, coinciding with times that the surface was covered by vines that had dropped their leaf cover. Finally, we demonstrated that the runoff threshold, soil loss and overland flow showed high variability depending on the stoniness, soil texture, and antecedent conditions of tillage.

Runoff Water Quality Characteristics Following Swine Slurry Application under Broadcast and Injected Conditions

This study was conducted to measure the effects of swine slurry application method, time following slurry application, and runoff rate on selected water quality characteristics. Slurry from a commercial swine operation was broadcast or injected on field plots at a rate required to meet annual nitrogen requirements for corn. Rainfall simulation tests were conducted at five varying periods following slurry application. During each study period, three simulated rainfall events, separated by 24 h intervals, were applied for 30 min duration at an intensity of approximately 70 mm h-1. Following the third rainfall simulation event, inflow was applied at the top of the plots in four successive increments to simulate greater plot lengths. Runoff samples were collected for analyses of dissolved P (DP), particulate P, total P (TP), NO3-N, NH4-N, total N, electrical conductivity, pH, and soil loss. The DP and TP loads of 0.35 and 0.46 kg ha-1 measured for the broadcast treatment were significantly greater than the 0.13 and 0.19 kg ha-1 obtained for the injection treatment. As time following slurry application increased from 1 to 3 days to 43 to 45 days, DP, TP, and NH4-N loads decreased from 0.35 to 0.14, from 0.52 to 0.18, and from 2.17 to 0.14 kg ha-1, respectively. Runoff rate significantly affected each of the measured water quality parameters. Runoff loads of DP, TP, and NH4-N increased from 10.1 to 29.8, from 12.9 to 35.5, and from 13.9 to 25.1 g ha-1 min-1, respectively, as overland flow rate increased from 2.3 to 12.6 L min-1. Application method, time following slurry application, and runoff rate are important variables influencing water quality characteristics of runoff.

Rill erosion in burned and salvage logged western montane forests: Effects of logging equipment type, traffic level, and slash treatment

Following wildfires, forest managers often consider salvage logging burned trees to recover monetary value of timber, reduce fuel loads, or to meet other objectives. Relatively little is known about the cumulative hydrologic effects of wildfire and subsequent timber harvest using logging equipment. We used controlled rill experiments in logged and unlogged (control) forests burned at high severity in northern Montana, eastern Washington, and southern British Columbia to quantify rill overland flow and sediment production rates (fluxes) after ground-based salvage logging. We tested different types of logging equipment—feller-bunchers, tracked and wheeled skidders, and wheeled forwarders—as well as traffic levels and the addition of slash to skid trails as a best management practice. Rill experiments were done at each location in the first year after the fire and repeated in subsequent years. Logging was completed in the first or second post-fire year. We found that ground-based logging using heavy equipment compacted soil, reduced soil water repellency, and reduced vegetation cover. Vegetation recovery rates were slower in most logged areas than the controls. Runoff rates were higher in the skidder and forwarder plots than their respective controls in the Montana and Washington sites in the year that logging occurred, and the difference in runoff between the skidder and control plots at the British Columbia site was nearly significant (p=0.089). Most of the significant increases in runoff in the logged plots persisted for subsequent years. The type of skidder, the addition of slash, and the amount of forwarder traffic did not significantly affect the runoff rates. Across the three sites, rill sediment fluxes were 5–1900% greater in logged plots than the controls in the year of logging, and the increases were significant for all logging treatments except the low use forwarder trails. There was no difference in the first-year sediment fluxes between the feller-buncher and tracked skidder plots, but the feller-buncher fluxes were lower than the values from the wheeled skidder plots. Manually adding slash after logging did not affect sediment flux rates. There were no significant changes in the control sediment fluxes over time, and none of the logging equipment impacted plots produced greater sediment fluxes than the controls in the second or third year after logging. Our results indicate that salvage logging increases the risk of sedimentation regardless of equipment type and amount of traffic, and that specific best management practices are needed to mitigate the hydrologic impacts of post-fire salvage logging.

Surface Runoff in Watershed Modeling—Turbulent or Laminar Flows?

Determination of overland sheet flow depths, velocities and celerities across the hillslope in watershed modeling is important towards estimation of surface storage, travel times to streams and soil detachment rates. It requires careful characterization of the flow processes. Similarly, determination of the temporal variation of hillslope-riparian-stream hydrologic connectivity requires estimation of the shallow subsurface soil hydraulic conductivity and soil-water retention (i.e., drainable porosities) parameters. Field rainfall and runoff simulation studies provide considerable information and insight into these processes; in particular, that sheet flows are likely laminar and that shallow hydraulic conductivities and storage can be determined from the plot studies. Here, using a 1 m by 2 m long runoff simulation flume, we found that for overland flow rates per unit width of roughly 30–60 mm2/s and bedslopes of 10%–66% with varying sand roughness depths that all flow depths were predicted by laminar flow equations alone and that equivalent Manning’s n values were depth dependent and quite small relative to those used in watershed modeling studies. Even for overland flow rates greater than those typically measured or modeled and using Manning’s n values of 0.30–0.35, often assumed in physical watershed model applications for relatively smooth surface conditions, the laminar flow velocities were 4–5 times greater, while the laminar flow depths were 4–5 times smaller. This observation suggests that travel times, surface storage volumes and surface shear stresses associated with erosion across the landscape would be poorly predicted using turbulent flow assumptions. Filling the flume with fine sand and conducting runoff studies, we were unable to produce sheet flow, but found that subsurface flows were onflow rate, soil depth and slope dependent and drainable porosities were only soil depth and slope dependent. Moreover, both the sand hydraulic conductivity and drainable porosities could be readily determined from measured capillary pressure displacement pressure head and assumption of pore-size distributions (i.e., Brooks-Corey lambda values of 2–3).

Response of a soil‐mantled experimental landscape to exogenic forcing

Soil erosion can severely degrade landscapes, and concentrated flows such as rills and gullies can be the dominant contributor to the soil losses. This paper examines the growth, development, and spatiotemporal evolution of rills and rill networks using a soil‐mantled experimental landscape subjected to simulated rain and downstream base level lowering. Rill incision and network development and extension occurred due to actively migrating headcuts formed at the flume outlet by base level lowering. The communication of this wave of degradation due to this exogenic forcing occurred very quickly in space, and resulted in nearly the same amount of bed incision throughout the network. Rill incision, channel development, and peaks in sediment efflux occurred episodically, yet these were in direct response to the downstream base level adjustments. Although flows were supply limited, most of the sediment efflux was genetically linked to headcut development and migration. The geometry of the eroded rills and the rates of headcut migration were well correlated to overland flow rate. These findings have important implications for the prediction of soil loss, rill network development, and landscape evolution where headcut erosion can occur.

ブナ林における林床合計被覆率の変化が地表流流出率に与える影響

神奈川県東丹沢堂平地区のブナ林では, シカの採食圧により林床植生が衰退し, 土壌侵食が広範囲にわたり進行しており, 森林および渓流の環境に大きな影響を与えている。既往の研究により堂平地区においては土壌侵食の中で布状侵食の占める割合は雨滴侵食よりも大きく, 流出土砂は主として地表流により生産されていることが報告されている。本研究では2004∼2009年の間, 堂平地区のブナ林斜面において幅2 m, 長さ5 mの林床植生被覆率の異なる試験プロット4個を設置し, 樹冠通過雨量, 地表流流出量, 林床合計被覆率 (林床植生被覆率+リター被覆率), 土壌侵食量等を測定し, ブナ林斜面における林床合計被覆率が地表流流出率に与える影響について検討した。その結果, 林床合計被覆率 (Cm) と地表流流出率 (Rem) にはRem=−18.9 ln (Cm) + 86.4で表される高い負の相関があることがわかった。このため, 地表流流出率は季節により大きく変化し, リター堆積量が少ない夏季 (7∼9月) はリター堆積量が多い春季 (4∼6月) や秋季 (10∼11月) に比べて地表流流出率が増加することが明らかになった。

Wheat Strip Effects on Nutrient Loads Following Variable Manure Applications

Vegetative filters have been found to significantly reduce nutrient loads in runoff. This study was conducted to: (1) evaluate the effects of a narrow wheat strip, varying manure application rates, and different overland flow rates on runoff nutrient loads following application of beef cattle manure; (2) determine the upper capacity of a narrow wheat strip to reduce nutrient loads by applying excessive amounts of manure; and (3) compare the effectiveness of narrow wheat strips and grass hedges in reducing runoff nutrient loads. A 1.4 m wide strip of actively growing winter wheat was located at the bottom of selected 0.75 m wide by 4.0 m long plots. Three 30 min simulated rainfall events, separated by 24 h intervals, were applied at an intensity of 70 mm h-1 to the plots. The wheat strips were effective in reducing runoff loads of NO3-N, NH4-N, and total nitrogen (TN). Runoff loads of dissolved reactive phosphorus (DP), particulate phosphorus (PP), total phosphorus (TP), NH4-N, and TN were significantly influenced by manure application rate. The application of manure to meet a 2-year rather than a 1-year corn P requirement did not significantly increase DP, PP, or TP loads. However, application of manure to meet a 4-year P requirement resulted in DP, PP, and TP loads that were significantly greater than those obtained for a 2-year P requirement. Runoff rate significantly affected each of the measured water quality parameters. The actively growing wheat strips were much less effective than grass hedges in reducing runoff nutrient loads.

The effect of ant mounds on overland flow and soil erodibility following a wildfire in eastern Spain

AbstractThis study examines the soil hydrological and erosional effects of ant mounds during summer and winter conditions following a wildfire in scrub terrain in eastern Spain. Forty rainfall simulations (1 m2 plots, 1 h duration, 78 mm h−1 intensity) were carried out over plots with mounds (n = 20) and mound‐free control plots (n = 20) in August 2002, and repeated in December. By winter, some of the mound material had been removed and some vegetation regrowth occurred. Overall, mound presence increased soil erodibility in summer and winter due to the availability of highly erodible mound material. However, mound plots showed higher mean overland flow rates in summer (10·1 vs 6·9% for control plots) but lower rates in winter (59·3 vs 74·4% for control plots). This reversal is suggested to be caused by (i) post‐fire changes in ground cover and (ii) seasonal differences in antecedent soil moisture. First, immediately after burning, an absorbent ash layer covered much of the soil surface, but this was partially masked by less permeable mound material, reducing infiltration on mound plots. By winter, the ash layer had been largely removed and ash particles clogged soil surface pores, reducing overall infiltration on control plots compared to mound plots. Second, during summer, the dry soil underneath the absorbent ash cover has a high infiltration capacity. In winter, soils are near saturation, resulting in a much greater runoff coefficient. Under these wet soil conditions, the presence of macropores (i.e. nest entrances) becomes important, reducing overland flow in mound plots. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Using beryllium‐7 to monitor the relative proportions of interrill and rill erosion from loessal soil slopes in a single rainfall event

AbstractQuantifying the relative proportions of soil losses due to interrill and rill erosion processes during erosion events is an important factor in predicting total soil losses and sediment transport and deposition. Beryllium‐7 (7Be) can provide a convenient way to trace sediment movement over short timescales providing information that can potentially be applied to longer‐term, larger‐scale erosion processes. We used simulated rainstorms to generate soil erosion from two experimental plots (5 m × 4 m; 25° slope) containing a bare, hand‐cultivated loessal soil, and measured 7Be activities to identify the erosion processes contributing to eroded material movement and/or deposition in a flat area at the foot of the slope. Based on the mass balance of 7Be detected in the eroded soil source and in the sediments, the proportions of material from interrill and rill erosion processes were estimated in the total soil losses, the deposited sediments in the flat area, and in the suspended sediments discharged from the plots. The proportion of interrill eroded material in the discharged sediment decreased over time as that of rill eroded material increased. The amount of deposited material was greatly affected by overland flow rates. The estimated amounts of rill eroded material calculated using 7Be activities were in good agreement with those based on physical measurements of total plot rill volumes. Although time lags of 45 and 11 minutes existed between detection of sediment being removed by rill erosion, based on 7Be activities, and observed rill initiation times, our results suggest that the use of 7Be tracer has the potential to accurately quantify the processes of erosion from bare, loessal cultivated slopes and of deposition in flatter, downslope areas that occur in single rainfall events. Such measurements could be applied to estimate longer‐term erosion occurring over larger areas possessing similar landforms. Copyright © 2010 John Wiley &amp; Sons, Ltd.